Robert Gibson Griswold

IV. Lathe Tools and Their Cutting Action.

The essential requirements for lathe tools, especially those used for general work, are hardness of cutting edge combined with a toughness in the metal. There are many special uses to which lathe tools are put that require a very hard edge, but such an edge must have very little clearance or the pressure of the work would soon crumble it off. This degree of hardness can be increased by special methods of hardening, to almost that of a diamond, but for the general line of work pursued by the amateur this would result in a great loss of time at the wheel, for the edges will chip off if at all thin. Especially is this true of the self-hardening steels. It is very difficult to put on them a finishing edge that will stay and give to the work a smooth finished appearance, although for the roughing cut they cannot be excelled.

For this reason a great many machinists do the roughing work with self-hardening steels and take the finishing cut with a high-carbon steel such as Jessop's. These tools hold a beautifully fine edge, keen and smooth, especially after being oil-stoned.

The tempering of lathe tools is generally done in the yellows, as noted in the table given in the first chapter. It must be remembered that the soft metals will pull a fine, keen edge into them if much rake or clearance is allowed, and for this reason they should not be left very hard, as this strain will snap off the edge.

The amateur will have need of as large a set of tools as he can afford. In fact, his set should comprise a form for every ordinary operation, to be time-saving. A great deal of time can be lost by attempting to force one tool to perform the work that should be done by one of another form. Thus, a diamond point tool can hardly be used as a thread tool and result in a very fine piece of work. Neither can a side tool be forced to do cutting off satisfactorily.

It has been the aim of the writer to illustrate in this article only the most frequently used and necessary tools, commencing with those used for the more common operations. Other forms will be taken up under the head of special tools.

In the first place, it will be well to consider the grinding of the tool and the terms " rake" and "clearance " and their effect on the resultant work.

In Fig. 1 is shown the action of a tool with a perfect-ly square edge. It requires a great pressure to force it through the work, and instead of cutting crushes the metal in front of the tool. The action of all cutting tools depends on a wedge action, and the keener the wedge the easier does it become to push it through the work.

If we decrease the angle o in Fig 1, the cutting angle o begins to approach a wedge and the advancing face falls away from the perpendicular, making the angle r, Fig. 2, less acute. Upon this angle r depends the ease of cutting, as the material is then cut off instead of being crushed, as in Fig. 1. But when the angle becomes as acute as in Fig. 3, there is great danger of the edge being cracked off by being forced downward by the pressure of the chip above, as shown by dotted lines, somewhat exaggerated. The angle r is known as the "angle of rake."

But when the tool is lying flat on the work, as in Figs. 1, 2 and 3, it is impossible to press the cutting edge into the work, and the chip will continue as started, or the tool will have a tendency to work out. If the metal under the tool is now removed, as in Fig. 4, the cutting edge is relieved and can be pressed into the work, the cutting action remaining the same.

This small angle c is called the "clearance angle" and is given to all tools so that the edge may be made to cut as it is fed to the work. But this edge must not be made with too great a clearance or the result will be a tool something like Fig. 5, in which both the rake and clearance are excessive. It can be readily seen how very little support the cutting edge of this tool really has, and with a chip of any thickness the edge would be bent downward and snapped off.

Let us now consider the above principles as applied to the lathe tool. In Fig. 6 is shown the action of a square edge tool when acting upon a circular piece in the lathe. When acting on the center line a d it may be fed into the work by exerting a great deal of pressure, which will have a tendency to spring the work. If the tool is raised any above this, line it will not cut at all because the edge is above the point of contact, as shown in the dotted position b. On the other hand, if this tool is lowered below the center line its action will be that of scraping and the nearer it approaches the position indicated by c the greater will be the tendency for the work to springup and "walk" out onto the tool. This action is especially evident with cutting off tools when the cutting edge is very wide and the stock not stiff enough to hold the work in place. In Fig. 7 the tool has been given zoprake so that the chip is cut off more easily, and it also has been given clearance so that it will feed into the work easily. This form of cutting edge is easily preserved by grinding. In Fig. 8 the same tool is shown placed above and below the center line. When placed above the center line the cutting action is very smooth but it will cut only as long as the point touches the periphery of the piece below the normal to the front of the tool, or the line oe. For this reason the tool will not cut to the center unless its position is changed. When the tool is placed below the center line its action becomes a scraping rather than a cutting one as the upper face approaches the perpendicular to the surface, and the condition is similar to that shown in Figs. 1 and 6. The point will surely go in this case if much pressure is applied, as there is no metal beneath to support it.