Principle of the Thread Indicator.—The principle upon which the thread indicator operates is as follows: The number of teeth in worm-wheel W is some multiple of the number of threads per inch of the lead-screw, and the number of teeth in the worm-wheel, divided by the pitch of the screw, equals the number of graduations on the dial. For example, if the lead-screw has six threads per inch, the worm-wheel could have twenty-four teeth, in which case the dial would have four divisions, each representing an inch of carriage travel, and by sub-dividing the dial into eighths (as shown) each line would correspond to 1/2 inch of travel. The dial, therefore, would enable the carriage to be engaged with the lead-screw at points equal to a travel of one-half inch. To illustrate the advantage of this suppose ten threads per inch are being cut and (with the lathe stationary) the carriage is disengaged and moved 1/6 inch or one thread on the lead-screw; the tool point will also have moved 1/6 inch, but it will not be opposite the next thread groove in the work as the pitch is 1/10 inch. If the carriage is moved another thread on the lead-screw, or 2/6 inch, the tool will still be out of line with the thread on the work, but when it has moved three threads, or 1/2 inch, the tool will then coincide with the original cut because it has passed over exactly five threads. This would be true for any number of threads per inch that is divisible by 2. If the thread being cut had nine threads per inch or any other odd number, the tool would only coincide with the thread at points 1 inch apart. Therefore, the carriage can only be engaged when one of the four graduations representing an inch of travel is opposite the arrow, when cutting odd threads; whereas even numbers can be “caught” by using any one of the eight lines.
This indicator can also be used for “catching” fractional threads. As an illustration, suppose 111/2 threads per inch are to be cut, and the carriage is engaged for the first cut when graduation line 1 is opposite the arrow; engagement would then be made for each successive cut, when either line 1 or 3 were opposite the arrow, or in other words at spaces equal to a carriage movement of 2 inches. As the use of the indicator when cutting fractional threads is liable to result in error, it is better to keep the half-nuts in engagement and return the carriage by reversing the lathe.
Replacing Sharpened Thread Tool.—If it is necessary to sharpen the thread tool before the thread is finished, it should be reset square with the work by testing with the thread gage as at B, [Fig. 1]. The carriage is then engaged with the lead-screw and the lathe is turned forward to bring the tool opposite the partly finished thread and also to take up any backlash or lost motion in the gears or half-nut. If the tool-point is not in line with the thread groove previously cut, it can be shifted sidewise by feeding the compound rest E in or out, provided the latter is set in an angular position as shown in the plan view, [Fig. 2].
If the thread tool is ground flat on the top as at A, [Fig. 4], it is not a good tool for removing metal rapidly as neither of its two cutting edges has any slope. In order to give each cutting edge a backward slope, it would be necessary to grind the top surface hollow or concave, which would be impracticable. When a course thread is to be cut, a tool shaped as at B can be used to advantage for rough turning the thread groove, which is afterward finished to the correct depth and angle by tool A. This roughing tool is ground with a backward slope from the point and the latter is rounded to make it stronger.
Fig. 7. Cutting Thread by using Compound Rest