(1.) In comparing steam-hammers with trip or crank hammers what mechanism does steam supplant or represent?—(2.) What can be called the chief distinction between steam and other hammers?—(3.) Under what circumstances is an automatic valve motion desirable?—(4.) Why is a dead or uncushioned blow most effective?—(5.) Will a hammer operate with air the same as with steam?
(1.) In comparing steam-hammers with trip or crank hammers what mechanism does steam supplant or represent?—(2.) What can be called the chief distinction between steam and other hammers?—(3.) Under what circumstances is an automatic valve motion desirable?—(4.) Why is a dead or uncushioned blow most effective?—(5.) Will a hammer operate with air the same as with steam?
Another principle to be noticed in connection with hammers and forging processes is that of the inertia of the piece operated upon—a matter of no little importance in the heavier kinds of work.
When a piece is placed on an anvil, and struck on the top side with a certain force, the bottom or anvil side of the piece does not receive an equal force. A share of the blow is absorbed by the inertia of the piece struck, and the effect on the bottom side is, theoretically, as the force of the blow, less the cushioning effect and the inertia of the pieces acted upon.
In practice this difference of effect on the top and bottom, or between the anvil and hammer sides of a piece, is much greater than would be supposed. The yielding of the soft metal on the top cushions the blow and protects the under side from the force. The effect produced by a blow struck upon hot iron cannot be estimated by the force of the blow; it requires, to use a technical term, a certain amount of force to "start" the iron, and anything less than this force has but little effect in moving the particles and changing the form of a piece.
From this it may be seen that there must occur a great loss of power in operating on large pieces, for whatever force is absorbed by inertia has no effect on the underside. By watching a smith using a hand hammer it will be seen that whenever a piece operated upon is heavier than the hammer employed, but little if any effect is produced on the anvil or bottom surface, nor is this loss of effect the only one. The expense of heating, which generally exceeds that of shaping forgings, is directly as the amount of shaping that may be done at each heat; and consequently, if the two sides of a piece, instead of one, can be equally acted upon, one-half the heating will be saved.
Another object gained by equal action on both sides of large pieces is the quality of the forgings produced, which is generally improved by the rapidity of the shaping processes, and injured by too frequent heating.
The loss of effect by the inertia of the pieces acted upon increases with the weight of the work; not only the loss of power, but also the expense of heating increases with the size of the pieces. There is, however, such a difference in the mechanical conditions between light and heavy forging that for any but a heavy class of work there would be more lost than gained in attempting to operate on both sides of pieces at the same time.
To attain a double effect, and avoid the loss pointed out, Mr Ramsbottom designed what may be called compound hammers, consisting of two independent heads or rams moving in opposite directions, and acting simultaneously upon pieces held between them.
It would be inferred that the arrangement of these double acting hammers must necessarily be complicated and expensive, but the contrary is the fact. The rams are simply two masses of iron mounted on wheels that run on ways, like a truck, and the impact of the hammers, so far as not absorbed in the work, isneutralised by each other. No shock or jar is communicated to framing or foundations as in the case of single acting hammers that have fixed anvils. The same rule applies in the back stroke of the hammers as the links which move them are connected together at the centre, where the power is applied at right angles to the line of the hammer movement. The links connecting the two hammers constitute, in effect, a toggle joint, the steam piston being attached where they meet in the centre.
The steam cylinder which moves the hammers is set in the earth at some depth below the plane upon which they move, and even when the heaviest work is done there is no perceptible jar when one is standing near the hammers, as there always is with those which have a vertical movement and are single acting.
(1.) Why is the effect produced different on the top and bottom of a piece when struck by a hammer?—(2.) Why does not a compound hammer create jar and concussion?—(3.) What would be a mechanical difficulty in presenting the material to such hammers?—(4.) Which is most important, speed or weight, in the effect produced on the under side of pieces, when struck by single acting hammers?
(1.) Why is the effect produced different on the top and bottom of a piece when struck by a hammer?—(2.) Why does not a compound hammer create jar and concussion?—(3.) What would be a mechanical difficulty in presenting the material to such hammers?—(4.) Which is most important, speed or weight, in the effect produced on the under side of pieces, when struck by single acting hammers?
