In machinery the ruling form is cylindrical; in structures other than machinery, those which do not involve motion, the ruling form is rectangular.
Machine motion is mainly rotary; and as rotary motion is accomplished by cylindrical parts such as shafts, bearings, pulleys and wheels, we find that the greater share of machine tools are directed to preparing cylindrical forms. If we note the area of the turned, bored and drilled surface in ordinary machinery, and compare with the amount of planed surface, we will find the former not less than as two to one in the finer class of machinery, and as three to one in the coarser class; from this may be estimated approximately the proportion of tools required for operating on cylindrical surfaces and plane surfaces; assuming the cutting tools to have the same capacity in the two cases, the proportion will be as three to one. This difference between the number of machines required for cylindrical and plane surfaces is farther increased, when we consider that tools act continually on cylindrical surfaces and intermittently on plane surfaces.
In practice, the truth of this proposition is fully demonstrated by the excess in the number of lathes and boring tools compared with those for planing.
An engine lathe is for many reasons called the master tool in machine fitting. It is not only the leading tool so far as performing a greater share of the work; but an engine lathe as an organised machine combines, perhaps, a greater number of useful and important functions, than any machine which has ever been devised. A lathe may be employed to turn, bore, drill, mill, or cut screws, and with a strong screw-feed may be employed to some extent for planing; what is still more strange, notwithstanding these various functions, a lathe is comparatively a simple machine without complication or perishable parts, and requires no considerable change in adapting it to the various purposes named.
For milling, drilling or boring ordinary work within its range, a lathe is by no means a makeshift tool, but performs these various operations with nearly all the advantages of machines adapted to each purpose. An ingenious workman who understands the adaptation of a modern engine lathe can make almost any kind of light machinery without other tools, except for planing, and may even perform planing when the surfaces are not too large; in this way machinery can be made at an expense not much greater than if a full equipment of different tools is employed. This of course can only be when no division of labour is required, and when one man is to perform all the several processes of turning, drilling, and so on.
The lathe as a tool for producing heliacal forms would occupy a prominent place among machine tools, if it were capable of performing no other work; the number of parts of machinery which have screw-threads is astonishing; clamping-bolts to hold parts together include a large share of the fitting on machinery of all kinds, while screws are the most common means for increasing power, changing movements and performing adjustments.
A finisher's engine lathe consists essentially of a strong inflexible shear or frame, a running spindle with from eight to sixteen changes of motion, a sliding head, or tail stock, and a sliding carriage to hold and move the tools.
For a half century past no considerable change has been made in engine lathes, at least no new principle of operation has been added, but many improvements have been made in their adaptation and capacity for special kinds of work. Improvements have been made in the facilities for changing wheels in screw cutting and feeding, by frictional starting gear for the carriages, an independent feed movement for turning, arrangements to adjust tools, cross feeding and so on, adding something, no doubt, to the efficiency of lathes; but the improvements named have been mainly directed to supplanting the skill of lathemen.
A proof of this last proposition is found in the fact that a thorough latheman will perform nearly as much work and do it as well on an old English lathe with plain screw feed, as can be performed on the more complicated lathes of modern construction; but as economy of skill is sometimes an equal or greater object than a saving of manual labour, estimates of tool capacity should be made accordingly. The main points of a lathe, such as may most readily affect its performance, are first—truth in the bearings of the running spindle which communicates a duplicate of its shape to pieces that are turned,—second, coincidence between the line of the spindle and the movement of the carriage,—third, a cross feed of the tool at a true right angle to the spindle and carriage movement,—fourth, durability of wearing surfaces, especially the spindle bearings and sliding ways. To these may be added many other points, such as the truth of feeding screws, rigidity of frames, and so on, but such requirements are obvious.
To avoid imperfection in the running spindles of lathes, or any lateral movement which might exist in the running bearings, there have been many attempts to construct lathes with still centres at both ends for the more accurate kinds of work. Such an arrangement would produce a true cylindrical rotation, but must at the same time involve mechanical complication to outweigh the object gained. It has besides been proved by practice that good fitting and good material for the bearings and spindles of lathes will insure all the accuracy which ordinary work demands.
It may be noticed that the carriages of some lathes move on what are termed V tracks which project above the top of lathe frames, and that in other lathes the carriages slide on top of the frames with a flat bearing. As these two plans of mounting lathe carriages have led to considerable discussion on the part of engineers, and as its consideration may suggest a plan of analysing other problems of a similar nature, I will notice some of the conditions existing in the two cases, calling the different arrangements by the names of flat shears and track shears.
These different plans will be considered first in reference to the effect produced upon the movement of carriages; this includes friction, endurance of wear, rigidity of tools, convenience of operating and the cost of construction. The cutting point in both turning and boring on a slide lathe is at the side of a piece, or nearly level with the lathe centres, and any movement of a carriage horizontally across the lathe affects the motion of the tool and the shape of the piece acted upon, directly to the extent of such deviation, so that parallel turning and boring depend mainly upon avoiding any cross movement or side play of a carriage. This, in both theory and practice, constitutes the greatest difference between flat top and track shears; the first is arranged especially to resist deviation in a vertical plane, which is of secondary importance, except in boring with a bar; the second is arranged to resist horizontal deviation, which in nine-tenths of the work done on lathes becomes an exact measure of the inaccuracy of the work performed.
A true movement of carriages is dependent upon the amount or wearing power of their bearing surface, how this surface is disposed in reference to the strain to be resisted, and the conditions under which the sliding surfaces move; that is, how kept in contact. The cutting strain which is to be mainly considered, falls usually at an angle of thirty to forty degrees downward toward the front, from the centre of the lathe. To resist such strain a flat top shear presents no surface at right angles to the strain; the bearings are all oblique, and not only this, but all horizontal strain falls on one side of the shear only; for this reason, flat top shears have to be made much heavier than would be required if the sum of their cross section could be employed to resist transverse strain. This difficulty can, however, be mainly obviated by numerous cross girts, which will be found in most lathe frames having flat tops.
A carriage moving on angular ways always moves steadily and easily, without play in any direction until lifted from its bearing, which rarely happens, and its lifting is easily opposed by adjustable gibs. A carriage on a flat shear is apt to have play in a horizontal direction because of the freedom which must exist to secure easy movement. In the case of tracks, it may also be mentioned that the weight of a carriage acts as a constant force to hold it steady, while with a flat shear the weight of a carriage is in a sense opposed to the ways, and has no useful effect in steadying or guiding. The rigidity and steadiness of tool movement is notoriously in favour of triangular tracks, so much so that nearly all American machine tool-makers construct lathes in this manner, although it adds no inconsiderable cost in fitting.
It may also be mentioned that lathes constructed with angular guides, have usually such ways for the moving heads as well as for the carriages; this gives the advantage of firmly binding the two sides of the frame together in fastening the moving head, which in effect becomes a strong girt across the frame; the carriages also have an equal and independent hold on both sides of a shear. In following this matter thus far, it may be seen how many conditions may have to be considered in reasoning about so apparently simple a matter as the form of ways for lathe carriages; we might even go on to many more points that have not been mentioned; but what has been explained will serve to show that the matter is not one of opinion alone, and that without practical advantages, machine tool-makers will not follow the most expensive of these two modes of mounting lathe carriages.
Lathes in common use for machine fitting are screw-cutting engine lathes, lathes for turning only, double-geared, single-geared, and back-geared lathes, lathes for boring, hand-lathes, and pulley-turning lathes; also compound lathes with double heads and two tool carriages.
These various lathes, although of a widely varied construction and adapted to uses more or less dissimilar, are still the engine lathe either with some of its functions omitted to simplify and adapt it to some special work, or with some of the operative parts compounded to attain greater capacity.
In respect to lathe manipulation, which is perhaps the most difficult to learn of all shop operations, the following hints are given, which may prove of service to a learner: At the beginning the form of tools should be carefully studied; this is one of the great points in lathe work; the greatest distinction between a thorough and indifferent latheman is that one knows the proper form and temper of tools and the other does not. The adjustment and presenting of tools is soon learned by experience, but the proper form of tools is a matter of greater difficulty. One of the first things to study is the shape of cutting edges, both as to clearance below the edge of the tool, and the angle of the edge, with reference to both turning and boring, because the latter is different from turning. The angle of lathe tools is clearly suggested by diagrams, and there is no better first lesson in drawing than to construct diagrams of cutting angles for plane and cylindrical surfaces.
