Use of Compound Rest for Thread Cutting.—Another form of thread tool is shown at A, [Fig. 7], which is very good for cutting V-threads especially of coarse pitch. When this tool is used, the compound rest E is set to an angle of 30 degrees, as shown, and it is fed in for the successive cuts by handle w in the direction indicated by the arrow. It will be seen that the point a of the tool moves at an angle of 60 degrees with the axis of the work, thus forming one side of the thread, and the cutting edge a—b, which can be set as shown at B, forms the opposite side and does all the cutting. As this edge is given a backward slope, as shown, it cuts easily and enables threading operations to be performed quickly. Threads cut in this way are often finished by taking a light cut with a regular thread tool. The cutting edge a—b is ground to an angle of 60 degrees (or slightly less, if anything) with the side, as shown by sketch A.
When cutting threads in steel or wrought iron, some sort of lubricant is usually applied to the tool to preserve the cutting end and give a smooth finish to the thread. Lard oil or a mixture of equal parts of lard oil and paraffin oil are often used for this purpose. If the thread is small, the lubricant may be applied from an ordinary oil can, but when cutting comparatively large threads, it is better to have a stream of oil constantly playing upon the tool-point. This constant flow may be obtained by mounting a can having a spout leading to the tool, on a bracket at the rear of the carriage.
Fig. 8. (A) V-thread.
(B) U. S. Standard Thread.
(C) Square Thread.
(D) Left-hand Thread.
(E) Double Square Thread.
(F) Triple Square Thread
Threads Commonly Used.—Three forms of threads or screws which are in common use are shown in [Fig. 8]; these are the V-thread (A), the U. S. standard (B), and the square thread (C). The shapes of these threads are shown by the sectioned parts. The V-thread has straight sides which incline at an angle of 60 degrees with each other and at the same angle with the axis of the screw. The U. S. standard thread is similar to the V-thread except that the top of the thread and bottom of the groove is left flat, as shown, and the width of these flats is made equal to 1/8 of the pitch. The square thread is square in section, the width a, depth b and space c being all equal. All of these threads are right-hand, which means that the grooves wind around to the right so that a nut will have to be turned toward the right to enter it on the thread. A left-hand thread winds in the other direction, as shown at D, and a nut is screwed on by turning it to the left.
Multiple Threads.—Threads, in addition to being right-and left-handed, are single, as at A, B, C and D, double, as at E, and triple, as at F, and for certain purposes quadruple threads or those of a higher multiple are employed. A double thread is different from a single thread in that it has two grooves, starting diametrically opposite, whereas a triple thread has three grooves cut as shown at F. The object of these multiple threads is to obtain an increase in lead without weakening the screw. For example, the threads shown at C and E have the same pitch p but the lead l of the double-threaded screw is twice that of the one with a single thread so that a nut would advance twice as far in one revolution, which is often a very desirable feature. To obtain the same lead with a single thread, the pitch would have to be double, thus giving a much coarser thread, which would weaken the screw, unless its diameter were increased. (The lead is the distance l that one thread advances in a single turn, or the distance that a nut would advance in one turn, and it should not be confused with the pitch p, which is the distance between the centers of adjacent threads. Obviously the lead and pitch of a single thread are the same.)