The remarks of the above eminent mechanic upon this subject, as also those of Professor Willis and Mr. Babbage have been embodied in a very excellent paper by Dodsworth Haydon, Esq., an amateur, and will be found in the Appendix to this work. The whole principle of the formation and application of cutting tools is explained in that paper, so that it only remains to treat briefly of a few special forms of tools which are required for metal-turning in the lathe, whether by hand or by the aid of the slide-rest. In the first place, however, a word or two may be necessary as to the kind or quality of steel required for such tools. What is called Blister steel may be at once passed over as unfit for the formation of tools—it is, however, the raw material (so to speak) from which, by the process of reheating and welding, the next quality, called Shear steel, is made. When bars of this are similarly heated and again welded into a homogeneous mass under the tilt-hammer or between rollers, double shear-steel is made, which is of extensive use for cutting tools, and must, moreover, of necessity be used in almost every case where there is to be an iron shank, for economy's sake, the steel being then welded to the iron, and forming that part of the tool intended for the edge. The third and best kind of all is Cast-steel, formed of blister steel, melted at an intense heat and run into iron moulds. This, however, can be welded only with great difficulty, and hence the whole tool, whatever its length, must be of the same material. This can be purchased in bars of a convenient size of round, triangular, square, or other section, and needs only the careful use of the hammer, file, and grindstone to become a tool of any required pattern. It would be very advantageous to an amateur to master the art of forging in a small way to enable him to make his own tools, for he may sometimes require them of unusual form, and if he lives far from a manufacturing town he will find it very difficult to get them fashioned to his liking. Cast-steel will not allow of the welding heat applied to iron—it will burn, and cannot then be made to recover its proper consistency, and is for ever useless for the purpose in question. Double shear will take a moderate white heat, while cast-steel must not be brought to a higher temperature than that indicated by bright red—a point never to be forgotten when shaping a tool at the forge. There are in every workshop a number of files laid aside as worn out. These being made of the best cast-steel, are invaluable to the turner in metal, as they supply the best material for his tools at no cost whatever. To begin with the saw files (called, by a horrible perversion of mathematical definition, "three square"). Here you possess a tool at once for the mere trouble of grinding off the teeth and reducing the sides to a smooth surface. Each angle is equally useful—each 60°, which, as the paper above alluded to demonstrates, is the best angle for cutting iron. On brass, however, its use is by no means to be recommended, being, as Holtzapffel remarks, "too penetrative and disposed to dig into the work." It is to be used upon iron in the position shown in [Fig. 110], where A is the rest, B the cross section of the tool, C the diametrical line. The side, of which D is a continuation, is to form very nearly a tangent to the circumference of the work, being, as explained in the Appendix, only 3° from that position. Worn out square files being of rectangular section, are exactly suited for brass turning, for which metal a cutting edge is required of 80° to 90°, the former for the first or roughing down cut, the latter for finishing. The position of such tool is shown in [Fig. 111].
Figs. 110, 111.
The flat files are not altogether so useful for making turning tools as those alluded to; they are too thin in proportion to their breadth. Their sides, moreover, generally speaking, are not rectilineal, but curved, so that they are more fit for grinding off at the ends to form brass-turning tools with rectangular edges. They may, however, by careful forging, be made to assume a square section, and thence be formed as desired; but a rectangular bar of steel, ready-made, is then to be preferred. A round or rat's tail file is of far greater service than the last-named—not, indeed, in its own shape, for the reasons stated by Mr. Haydon, but as being capable, with very little labour at the forge, of being converted into a bar of square section. The first tool to be made from such a bar is the graver, the tool of the watchmaker, and not less useful to the general mechanician. This is formed by grinding the end of a square bar diagonally, so as to produce a lozenge-shaped face. The angle to be preferred in this operation is 45°, which will give two cutting edges of 60°. The latter may be varied at pleasure by varying the angle at which this face is ground, as explained in the chapter which treats upon this question. The graver will do all kinds of outside work, light or heavy, as it may be made of any size. It is represented in [Fig. 112]. [Fig. 113] is the heel-tool shown in position for work; [114 and 115], two forms of nail-head tool very commonly used, but both requiring great attention to the angles of the cutting edge to become effective. All the above are for outside work, and are to be so held that the side next to the work—the sole of the tool in [Fig. 113]—forms very nearly a tangent to the work—a position, as Holtzapffel remarks, strangely similar to that required by the soft wood chisel and similar tools. The heel-tool, indeed, if more keenly sharpened, will cut soft wood (on the face) with great rapidity, and is in principle similar to the broads used for that purpose. It is, however, an unsafe tool, owing to its great tendency to dig into the work. It is a good plan to make extensive use of various shapes of hand-tool before passing to the slide-rest, because the hand feels exactly the resistance which the tool meets with, and the best form and position is thus practically tested, and will be found to bear out to the utmost the theory advanced in this work, and founded on mathematical truths worked out and applied by Willis, Babbage, and others.
