This article first appeared in Practical Engineering 1940 Vol1 No24. The content within the article is accurate as of 1940. This article describes developments in Engineering at the time.
Here is a new and economical method of making multiple-start threads, especially useful in the production of components which have rolled threads. It is necessary to know a little about thread-rolling to realise the advantages of this method.
Thread-rolling is set in various classes according to the diameter of the component to be threaded, and the pitch or number of threads per in. The diameter of the component will eventually determine the ratio of the rolling chucks, and therefore the number of starts the male member of the rolling chucks contain.
It will be easily understood that thread-rolling is performed in a special-purpose machine equipped with cams for opening and closing the rolling chucks, gears for altering the ratio speeds of the top and bottom chuck spindles and fine adjustment to obtain the required depth of thread. The spindles of the machine have a maximum and a minimum distance between the centres, therefore the tolling chucks are confined within these limits. As an example, the mean distance between centres is 2in.
The object is to arrange the ratio of the rolling chucks to maintain the mean centres. If the female or bottom chuck is lin. in diameter, the top chuck would be approximately 4in. diameter, which is 2in. when the chucks are engaged. Incidentally, the ratio for gearing would be 4 to 1, and the multiple starts on the top tool would be four. The component must go over the bottom chuck with sufficient allowance for stripping off after threading, so the component will govern the actual sizes and ratios.
It will now be apparent that if components of varying diameter are to be threaded with the same pitch of thread, the chucks will be of different sizes and have a different ratio and number of starts. For example, a 1in. component will have 1in. and 4in. chucks with four starts on the top, a 3/4in. component will have 3/4in. and 4 1/2in. chucks with six starts on the top, while a 2 1/2in. component will have chucks almost the same size with a 1 to 1 ratio. The bottom or female chuck is always threaded with the same pitch of thread as the component. (Sizes are approximate only.)
This new method of multiple-start thread cutting will cut any number of starts on almost any diameter provided that the same pitch is used for all the products. This means that 1-2-3-4-5-6-7 start threads can be cut on the top rolling chuck with the same tool and in one operation, if the component has to be threaded with eight threads to the inch.
All toolmakers and engineers know by heart the common method of cutting multiple threads which entails the gearing up of a lathe, making sure that the first driver will divide equally by the number of starts required, marking the gear tooth and the corresponding space it occupies cutting the first. single thread, disengaging the gears and counting round the first portion of the required multiple marking and engaging the gears, cutting the second thread, and continuing this procedure until the whole of the threads have been cut. Most operators continue one more thread than is required. The extra thread should go exactly into the first one, thus proving the correct spacing of the threads, although if a mistake occurs in the counting it is not always possible to find out until the thread has been cut.
The requirements for this new method are as follows: First obtain a pad of tool steel, Vital is preferred, 2in. in diameter and lin. thick, face the pad and bore a 1/2in. diameter hole through the centre. Mount the pad on a mandrel and turn over the top to clean it up, thread the pad using an ordinary single-form thread-cutting tool, gearing the lathe to cut 8 t.p.i. Finish this thread to the correct depth and form, because this is the master cutter and on its accuracy depends all the multiple-thread tools made by it later.
Remove the master thread from the mandrel and drill a small hole a little way from the centre, as shown in the accompanying illustration, and then saw out a sector. Next comes the hardening. Care must be taken to ensure the correct temperatures for the tool steel being used. If Vital is used, harden at 920 deg. C, and temper at 230 deg. C, using whale oil or other suitable oil. After the hardening, grind the cutting edges of the threads and place in the holder.. It will be seen from the illustration that the holder is a bar of case-hardened mild steel of suitable proportions, with two swan-neck adjusting pieces attached.
The advantage of the swan necks is that they make it possible to move the master in wide arc and still keep the cutting edges square. The master is now ready and should be placed in the lathe tool post with the cutting edge set slightly below the centre line and firmly tightened up. The lathe gearing must next be considered. It is equal to threads per i. on master cutter divided by no. of starts to be cut.
If the master cutter is 8 t.p.i. and the number of starts is five, the gearing would be 8/5=1 3/5. Seven starts would be 8/7 = l 1/7 t.p.i. With the lathe geared and the master set, and a top tool blank on a mandrel between centres, everything is ready for the first cut. If the lathe is reversible there is no need to disengage the screw-cutting nut after once having started; merely draw out the tool after each cut. If the lathe is not reversible, the procedure is the same as for cutting an ordinary single thread screw. It will be found that if five starts are cut the first five teeth on the master will each cut a separate thread in perfect pitch, and that the sixth and seventh teeth will follow in the groove made by the first two.
Any toolmaker or operator knows that there are only seven full threads in lin. of length, cut at 8 t.p.i., the first and last being half each. If more threads are required than teeth on the master, either a new and wider master must be made, or the first seven starts must be cut to full depth and then, without disengaging the screw-cutting nut, the master must be moved forward by moving the slide rest until the extra number of teeth are free. The remaining teeth should carefully be set in mesh with the threads already cut and the same procedure followed as when cutting a new thread; the free teeth will cut the remaining starts.
Although reference has been made only to 8 t.p.i. this method of multiple-thread cutting is not confined to it, having merely served as an example. A separate master must be made to correspond with the required pitch. Most thread-rolling for containers is within the range of 4 to 8 t.p.i., and a small number of masters is required; once made they will last indefinitely. All top tools for threading right-hand on components are threaded left-hand.