To facilitate setting the height of the tool, a gauge such as shown in [Fig. 985] may be used, the height of the line a from the base equalling the height or distance between the top surface of the cross slides and the axial line of the lathe centres. If the lathe, however, have an elevating slide rest, the rest must be set level before applying the gauge. Or in place of using the gauge, the tool stool or tool holder, as the case may be, may be made of such height that when level in the tool post its top face points to the axis of the lathe centre, the tool being sharpened on the angles and not ground on the top face.
But in the case of a tool holder, or of a chaser holder, the tool may be ground on the top face, and adjusted for height by any suitable means, the top of the holder serving as a guide to set the tool by.
Fig. 986.
Fig. 987.
The line of the cutting edge of the tool must, to obtain correct results, be presented to the work in the same manner as it was presented to the gauge to which its angles were ground, so that if the tool were in position in the tool post, and the gauge were applied, it would point to the axis of the lathe centre, for if this is not the case the thread cut will not be of correct angle or depth. Thus, in [Figs. 986] and [987] the tool t would cut threads too shallow, although placed at the correct height, because the cutting edges are at an angle to the radial lines c c.
It becomes obvious, then, that it is improper to set the height of a screw-cutting tool by means of any tool elevating or setting-device that throws it out of the horizontal position. To enable the correct setting of threading tools, and to avoid having to grind the angles correct to gauge every time the tool requires sharpening, various kinds of tool holders have been designed by means of which the tool may be ground on the top face, and set at correct height and in the proper plane.