Figs. 112, 113, 114, 115.
Inside tools must of course be made upon the same principles as the last, the particular form alone being modified to enable the cutting edges to penetrate into the various nooks and corners that may occur in such work. The inside tool for iron ([Fig. 116]), with cutting angles of 60°, is of a general and useful pattern. It must be so curved and so placed that both cutting edges come into action, one on the face and one on the side of the cut, a condition explained in the Appendix as essential to all good work. It being quite impossible to cut metal like wood, and necessary to allow sufficient time for such work, a pointed tool is in most cases preferable to one of a semicircular or rectangular form of edge, and the greater part of the heavy work done in large factories is thus executed. Inasmuch, however, as such a tool, well formed on correct principles, may be made to take a tolerably deep cut, the shaving detached will be of sufficient thickness, and consequently sufficient width to reduce work a satisfactory quantity at each cut. The cut, too, must be continuous wherever possible, the tool being slowly and steadily advanced the whole of the acquired distance without being removed, and then re-entered for a second cut. The result should be a smooth, even surface, and that great exertion is not required when thus working with hand-tools is sufficiently evidenced by a remark of Mr. Haydon to the writer, "I have often detached, with a graver alone, a tolerably thick shaving of iron, two feet and more in length." From what has been said, it will be understood that in hollowing out a piece of metal such as a chuck, the tool should not be made with a long cutting edge, such as would be used if it were intended to scrape the whole depth of the side. A broad shaving is not to be thus aimed at, nor is the inside only to be thus attacked, but the tool advanced gradually inwards from the face of the work till it reaches the bottom, thus (to repeat the important point again) cutting at the same time the front and side of the shaving. The half-round or cylinder-boring bit already described is, of course, an inside turning tool, but is used with the aid of the back centre. In principle it follows other inside tools, the end being bevelled or sloped off 3°, and the side being 90°. The latter is to be regarded as a blunt cutting tool, being the largest angle that can be used; but, nevertheless, this bit must be regarded as cutting in the two required directions—forward and sideways. If a regular Goniometer for measuring angles is not to be obtained, nor any apparatus for grinding a tool to the required bevel, an addition to Nasmyth's tool-gauge, described in the Appendix, may be made by constructing in tin a set of templates, with the angles marked upon them. The easiest way is to mount on the lathe a few round sheets of tin, and mark the degrees by the division-plate, the outer circle of which contains 360°. The tin may then have pieces cut out, as shown in [Fig. 117], to be applied as gauges over the ends of the tool, or solid pieces of the required sections (those which are removed in forming the above) may be retained. It will, perhaps, be as well to finish up both neatly, taking care to mark the angles on each. The degrees most required are, as explained, 90°, 60°, 80°, and 3°; but intermediate numbers may be prepared, and will often be found convenient. An inside tool for brass must retain the angles 80° to 90°, the latter acting as a scraping tool to put a finish to the interior of work roughed out by a tool of the lesser angle. There are not many forms in general vogue, for brass work, whether internal or external. The round, the flat, and the point tools ([Figs. 118 to 121]) are more or less capable of hollowing out work, as well as surfacing. With the first ground to an angle of 80°, brass chucks can be hollowed, and, with the second at 90°, finished; and if there chance to be any internal angles out of reach by the point-tool, it is only necessary to use a similar one bent round at the end towards the left. Much of the turner's success in brass work depends upon the quality of the metal, which is often very hard and unequal in texture and perhaps blown and honey-combed. It is always the best plan to send patterns of any work of importance to some well-known firm, instead of trusting to a country foundry, whose business, if worthy the name, is generally iron work, and who only run brass once in a way, and make a terrible mess of it, too. The same may be said respecting iron castings. It is worth while, as an experiment, to test with turning tools the quality of a country casting, pulley, or what not, by the side of a similar work really made of malleable iron, such as is now so extensively used by the sewing-machine makers. Tools that will stand the first and make good work deserve a place in the British Museum, with a portrait of the turner!
Figs. 116, 117.