The complete practical machinist

In presenting to machinists a thoroughly revised edition of this book, the author would be ungrateful did he omit to express his sincere thanks for the unusual welcome which it has met at the hands of American machinists, and also for the large number of kindly letters of appreciation from the many, on both sides of the Atlantic, with whom he has become acquainted through the medium of it.

This revision brings the work thoroughly up to date, while maintaining its chief characteristic of being as intelligible to the student and apprentice as it is to the skillful machinist.

JOSHUA ROSE.

The lathe is the most important of all metal-cutting machines, or machine tools as they are termed, not only he cause of the comparative rapidity of its action, but also from the wide range and variety of operations that may be performed in it. He who is an expert lathe hand, or turner, will find but little difficulty in operating any other metal- cutting machine tool, because the methods of holding work and the shapes of the tools for other metal-cutting machines are similar, and are governed by the same principles as in the case of lathe work ; hence, in this book all tools that are used in the lathe will be discussed under the head of lathe tools, notwithstanding that they may be also used in other machines.

Cutting tools for lathes, planing machines, etc., etc., are made of a special grade of cast-steel known as tool steel. The tool is first forged to shape, and then hardened by heating it to a red heat and dipping it in water.

Lathe-cutting tools may be divided into two principal classes, viz., slide-rest tools and hand tools. The latter, however, have lost their former importance, because even small lathes are now provided with means to traverse the tools to the cut.

Slide- rest tools may be subdivided into two classes, those for inside or internal work, and those for external or outside work. They are designated from either the nature of the duty they perform, or from some characteristic peculiar to the tool itself. Thus a side tool is one that cuts upon a side or end face; a front tool is one that cuts in front ; a spring tool is one that admits of deflection or spring, and so on.

In forming cutting tools it will be found that a very slight variation of shape, or of presentment to the work, causes appreciable difference in its cutting capacity, whether for smoothness or in taking off a quantity of metal. Furthermore, the shape of the tool mr.st not only be varied for different kinds of metal, but also for extreme differences of hardness in the same kind of metal, more notably in the case of steel, some of which is almost as soft as wrought-iron, while the finer grades are exceedingly hard, especially when cut from the bar and not annealed or softened by being brought to a red heat and left to cool slowly. Cast-iron also is sometimes exceptionally hard, requiring a special shape of tool, while wrought-iron and. brass vary but very little in their degree of hardness.

The manner in which a lathe tool cuts the metal from the work when fed along it is shown in Fig. 1, which represents a tool feeding a cut along a piece of wrought-iron, and it will be seen that the cutting comes off in a spiral. The diameter and the openness of this spiral depend entirely upon the shape of the tool, so that from the appearance of the cutting the quality of the tool may be judged.

The principles which govern the shape of tool necessary to cut a piece of metal under any given condition are general in their application ; so that when these conditions are clearly understood it becomes a comparatively easy matter to shape a tool suitable for them.

The principal consideration in determining the proper shape of a cutting tool, for use in a lathe or planer, is where it shall have the rake necessary to make it keen enough to cut well, and yet be kept as strong as possible; and this is governed, in a large degree, by the nature of the work on which it is to be used. It is always desirable, circumstances permitting, to place nearly all the rake or keenness on the top face of the tool, as shown in Fig. 2, in which D is the top face, and B the bottom one; lines
AA and EE representing the level of the top and bottorn of the tool steel, and C a line at a right angle to E, or what is the same thing, to A. The tool in Fig. 3 corresponds to that in Fig. 2 so far as its cutting qualifications are concerned, there being merely a slight difference in the forged shape, but not in the cutting edges. That shown in Fig. 3 is called a "diamond point," from the diamond shape of its top face, while that in Fig. 2 is called a " front tool;" the former being more suitable for small, and the latter for large work.

Referring now to the top face a, its angle or rake is its incline in the direction of the arrow in Fig. 4. In those (to be hereafter specified) in which top rake is, from the nature of the work to be cut, impracticable, it must be taken off and the tool given the necessary keenness by increasing the rake or angle of the bottom or side faces in the direction shown in Fig. 5, in which letter b represents a side or bottom face of the tool, its amount of rake being denoted by its angle in the direction of the arrow.

These top and side faces, taken one in conjunction with the other, form a wedge, and all machine tools are nothing more than cutting wedges, the duty performed by the respective faces depending, first, upon the keenness of the general outline of the top and bottom faces, and secondly, upon the position, relative to the work, in which the tool is held and applied.

The strain sustained by the top face is not alone that due to the severing of the metal, but that, in addition, which is exerted to break or curl the shaving, which would, if not obstructed by the top face, come off in a straight line, like a piece of cord being unrolled from a cylinder; button coming into contact with the face of the tool (immediately after it has left the cutting edge), it is forced, by that face, out of the straight line and takes circular form of more or less diameter according to the amount of top rake possessed by the tool. The direction of the whole strain upon the top face is at a right angle to it, as denoted in Fig. 6 by the line D, d being the work, B the tool, and C the shaving. It will be readily perceived, then, that if a tool possessing so much top rake is held far out from the tool post or clamp, or is slight in body, any springing of the body of the tool, arising from the pressure due to the cut, will cause the tool point to take a deeper cut, and that the tendency of the strain upon the top face is to draw the tool deeper into its cut.