Cutting metric and other complex pitches on an Imperial lathe, and similarly Imperial pitches on a metric lathe 07

Harold Hall

Workshop Processes


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.


Three Choices

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%



Metric  Gearbox

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 results.