Machine motion is mainly rotary3; and as rotary motion is accomplished4 by cylindrical parts such as shafts5, 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 intermittently6 on plane surfaces.
In practice, the truth of this proposition is fully7 demonstrated by the excess in the number of lathes9 and boring tools compared with those for planing.
An engine lathe8 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 [122] 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 perishable10 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 essentially11 of a strong inflexible12 shear13 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 supplanting15 the skill of lathemen.
[123]
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, durability16 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, rigidity17 of frames, and so on, but such requirements are obvious.
To avoid imperfection in the running spindles of lathes, or any lateral18 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 rotation19, but must at the same time involve mechanical complication to outweigh20 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 shears21 and track shears.
These different plans will be considered first in reference to the effect produced upon the movement of carriages; this includes friction14, 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 [124] and the shape of the piece acted upon, directly to the extent of such deviation22, 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 vertical23 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 oblique24, 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 obviated25 by numerous cross girts, which will be found in most lathe frames having flat tops.
A carriage moving on angular ways always moves steadily26 and easily, without play in any direction until lifted from its bearing, which rarely happens, and its lifting is easily opposed by adjustable27 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 triangular28 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 binding29 the [125] 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 apparently30 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 varied31 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 attain32 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 clearance33 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 [126] 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 pertaining34 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 apprentice35 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 [127] 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 computing37 wheels for screw cutting, when there is but one train of wheels. Wheels for screw cutting may be computed38 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 brass39; the relative speed of the belt cones40, whether the changes are by a true ascending42 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 cone41 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 dealing43 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 evade44 hand tool work, and in many cases effect no saving except in skill. A latheman who is skilful45 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 uncommon46 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 applied36 to the finer class of lathes?—(4.) How may the alignment47 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 [128] ways of a lathe and the corresponding grooves48 in a carriage be planed to fit without employing gauges49?—(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?
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1
machinery
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| n.(总称)机械，机器；机构 | |
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cylindrical
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| adj.圆筒形的 | |
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rotary
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| adj.(运动等)旋转的；轮转的；转动的 | |
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accomplished
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| adj.有才艺的；有造诣的；达到了的 | |
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shafts
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| n.轴( shaft的名词复数 )；（箭、高尔夫球棒等的）杆；通风井；一阵（疼痛、害怕等） | |
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intermittently
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| adv.间歇地；断断续续 | |
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fully
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| adv.完全地，全部地，彻底地；充分地 | |
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lathe
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| n.车床，陶器，镟床 | |
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lathes
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| 车床( lathe的名词复数 ) | |
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perishable
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| adj.（尤指食物）易腐的，易坏的 | |
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essentially
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| adv.本质上，实质上，基本上 | |
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inflexible
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| adj.不可改变的，不受影响的，不屈服的 | |
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shear
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| n.修剪，剪下的东西，羊的一岁；vt.剪掉，割，剥夺；vi.修剪，切割，剥夺，穿越 | |
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friction
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| n.摩擦，摩擦力 | |
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supplanting
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| 把…排挤掉，取代( supplant的现在分词 ) | |
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durability
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| n.经久性，耐用性 | |
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rigidity
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| adj.钢性，坚硬 | |
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lateral
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| adj.侧面的，旁边的 | |
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rotation
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| n.旋转；循环，轮流 | |
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outweigh
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| vt.比...更重，...更重要 | |
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shears
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| n.大剪刀 | |
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deviation
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| n.背离，偏离；偏差，偏向；离题 | |
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vertical
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| adj.垂直的，顶点的，纵向的；n.垂直物，垂直的位置 | |
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oblique
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| adj.斜的，倾斜的，无诚意的，不坦率的 | |
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obviated
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| v.避免，消除（贫困、不方便等）( obviate的过去式和过去分词 ) | |
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steadily
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| adv.稳定地；不变地；持续地 | |
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adjustable
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| adj.可调整的，可校准的 | |
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triangular
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| adj.三角（形）的，三者间的 | |
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binding
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| 有约束力的，有效的，应遵守的 | |
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apparently
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| adv.显然地；表面上，似乎 | |
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varied
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| adj.多样的，多变化的 | |
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attain
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| vt.达到，获得，完成 | |
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clearance
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| n.净空；许可(证)；清算；清除，清理 | |
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pertaining
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| 与…有关系的，附属…的，为…固有的(to) | |
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apprentice
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| n.学徒，徒弟 | |
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applied
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| adj.应用的；v.应用，适用 | |
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computing
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| n.计算 | |
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computed
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| adj.[医]计算的，使用计算机的v.计算，估算( compute的过去式和过去分词 ) | |
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brass
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| n.黄铜；黄铜器，铜管乐器 | |
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cones
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| n.（人眼）圆锥细胞；圆锥体( cone的名词复数 )；球果；圆锥形东西；（盛冰淇淋的）锥形蛋卷筒 | |
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cone
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| n.圆锥体，圆锥形东西，球果 | |
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ascending
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| adj.上升的，向上的 | |
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dealing
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| n.经商方法，待人态度 | |
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evade
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| vt.逃避，回避；避开，躲避 | |
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skilful
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| (=skillful)adj.灵巧的，熟练的 | |
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uncommon
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| adj.罕见的，非凡的，不平常的 | |
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alignment
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| n.队列；结盟，联合 | |
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48
grooves
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| n.沟( groove的名词复数 )；槽；老一套；（某种）音乐节奏v.沟( groove的第三人称单数 )；槽；老一套；（某种）音乐节奏 | |
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49
gauges
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| n.规格( gauge的名词复数 )；厚度；宽度；标准尺寸v.（用仪器）测量( gauge的第三人称单数 )；估计；计量；划分 | |
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