Rivett-Dock Threading Tool.—A special form of thread tool, which overcomes a number of disadvantages common to an ordinary single-point thread tool, is shown in [Fig. 27]. This tool has a circular-shaped cutter C, having ten teeth around its circumference, which, beginning with tooth No. 1, gradually increase in height, cutter No. 2 being higher than No. 1, etc. This cutter is mounted on a slide S, that is fitted to the frame F, and can be moved in or out by lever L. The hub of this lever has an eccentric stud which moves slide S and locks it when in the forward or cutting position. The action of the lever in moving the slide engages the cutter with pawl P, thus rotating the cutter one tooth at a time and presenting a different tooth to the work for each movement of the lever. When the slide is moved forward, the heel or underside of the tooth which is in the working position rests on a stop that takes the thrust of the cut.
When the tool is in use, it is mounted on the tool-block of the lathe as shown in the illustration. The cutter is set for height by placing a tooth in the working position and setting the top level with the lathe center. The cutter is also set square with the work by using an ordinary square, and it is tilted slightly from the vertical to correspond with the angle of the thread to be cut, by adjusting frame F. At first a light cut is taken with lever L moved forward and tooth No. 1 on the stop. After this cut is completed, the lever is reversed which rotates the cutter one tooth, and the return movement places tooth No. 2 in the working position. This operation is repeated until the tenth tooth finishes the thread. It is often necessary, when using a single-point thread tool, to re-sharpen it before taking the finishing cut, but with a circular tool this is not necessary, for by using the different teeth successively, the last tooth, which only takes finishing cuts, is kept in good condition.
Cutting Screws to Compensate for Shrinkage.—Some tool steels are liable to shrink more or less when they are hardened; consequently if a very accurate hardened screw is required, it is sometimes cut so that the pitch is slightly greater than standard, to compensate for the shrinkage due to the hardening operation. As the amount of contraction incident to hardening is very little, it is not practicable to use change gears that will give the exact pitch required. A well-known method of obtaining this increase of pitch is by the use of a taper attachment.
Fig. 28. Diagram Illustrating Method of Cutting a Thread to
Compensate for the Error in Pitch due to Shrinkage in Hardening
For example, suppose a tap having 8 threads per inch is to be threaded, and, owing to the contraction of the steel, the pitch must be 0.12502 inch instead of 0.125 inch. The lathe is geared to cut 8 threads per inch or 0.125 inch pitch, and then the taper attachment is set to an angle a, [Fig. 28], the cosine of which equals 0.125÷0.12502; that is, the cosine of angle a equals the pitch required after hardening, divided by the pitch necessary to compensate for shrinkage. The angle is then found by referring to a table of cosines. The tap blank is also set to the same angle a by adjusting the tailstock center, thus locating the axis of the work parallel with the slide of the taper attachment. When the carriage moves a distance x, the tool point will have moved a greater distance y along the work, the difference between x and y depending upon angle a; hence the tool will cut a thread of slightly greater pitch than the lathe is geared to cut.
To illustrate by using the preceding example, cosine of angle a = 0.125÷0.12502 = 0.99984. By referring to a table of cosines, we find that 0.99984 is the cosine of 1 degree, approximately; hence, the taper attachment slide and the work should be set to this angle. (The angle a in [Fig. 28] has been exaggerated in order to more clearly illustrate the principle.)