The cup-chuck is the last required. It is sketched at P, and is sometimes of iron, but generally of brass. There are several sizes made and sold with lathes, but you need not have at most more than one or two, as I shall show you how to make wooden ones, which answer as well, if not better. The flat plates, R, R², can scarcely be called chucks, but they generally come into the list of such. The latter has five projecting points, which, sticking into such a thing as a flat-board (like a bread-platter, or round pulley), hold it sufficiently firm when the back centre is brought up against the other side of the piece, to allow of its being turned. The other is merely a flat plate with holes in it, through which screws can be passed from behind into any odd bit of wood of 2 or 3 inches in thickness, whereby a chuck can be quickly made to suit any required purpose. Two or three of these would be convenient, one of which should be nearly as large as the lathe will carry; and in this one a great many holes and slots should be made. This is called a face-plate, and, in addition to the ordinary screws, whereby pieces of wood are attached to it, it is fitted with clamps and bolts of various forms, for the purpose of holding securely upon its face all kinds of flat works in wood or metal,—such as cog-wheels, which have to be bored out and faced. The young model-maker will find a face-plate of great service. The larger one should be of iron, as it will be cheaper than brass.
We now pass on to chucks for metal turning. These are of various shapes. First in order comes the centre chuck and dog, for holding rods of iron which can be supported at both ends. The commonest form is represented in Fig. 41, S, T. S is such a face-plate almost as I have described, but it has a pin projecting from it, and also a steel centre-point. The latter is often made to screw out and in, which is the best plan. The pin can be slid to any point in the face-plate, and clamped by a nut at the back. T is called a dog, and of these two at least will be required, if the young mechanic intends to work in metal.
The way of using these is shown at T². The rod of iron has a hole drilled at each end, as nearly in the centre as possible. It is first indented with a punch, then a drill is put into the drill chuck, and one end of the rod brought against it as it revolves, while the back poppit centre-point is screwed against the indentation at the other end. A little oil is applied to the drill to assist its working, and the rod itself is prevented from turning round either by grasping it with the hand or screwing a hand-vice upon it, so that this comes against the bed or the rest; or it can be held in the hand, which has one advantage, namely, that the operator can feel exactly what is the resistance caused by the drill, and can regulate the pressure accordingly. The screw of the poppit is, of course, to be very slowly and steadily advanced during the process. All drilling in the lathe is done in this way, but in boring out long holes, the action is often reversed, the work being kept in motion while the tool is advanced, without being allowed to revolve. You need not bore more than one-eighth of an inch for light work, but must do the same at each end of the rod. The holes thus made should be of such a size as not to let the extreme end of the back centre-point touch the bottom, or it will soon be worn down and blunted;—remember this in all future work.
Supposing the rod to be thus bored at each end, place the centre-chuck upon the mandrel, instead of the drill-chuck, and mount the bar between this and the point of the back-centre. Thus placed, it will be accurately supported, but if the lathe is put in motion, it will not turn round. Now come into use the little dogs. Remove the bar, and choosing a dog of which the open part is tolerably near the size of it, slip it over the end about half an inch, and there fix it by tightening the little screw, which, you observe, will drive the bar as far as possible towards the smaller part of the opening, and when it can go no farther, will secure it as in a vice. It is a good plan to file a slight flat upon the bar, just where the screw of the carrier will come. Now replace the bar, and when the lathe is put in motion, the tail of the carrier should come against the projecting pin in the face of the face-plate, which will compel the iron to go round with it. This is the way all bars of metal are mounted. I shall not tell you yet how they are to be turned, because this would interfere with the order of my description.
To mount in the lathe such pieces as cylinders of engines, which require to be bored, or any other objects which have to be turned on one or both faces, the young mechanic must make wooden chucks, and bore them out exactly to fit the article and hold it securely. There are metal chucks expressly made to take all work of this kind, and which are so contrived that they will also hold it truly central, but they are costly, and need not be obtained with the first lathe—at any rate, not until absolutely required, and that will be, I know, a long time hence; ay, a very long time, for many good workmen have never even seen, much less possessed one of them. Perhaps I may draw and explain one in a future page, as well as some other chucks, which it is not necessary to notice here.
The chucks then absolutely necessary are these—
1. Square Hole Chuck, which will take the prong, the cross, the drills, and short bits of iron to be turned.
2. The Taper Screw.
3. Flange or Face Chucks, one with five points, and two with holes for screws, also one larger for a face-plate.
4. Two or three Cup-chucks (I can, however, scarcely call these absolutely necessary).