Re: Metric Thread Dial

Re-engaging the half nuts at the same position on the bed using a suitable stop and at the same thread dial number won't work when cutting metric threads with an imperial leadscrew.

The rotational alignment of spindle and leadscrew have to be maintained as well as longitudinal (distance along the bed) alignment if the next pass is to correspond with the previous one. Theoretically possible if you wait long enough but it could be hundreds, maybe even thousands, of spindle turns before sufficiently accurate alignment recurs. Its a matter of common integer factors between screw pitches and gear ratios. Telling the difference between darn close but no cigar and dead nuts right would be challenging.

The practical answer is to use a single tooth dog clutch in the drive from spindle to drop gear train. The clutch disengages at the end of each threading pass and stops the leadscrew. Because the screw is stopped the half nuts can be re-engaged at any position along the bed without loosing longitudinal feed alignment. As the dog clutch has only one tooth and is directly geared to the spindle rotational alignment is maintained because it always picks up the drive at the same spindle position.

My big lathe, a P&W Model B 12 x 32, has this system. It has a quick withdraw on the cross slide feed too. Threading is almost too easy. Whacked out a left hand 5/16 x 24 UN thread this afternoon at 250 rpm "just like that". Only easier way would be my Coventry Die Head. No way am I buying dies for one job tho', but I do have most of the smaller sizes so screw-cutting is rare for me now.


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