A set of lathe tools should consist of all that are required for every variety of work performed, so that no time will be lost by waiting to prepare tools after they are wanted. An ordinary engine lathe, operating on common work not exceeding twenty inches of diameter, will require from twenty-five to thirty-five tools, which will serve for every purpose if they are kept in order and in place. A workman may get along with ten tools or even less, but not to his own satisfaction, nor in a speedy way. Each tool should be properly tempered and ground, ready for use 'when put away;' if a tool is broken, it should at once be repaired, no matter when it is likely to be again used. A workman who has pride in his tools will always be supplied with as many as he requires, because it takes no computation to prove that fifty pounds of extra cast steel tools, as an investment, is but a small matter compared to the gain in manipulation by having them at hand.
To an experienced mechanic a single glance at the tools on a lathe is a sufficient clue to the skill of the operator. If the tools are ground ready to use, of the proper shape, and placed in order so as to be reached without delay, the latheman may at once be set down as having two of the main qualifications of a first-class workman, which are order, and a knowledge of tools; while on the contrary, a lathe board piled full of old waste, clamp bolts, and broken tools, shows a want of that system and order, without which no amount of hand skill can make an efficient workman.
It is also necessary to learn as soon as possible the technicalities pertaining to lathe work, and still more important to learn the conventional modes of performing various operations. Although lathe work includes a large range of operations which are continually varied, yet there are certain plans of performing each that has by long custom become conventional; to gain an acquaintance with these an apprentice should watch the practice of the best workmen, and follow their plans as near as he can, not risking any innovation or change until it has been very carefully considered. Any attempt to introduce new methods, modes of chucking work, setting and grinding tools, or other of the ordinary operations in turning, may not only lead to awkward mistakes, but will at once put a stop to useful information that might otherwise be gained from others. The technical terms employed in describing lathe work are soon learned, generally sooner than they are needed, and are often misapplied, which is worse than to be ignorant of them.
In cutting screws it is best not to refer to that mistaken convenience called a wheel list, usually stamped on some part of engine lathes to aid in selecting wheels. A screw to be cut is to the lead screw on a lathe as the wheel on the screw is to the wheel on the spindle, and every workman should be familiar with so simple a matter as computing wheels for screw cutting, when there is but one train of wheels. Wheels for screw cutting may be computed not only quite as soon as read from an index, but the advantage of being familiar with wheel changes is very important in other cases, and frequently such combinations have to be made when there is not an index at hand.
The following are suggested as subjects which may be studied in connection with lathes and turning: the rate of cutting movement on iron, steel, and brass; the relative speed of the belt cones, whether the changes are by a true ascending scale from the slowest; the rate of feed at different changes estimated like the threads of a screw at so many cuts per inch; the proportions of cone or step pulleys to insure a uniform belt tension, the theory of the following rest as employed in turning flexible pieces, the difference between having three or four bearing points for centre or following rests; the best means of testing the truth of a lathe. All these matters and many more are subjects not only of interest but of use in learning lathe manipulation, and their study will lead to a logical method of dealing with problems which will continually arise.
The use of hand tools should be learned by employing them on every possible occasion. A great many of the modern improvements in engine lathes are only to evade hand tool work, and in many cases effect no saving except in skill. A latheman who is skilful with hand tools will, on many kinds of light work, perform more and do it better on a hand lathe than an engine lathe; there is always more or less that can be performed to advantage with hand tools even on the most elaborate engine lathes.
It is no uncommon thing for a skilled latheman to lock the slide rest, and resort to hand tools on many kinds of work when he is in a hurry.
(1.) Why does machinery involve so many cylindrical forms?—(2.) Why is it desirable to have separate feed gear for turning and screw cutting?—(3.) What is gained by the frictional starting gearing now applied to the finer class of lathes?—(4.) How may the alignment of a lathe be tested?—(5.) What kind of deviation with a lathe carriage will most affect the truth of work performed?—(6.) How may an oval hole be bored on a common slide lathe?—(7.) How can the angular ways of a lathe and the corresponding grooves in a carriage be planed to fit without employing gauges?—(8.) Give the number of teeth in two wheels to cut a screw of ten threads, when a leading screw is four threads per inch?
The term planing should properly be applied only to machines that produce planes or flat surfaces, but the technical use of the term includes all cutting performed in right lines, or by what may be called a straight movement of tools.
As no motion except rotary can be continuous, and as rotary movement of tools is almost exclusively confined to shaping cylindrical pieces, a proper distinction between machine tools which operate in straight lines, and those which operate with circular movement, will be to call them by the names of rotary and reciprocating.
It may be noticed that all machines, except milling machines, which act in straight lines and produce plane surfaces have reciprocating movement; the class includes planing, slotting and shaping machines; these, with lathes, constitute nearly the whole equipment of an ordinary fitting shop.
It is strange, considering the simplicity of construction and the very important office filled by machines for cutting on plane surfaces, that they were not sooner invented and applied in metal work. Many men yet working at finishing, can remember when all flat surfaces were chipped and filed, and that long after engine lathes had reached a state of efficiency and were generally employed, planing machines were not known. This is no doubt to be accounted for in the fact that reciprocal movement, except that produced by cranks or eccentrics, was unknown or regarded as impracticable for useful purposes until late years, and when finally applied it was thought impracticable to have such movements operate automatically. This may seem quite absurd to even an apprentice of the present time, yet such reciprocating movement, as a mechanical problem, is by no means so simple as it may at first appear.
A planing machine platen, for instance, moves at a uniform rate of speed each way, and by its own motion shifts or reverses the driving power at each extreme of the stroke. Presuming that there were no examples to be examined, an apprentice would find many easier problems to explain than how a planing machine can shift its own belts. If a platen or table disengages the power that is moving it, the platen stops; if the momentum carries it enough farther to engage or connect other mechanism to drive the platen in the opposite direction, the moment such mechanism comes into gear the platen must stop, and no movement can take place to completely engage clutches or shift belts. This is a curious problem that will be referred to again.
Reciprocating tools are divided into those wherein the cutting movement is given to the tools, as in shaping and slotting machines, and machines wherein the cutting movement is given to the material to be planed, as in a common planing machine. Very strangely we find in general practice that machine tools for both the heaviest and the lightest class of work, such as shaping, and butting, operate upon the first principle, while pieces of a medium size are generally planed by being moved in contact with stationary tools.
This problem of whether to move the material or to move the tools in planing, is an old one; both opinion and practice vary to some extent, yet practice is fast settling down into constant rules.
Judged upon theoretical grounds, and leaving out the mechanical conditions of operation, it would at once be conceded that a proper plan would be to move the lightest body; that is, if the tools and their attachments were heavier than the material to be acted upon, then the material should be moved for the cutting action, and vice versa. But in practice there are other conditions to be considered more important than a question of the relative weight of reciprocating parts; and it must be remembered that in solving any problem pertaining to machine action, the conditions of operation are to be considered first and have precedence over problems of strain, arrangement, or even the general principles of construction; that is, the conditions of operating must form a base from which proportions, arrangements, and so on, must be deduced. A standard planing machine, such as is employed for most kinds of work, is arranged with a running platen or carriage upon which the material is fastened and traversed beneath the cutting tools. The uniformity of arrangement and design in machines of this kind in all countries wherever they are made, must lead to the conclusion that there are substantial reasons for employing running platens instead of giving a cutting movement to the tools.
A planing machine with a running platen occupies nearly twice as much floor space, and requires a frame at least one-third longer than if the platen were fixed and the tools performed the cutting movement. The weight which has to be traversed, including the carriage, will in nearly all cases exceed what it would be with a tool movement; so that there must exist some very strong reasons in favour of a moving platen, which I will now attempt to explain, or at least point out some of the more prominent causes which have led to the common arrangement of planing machines.