Tempering may be called a mystery of the smith-shop; this operation has that attraction which characterises every process that is mysterious, especially such as are connected with, or belong to mechanical manipulation. A strange and perhaps fortunate habit of the mind is to be greatly interested in what is not well understood, and to disregard what is capable of plain demonstration.
An old smith who has stood at the forge for a score of years will take the same interest in tempering processes that a novice will. When a piece is to be tempered which is liable to spring or break, and the risk is great, he will enter upon it with the same zeal and interest that he would have done when learning his trade.
No one has been able to explain clearly why a sudden change of temperature hardens steel, nor why it assumes various shades of colour at different degrees of hardness; we only know the fact, and that steel fortunately has such properties.
Every one who uses tools should understand how to temper them, whether they be for iron or wood. Experiments with tempered tools is the only means of determining the proper degree of hardness, and as smiths, except with their own tools, have to rely upon the explanations of others as to proper hardening, it follows that tempering is generally a source of complaint.
Tempering, as a term, is used to comprehend both hardening and drawing; as a process it depends mainly upon judgment instead of skill, and has no such connection with forging as to be performed by smiths only. Tempering requires a different fire from those employed in forging, and also more care and precision than blacksmiths can exercise, unless there are furnaces and baths especially arranged for tempering tools.
A difficulty which arises in hardening tools is because of the contraction of the steel which takes place in proportion to the change of temperature; and as the time of cooling is in proportion to the thickness or size of a piece, it follows, of course, that there is a great strain and a tendency to break the thinner parts before the thicker parts have time to cool; this strain may take place either from cooling one side first, or more rapidly than another.
The following propositions in regard to tempering, comprehend the main points to be observed:
The permanent contraction of steel in tempering is as the degree of hardness imparted to it by the bath.
The time in which the contraction takes place is as the temperature of the bath and the cross section of the piece; in other words the heat passes off gradually from the surface to the centre.
Thin sections of steel tools being projections from the mass which supports the edges, are cooled first, and if provision is not made to allow for contraction they are torn asunder.
The main point in hardening and the most that can be done to avoid irregular contraction, is to apply the bath so that it will act first and strongest on the thickest parts. If a piece is tapering or in the form of a wedge, the thick end should enter the bath first; a cold chisel for instance that is wide enough to endanger cracking should be put into the bath with the headdownward.
The upflow of currents of warmed water are a common cause of irregular cooling and springing of steel tools in hardening; the water that is heated, rises vertically, and the least inclination of a piece from a perpendicular position, allows a warm current to flow up one side.
The most effectual means of securing a uniform effect from a tempering bath is by violent agitation, either of the bath or the piece; this also adds to the rapidity of cooling.
The effect of tempering baths is as their conducting power; chemicals except as they may contribute to the conducting properties of a bath, may safely be disregarded. For baths, cold or ice water loaded with salt for extreme hardness, and warm oil for tools that are thin and do not require to be very hard, are the two extremes outside of which nothing is required in ordinary practice.
In the case of tools composed partly of iron and partly of steel, steel laid as it is called, the tendency to crack in hardening may be avoided in most cases by hammering the steel edge at a low temperature until it is so expanded that when cooled in hardening it will only contract to a state of rest and correspond to the iron part; the same result may be produced by curving a piece, giving convexity to the steel side before hardening.
Tools should never be tempered by immersing their edges or cutting parts in the bath, and then allowing the heat to "run down" to attain a proper temper at the edge. I am well aware that this is attacking a general custom, but it is none the less wrong for that reason. Tools so hardened have a gradually diminishing temper from their point or edge, so that no part is properly tempered, and they require continual re-hardening, which spoils the steel; besides, the extreme edge, the only part which is tempered to a proper shade, is usually spoiled by heating and must be ground away to begin with. No latheman who has once had a set of tools tempered throughout by slow drawing, either in an oven, or on a hot plate, will ever consent to point hardening afterwards. A plate of iron, two to two and one-half inches thick, placed over the top of a tool dressing fire, makes a convenient arrangement for tempering tools, besides adding greatly to the convenience of slow heating, which is almost as important as slow drawing. The writer has by actual experiment determined that the amount of tool dressingand tempering, to say nothing of time wasted in grinding tools, may in ordinary machine fitting be reduced one-third by "oven tempering."
