Fig. 10. Illustration showing the Meaning of Terms used
in Tool Grinding as applied to Tools of Different Types
In [Fig. 10] a few of the different types of tools which are used in connection with lathe work are shown. This illustration also indicates the meaning of the various terms used in tool grinding. As shown, the clearance of the tool is represented by the angle α, the back slope is represented by the angle β, and the side slope by the angle γ. The angle δ for a tool without side slope is known as the lip angle or the angle of keenness. When, however, the tool has both back and side slopes, this lip angle would more properly be the angle between the flank f and the top of the tool, measured diagonally along a line z—z. It will be seen that the lines A—B and A—C from which the angles of clearance and back slope are measured are parallel with the top and sides of the tool shank, respectively. For lathe tools, however, these lines are not necessarily located in this way when the tool is in use, as the height of the tool point with relation to the work center determines the position of these lines, so that the effective angles of back slope, clearance and keenness are changed as the tool point is lowered or raised. The way the position of the tool affects these angles will be explained later.
While tools must, of necessity, be varied considerably in shape to adapt them to various purposes, there are certain underlying principles governing their shape which apply generally; so in what follows we shall not attempt to explain in detail just what the form of each tool used on the lathe should be, as it is more important to understand how the cutting action of the tool and its efficiency is affected when it is improperly ground. When the principle is understood, the grinding of tools of various types and shapes is comparatively easy.
Fig. 11. Plan View of Lathe Turning and Threading Tools
Shape or Contour of Cutting Edge.—In the first place we shall consider the shape or contour of the cutting edge of the tool as viewed from the top, and then take up the question of clearance and slope, the different elements being considered separately to avoid confusion. The contour of the cutting edge depends primarily upon the purpose for which the tool is intended. For example, the tool A, in [Fig. 11], where a plan view of a number of different lathe tools is shown, has a very different shape from that of, say, tool D, as the first tool is used for rough turning, while tool D is intended for cutting grooves or severing a turned part. Similarly, tool E is V-shaped because it is used for cutting V-threads. Tools A, B and C, however, are regular turning tools; that is, they are all intended for turning plain cylindrical surfaces, but the contour of the cutting edges varies considerably, as shown. In this case it is the characteristics of the work and the cut that are the factors which determine the shape. To illustrate, tool A is of a shape suitable for rough-turning large and rigid work, while tool B is adapted for smaller and more flexible parts. The first tool is well shaped for roughing because experiments have shown that a cutting edge of a large radius is capable of higher cutting speed than could be used with a tool like B, which has a smaller point. This increase in the cutting speed is due to the fact that the tool A removes a thinner chip for a given feed than tool B; therefore, the speed may be increased without injuring the cutting edge to the same extent. If, however, tool A were to be used for turning a long and flexible part, chattering might result; consequently, a tool B having a point with a smaller radius would be preferable, if not absolutely necessary.
The character of the work also affects the shape of tools. The tool shown at C is used for taking light finishing cuts with a wide feed. Obviously, if the straight or flat part of the cutting edge is in line with the travel of the tool, the cut will be smooth and free from ridges, even though the feed is coarse, and by using a coarse feed the cut is taken in less time; but such a tool cannot be used on work that is not rigid, as chattering would result. Therefore, a smaller cutting point and a reduced feed would have to be employed. Tools with broad flat cutting edges and coarse feeds are often used for taking finishing cuts in cast iron, as this metal offers less resistance to cutting than steel, and is less conducive to chattering.
The shape of a tool (as viewed from the top) which is intended for a more specific purpose than regular turning, can be largely determined by simply considering the tool under working conditions. This point may be illustrated by the parting tool D which, as previously stated, is used for cutting grooves, squaring corners, etc. Evidently this tool should be widest at the cutting edge; that is, the sides d should have a slight amount of clearance so that they will not bind as the tool is fed into a groove. As the tool at E is for cutting a V-thread, the angle α between its cutting edges must equal the angle between the sides of a V-thread, or 60 degrees. The tool illustrated at F is for cutting inside square threads. In this case the width w should be made equal to one-half the pitch of the thread (or slightly greater to provide clearance for the screw), and the sides should be given a slight amount of side clearance, the same as with the parting tool D. So we see that the outline of the tool, as viewed from the top, must conform to and be governed by its use.