Strains caused by cutting action, in planing or other machines, fall within and are resisted by the framing; even when the tools are supported by one frame and the material by another, such frames have to be connected by means of foundations which become a constituent part of the framing in such cases.
Direct action and reaction are equal; if a force is exerted in any direction there must be an equal force acting in the opposite direction; a machine must absorb its own strains.
Keeping this in view, and referring to an ordinary planing machine with which the reader is presumed to be familiar, the focal point of the cutting strain is at the edge of the tools, and radiates from this point as from a centre to the various parts of the machine frame, and through the joints fixed and movable between the tools and the frame; to follow back from this cutting point through the mechanism to the frame proper; first starting with the tool and its supports and going to the main frame; then starting from the material to be planed, and following back in the other direction, until we reach the point where the strains are absorbed by the main frame, examining the joints which intervene in the two cases, there will appear some reasons for running carriages.
Beginning at the tool there is, first, a clamped joint between the tool and the swing block; second, a movable pivoted joint between the block and shoe piece; third, a clamped joint between the shoe piece and the front saddle; fourth, a moving joint where the front saddle is gibed to the swing or quadrant plate; fifth, a clamp joint between the quadrant plate and the main saddle; sixth, a moving joint between the main saddle and the cross head; seventh, a clamp joint between the cross head and standards; and eighth, bolted joints between the standards and the main frame; making in all eight distinct joints between the tool and the frame proper, three moving, four clamped, and one bolted joint.
Starting again from the cutting point, and going the other way from the tool to the frame, there is, first, a clamped and stayed joint between the material and platen, next, a running joint between the platen and frame; this is all; one joint that is firm beyond any chance of movement, and a moving joint that is not held by adjustable gibs, but by gravity; a force which acts equally at all times, and is the most reliable means of maintaining a steady contact between moving parts.
Reviewing these mechanical conditions, we may at once see sufficient reasons for the platen movement of planing machines; and that it would be objectionable, if not impossible, to add a traversing or cutting action to tools already supported through the medium of eight joints. To traverse for cutting would require a moving gib joint in place of the bolted one, between the standards and main frame, leading to a complication of joints and movements quite impracticable.
These are, however, not the only reasons which have led to a running platen for planing machines, although they are the most important.
If a cutting movement were performed by the tool supports, it would necessarily follow that the larger a piece to be planed, and the greater the distance from the platen to the cutting point, the farther a tool must be from its supports; a reversal of the conditions required; because the heavier the work the greater the cutting strain will be, and the tool supports less able to withstand the strains to be resisted.
It may be assumed that the same conditions apply to the standards of a common planing machine, but the case is different; the upright framing is easily made strong enough by increasing its depth; but the strain upon running joints is as the distance from them at which a force is applied, or to employ a technical phrase, as the amount of overhang. With a moving platen the larger and heavier a piece to be planed, the more firmly a platen is held down; and as the cross section of pieces usually increases with their depth, the result is that a planing machine properly constructed will act nearly as well on thick as thin pieces.
The lifting strain at the front end of a platen is of course increased as the height at which the cutting is done above its top, but this has not in practice been found a difficulty of any importance, and has not even required extra length or weight of platens beyond what is demanded to receive pieces to be planed and to resist flexion in fastening heavy work. The reversing movement of planing machine platens already alluded to is one of the most complex problems in machine tool movement.
Platens as a rule run back at twice the forward or cutting movement, and as the motion is uniform throughout each stroke, it requires to be stopped at the extremes by meeting some elastic or yielding resistance which, to use a steam phrase, "cushions" or absorbs the momentum, and starts the platen back for the return stroke.
This object is attained in planing machines by the friction of the belts, which not only cushions the platen like a spring, but in being shifted opposes a gradually increasing resistance until the momentum is overcome and the motion reversed. By multiplying the movement of the platen with levers or other mechanism, and by reason of the movement that is attained by momentum after the driving power ceases to act, it is found practicable to have a platen 'shift its own belts,' a result that would never have been reached by theoretical deductions, and was no doubt discovered by experiment, like the automatic movement of engine valves is said to have been.
It is not intended to claim that this platen-reversing motion cannot, like any other mechanical movement, be resolved mathematically, but that the mechanical conditions are so obscure and the invention made at a time that warrants the supposition of accidental discovery.
In the driving gearing of planing machines, conditions which favour the reversing movement are high speed and narrow driving belts. The time in which belts may be shifted is as their speed and width; to be shifted a belt must be deflected or bent edgewise, and from this cause wind spirally in order to pass from one pulley to another. To bend or deflect a belt edgewise there will be required a force in proportion to its width, and the time of passing from one pulley to another is as the number of revolutions made by the pulleys.
Planing machines of the most improved construction are driven by two belts instead of one, and many mechanical expedients have been adopted to move the belts differentially, so that both should not be on the driving pulley at the same time, but move one before the other in alternate order. This is easily attained by simply arranging the two belts with the distance between them equal to one and one-half or one and three-fourth times the width of the driving pulley. The effect is the same as that accomplished by differential shifting gearing, with the advantage of permitting an adjustment of the relative movement of the belts.
Another principle in planing machines which deserves notice is the manner of driving carriages or platens; this is usually performed by means of spur wheels and a rack. A rack movement is smooth enough, and effective enough so far as a mechanical connection between the driving gearing and a platen, but there is a difficulty met with from the torsion and elasticity of cross-shafts and a train of reducing gearing. In all other machines for metal cutting, it has been a studied object to have the supports for both the tools and the material as rigid as possible; but in the common type of planing machines, such as have rack and pinion movement, there is a controversion of this principle, inasmuch as a train of wheels and several cross-shafts constitute a very effective spring between the driving power and the point of cutting, a matter that is easily proved by planing across the teeth of a rack, or the threads of a screw, on a machine arranged with spur wheels and the ordinary reducing gearing. It is true the inertia of a platen is interposed and in a measure overcomes this elasticity, but in no degree that amounts to a remedy.
A planing machine invented by Mr Bodmer in 1841, and since improved by Mr William Sellers of Philadelphia, is free from this elastic action of the platen, which is moved by a tangent wheel or screw pinion. In Bodmer's machine the shaft carrying the pinion was parallel to the platen, but in Sellers' machine is set on a shaft with its axis diagonal to the line of the platen movement, so that the teeth or threads of the pinion act partly by a screw motion, and partly by a progressive forward movement like the teeth of wheels. The rack on the platen of Mr Sellers' machine is arranged with its teeth at a proper angle to balance the friction arising from the rubbing action of the pinion, which angle has been demonstrated as correct at 5°, the ordinary coefficient of friction; as the pinion-shaft is strongly supported at each side of the pinion, and the thrust of the cutting force falls mainly in the line of the pinion shaft, there is but little if any elasticity, so that the motion is positive and smooth.
The gearing of these machines is alluded to here mainly for the purpose of calling attention to what constitutes a new and singular mechanical movement, one that will furnish a most interesting study, and deserves a more extended application in producing slow reciprocating motion.
(1.) Can the driving power be employed directly to shift the belts of a planing machine?—(2.) Why are planing machines generally constructed with a running carriage instead of running tools?—(3.) What objection exists in employing a train of spur wheels to drive a planing machine carriage?—(4.) What is gained by shifting the belts of a planing machine differentially?—(5.) What produces the screeching of belts so common with planing machines?—(6.) What conditions favour the shifting of planing machine belts?
Slotting machines with vertical cutting movement differ from planing machines in several respects, to which attention may be directed. In slotting, the tools are in most cases held rigidly and do not swing from a pivot as in planing machines. The tools are held rigidly for two reasons; because the force of gravity cannot be employed to hold them in position at starting, and because the thrust or strain of cutting falls parallel, and not transverse to the tools as in planing. Another difference between slotting and planing is that the cutting movement is performed by the tools and not by movement of the material. The cutting strains are also different, falling at right angles to the face of the table, in the same direction as the force of gravity, and not parallel to the face of the table, as in planing and shaping machines.
