This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
An apparatus called a boring collar, somewhat similar to that just described, is used for supporting the ends of pieces of which the ends are to be bored, and which are too long to be held by the cup-chuck alone. It consists of a plate similar to a face-chuck, Fig. 118G, through which a number of conical holes are bored, whose centres are equidistant from the centre of the plate, so that when the latter is turned on its axis any hole can be brought exactly in a line with the 2 centres. The plate may be attached to a standard similar to either of the foregoing.
It may sometimes occur that the work to be turned, as a wheel, the foot of a stand, and so on, may be rather too large for the lathe; in this case it is convenient to have frames truly planed and fitted. Such a frame is shown at Fig. 1187. It is made of cast iron, the top being fitted to the bottom of the poppet, and the bottom being fitted to the lathe-bed, care being taken that the mandrel is retained parallel to the lathe-bed. The rest may be blocked up in a similar manner, or a temporary rest may be made of a piece of bar iron bent to a suitable form.
In some cases it will be convenient to have a self-acting slide-rest, as for turning large screws, spirals, and so on. The slide-rest is shown in Fig. 1188 (elevation) and Fig. 1189 (plan), a is a slide which fits the lathe-bed very accurately, but will yet slide freely upon it, and in a direction exactly parallel to the axis of the object to be turned. b is another slide fitted to the lower one and sliding upon it in a direction at right angles to the lathe-bed. It is worked by a screw attached to the lower slide, which gears into a nut fixed to the bottom of the slide b. Upon the slide b is fitted a small slide c, upon which the turning tool is fixed by means of a clamp. This slide is moved in a direction parallel to the lathe-bed by means of a screw attached to the slide b, gearing in a similar manner to that in the slide a. The whole slide may be moved along the bed either by hand or by means of a screw running along the side of the bed and gearing into a nut made in 2 halves, so that it may be thrown into or out of gear by closing or opening the nut.
The use of this screw, which is called the leading screw, requires a different form of fixed poppet-head, and constitutes what is called a screw-cutting lathe, on account of its suitability to that process.
The poppet-head generally fitted to self-acting lathes is represented in Figs. 1190 to 1192. a is a side elevation, b a plan, and c a front elevation. This head is fitted with speed pulleys /, which may be made fast to the mandrel, so as to drive it direct or loosened, and geared by a tooth-wheel with the shaft g, which again gears into the mandrel, which is supported in bearings at each end. The wheels on the shaft g are thrown out of gear with those on the mandrel by sliding the shaft endwise in its bearings. It is retained in or out of gear by a pin passing into the bearing, which rests against a groove turned on the shaft g. On the end e of the mandrel a toothed wheel is slid and retained there by a nut. This wheel may act directly upon another placed on the end of the leading screw, or may be connected with it by means of one or two intermediate wheels, according to the speed required and the direction of the intended screw.
It is evident from this arrangement that any ratio between the speeds of the mandrel and leading screw may be obtained either for cylindrical turning or screw cutting.
Fig. 1193 is a very complete double-gear foot-lathe, with planed bed, standards, antifriction treadle, with chain, crank, and driving wheel, hand-rest, face-plate, drill-chuck, and 2 centres.
Fig. 1194 is a single-gear foot-lathe, with planed bed, standards, anti-friction treadle, with chain, crank, and driving wheel, hand-rest, face-plate, drill-chuck, and 2 centres.
Fig. 1195 is a compound slide-rest; another arrangement of compound slide-rest is shown in Fig. 1196.
With reference to lathe manipulation, which is perhaps the most difficult of all shop operations to learn, the following hints are given by Richards in his excellent manual on ' Workshop Manipulation.'
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 an 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 20 in. of diameter, will require 25-35 tools, which will serve for every purpose if they are kept in order and in place. A workman may get along with 10 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 50 lb. of extra cast steel tools, as an in-vetstment, 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.