Harold Hall

Workshop Processes


Screw Cutting Complex Pitches

For most viewers having a lathe calibrated in imperial units, screw cutting will be largely confined to those straight forward values that can be read off the lathes changewheel chart, 8, 10, 12 TPI, etc. However, the occasional odd value will

surface but because of its infrequent occurrence may be attempted with a degree of doubt regarding the task.These pages seek to fill in some of the gaps the viewer may have in understanding the situation.


Whilst this is written for the viewer who has an imperial lathe and requires to cut

a metric thread, or worm gear, the viewer coming in the other direction, metric to imperial, should find some benefit in reading these pages.


Metric Threads

Wishing to screw cut a metric thread on a lathe with an imperial leadscrew will be for a number of reasons. Top of the list probably, and the one most crucial, is where one half of the assembly exists and a mating thread has to be made. In this case there are no alternatives and to cut a thread with a metric pitch is the only option.


It maybe that you are restoring some item and whilst you are making both internal and external threads you want the final result to be authentic and follow the original design. In another case, you may be making some item whose design is dimensioned using metric values, and wish to follow the drawings precisely.


Without going into the mathematics at this stage there is only one relatively simple method of creating a precise metric thread, that is to use a changewheel having 127 teeth.

With there being 25.4 mm to one inch that is 254/2 = 127. As this is a large gear there may be difficulty in fitting it to the lathe's standard quadrant and if possibly for a one off application it would be an expensive purchase. What then is the alternative? Fortunately, even with standard changewheels, there are combinations that will get extremely close to the required pitch and are likely to be close enough for all but the most critical applications.


Unfortunately, in this respect, almost all charts that list combinations to use to cut a metric thread neither indicate that the result is imprecise nor the degree of the error. In fairness, it should be stated, that in most cases the result is very close, even so, knowing it will not be exact it would be nice to know what the error will be, helping to increase the users knowledge of the subject.


If the item being cut is a leadscrew and or mating nut for a lathe or milling machine, most will consider any error to be unacceptable and the 127 tooth changewheel is a must.


It may be though that it is just a nut or similar to replace one that has been lost of has had its hexagon severely damaged. In this case, a very small pitch error would no doubt be acceptable, depending of course on the length of the engaged thread. The shorter the engagement the more likely that the pitch error would not cause a problem.


If though both threads are being cut then the pitch error would be of no consequence unless you are being very demanding regarding the end result being authentic.