As to the shades that appear in drawing temper, or tempering it is sometimes called, it is quite useless to repeat any of the old rules about "straw colour, violet, orange, blue," and so on; the learner knows as much after such instruction as before. The shades of temper must be seen to be learned, and as no one is likely to have use for such knowledge before having opportunities to see tempering performed, the following plan is suggested for learning the different shades. Procure eight pieces of cast steel about two inches long by one inch wide and three-eighths of an inch thick, heat them to a high red heat and drop them into a salt bath; preserve one without tempering to show the white shade of extreme hardness, and polish one side of each of the remaining seven pieces; then give them to an experienced workman to be drawn to seven varying shades of temper ranging from the white piece to the dark blue colour of soft steel. On the backs of these pieces labels can be pasted describing the technical names of the shades and the general uses to which tools of corresponding hardness are adapted.
This will form an interesting collection of specimens and accustom the eye to the various tints, which after some experience will be instantly recognised when seen separately.
It may be remarked as a general rule that the hardness of cutting tools is "inverse as the hardness of the material to be cut," which seems anomalous, and no doubt is so, if nothing but the cutting properties of edges is considered; but all cutting edges are subjected to transverse strain, and the amount of this strain is generally as the hardness of the material acted upon; hence the degree of temper has of necessity to be such as to guard against breaking the edges. Tools for cutting wood, for example, can be much harder than for cutting iron, or to state it better, tools for cutting wood are harder than those usually employed for cutting iron; for if iron tools were always as carefully formed and as carefully used as those employed in cutting wood, they could be equally hard.
Forges, pneumatic machinery for blast, machinery for handling large pieces, and other details connected with forging, are easily understood from examples.
(1.) What causes tools to bend or break in hardening?—(2.) What means can be employed to prevent injury to tools in hardening?—(3.) Can the shades of temper be produced on a piece of steel without hardening?—(4.) What forms a limit of hardness for cutting tools?—(5.) What are the objects of steel-laying tools instead of making them of solid steel?
(1.) What causes tools to bend or break in hardening?—(2.) What means can be employed to prevent injury to tools in hardening?—(3.) Can the shades of temper be produced on a piece of steel without hardening?—(4.) What forms a limit of hardness for cutting tools?—(5.) What are the objects of steel-laying tools instead of making them of solid steel?
The fitting or finishing department of engineering establishments is generally regarded as the main one.
Fitting processes, being the final ones in constructing machinery, are more nearly in connection with its use and application; they consist in the organisation or bringing together the results of other processes carried on in the draughting room, pattern shop, foundry, and smith shop.
To the unskilled, or to those who do not take a comprehensive view of an engineering business as a whole, the finishing and fitting department seems to constitute the whole of machine manufacture—an impression which a learner should guard against, because nothing but a true understanding of the importance and relations of the different divisions of an establishment can enable them to be thoroughly or easily learned.
Finishing, therefore, it must be borne in mind, is but one among several processes, and that the fitting department is but one out of four or more among which attention is to be divided.
Finishing as a process is a secondary and not always an essential one; many parts of machinery are ready for use when forged or cast and do not require fitting; yet a finishing shop must in many respects be considered the leading department of an engineering establishment. Plans, drawings and estimates are always based on finished work, and when the parts have accurate dimensions; hence designs, drawings and estimates may be said to pass through the fitting shop and follow back to the foundry and smith shop, so that finishing, although the last process in the order of the work, is the first one after the drawings in every other sense; even the dimensions in pattern-making which seems farthest removed from finishing, are based upon fitting dimensions, and to a great extent must be modifiedby the conditions of finishing.
