with an acme thread form inside such a small bore will be beyond my meagre talents. I hope to use a 'single pass' tap as getting a threading tool. Hopper, thank you for your clear and very helpful explanation, I am much obliged. Whenever i have needed to cut a female thread acme/trapisiodal I have never been able to cut with a tap I've always had to single point it before and use the tap to clean up.even in small sizes Nevertheless thanks for the heads up, measure twice, cut once., but I keep getting it backwards! I must admit I have also assumed that the leadscrew will be an acme thread, it is 3/8' diameter and 10 tpi and the lathe seems to be a myford ML7 clone. Hi, I meant the tops of the threads seem equal in width and not narrower on the middle of the screw where In assume most wear takes place.Whereas the nut threads look more pointed on the crests. The 90 per cent you refer to is the amount of thread engagement, ie there is 10 per cent clearance top and bottom of the threads so it does not bind on the OD. Thread depth is half of the pitch, then deduct whatever clearance you want between the tops of the threads and the mating roots.

Are you sure you are dealing with ACME thread? Or is it a square thread? Better to confirm now than get it wrong! I am not sure what the 90% relates to and whether or not this drill would give the right or best tolerance for sixty year old lead screw, although the threads on it appear to be 'equally square' along the length and the presumabley unworn portions at the limits of travel. Can anyone tell me the correct size of tapping drill for this thread, I have been unable to find anything on line, although one chart I found referrred to a 90% thread and a tapping drill of. Hello all, Please may I tap the knowledgw base again? I neeed to cut a 3/8 x 10 right hand thread for a new crosslide nut on the Hobson H7 lathe I am in the process of refurbishing.

Acme tap drill size chart