Is there anyone out there who can give a simple answer to a simple question? Is there a formula or method by which you can determine the bar diameter required for a given thread size, by this I mean turn a length of bar on a lathe to a pre determined diameter such that when you place the die on it to cut the thread, it happens. I've searched high and low on the net, YT etc and had various methods put before me, I've then gone and turned to a diameter using the method described and each time the diameter is too big. The mic I use is good for half a thou so I'm pretty sure diameters are correct, I even put a nice taper at the start. So, can anyone out there unlock the box? Please.

All I want is an M10 x 1.25 on a bit of bar, by hand.

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.

I would suggest you mike up a M10 thread, ..... and when you find it is undersize you will have learnt all about thread form reducing the peaks by rounding as also the thread root is greater than the theoretical minimum dia due to root radiusing. Interesting that the crest and roots of metric threads are not as pronounced as Whitworth form with the exception of the metric 'MJ' series thread which was devised to replicate the Whitworth form for use in aerospace applications.

As for problems with starting a thread can I ask if you are aware of the correct side to use when using a die? Usually it is the side with the die name and details stamped on it, this was the 'norm' but I have seen dies of Far Eastern manufacture that do not follow this convention, in fact they do not follow any convention that I can ascertain!

Edited By Oily Rag on 01/01/2021 21:20:38

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.

One formula I've tried is 'Die Blank Size is the Major Diameter less 10% of the Pitch', and this is on a quite well known engineering web site it even works it out for. However, it's too big a diameter. I must admit that the dies are foreign and a few years old, but I do have it the correct way round with the start taper at the bottom.

I can't use 10mm stock as the thread is the final part of a bigger bit of metal.

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

10 x 1.25 pitch can be truncated down to a diameter of about 9.8mm dia, that will make threading with a die a little bit easier.

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

Get yourself a Zeus book

I got one

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.

For 10mm threads cut with a stocks and die, I use 10mm rod, am I doing something wrong ?

Just asking.

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

Unless I have missed it, nobody has suggested pressing the die to the work whilst turning the diestock or workpiece. I find that pressing the die to the work gives a far better lead in and if you use the tailstock, it keeps the die true to the axis of the work. Put a piece of rag up the morse taper to catch swarf etc, better still have a blank MT that you can use for this exercise. Even better is to use a tailstock die holder.

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.

I'm with Speedy Builder5 on this one. I've never machined bar down in size from nominal and usually use a die holder in the tailstock with backup from the tailstock handwheel.

Brian

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

That's about what I do if I'm turning from a larger diameter prior to threading. If I'm starting from 10mm stock it'll usually be a gnat's undersize at about 9,95 anyway if it's drawn bar.

If I'm using a hand diestock, I'll push it against the work with the front face of the tailstock quill or the face of the drill chuck jaws if it's small; then rotate the main lathe chuck by hand, advancing the tailstock until the die's properly engaged. A bit of Rocol or suchlike thickish lubricant will usually help quite a lot.

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