POSITION OF CUTTING TOOLS.
We must now consider the mode of applying the edge of a tool to the work, so as to produce the best effect. First, we will consider the case of a gouge and chisel acting upon soft wood.
Fig. 48.
In Fig. 48, A represents a piece of wood in the lathe, as you would see it if you stood at one end of it, and a chisel is being held against it. The arrow shows the direction in which the wood is supposed to be revolving. Held thus, the chisel would scrape, and its edge would be carried off at once; it could not possibly cut. But, held as at B, it would cut off a clean and continuous shaving as the wood revolved against it, and this shaving would slide off along the upper face, b, of the tool, so that you can see that this face ought to offer the least possible resistance to it. The tool acts, in fact, like a very thin, sharp wedge, which divides the material by pressure, which has to be great or slight according as the edge is sharp and thin or the contrary. Now, if you again look at A, you will see that this wedge-like action cannot take place, so that the tool is in its worst possible position.
Between the two positions, however, here shown, are several others at a greater or less angle to the surface of the wood; but the smallest possible angle it can make is the best, so long as the thickness of shaving removed will suffice for your purpose. This rule holds good with all tools, whether carpenters’ or turners’, which are made with sharp-cutting edges. Care must be taken, however, that the lower face of the tool does not rub against the work, which, again, it is evident, limits to a given degree the angle at which the cutting edge is to be applied to the work.
We now pass on to C, which represents the ordinary tool for turning iron, held flat upon the rest, the position it usually occupies. We see at once that in this case also we have a scraping tool only, and that, although the angle of the edge is far greater than that of the chisel, it must soon be ground off by the action of the metal to which it is applied, or of the hard wood, which is also cut in this way. But with this form of tool we shall find it impossible to apply it so as to cut in the best way; because if we lower the handle, as we did that of the chisel, the part below the edge will rub against the work, while the edge itself will be moved out of contact with it. Thus we are obliged to hold the tool in the position first shown; but we may therefore conclude that the tool itself is a badly formed one for the intended purpose; and so it is, although you will see it in almost every workshop in the kingdom. Let us see what can be done to improve it. At D, I have represented the same tool, but the blackened part shows what has been filed away from the upper face, and the dotted lines show that, when this has been done, a tool is made very similar to the chisel for wood, and that it is also now in a good position for cutting (not scraping), although it is still held horizontally upon the rest. Shavings of iron curl off the upper face of this, as wood shavings curl off upon a chisel.
If the angle, however, is too small, the edge will soon be broken off, and the tool will dig into the work; hence the necessity of knowing at what angle a tool ought to be ground to cut any particular metal successfully.
Such a tool as the last named, which is intended only to cut with the front edge, and which is represented in E, is called a single-edged one, because it only cuts in one direction, but many others are double-edged, cutting the shaving at once on the flat and edge—that is, paring it off from the material below and also from the side. For instance, F is a cylinder of iron, from which a shaving is supposed to be in process of being cut. It has to be removed from the shoulder to which it is represented as still adhering, and also from the flat surface, e b, around which it was, as it were, once coiled. But this requires two cutting edges, both acting at the same time, but in different directions; and good mechanics therefore so form the tools, and so use them, as to cut in both directions, which leaves the work beautifully smooth and even.
These tools are mostly used in the slide-rest, where their true position, once determined, can be accurately maintained; and it is, perhaps, only with the slide-rest that perfect work can be done. There is, however, no reason why you should not use tools of all kinds intelligently, and understand exactly how they should be formed, and how held. Suppose you have a tool correctly made by the aid of the table of tool angles already explained, still looking at fig. F, you can see that the smaller part of the roller is that which is to be left finished, and that it ought to be quite smooth, but the shoulder at a is not of the same degree of importance. A tool fit for such work would evidently be shaped on its plan-angle or face, like H in fig. C or I; and, if held as seen, both edges would be brought into action at the same time, as will be at once evident on inspection. In practice, however, the two edges would not be allowed to touch for their whole length, or the angle on the right would leave a scratch upon the finished work; therefore it would be eased off a little, as at K, L. But this is evidently as nearly as possible the shape and position to be given to such a tool, and the edge which has to leave the finished surface should, as it were, follow the other; the right-hand angle being just and only just kept out of cut.
The hand-tools you will generally use are the heel-tool, M, held on the rest as shown, which, you see, brings the edge into cut at the least possible angle to the work, and the nail-head, which is in fact a heel-tool of four faces, or, if round, a heel-tool all edge, and which can be rolled over as it gets blunted. To these add the graver, of which I have already spoken. I have tried to show its position at O, with the bevel of the face pointed in the direction of the shoulder, and downwards; but it can be held face upwards also, and in one or two other positions. Always remember that the cutting edge is to be presented at a small angle with the work, and you cannot go wrong if the tool is well formed. The nail-head and heel-tools are single edged, and easily ground without the table of angles, but the graver is a double-edged tool, properly speaking, although only one edge may perhaps be used.
Having explained the principles upon which you have to work as regards grinding your tools and holding them when in use, I shall merely add a few remarks as to the action of the grindstone and oilstone, and the proper way of using them.
Always let the stone revolve towards you, as if you had to turn it smooth with the tool you have to sharpen, except when you cannot possibly do so without cutting grooves in it. Chisels, knives, axes, planes, and all similar tools with flat edges, are to be ground with the stone running in that direction, by which means you will avoid giving them a wire edge, as it is called (i.e., a ragged-looking edge), and it will instead be even and sharp; the filament of metal being, as it were, driven back into the substance of the tool, instead of drawn away from it. Gouges may be ground in the same way, but must be rolled about to keep up the form of edge. It is indeed the easiest way with these to hold them across the stone, in the same direction as its axis, and then, by rolling them over backwards and forwards, you can give a very good shape to the edge, which should run slightly to a point, or rather tend to one. They are never to be ground square across, like that of the carpenter.
It is generally necessary to have some sort of rest upon which to lay the tools during the operation of grinding, but do not trust to special contrivances for holding them at the precise angle needed; rather trust to your own skill, which will increase more and more by being severely exercised. Always remember to grind your tools to a sharper angle than will be ultimately required, that the final angle may be given by the oilstone. Of the latter there are many kinds. Nothing probably can surpass a Turkey stone, if good, but this varies considerably in hardness and other qualities. There is a very quick-cutting, slightly coarse stone from Nova Scotia, which is very serviceable, as it does this tedious work with great rapidity, not, however, putting on the tools a very fine edge, but one that admirably suits for such as are to be used on metal. With the rest, a rub or two on Turkey, or Arkansas, or Chorley Forest stone, will impart a finish. Arkansas stone, however, may be had coarse as well as fine; it is much liked by some, but I prefer the Nova Scotia, as it cuts more keenly, and even with the sharpest stone, setting tools is a most laborious process.
The young mechanic will find it very difficult at first to hold the tool steady, and to move it to and fro upon the oilstone so as not to give it any rolling movement, by which the edge and bevel would be rounded, as I before explained, which would in effect enlarge the angle of the cutting edge, besides preventing it from being held at a sufficiently small angle to the work to cut effectively. Nothing but practice will overcome this difficulty; I shall not therefore attempt to describe exactly how the tool should be held and the sharpening effected, such description being not only difficult, but, as experience has proved to me, impossible.
