This section is from the book "Lathe Design, Construction And Operation, With Practical Examples Of The Lathe Work", by Oscar E. Perrigo. Also available from Amazon: Lathe Design: Construction And Operation.
From these examples and remarks it will be seen that much depends on making the center holes of the right form if we expect to produce a good piece of turned work.
In centering large pieces of work it is sometimes the custom to hold one end of the shaft or forging in a chuck on the main spindle of the lathe, and the other end in a steady rest, or center rest. The lathe is started and a pointed tool set in the tool-post is brought against the work and the center scratched into it as it revolves. This is quicker and more accurate than the scribing method, particularly in the case of heavy and rough forgings.
The piece of work having been properly centered, we apply to it a dog which serves to drive it and suspend it between the centers, first carefully oiling the tail-stock center and setting it up just tight enough to hold the work closely and without end motion.
Lathe dogs are of various kinds. The most common kind is that shown at 1, in Fig. 209, which is fixed to the piece to be turned by the set-screw and the work is driven by the tail of the dog entering the driving slot in the small face-plate of the lathe. The clamp dog shown at 2 is useful for driving square or flat pieces, and is also frequently used for cylindrical work, which it is not so liable to mar as is the set-screw of the first form.
At 3 is shown another form known as a die dog, the jaws being movable and closed up by the set-screw. The jaws being threaded may be applied to threaded work which is of such form that a dog cannot be placed upon any part but that which is threaded.
At 4 is shown what is called a two-tailed dog, sometimes used on large work and driven from "drivers" placed against the two tails. These drivers may be made for the purpose and consist of a piece of round steel of sufficient length to reach from the front of the faceplate out to and across the dog, and be secured to the face-plate by a cap screw, with a washer under its head, and coming through the face-plate from the back and into the end of the driver. Or it may have a shoulder and be held by a nut.
More often, however, the driver is a bolt long enough for the purpose, with a sleeve made of a piece of gas pipe or a block of cast iron with a hole through it, which keeps the end of the bolt far enough to reach the dog.
In placing dogs on finished work a piece of brass or copper should be put under the points of the set-screws to prevent marring the work. In using the clamp dog at 2 on finished work the pieces of brass or copper should also be used.
Various other forms of dogs are used for special work and for very large work; as, for instance, two more or less curved bars and fastened together by bolts, somewhat in the form shown at 2, Fig. 209.
But in all cases the principle is the same, to clamp to the piece of work a device having formed upon it a projecting part, called the tail, by which the work may be rotated.
In some cases when the clamp dog shown at 2 is much used on taper work the heads of the clamp screws are made in the form of eyes, and the upper cross bar or clamp bar has trunnions or bearings turned on each end which enter into the holes or eyes of the bolts. By this means the clamp bar may turn in its bearings sufficiently to have its flat side set fairly on the inclined surface of the taper.
In driving bolts which are to be threaded and in which the marks of the center hole in the top of the head are not objectionable, a "bolt dog" is used. This is simply an offset plate fastened to the face-plate by a single bolt and its free end slotted so as to embrace the head of the bolt. This device is not much used at the present time as bolts and cap screws are usually made from a bar in the turret lathe at much less cost than is possible to produce them in an engine lathe.
Lathe work that is not held suspended between centers must be held by one of the following methods, namely: bolted or clamped to the face-plate; held entirely in a chuck; one end held in a chuck and the other in a center rest; or secured to the carriage, or some part of it, as in boring jobs. One exception is made to these statements. This is that work may be held against the head spindle center by any convenient means, and the other end supported in a center rest. This is usually only resorted to for such work as boring and reaming and, with the exception of the advantage derived from accurate centering by means of the head spindle center, is not a very advisable method of running work in a lathe, particularly when a chuck with truly concentric jaws is at hand.
What is ordinarily called center rest work is all kinds in which one end is supported in a center rest. Of course this does not include work held on centers and supported in the center or at any intermediate point by a center rest. In this case many machinists call it a " steady rest," rather than a center rest, and this function may be readily performed by a back rest or what is called by some manufacturers a steady rest, which has the three jaws of the center rest, although they are not placed equidistant around the circle and the supporting casting is left open in front instead of being provided with a hinged top segment.
Chuck work and face-plate work is very closely allied, and in fact very many face-plate jobs can readily be done in a chuck, and nearly all chuck jobs can be done if fixed to the face-plate in the usual manner. It is altogether probable that the first chuck made was simply a face-plate provided with jaws temporarily attached, and it is more than likely that these "jaws" consisted merely of blocks or studs fastened to the face-plate and provided with set-screws for holding the work.