| Fig. 30. Plain Hook Tool | Fig. 31. Plain Straight Tool [ToList] |
The two illustrations here given show a pair of plain bits, in which [Fig. 30] represents a hook-shaped formation, and [Fig. 31] a straight grind, without any top rake. The hooked bit would do for aluminum, or steel, but for cast iron the form shown in [Fig. 31] would be most serviceable.
Then the side bits, such as the round-nosed, [Fig. 32] and the square end, [Fig. 33], may be ground hooked, or with a top rake, or left flat.
The too common mistake is to grind the lower or clearance side at too great an angle. [Fig. 34][p. 39] shows the correct angle, and the dotted line A illustrates the common tendency to grind the clearance.
The Proper Angle for Lathe Tools.—Now there is a reason why the angle of from 10 to 15 should be maintained in the clearance. The point of the tool must have suitable support for the work it is required to do, so it will not chatter or yield in the slightest degree. A bit ground along the dotted line has a cutting edge which will spring down, and consequently break or produce a rough surface.
Then, again, the angle of the clearance acts as a guide, or rather, a guard, to prevent the tool from going in too far, as will now be explained.
Setting the Bit.—In order to understand the correct setting, examine the work A, in [Fig. 35.]
A is a cylinder being turned up in the lathe, and B the cutting tool, which approaches it on a hori[p. 40]zontal line, C, extending out from the center of the cylinder A. This setting is theoretically correct, and in practice has been found most advantageous.