The feed motion in slotting machines, because of the tools being held rigidly, has to operate differently from that of planing machines. The cross-feed of a planing machine may act during the return stroke, but in slotting machines, the feed movement should take place at the end of the up-stroke, or after the tools are clear of the material; so much of the stroke as is made during the feeding action is therefore lost; and because of this, mechanism for operating the feed usually has a quick abrupt action so as to save useless movement of the cutter bar.
The relation between the feeding and cutting motion of reciprocating machines is not generally considered, and forms an interesting problem for investigation.
(1.) Name some of the differences between planing and slotting machines.—(2.) Why should the feed motion of a slotting machine act abruptly?—(3.) To what class of work are slotting machines especially adapted?
Shaping machines as machine tools occupy a middle place between planing and slotting machines; their movements correspond more to those of slotting machines, while the operation of the tools is the same as in planing. Some of the advantages of shaping over planing machines for certain kinds of work are, because of the greater facilities afforded for presenting and holding small pieces, or those of irregular shape; the supports or tables having both vertical and horizontal faces to which pieces may be fastened, and the convenience of the mechanism for adjusting and feeding tools.
Shaping machines are generally provided with adjustable vices, devices for planing circular forms, and other details which cannot be so conveniently employed with planing machines. Another feature of shaping machines is a positive range of the cutting stroke produced by crank motion, which permits tools to be stopped with precision at any point; this admits of planing slots, keyways, and such work as cannot well be performed upon common planing machines.
Shaping machines are divided into two classes, one modification with a lateral feed of the tools and cutter bar, technically called "travelling head machines," the other class with a feed motion of the table which supports the work, called table-feeding machines. The first modification is adapted for long pieces to be planed transversely, such as toothed racks, connecting rods, and similar work; the second class to shorter pieces where much hand adjustment is required.
An interesting study in connection with modern shaping machines is the principle of various devices called 'quick return' movements. Such devices consist of various modifications of slotted levers, and what is known as Whitworth's quick return motion.
The intricacy of the subject renders it a difficult one to deal with except by the aid of diagrams, and as such mechanism may be inspected in almost any machine fitting shop, attention is called to the subject as one of the best that can be chosen for demonstration by diagrams. Problems of these variable speed movements are not only of great interest, but have a practical importance not found in many better known problems which take up time uselessly and have no application in a practical way.
The remarks, given in a former place, relating to tools for turning, apply to those for planing as well, except that in planing tools greater rigidity and strength are required.
(1.) Why are shaping machines better adapted than planing machines for planing slots, key-ways, and so on?—(2.) What objects are gained by a quick return motion of the cutter bar of shaping machines?
Boring, as distinguished from drilling, consists in turning out annular holes to true dimensions, while the term drilling is applied to perforating or sinking holes in solid material. In boring, tools are guided by axial support independent of the bearing of their edges on the material, while in drilling, the cutting edges are guided and supported mainly from their contact with and bearing on the material drilled.
Owing to this difference in the manner of guiding and supporting the cutting edges, and the advantages of an axial support for tools in boring, it becomes an operation by which the most accurate dimensions are attainable, while drilling is a comparatively imperfect operation; yet the ordinary conditions of machine fitting are such that nearly all small holes can be drilled with sufficient accuracy.
Boring may be called internal turning, differing from external turning, because of the tools performing the cutting movement, and in the cut being made on concave instead of convex surfaces; otherwise there is a close analogy between the operations of turning and boring. Boring is to some extent performed on lathes, either with boring bars or by what is termed chuck-boring, in the latter the material is revolved and the tools are stationary.
Boring may be divided into three operations as follows: chuck-boring on lathes; bar-boring, when a boring bar runs on points or centres, and is supported at the ends only; and bar-boring when a bar is supported in and fed through fixed bearings. The principles are different in these operations, each one being applicable to certain kinds of work. A workman who can distinguish between these plans of boring, and can always determine from the nature of a certain work which is the best to adopt, has acquired considerable knowledge of fitting operations.
Chuck-boring is employed in three cases; for holes of shallow depth, taper holes, and holes that are screw-threaded. As pieces are overhung in lathe-boring there is not sufficient rigidity neither of the lathe spindle nor of the tools to admit of deep boring. The tools being guided in a straight line, and capable of acting at any angle to the axis of rotation, the facilities for making tapered holes are complete; and as the tools are stationary, and may be instantly adjusted, the same conditions answer for cutting internal screw-threads; an operation corresponding to cutting external screws, except that the cross motions of the tool slide are reversed.
The second plan of boring by means of a bar mounted on points or centres is one by which the greatest accuracy is attainable; it is like chuck-boring a lathe operation, and one for which no better machine than a lathe has been devised, at least for the smaller kinds of work. It is a problem whether in ordinary machine fitting there is not a gain by performing all boring in this manner whenever the rigidity of boring bars is sufficient without auxiliary supports, and when the bars can pass through the work. Machines arranged for this kind of boring can be employed in turning or boring as occasion may require.
When a tool is guided by turning on points, the movement is perfect, and the straightness or parallelism of holes bored in this manner is dependent only on the truth of the carriage movement. This plan of boring is employed for small steam cylinders, cylindrical valve seats, and in cases where accuracy is essential.
The third plan of boring with bars resting in bearings is more extensively practised, and has the largest range of adaptation. A feature of this plan of boring is that the form of the boring-bar, or any imperfection in its bearings, is communicated to the work; a want of straightness in the bar makes tapering holes. This, of course, applies to cases where a bar is fed through fixed bearings placed at one or both ends of a hole to be bored. If a boring-bar is bent, or out of truth between its bearings, the diameter of the hole being governed by the extreme sweep of the cutters is untrue to the same extent, because as the cutters move along and come nearer to the bearings, the bar runs with more truth, forming a tapering hole diminishing toward the rests or bearings. The same rule applies to some extent in chuck-boring, the form of the lathe spindle being communicated to holes bored; but lathe spindles are presumed to be quite perfect compared with boring bars.
The prevailing custom of casting machine frames in one piece, or in as few pieces as possible, leads to a great deal of bar-boring, most of which can be performed accurately enough by boring bars supported in and fed through bearings. By setting up temporary bearings to support boring-bars, and improvising means of driving and feeding, most of the boring on machine frames can be performed on floors or sole plates and independent of boring machines and lathes. There are but few cases in which the importance of studying the principles of tool action is more clearly demonstrated than in this matter of boring; even long practical experience seldom leads to a thorough understanding of the various problems which it involves.
Drilling differs in principle from almost every other operation in metal cutting. The tools, instead of being held and directed by guides or spindles, are supported mainly by the bearing of the cutting edges against the material.
A common angular-pointed drill is capable of withstanding a greater amount of strain upon its edges, and rougher use than any other cutting implement employed in machine fitting. The rigid support which the edges receive, and the tendency to press them to the centre, instead of to tear them away as with other tools, allows drills to be used when they are imperfectly shaped, improperly tempered, and even when the cutting edges are of unequal length.
Most of the difficulties which formerly pertained to drilling are now removed by machine-made drills which are manufactured and sold as an article of trade. Such drills do not require dressing and tempering or fitting to size after they are in use, make true holes, are more rigid than common solid shank drills, and will drill to a considerable depth without clogging.
A drilling machine, adapted to the usual requirements of a machine fitting establishment, consists essentially of a spindle arranged to be driven at various speeds, with a movement for feeding the drills; a firm table set at right angles to the spindle, and arranged with a vertical adjustment to or from the spindle, and a compound adjustment in a horizontal plane. The simplicity of the mechanism required to operate drilling tools is such that it has permitted various modifications, such as column drills, radial drills, suspended drills, horizontal drills, bracket drills, multiple drills, and others.
Drilling, more than any other operation in metal cutting, requires the sense of feeling, and is farther from such conditions as admit of power feeding. The speed at which a drill may cut without heating or breaking is dependent upon the manner in which it is ground and the nature of the material drilled, the working conditions may change at any moment as the drilling progresses; so that hand feed is most suitable. Drilling machines arranged with power feed for boring should have some means of permanently disengaging the feeding mechanism to prevent its use in ordinary drilling.
