Stop for Thread Tools.—When cutting a thread, it is rather difficult to feed in the tool just the right amount for each successive cut, because the tool is moved in before it feeds up to the work. A stop is sometimes used for threading which overcomes this difficulty. This stop consists of a screw S, [Fig. 16], which enters the tool slide and passes through a block B clamped in front of the slide. The hole in the block through which the stop-screw passes is not threaded, but is large enough to permit the screw to move freely. When cutting a thread, the tool is set for the first cut and the screw is adjusted until the head is against the fixed block. After taking the first cut, the stop-screw is backed out, say one-half revolution, which allows the tool to be fed in far enough for a second cut. If this cut is about right for depth, the screw is again turned about one-half revolution for the next cut and this is continued for each successive cut until the thread is finished. By using a stop of this kind, there is no danger of feeding the tool in too far as is often done when the tool is set by guess. If this form of stop is used for internal threading, the screw, instead of passing through the fixed block, is placed in the slide so that the end or head will come against the stop B. This change is made because the tool is fed outward when cutting an internal thread.
Fig. 17. Gage for grinding and setting Acme Thread Tools
The Acme Standard Thread.—The Acme thread is often used, at the present time, in place of a square thread. The angle between the sides of the Acme thread is 29 degrees (see [Fig. 21]) and the depth is made equal to one-half the pitch plus 0.010 inch to provide clearance and insure a bearing upon the sides. The thread tool is ordinarily ground to fit a gage having notches representing different pitches. An improved form of Acme thread gage is shown in [Fig. 17]. The tool point is first ground to the correct angle by fitting it to the 29-degree notch in the end of the gage, as at A. The end is then ground to the proper width for the pitch to be cut, by testing it, as at B. The numbers opposite the shallow notches for gaging the width represent the number of threads per inch. With this particular gage, the tool can be set square by placing edge D against the turned surface to be threaded, and adjusting the tool until the end is in line with the gage, as at C. By placing the tool in this position, the angle between the side and the end can also be tested.
Fig. 18. Measuring Width of Acme Thread Tool with Vernier Gear-tooth Caliper
In case it should be necessary to measure the end width of an Acme thread tool, for a pitch not on the regular gage, this can be done by using a vernier gear-tooth caliper, as indicated in [Fig. 18]. If we assume that the caliper jaws bear on the sides of the tool at a distance A from the top, equal to 1/4 inch, then the width of the tool point equals the caliper reading (as shown by the horizontal scale) minus 0.1293 inch. For example, if the caliper reading was 0.315 inch, the width at the point would equal 0.315 - 0.1293 = 0.1857 inch, assuming that the sides were ground to the standard angle of 29 degrees. The constant to be subtracted from the caliper reading equals 2 A tan 14° 30' or, in this case, 2 × 0.25 × 0.2586 = 0.1293.