Fig. 1001.
The difficulty of adjusting the height of threading tools that are ground on their top faces to sharpen them is obviated in a very satisfactory manner by the tool holder patented by the Pratt and Whitney Company, and represented in [Figs. 1000] and [1001]. a is the body of the holder, c is the tool clamp, and b the set screw for c; d is a pin fast in a and projecting into c to adjust it square upon a. The threading tool g has a groove h, into which the projection e fits, so that the tool is held accurately in position. f is the screw which adjusts the height of the tool, being threaded into a and partly into g, as is shown at i. The holder once being set in correct position, the threading tool may be removed for grinding, and reset with accuracy. The face k of the holder is made at 30° to the front or leading face of the holder, so that the stem or body of the holder will be at an angle and out of the way of the work driver.
Fig. 1002.
If a chaser instead of a single-pointed tool be used to cut a thread, the thread requires to be gauged for its full diameter only, because both the angles of the thread sides and the thread depth are determined by the chaser itself. Chasers are also preferable to a single-pointed tool when the work does not require to be cut to an exact diameter, nor to have a fully developed thread clear up to a shoulder; but when such is the case a single-pointed tool is preferable, because if the leading tooth should happen to run against the shoulder the whole of the teeth dig into the work, and more damage is done to it than with a single-pointed tool. When the thread does not run up to a shoulder, or in cases where the thread may be permitted to run gradually out, and, again, where the thread is upon a part of enlarged diameter, a chaser may have its efficiency increased in two ways, the first of which is shown in [Fig. 1002]. When the chaser is set and formed as at a in the figure, the leading tooth takes all the cut, and the following tooth will only cut as it is permitted to do so from the wear of the leading bolt. This causes the tooth to wear, but the teeth may be caused to each take a proportion of the cut by chamfering them as at b in the figure, which will relieve the front tooth of a great part of its duty and let the following teeth perform duty, and thus preserve the sharpness of the cutting edges. We are limited in the degree of chamfer that may be given to the teeth, first, because as the cutting edge is broader and the strain of the cut is greater it causes the tool to spring or bend more under the cut pressure; and secondly, because if the tool be given many teeth in order to lengthen the chamfer, then the pitch is altered to a greater extent by reason of the expansion which accompanies the hardening of the chaser.
Fig. 1003.
A chaser thus chamfered may be set square in the tool post by placing a scale against the work as at s in [Fig. 1003], and setting the bottoms of the chaser teeth fair with the outer edge of the scale as in the figure.