I am well aware how far this opinion is at variance with practice, especially in England; yet careful observation in a workshop will prove that power feed in ordinary drilling effects no saving of time or expense.
(1.) What is the difference between boring and drilling?—(2.) Why will drills endure more severe use than other tools?—(3.) Why is hand feeding best suited for drills?—(4.) What is the difference between boring with a bar supported on centres and one fed through journal bearings?
Milling relates to metal cutting with serrated rotary cutters, and differs in many respects from either planing or turning. The movement of the cutting edges can be more rapid than with tools which act continuously, because the edges are cooled during the intervals between each cut; that is, if a milling tool has twenty teeth, any single tooth or edge acts only from a fifteenth to a twentieth part of the time; and as the cutting distance or time of cutting is rarely long enough to generate much heat, the speed of such tools may be one-half greater than for turning, drilling, or planing tools. Another distinction between milling and other tools is the perfect and rigid manner in which the cutting edges are supported; they are short and blunt, besides being usually carried on short rigid mandrils. A result of this rigid support of the tools is seen in the length of the cutting edges that can be employed, which are sometimes four inches or more in length. It is true the amount of material cut away in milling is much less than the edge movement will indicate when compared with turning or planing; yet the displacing capacity of a milling machine exceeds that of either a lathe or a planing machine. Theoretically the cutting or displacing capacity of any metal or wood cutting machine, is as the length of the edges multiplied into the speed of their cutting movement; a rule which applies very uniformly in wood cutting, and also in metal cutting within certain limits; but the strains that arise in metal cutting are so great that they may exceed all means of resisting them either in the material acted upon, or in the means of supporting tools, so that the length of cutting edges is limited. In turning chilled rolls at Pittsburg, tools to six inches wide are employed, and the effect produced is as the length of the edge; but the depth of the cut is slight, and the operation is only possible because of the extreme rigidity of the pieces turned, and the tools being supported without movable joints as in common lathes.
Under certain conditions a given quantity of soft iron or steel may be cut away at less expense, and with greater accuracy, by milling than by any other process.
A milling tool with twenty edges should represent as much wearing capacity as a like number of separate tools, and may be said to equal twenty duplicate tools; hence, in cutting grooves, notches, or similar work, a milling tool is equivalent to a large number of duplicate single tools, which cannot be made or set with the same truth; so that milling secures accuracy and duplication, objects which are in many cases more important than speed.
Milling, as explained, being a more rapid process than either planing or turning, it seems strange that so few machines of this kind are employed in engineering shops. This points to some difficulty to be contended with in milling, which is not altogether apparent, because economic reasons would long ago have led to a more extended use of milling processes, if the results were as profitable as the speed of cutting indicates. This is, however, not the case, except on certain kinds of material, and only for certain kinds of work.
The advantages gained by milling, as stated, are speed, duplication, and accuracy; the disadvantages are the expense of preparing tools and their perishability.
A solid milling cutter must be an accurately finished piece of work, made with more precision than can be expected in the work it is to perform. This accuracy cannot be attained by ordinary processes, because such tools, when tempered, are liable to become distorted in shape, and frequently break. When hardened they must be finished by grinding processes, if intended for any accurate work; in fact, no tools, except gauging implements, involve more expense to prepare, and none are so liable to accident when in use.
Such tools consist of a combination of cutting edges, all of which may be said to depend on each one; because if one breaks, the next in order will have a double duty to perform, and will soon follow—a reversal of the old adage, that 'union is strength,' if by strength is meant endurance.
In planing and turning, the tools require no exact form; they can be roughly made, except the edge, and even this, in most cases, is shaped by the eye. Such tools are maintained at a trifling expense, and the destruction of an edge is a matter of no consequence. The form, temper, and strength can be continually adapted to the varying conditions of the work and the hardness of material. The line of division between planing and milling is fixed by two circumstances—the hardness and uniformity of the material to be cut, and the importance of duplication. Brass, clean iron, soft steel, or any homogeneous metal not hard enough to cause risk to the tools, can be milled at less expense than planed, provided there is enough work of a uniform character to justify the expense of milling tools. Cutting the teeth of wheels is an example where milling is profitable, but not to the extent generally supposed. In the manufacture of small arms, sewing machines, clocks, and especially watches, where there is a constant and exact duplication of parts, milling is indispensable. Such manufactures are in some cases founded on milling operations, as will be pointed out in another chapter.
Milling tools large enough to admit of detachable cutters being employed, are not so expensive to maintain as solid tools. Edge movement can sometimes be multiplied in this way, so as to greatly exceed what a single tool will perform.
Milling tools are employed at Crewe for roughing out the slots in locomotive crank axles. A number of detachable tools are mounted on a strong disc, so that four to six will act at one time; in this way the displacement exceeds what a lathe can perform when acting continuously with two tools. Rotary planing machines constructed on the milling principle, have been tried for plane surfaces, but with indifferent success, except for rough work.
There is nothing in the construction or operation of milling machines but what will be at once understood by a learner who sees them in operation. The whole intricacy of the process lies in its application or economic value, and but very few, even among the most skilled, are able in all cases to decide when milling can be employed to advantage. Theoretical conclusions, aside from practical experience, will lead one to suppose that milling can be applied in nearly all kinds of work, an opinion which has in many cases led to serious mistakes.
(1.) If milling tools operate faster than planing or turning tools, why are they not more employed?—(2.) How may the effect produced by cutting tools generally be computed?—(3.) To what class of work are milling machines especially suited?—(4.) Why do milling processes produce more accurate dimensions than are attainable by turning or planing?—(5.) Why can some branches of manufacture be said to depend on milling processes?
The tools employed for cutting screw threads constitute a separate class among the implements of a fitting shop, and it is considered best to notice them separately.
Screw-cutting is divided into two kinds, one where the blanks or pieces to be threaded are supported on centres, the tools held and guided independently of their bearing at the cutting edges, called chasing; the other process is where the blanks have no axial support, and are guided only by dies or cutting tools, called die-cutting.
The first of these operations includes all threading processes performed on lathes, whether with a single tool, by dies carried positively by slide rests, or by milling.
The second includes what is called threading in America and screwing in England. Machines for this purpose consist essentially of mechanism to rotate either the blank to be cut or the dies, and devices for holding and presenting the blanks.
Chasing produces screws true with respect to their axis, and is the common process of threading all screws which are to have a running motion in use, either of the screw itself, or the nut.
Die-cutting produces screws which may not be true, but are still sufficiently accurate for most uses, such as clamping and joining together the parts of machinery or other work.
Chasing operations being lathe work, and involving no principles not already noticed, what is said further will be in reference to die-cutting or bolt-threading machines, which, simple as they may appear to the unskilled, involve, nevertheless many intricacies which will not appear upon superficial examination.
Screw-cutting machines may be divided into modifications as follows:—(1) Machines with running dies mounted in what is called the head; (2) Machines with fixed dies, in which motion is given to the rod or blank to be threaded; (3) Machines with expanding dies which open and release the screws when finished without running back; (4) Machines with solid dies, in which the screws have to be withdrawn by changing the motion of the driving gearing; making in all four different types.
If these various plans of arranging screw-cutting machines had reference to different kinds of work, it might be assumed that all of them are correct, but they are as a rule all applied to the same kind of work; hence it is safe to conclude that there is one arrangement better than the rest, or that one plan is right and the others wrong. This matter may in some degree be determined by following through the conditions of use and application.
Between a running motion of the dies, or a running motion of the blanks, there are the following points which may be noticed.
If dies are fixed, the clamping mechanism to hold the rods has to run with the spindle; such machines must be stopped while fastening the rods or blanks. Clamping jaws are usually as little suited for rotation on a spindle as dies are, and generally afford more chances for obstruction and accident. To rotate the rods, if they are long, they must pass through the driving spindle, because machines cannot well be made of sufficient length to receive long rods. In machines of this class, the dies have to be opened and closed by hand instead of by the driving power, which can be employed for the purpose when the dies are mounted in a running head.