In casting and forging operations the material is treated while in a heated and expanded condition; the nature of these operations is such that accurate dimensions cannot be attained, so that both forgings and castings require to be made enough larger than their finished dimensions to allow for shrinkage and irregularities. Finishing as a process consists in cutting away this surplus material, and giving accurate dimensions to the parts of machinery when the material is at its natural temperature. Finishing operations being performed as said upon material at its normal temperature permits handling, gauging and fitting together of the parts of machinery, and as nearly all other processes involve heating, finishing may be called the cold processes of metal work. The operations of a fitting shop consist almost entirely of cutting, and grinding or abrading; a proposition that may seem novel, yet these operations comprehend nearly all that is performed in what is called fitting.
Cutting processes may be divided into two classes: cylindrical cutting, as in turning, boring, and drilling, to produce circular forms; and plane cutting, as in planing, shaping, slotting and shearing, to produce plane or rectangular forms. Abrading or grinding processes may be applied to forms of any kind.
To classify further—cutting machines may be divided into those wherein the tools move and the material is fixed, and those wherein the material is moved and the tools fixed, and machines which involve a compound movement of both the tools and the material acted upon.
There is also a distinction between machine and hand cutting that may be noted. In machine cutting it is performed in true geometrical lines, the tools or material being moved by positive guides as in planing and turning; in hand operations, such as filing, scraping or chipping, the tools are moved without positive guidance, and act in irregular lines.
To attempt a generalisation of the operations of the fitting shop in this manner may not seem a very practical means of understanding them, yet the application will be better understood as we go farther on.
Cutting tools include nearly all that are employed in finishing; lathes, planing machines, drilling and boring machines, shaping, slotting and milling machines, come within this class. The machines named make up what are called standard tools, suchas are essential and are employed in all establishments where general machine manufacture is carried on. Such machines are constructed upon principles substantially the same in all countries, and have settled into a tolerably uniform arrangement of movements and parts.
Besides the machine tools named, there are special machines to be found in most works, machines directed to the performance of certain work; by a particular adaptation such machines are rendered more effective, but they are by suchadaptation unfitted for general purposes.
General engineering work cannot consist in the production of duplicate pieces, nor in operations performed constantly in the same manner as in ordinary manufacturing; hence there has been much effort expended in adapting machines to general purposes—machines, which seldom avoid the objections of combination, pointed out in a previous chapter.
The principal improvements and changes in machine fitting at the present time is in the application of special tools. A lathe, a planing machine, or drilling machine as a standard machine, must be adapted to a certain range of work, but it is evident that if such tools were specially arranged for either the largest or the smallest pieces that come within their capacity, more work could be performed in a given time and consequently at less expense. It is also evident that machine tools must be kept constantly at work in order to be profitable, and when there are not sufficient pieces of one kind to occupy a machine, it must be employed on various kinds of work; but whenever there are sufficient pieces of the same size upon which certain processes of a uniform character are to be performed, there is a gain by having machines constructed to conform as nearly as possible to the requirements of special work, and without reference to any other.
It is now proposed to review the standard tools of a fitting shop, noticing the general principles of their construction and especially of their operation; not by drawings nor descriptions to show what a lathe or a planing machine is, nor how some particular engineer has constructed such tools, but upon the plan explained in the introduction, presuming the reader to be familiar with the names and purposes of standard machine tools. If he has not learned this much, and does not understand the names and general objects of the several operations carriedon in a fitting shop, he should proceed to acquaint himself thus far before troubling himself with books of any kind.
(1.) Why cannot the parts of machinery be made to accurate dimensions by forging or casting?—(2.) What is the difference between hand tool and machine tool operation as to truth?—(3.) Why cannot hand-work be employed in duplicating the parts of machinery?—(4.) What is the difference between standard and special machine tools?
(1.) Why cannot the parts of machinery be made to accurate dimensions by forging or casting?—(2.) What is the difference between hand tool and machine tool operation as to truth?—(3.) Why cannot hand-work be employed in duplicating the parts of machinery?—(4.) What is the difference between standard and special machine tools?
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 beendevised. 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 tooland 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 thetwo 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 exceedingtwenty 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 tobe 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 angularways 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?
(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 angularways 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, andvice 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 jointwhere 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 morefirmly 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, andthe 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.