No doubt, to sharpen a tool which is in very bad order is a tedious and tiresome job; but it is not so wearisome an affair to keep tools in condition for work, after they have been once thoroughly sharpened by one who understands how to do it. Never, therefore, use a blunt tool, but at once go to the hone or grindstone with it, and put it in first-rate order. Time thus employed is never wasted, but rather saved; and the result will appear invariably in the work which you are engaged upon. You must, in the first place, understand precisely what it is you have to do; and although the following details may be by some considered more adapted for advanced students than for young mechanics, a little attention to the explanations will render the matter clear to any boy of age and intelligence to take in hand, with reasonable prospect of success, the tools of the carpenter, turner, and fitter. I can only say, that boys of this generation are wonderfully well off in having these things explained to them. Twenty years ago young mechanics had to grope along in the dark, ignorant to a great extent of the principles of work, and almost equally uninstructed in the practical part of it.
In Fig. 45 are represented similar angles to those already explained to you, and you will quickly understand how useful is a little knowledge of the elements of mathematics. Suppose A to be a tool, the angle of the point is a right angle, or 90°. B is another of 60° at the point, and I have drawn a line across to show you that the three sides of this figure (called a triangle) are equal. So remember that if you want an angle of 60°, you have only to draw a triangle of three equal sides, and each of these angles will be 60°. Again, I may as well remind you that three times 60° equals 180°, which is equal to two right angles, so we find here that the three angles of an equal-sided triangle equal two right angles, and even if the sides are not equal, the same thing is true. For instance, look at the first tool, across which I have also drawn a line to make a triangle. The point we know is 90°, and if the sides, a b, are equal (although the third line is not equal to either), the two small angles are each 45°, i.e., 90° between them, so the three angles again equal 180°.
Fig. 45.
The third tool (which we may suppose a turner’s chisel held edgewise) is shown to have an angle of 30°, and I have added one more which has an angle of 45°. Now all tools, if well ground, are ground to a certain known angle, according to the material which they are intended to cut. Tools intended to cut soft woods, like deal, are ground to an angle of 20° to 30°, like the chisel seen edgewise. I shall have a word to say presently as to the direction in which such tools are to be held, in order to make them cut as well as possible. A tool for hard wood is given next at E. The angle is now at least 40°, and it ranges up to 80°, giving a stronger, thicker edge, but not so keen a one. We have, therefore, more of a scraping tool than a cutting one,—at least, in the way it is usually held. Then we come to the tools with which iron is turned and steel also. Fig. F is one of these, and the usual angle is 60°, and thence it ranges to 90°. Thus you see, advancing from soft wood tools to those for hard wood, and thence to a substance still harder, we have increased the angle of the edge, beginning at 30° and ending with 80° or 90°. But now we come to a material which is harder than wood and not so hard as iron, yet we use tools with an angle of 90°, which is still greater, and 70° is the least angle ever used for this metal.
Experience only has taught the proper angle for tools, and it is found, that if brass and gun-metal are turned with tools of a less angle than 70°, they only catch into the material, and do not work at all satisfactorily. You can, however, scrape brass, as a finish, with the thin edge of a common chisel; but then the tool is held so as to scrape very lightly and polish; and its edge will not remain many minutes, unless the maker (intending it to be so used) has made it much harder than he would make it for soft wood cutting.
If you buy your tools at any good shop, you will find that they are already ground to nearly the angles named, and when you re-grind them, you must endeavour to keep them to the same. The bevel, as it is called, of many tools need not be ground at all, as they may be sharpened solely by rubbing the upper face on a hone, or grinding it, holding it so that the stone shall act equally on all parts of it. If, however, the tool should become notched, you must grind the bevel of it, and then you must try and keep the intended angle. One tool, however, or rather one pair of tools, viz., turning-gouges and chisels, are very seldom ground with a sufficiently long bevel when they first come from the maker. The usual shape of the edge is like G, whereas the angle should be much less, as seen at H. This you must correct when you first grind the tools for use, and keep the same long bevel and small angle of edge continually afterwards, for you will never make good work on soft wood if your chisels and gouges are ground with too short a bevel.
I must also guard you against another common error, which, however, is very difficult to avoid at first, and only long practice will enable you entirely to overcome it. I, is the chisel (held edgewise as before) ground as it ought to be; K is the same tool ground as it generally is by young hands, or, even if it is correctly formed at the grindstone, one or two applications to the oilstone almost invariably round it off as shown. The bevel of all tools must be kept quite flat and even, and when the tool is afterwards rubbed on the oilstone to give a finish to the edge, another flat, even bevel should be made. In the same figure at L is an exaggerated view of the chisel, with its first long bevel formed at the grindstone, and the second very small bright bevel seen at the extreme edge of all such tools when they have been set upon the oilstone. This second bevel, slight as it is, you will at once understand makes the angle of the edge a little larger, therefore you must allow for it, and grind a little keener edge than you really require.