With the exception of pipe threads I always make the bar the nominal diameter so for your M10 x 1.25 I would turn to 10mm or just thread 10mm dia rod, if it were say 3/8" BSF I would turn to 0.375 or use 3/8" rod.

If you have split dies, they can be opened up for the first pass and then closed to get a good fit and better finish. Dies that are not adjustable are ok for cleaning up an existing thread, but not much else. Single point threading on the lathe if better if you have the tooling to do it. The higher quality tight tolerance threads come closest to the nominal diameter. If I wanted to produce a 10mm thread, I would start with 10mm diameter stock, not some other size.

How do you intend to produce 10mm on the end of a bigger bit of metal? What about getting a good quality 10 mm die and chuck that old one in the bin. Make sure it is a split die and make sure the diestock has the clearance to allow it to expand about 0.3mm and threading will be easy. If the work is turned to 10mm in the lathe and a slight lead in taper is on the end, then use the chuck in the tailstock (with the jaws retracted) to hold the die square and push at the same time as you start the thread.

Edited By old mart on 01/01/2021 21:55:37

Threading with a worn out die, you mean? And ending up with a crap thread.

Get yourself a Zeus book

In that case get a copy of 'Guide to the World of Threads' by P.A Sidders – gives you the formulae for every thread you can ever imagine – and some that you can't.

I always reduce stock diameter by 1/10 of the pitch, and this is also what Dormer recommend, so reducing stock diam to 9.88mm is OK.

Jan

Can you be more specific about what is happening when you attempt to thread?

Are you getting the thread to start, but then finding cutting hard going, or are you not even getting the thread started?

As others have said, a good quality new split-die would give you the best chance of success.

Trust nothing! What's that 'bit of bar' made of?

Typically in free-cutting mild-steel and brass, I put M10 straight on to 10mm diameter stock, untouched apart from a starting chamfer on the end. Other materials are worth reducing slightly, 9.9mm feels about right.

But! Could this 'bit of bar' be something nasty like work-hardening stainless? If so, the bar could be too much for the die which may have been blunted by it!

No mention of cutting fluid either, and disadvantages add up. I fear the die has had it.

Dave

Good advice here.

"Help" the die as it engages. Particularly, fine threads don't like having to drag a Tailstock along with the Die holder. (I try to set my Die to a commercial bolt, if possible ) A ME 40 will strip rather than drag the Tailstock! along

This is the advantage of a sliding Die holder, minimal load on the new thread, and the little bit of clearance helps the Die to follow the work, reducing the risk of cutting then thread off centre.

Lubrication is important, Trefolex or Rocol RTD, you are usually cutting the thread in just one pass.

Howard