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.
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
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
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
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.