It is worth mentioning here that metric threads are not the only complex pitches
(in imperial terms) that may need to be cut, wormwheels have similar requirements.
Because of this it will probably be best to be prepared to purchase the gears on
an as required basis. If you already have the gears available, I would still suggest
you purchase the 21 tooth gear.
1. Having the need to cut a metric thread you have three choices. One is to set the
gearbox to the position for cutting an 8TPI thread, in which position the gearbox
is the equal of just a through shaft. With this done, any of the combinations given
earlier could be set up and the gearbox ignored.
2. Another possibility is to set up a gear chain and use the gearbox in combination
with this to provide a number of different threads.
3. Finally, changing the final gear of the lathe's tumbler assembly can very easily
produce some values, albeit not so accurate as the other two methods.
Method 1 needs no further comment but as an example of method 2, if you set up the
gears to give a 2mm pitch with the gearbox in the 8TPI position you will also have
available 1mm at the 16 TPI, 0.8mm at 20TPI and 0.5mm at 32TPI
If you compare the gears required for both 2mm and 1.5mm pitches you will find that
there is only one difference, that in the drivers that can be accommodated by fitting
the three common ones on the quadrant and the variable one as the first driver. This
would be easy to change and only the quadrant needing to be swung to achieve the
required mesh with the alternative gear.
When removing the quadrant to return to normal use, the gears could remain fitted
making it quick to set up for future requirements. Table five should make the suggestion
clear. Incidentally, whilst those in the table are not the most accurate, they are
amongst the best, all having an error of + 0.012%
Pitch setting DR1 DR2 DN1 DN2
2mm 8TPI 60 21 40 50
1mm 16TPI 60 21 40 50
0.8mm 20TPI 60 21 40 50
0.5mm 32TPI 60 21 40 50
1.5mm 8TPI 45 21 40 50
1mm 12TPI 45 21 40 50
0.75mm 16TPI 45 21 40 50
0.6mm 20TPI 45 21 40 50
0.5mm 24TPI 45 21 40 50
For other values there are still many options but the easiest would be to set the
gearbox at 8TPI and set up one of the gear chains listed above for the pitch required.
Method 3. An easier option
Much better results can be achieved for some pitches by changing only the final gear
on the tumbler assembly, though the accuracy is rather less than those above. This
method requires just two special gears having 33 and 34 teeth to replace the 24 tooth
normally fitted with the gearbox quadrant.
This method will only work though if the quadrant gear is 24 tooth and I suspect
that this may not apply to all lathes, maybe only those made by Myford. If your lathe
has a different size gear it would be worth investigating if other alternative gears
would provide similar benefits. Table six shows the setting and the accuracy that