With running dies, blanks may be clamped when a machine is in motion, and as the blank does not revolve, it may, when long, be supported in any temporary manner. The dies can be opened and closed by the driving power also, and no stopping of a machine is necessary; so that several advantages of considerable importance may be gained by mounting the dies in a running head, a plan which has been generally adopted in late years by machine tool makers both in England and America.
In respect to the difference between expanding and solid dies it consists mainly in the time required to run back, and the injury to dies which this operation occasions. Uniformity of size is within certain limits insured by solid dies, but they are more liable to derangement and less easy to repair than expanding or independent dies.
Another difference between solid and expanding dies, which may be pointed out, is in the firmness with which the cutting edges are held. With a solid die, the edges or teeth being all combined in one solid piece, are firmly held in a fixed position; while with expanding dies their position has to be maintained by mechanical devices which are liable to yield under the pressure which arises in cutting. The result is, that the precision with which a screwing machine with movable dies will act, is dependent upon the strength of the 'abutment' behind the dies, which should be a hard unyielding surface with as much area as possible.
Connected with screw dies, there are various problems, such as clearance behind the cutting edge; whether an odd or even number of edges are best; how many threads require to be bevelled at the starting point; and many other matters about which there are no determined rules. The diversity of opinion that will be met with on these points, and in reference to taps, the form of screw-threads, and so on, will convince a learner of the intricacies in this apparently simple matter of cutting screw-threads.
(1.) Describe the different modifications of screw-cutting machines.—(2.) What is gained by revolving the dies instead of the rod?—(3.) What is gained by expanding dies?—(4.) What is the difference between screws cut by chasing and those cut on a screw-cutting machine?
Machines are composed of parts connected together by rigid and movable joints; rigid joints are necessary because of the expense, and in most cases the impossibility, of constructing framing and other fixed detail in one piece.
All moving parts must of course be independent of fixed parts, the relation between the two being maintained by what has been called running joints.
It is evident that when the parts of a machine are joined together, each piece which has contact on more than one side must have specific dimensions; it is farther evident that as many of the joints in a machine as are to accommodate the exigencies of construction must be without space, that is, they represent continued sections of what should be solid material, if it were possible to construct the parts in that manner. This also demands specific dimensions.
In arranging the details of machines, it is impossible to have a special standard of dimensions for each case, or even for each shop; the dimensions employed are therefore made to conform to some general standard, which by custom becomes known and familiar to workmen and to a country, or as we may now say to all countries.
A standard of lineal measures, however, cannot be taken from one country to another, or even transferred from one shop to another without the risk of variation; and it is therefore necessary that such a standard be based upon something in nature to which reference can be made in cases of doubt.
In ages past, various attempts were made to find some constant in nature on which measures could be based. Some of these attempts were ludicrous, and all of them failures, until the vibrations of a pendulum connected length and space with time. The problem was then more easy. The changes of seasons and the movement of heavenly bodies had established measures of time, so that days, hours, and minutes became constants, proved and maintained by the unerring laws of nature.
A pendulum vibrating in uniform time regardless of distance, but always as its length, if arranged to perform one vibration in a given time, gave a constant measure of length. Thus lineal measure comes from time; cubic or solid measures from lineal measure, and standards of weight from the same source; because when a certain quantity of a substance of any kind could be determined by lineal measurement, and this quantity was weighed, a standard of weight would be reached, provided there was some substance sufficiently uniform, to which reference could be made in different countries. Such a substance is sea or pure water; weighed in vacuo, or with the air at an assumed density, water gives a result constant enough for a standard of weight.
It is a strange thought that with all the order, system, and regularity, existing in nature, there is nothing but the movements of the heavenly bodies constant enough to form a base for gauging tests. The French standard based upon the calculated length of the meridian may be traced to this source.
Nothing animate or inanimate in nature is uniform; plants, trees, animals, are all different; even the air we breathe and the temperature around us is constantly changing; only one thing is constant, that is time, and to this must we go for all our standards.
I am not aware that the derivation of our standard measures has been, in an historical way, as the foregoing remarks will indicate, nor is it the purpose here to follow such history. A reader, whose attention is directed to the subject, will find no trouble in tracing the matter from other sources. The present object is to show what a wonderful series of connections can be traced from so simple a tool as a measuring gauge, and how abstruse, in fact, are many apparently simple things, often regarded as not worth a thought beyond their practical application.
(1.) Why are machine frames constructed in sections, instead of being in one piece?—(2.) Why must parts which have contact on opposite sides have specific dimensions?—(3.) What are standards of measure based upon in England, America, and France?—(4.) How can weight be measured by time?—(5.) Has the French metre proved a standard admitting of test reference?
Among the improvements in machine fitting which have in recent years come into general use, is the employment of standard gauges, by means of which uniform dimensions are maintained, and within certain limits, an interchange of the parts of machinery is rendered possible.
Standard gauging implements were introduced about the year 1840, by the celebrated Swiss engineer, John G. Bodmer, a man who for many reasons deserves to be considered as the founder of machine tool manufacture. He not only employed gauges in his works to secure duplicate dimensions, but also invented and put in use many other reforms in manipulation; among these may be mentioned the decimal or metrical division of measures, a system of detail drawings classified by symbols, the mode of calculating wheels by diametric pitch, with many other things which characterise the best modern practice.
The importance of standard dimensions, and the effect which a system of gauging may have in the construction of machines, will be a matter of some difficulty for a learner to understand. The interchangeability of parts, which is the immediate object in employing gauges, is plain enough, and some of the advantages at once apparent, yet the ultimate effects of such a system extend much farther than will at first be supposed.
The division of labour, that system upon which we may say our great industrial interests are founded, is in machine fitting promoted in a wonderful degree by the use of gauging implements. If standard dimensions can be maintained, it is easy to see that the parts of a machine can be constructed by different workmen, or in different shops, and these parts when assembled all fit together, without that tedious and uncertain plan of try-fitting which was once generally practised. There are, it is true, certain kinds of fitting which cannot well be performed by gauges; moving flat surfaces, such as the bearings of lathe slides or the faces of steam engine valves, are sooner and better fitted by trying them together and scraping off the points of contact; but even in such cases the character of the work will be improved, if one or both surfaces have been first levelled by gauging or surface plates.
In cylindrical fitting, which as before pointed out, constitutes the greater part in machine fitting, gauges are especially important, because trial-fitting is in most cases impossible.
Flat or plane joints nearly always admit of adjustment between the fitted surfaces; that is, the material scraped or ground away in fitting can be compensated by bringing the pieces nearer together; but parallel cylindrical joints cannot even be tried together until finished, consequently, there can be nothing cut away in trying them together. Tapering, or conical joints, can of course be trial-fitted, and even parallel fits are sometimes made by trial, but it is evident that the only material that can be cut away in such cases, is what makes the difference between a fit too close, and one which will answer in practice.
As to the practical results which may be attained by a gauging system, it may be said that they are far in advance of what is popularly supposed, especially in Europe, where gauges were first employed.
The process of milling, which has been so extensively adopted in the manufacture of guns, watches, sewing-machines, and similar work in America, has, on principles explained in the chapter on milling, enabled a system of gauging which it is difficult to comprehend without seeing the processes carried on. And so important is the effect due to this duplicating or gauging system, that several important branches of manufacture have been controlled in this way, when other elements of production, such as the price of labour, rent, interest, and so on, have been greatly in favour of countries where the trying system is practised.
As remarked, the gauging system is particularly adapted to, or enabled by milling processes, and of course must have its greatest effect in branches of work directed to the production of uniform articles, such as clocks, watches, sewing-machines, guns, hand tools, and so on. That is, the direct effect on the cost of processes will be more apparent and easily understood in such branches of manufacture; yet in general engineering work, where each machine is more or less modified, and made to special plans, the commercial gain resulting from the use of gauges is considerable.
In respect to repairing alone, the consideration of having the parts of machinery fitted to standard sizes is often equal to its whole value.
