Chapter IX.
There is no operation in which the young mechanic is so much at fault as in that of grinding and setting in order the various tools he has to use. Nevertheless he will never become either an independent workman or a good one, if he has to depend upon others for this necessary labour.
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.
Now, all this is very simple and easy to understand, and when you have mastered this much, you will be in a fair way to understand more. The second part of the subject, nevertheless, requires very close attention, and very likely may not become quite clear to you when explained. I shall therefore draw a line here, and make this lesson a special paragraph, which you can look back to some other day, when you are grown from a boy-mechanic to a man, and have had more experience in cutting and turning wood and metal.
The tools above described have their cutting edges formed by the meeting of two planes at a given angle,—these planes being the flat bevels (or the flat top and one bevel) formed by the grindstone. But in some tools three planes meet to form an edge instead of two, and the angle of the cutting edge is not the same as that of either of these, although it depends upon them, and can be nicely calculated. This calculation, however, requires a knowledge of some higher branches of mathematics than the young mechanic is supposed to be acquainted with, and therefore a table is added instead, by which, when the angles of two of these planes are known, the third may be at once seen, which last determines, of course, the angle of the edge.
As an example, take the graver, of which you will find a drawing among the other tools, but which I give again in this place. M, Fig. 45, is the tool, looking at the face or bevel which has been ground upon it, making a lozenge-shape or diamond. But this face is a third plane, and the cutting edges, a and b, depend for their angles upon all three of these. Now, for iron we want an angle of 60°. How are we to make the edges, a b, of that exact size? The bar is first of all square in section, like N, which would be its shape before the third face or bevel is ground, and all the angles are now right angles of 90° each. But instead of this, we want two of them 60°, the other two being of no importance. We simply proceed thus:—Determine which angle is to become the point of the tool (it is no matter in the present case, as all are alike), then grind away underneath till the new bevel forms an angle of 45° with the back (by which I mean the edge which runs along from the sharp point towards the handle—the edge x in fig. O). Trigonometry enables us to find out that an angle of 45° is the one required, but you will find it in the table annexed to this chapter, and an explanation of this table is also given to enable you to use it easily. Thus ground, the edges a b of fig. O will be each formed of two planes meeting at an angle of 60°. You can make a gauge of card or tin, P, to work by, of the required angle.
Fig. 46.
Fig. 47.
In order to understand the use of this table, it is necessary to give names to the several angles of a tool. That upon the front or face of the tool, as A of the point-tool, is called the plan-angle; that made by the upper surface and the front edge, as B (a, being the angle in question), is called the section angle, because, if you were to saw right through the central line lengthwise, this is the angle that would appear at the point, viewing it sideways. Now, if we look at C, Fig. 47, we shall be able to understand how the front line, b c, is obtained, which constitutes one side of the section angle of a tool. It results from the meeting of the two diamond-shaped planes at the sides formed by the grindstone, but is dependent also on the plan-angle. These two side-planes are to be generally ground at an angle of about 3° from the vertical, which is to give the clearance of the tool if held in a fixed position, as in the tool-holder of a slide-rest, the tool being supposed horizontal. This is in accordance with what I have before told you, viz., that the cutting edge should be presented to the work at the smallest possible angle, 3° being very small indeed. This angle is generally measured by placing the side ground in contact with a cone of wood or metal, turned to an angle of 3°, such as D,—k being a tool the front of which is evidently 3°; or a piece of tin, l, cut to the same angle, and stood on its edge, will answer the same purpose. By 3°, I mean an angle of 3° measured on the circumference of a circle, as I have already explained in a former page, such angle being of course at the centre of the circle where the lines drawn from the several degrees on the circumference meet.
Now, when you have ground these two surfaces, the line b c of B (or C) will have a certain slope or inclination depending on the plan-angle of the point. The exact inclination of it may be therefore said to be accidental; but, whatever it is, it becomes of great importance in the final result, being one side of the angle which will give any particular angle of cutting edge. And here the table comes into use:—Suppose I wish to have an edge of 60°, for cutting iron. Measure the plan-angle,—say it is 90°, which is that of the graver; then, on the table, under the words “plan angle,” you will see 90°, and opposite, above 60° of “cutting edges,” you will see 45°. You have only to grind back the upper face of the tool, until it makes an angle of 45° (section angle) with the front edge or line, b c, and the edges x x will be angles of 60°. Or take the tool E, of which the plan angle is 120°, and suppose you want cutting edges of 80°, for brass, opposite 120°, and above 80°, is 78° 5″. Grind back the top face to an angle of 78° 5″ (or 78½) with the point line, and it is done.
Until you have practically proved it, you can have no idea of the vast importance of having correctly-formed cutting edges, and of placing them within a hair’s-breadth of the proper position. But it is in slide-rest work especially, and in cutting metal with tools held rigidly in one position, that this is of such paramount importance. It makes all the difference between cutting off a clean shaving, and tearing from the material by main force a quantity of disjointed particles, the latter process leaving a rough unfinished surface, the former producing one as smooth and polished as a sheet of glass; and the advantage of this short table is, that you can at any time shape your own tools for the particular work in hand.
After you have had some practice in turning, you should certainly learn to shape your tools from square bars of steel, worn files, and broken steel tools of various kinds; and before you have arrived at sufficient dexterity to do this entirely by yourself, you will get them roughly shaped for you by the blacksmith, and then with grindstone and file you will further perfect the angles for use. Steel does not require, and must on no account be subjected to, a white heat, or you will spoil it hopelessly; and you can always heat it in a common fire, or in the little stove that I shall describe in a subsequent chapter, to a temperature that will allow you to bend it into any required form with the hammer and anvil—a bright red being the utmost heat it must be brought to.