Machinery subjected to destructive wear, and to be operated at a distance from machine shops—locomotive engines for example—if not constructed with standard dimensions, may, by the detention due to repairing, cause a loss and inconvenience equal to their value; if a shaft wheel bearing, or even a fitted screw bolt is broken, time must be allowed to make the parts new; and in order to fit them, the whole machine, or such of its details as have connection with the broken parts, must be taken to a shop in order to fit by trial.
The duplicate system has gradually made its way in locomotive engineering, and will no doubt extend to the whole of railway equipment, as constants for dimensions are proved and agreed upon.
The gauging system has been no little retarded by a selfish and mistaken opinion that an engineering establishment may maintain peculiar standards of its own; in fact, relics of this spirit are yet to be met with in old machines, where the pitch of screw-threads has been made to fractional parts of an inch, so that engineers, other than the original makers, could not well perform repairing, or replace broken parts.
One of the effects of employing gauges in machine fitting is to inspire confidence in workmen. Instead of a fit being regarded as a mysterious result more the work of chance than design, men accustomed to gauges come to regard precision as something both attainable and indispensable. A learner, after examining a set of well fitted cylindrical gauges, will form a new conception of what a fit is, and will afterwards have a new standard fixed in his mind.
The variation of dimensions which are sensible to the touch at one ten-thousandth part of an inch, furnishes an example of how important the human senses are even after the utmost precision attainable by machine action. Pieces may pass beneath the cutters of a milling machine under conditions, which so far as machinery avails will produce uniform sizes, yet there is no assurance of the result until the work is felt by gauges.
The eye fails to detect variations in size, even by comparison, long before we reach the necessary precision in common fitting. Even by comparison with figured scales or measuring with rules, the difference between a proper and a spoiled fit is not discernible by sight.
Many of the most accurate measurements are, however, performed by sight, with vernier calipers for example, the variation being multiplied hundreds or thousands of times by mechanism, until the least differences can be readily seen.
In multiplying the variations of a measuring implement by mechanism, it is obvious that movable joints must be employed; it is also obvious that no positive joint, whether cylindrical or flat, could be so accurately fitted as to transmit such slight movement as occurs in gauging or measuring. This difficulty is in most measuring instruments overcome by employing a principle not before alluded to, but common in many machines, that of elastic compensation.
A pair of spring calipers will illustrate this principle. The points are always steady, because the spring acting continually in one direction compensates the loose play that may be in the screw. In a train of tooth wheels there is always more or less play between the teeth; and unless the wheels always revolve in one direction, and have some constant resistance offered to their motion, 'backlash' or irregular movement will take place; but if there is some constant and uniform resistance such as a spring would impart, a train of wheels will transmit the slightest motion throughout.
The extreme nicety with which gauging implements are fitted seems at first thought to be unnecessary, but it must be remembered that a cylindrical joint in ordinary machine fitting involves a precision almost beyond the sense of feeling, and that any sensible variation in turning gauges is enough to spoil a fit.
Opposed to the maintenance of standard dimensions are the variations in size due to temperature. This difficulty applies alike to gauging implements and to parts that are to be tested; yet in this, as in nearly every phenomenon connected with matter, we have succeeded in turning it to some useful purpose. Bands of iron, such as the tires of wheels when heated, can be 'shrunk' on, and a compressive force and security attained, which would be impossible by forcing the parts together both at the same temperature. Shrinking has, however, been almost entirely abandoned for such joints as can be accurately fitted.
(1.) How may gauging implements affect the division of labour?—(2.) In what way do standard dimensions affect the value of machinery?—(3.) Why cannot cylindrical joints be fitted by trying them together?—(4.) Under what circumstances is it most important that the parts of machinery should have standard dimensions?—(5.) Which sense is most acute in testing accurate dimensions?—(6.) How may slight variations in dimensions be made apparent to sight?
It will scarcely be expected that any part of the present work, intended mainly for apprentice engineers, should relate to designing machines, yet there is no reason why the subject should not to some extent be treated of; it is one sure to engage more or less attention from learners, and the study of designing machines, if properly directed, cannot fail to be of advantage.
There is, perhaps, no one who has achieved a successful experience as an engineer but will acknowledge the advantages derived from early efforts to generate original designs, and none who will not admit that if their first efforts had been more carefully directed, the advantages gained would have been greater.
It is exceedingly difficult for an apprentice engineer, without experimental knowledge, to choose plans for his own education, or to determine the best way of pursuing such plans when they have been chosen; and there is nothing that consumes so much time, or is more useless than attempting to make original designs, if there is not some systematic method followed.
There is but little object in preparing designs, when their counterparts may already exist, so that in making original plans, there should be a careful research as to what has been already done in the same line. It is not only discouraging, but annoying, after studying a design with great care, to find that it has been anticipated, and that the scheme studied out has been one of reproduction only. For this reason, attempts to design should at first be confined to familiar subjects, instead of venturing upon unexplored ground.
Designing is in many respects the same thing as invention, except that it deals more with mechanism than principles, although it may, and often does include both. Like invention, designing should always be attempted for the attainment of some definite object laid down at the beginning, and followed persistently throughout.
It is not always an easy matter to hit upon an object to which designs may be directed; and although at first thought it may seem that any machine, or part of a machine, is capable of improvement, it will be found no easy matter to detect existing faults or to conceive plans for their remedy.
A new design should be based upon one of two suppositions—either that existing mechanism is imperfect in its construction, or that it lacks functions which a new design may supply; and if those who spend their time in making plans for novel machinery would stop to consider this from the beginning, it would save no little of the time wasted in what may be called scheming without a purpose.
After determining the ultimate objects of an improvement, and laying down the general principles which should be followed in the preparation of a design, there is nothing connected with constructive engineering that can be more nearly brought within general rules than arranging details. I am well aware of how far this statement is at variance with popular opinion among mechanics, and of the very thorough knowledge of machine application and machine operation required in making designs, and mean that there are certain principles and rules which may determine the arrangement and distribution of material, the position and relation of moving parts, bearings, and so on, and that a machine may be built up with no more risk of mistakes than in erecting a permanent structure.
Designing machines must have reference to adaptation, endurance, and the expense of construction. Adaptation includes the performance of machinery, its commercial value, or what the machinery may earn in operating; endurance, the time that machines may operate without being repaired, and the constancy of their performance; expense, the investment represented in machinery.
The adaptation, endurance, and cost of machines in designing become resolved into problems of movements, the arrangement of parts, and proportions.
Movements and strains may be called two of the leading conditions upon which designs for machines are based: movements determine general dimensions, and strains determine the proportions and sizes of particular parts. Movement and strain together determine the nature and area of bearings or bearing surfaces.
The range and speed of movement of the parts of machines are elements in designing that admit of a definite determination from the work to be accomplished, but arrangement cannot be so determined, and is the most difficult to find data for. To sum up these propositions we have:—
1. A conception of certain functions in a machine, and some definite object which it is to accomplish.
2. Plans of adaptation and arrangement of the component parts of the machinery, or organisation as it may be called.
3. A knowledge of specific conditions, such as strains, the range and rate of movements, and so on.
4. Proportions of the various parts, including the framing, bearing surfaces, shafts, belts, gearing, and other details.
5. Symmetry of appearance, which is often more the result of obvious adaptation than ornamentation.
To illustrate the practical application of what has preceded, let it be supposed, for example, that a machine is to be made for cutting teeth in iron racks ? in. pitch and 3 in. face, and that a design is to be prepared without reference to such machines as may already be in use for the purpose.
It is not assumed that an actual design can be made which by words alone will convey a comprehensive idea of an organised machine; it is intended to map out a course which will illustrate a plan of reasoning most likely to attain a successful result in such cases.
The reader, in order to better understand what is said, may keep in mind a common shaping machine with crank motion, a machine which nearly fills the requirements for cutting tooth racks.
Having assumed a certain work to do, the cutting of tooth racks ? in. pitch, and 3 in. face, the first thing to be considered will be, is the machine to be a special one, or one of general adaptation? This question has to do, first, with the functions of the machine in the way of adapting it to the cutting of racks of various sizes, or to performing other kinds of work, and secondly, as to the completeness of the machine; for if it were to be a standard one, instead of being adapted only to a special purpose, there are many expensive additions to be supplied which can be omitted in a special machine. It will be assumed in the present case that a special machine is to be constructed for a particular duty only.
The work to be performed consists in cutting away the metal between the teeth of a rack, leaving a perfect outline for the teeth; and as the shape of teeth cannot well be obtained by an adjustment of tools, it must be accomplished by the shape of the tools. The shape of the tools must, therefore, be constantly maintained, and as the cross section of the displaced metal is not too great, it may be assumed that the shape of the tools should be a profile of the whole space between two teeth, and such a space be cut away at one setting or one operation. By the application of certain rules laid down in a former place in reference to cutting various kinds of material, reciprocating or planing tools may be chosen instead of rotary or milling tools.
Movements come next in order, and consist of a reciprocating cutting movement of the tools or material, a feed movement to regulate the cutting action, and a longitudinal movement of the rack, graduated to pitch or space, the distance between the teeth.
The reciprocating cutting movement being but four inches or less, a crank is obviously the best means to produce this motion, and as the movement is transverse to the rack, which may be long and unwieldy, it is equally obvious that the cutting motion should be performed by the tools instead of the rack.
The feed adjustment of the tool being intermittent and the amount of cutting continually varying, this movement should be performed by hand, so as to be controlled at will by the sense of feeling. The same rule applies to the adjustment of the rack for spacing; being intermittent and irregular as to time, this movement should also be performed by hand. The speed of the cutting movement is known from ordinary practice to be from sixteen feet to twenty feet a minute, and a belt two and a half inches wide must move two hundred feet a minute to propel an ordinary metal cutting tool, so that the crank movement or cutter movement must be increased by gearing until a proper speed of the belt is reached; from this the speed of intermediate movers will be found.
Arrangement comes next; in this the first matter to be considered is convenience of manipulation. The cutting position should be so arranged as to admit of an easy inspection of the work. An operator having to keep his hand on the adjusting or feed mechanism, which is about twelve inches above the work, it follows that if the cutting level is four feet from the floor, and the feed handle five feet from the floor, the arrangement will be convenient for a standing position. As the work requires continual inspection and hand adjustments, it will for this reason be a proper arrangement to overhang both the supports for the rack and the cutting tools, placing them, as we may say, outside the machine, to secure convenience of access and to allow of inspection. The position of the cutting bar, crank, connections, gearing, pulleys, and shafts, will assume their respective places from obvious conditions, mainly from the position of the operator and the work.
Next in order are strains. As the cutting action is the source of strains, and as the resistance offered by the cutting tools is as the length or width of the edges, it will be found in the present case that while other conditions thus far have pointed to small proportions, there is now a new one which calls for large proportions. In displacing the metal between teeth of three-quarters of an inch pitch, the cutting edge or the amount of surface acted upon is equal to a width of one inch and a half. It is true, the displacement may be small at each cut, but the strain is rather to be based upon the breadth of the acting edge than the actual displacement of metal, and we find here strains equal to the average duty of a large planing machine. This strain radiates from the cutting point as from a centre, falling on the supports of the work with a tendency to force it from the framing. Between the rack and the crank-shaft bearing, through the medium of the tool, cutter bar, connection, and crank pin, and in various directions and degrees, this strain may be followed by means of a simple diagram. Besides this cutting strain, there are none of importance; the tension of the belt, the side thrust in bearings, the strain from the angular thrust of the crank, and the end thrust of the tool, although not to be lost sight of, need not have much to do with problems of strength, proportion, and arrangement.
Strains suggest special arrangement, which is quite a distinct matter from general arrangement, the latter being governed mainly by the convenience of manipulation. Special arrangement deals with and determines the shape of framing, following the strains throughout a machine. In the present case we have a cutting strain which may be assumed as equal to one ton, exerted between the bracket or jaws which support the work, and the crank-shaft. It follows that between these two points the metal in the framing should be disposed in as direct a line as possible, and provision be made to resist flexion by deep sections parallel with the cutting motion.
Lastly, proportions; having estimated the cutting force required at one ton, although less than the actual strain in a machine of this kind, we proceed upon this to fix proportions, beginning with the tool shank, and following back through the adjusting saddle, the cutting bar, connections, crank pins, shafts, and gear wheels to the belt. Starting again at the tool, or point of cutting, following through the supports of the rack, the jaws that clamp it, the saddle for the graduating adjustment, the connections with the main frame, and so on to the crank-shaft bearing a second time, dimensions may be fixed for each piece to withstand the strains without deflection or danger of breaking. Such proportions cannot, I am aware, be brought within the rules of ordinary practice by relying upon calculation alone to fix them, and no such course is suggested; calculation may aid, but cannot determine proportions in such cases; besides, symmetry, which cannot be altogether disregarded, modifies the form and sometimes the dimensions of various parts.
I have in this way imperfectly indicated a methodical plan of generating a design, as far as words alone will serve, beginning with certain premises based upon a particular work to be performed, and then proceeding to consider in consecutive order the general character of the machine, mode of operation, movements and adjustments, general arrangement, strains, special arrangement, and proportions.
With a thorough knowledge of practical machine operation, and an acquaintance with existing practice, an engineer proceeding upon such a plan, will, if he does not overlook some of the conditions, be able to generate designs which may remain without much modification or change, so long as the purpose to which the machinery is directed remains the same.
Perseverance is an important trait to be cultivated in first efforts at designing; it takes a certain amount of study to understand any branch of mechanism, no matter what natural capacity may be possessed by a learner. Mechanical operations are not learned intuitively, but are always surrounded by many peculiar conditions which must be learned seriatim, and it is only by an untiring perseverance at one thing that there can be any hope of improving it by new designs.
A learner who goes from gearing and shafts to steam and hydraulics, from machine tools to cranes and hoisting machinery, will not accomplish much. The best way is to select at first an easy subject, one that admits of a great range of modification, and if possible, one that has not assumed a standard form of construction. Bearings and supports for shafts and spindles, is a good subject to begin with.
In designing supports for shafts the strains are easily defined and followed, while the vertical and lateral adjustment, lubrication of bearings, symmetry of supports and hangers, and so on, will furnish grounds for endless modification, both as to arrangement and mechanism.
In making designs it is best to employ no references except such as are carried in the memory. The more familiar a person is with machinery of any class, the more able he may be to prepare designs, but not by measuring and referring to other people's plans. Dimensions and arrangement from examples are, by such a course, unconsciously carried into a new drawing, even by the most skilled; besides, it is by no means a dignified matter to collect other people's plans, and by a little combination and modification produce new designs. It may be an easy plan to acquire a certain kind of proficiency, but will most certainly hinder an engineer from ever rising to the dignity of an original designer.
Symmetry, as an element in designs for machinery, is one of those unsettled matters which may be determined only in connection with particular cases; it may, however, be said that for all engineering implements and manufacturing machinery of every kind, there should be nothing added for ornament, or anything that has no connection with the functions of the machinery.
Modern engineers of the abler class are so thoroughly in accord in this matter of ornamentation, both in opinion and practice, that the subject hardly requires to be mentioned, and it will be no disadvantage for a learner to commence by cultivating a contempt for whatever has no useful purpose. Of existing practice it may be said, that in what may be called industrial machinery, the amount of ornamentation is inverse as the amount of engineering skill employed in preparing designs.
A safe rule will be to assume that machinery mainly used and seen by the skilled should be devoid of ornament, and that machinery seen mainly by the unskilled, or in public, should have some ornament. Steam fire engines, sewing machines, and works of a similar kind, which fall under the inspection of the unskilled, are usually arranged with more or less ornament.
As a rule, ornament should never be carried further than graceful proportions; the arrangement of framing should follow as nearly as possible the lines of strain. Extraneous decoration, such as detached filagree work of iron, or painting in colours, is so repulsive to the taste of the true engineer and mechanic that it is unnecessary to speak against it.
(1.) Name some of the principal points to be kept in view in preparing designs?—(2.) Why should attempts at designing be confined to one class of machinery?—(3.) What objection exists to examining references when preparing designs?