I dont do metric but thats pretty much 5ba…I wouldnt wind it by hand, I would use the lathe back gear. If its stripping threads you need to open the split die as far as it will go and try a run. Always use lubricant. The dia of rod is dependant upon your application but it is not unusual to aim for a 3/4 thread engagement. For such small threads it is often much easier to buy threaded bar and locate into a tapped hole with loctite. Good luck.

As DG1 has already mentioned, it is important that the bore of the diestock is loose enough on the die to allow the die to be opened up for the initial cut. For that small size, about 0.2mm difference in diameters. A slight lead in chamfer helps, as does using the face of the tailstock chuck (with the jaws retracted) to keep the diestock square. A little cutting oil should be used. If you still cannot get a reasonable thread, the die may well be to blame, there are a lot of poor ones out there, and persisting with one of them is like polishing a turd.

I use size to minus for material and cut with hand turning n the lathe using the tail stock to keep square. Use cutting oil and back off frequently with the die set to maximum open. Turning the material through the die too quickly and not backing off will cause the swarf to strip the thread to a degree. I cut metric down to M2 with no problems in steel and brass.

I don't cut many external threads using dies on the centre lathe, but M3 should be no problem. Stock size can be nominal, ie, 3mm or a thou or two less.There shouldn't be any problem starting the die and the thread can be cut in one pass. Given it isn't working there are two likely issues. Since the parentage of the die is reputedly suspect I'd bin it and buy a known make. Second what is the parentage of the mild steel? If the material is unknown bin it and buy known material. Even known materials can be difficult to thread; EN3 has a propensity to tear for instance.

I have used a split die on my repetition lathe once, as I'd misplaced the M4 Coventry dies. The M4 threads were cut in one pass at 500rpm in EN1A. So with the right material and cutting tool there should be no problem. I think the M4 die was Dormer.

Andrew

Probably most of the main points about die setting have been covered Stuart.

One other thought though, if you have a brought-in die holder, just check that the central grub screw has a point (not a flat end) on it or it won't open the die properly (and yes, it took me a while to figure it out).

Personally, I don't thread with dies under power – I use a mandrel handle to turn the work. I'm sure it works but it's not what I do myself.

For larger diameter fine threads (1/4"/40tpi for instance) I tend to part screw-cut the thread (again using the handle for short/fine threads) to get it going, just finishing with the die. Where a short thread is required (2-3 threads) – I make the thread a bit longer and cut off the surplus after threading – it's much easier that way.

I always use material turned to the nominal thread size (not under) – 3mm in your case – and as Brian says, get yourself a tin of the magic green stuff (Trefolex) – it will last you a very long time…and you will eventually get to like the smell…

IanT

some dies will only cut from one side, usually the one with the writing on, been cought out by this before now.

not found the need for split dies on small threads 2, 2.5,3,and 4 mm, no need to go undersize i.e 3mm thread use 3mm material or machine to 3mm, does not matter if slightley under, also helps if there is a slight champher for the die to engage on.

usually turn the lathe by hand using spindle handle and tailstock up against the die holder. you can put slight preasure on the die using the tailstock handle.

ordinary cutting fluid usually used, nothing fancy.

rule number one – nevoer overlook the obvious!

Crikey, you must have a grip like Geoff Capes!

Neil

Coming in late no one has mentioned that the Die Holder needs to slide on an arbor. A small thread (under 6mm ) is unlikely to be strong enough to drag a Tailstock along the lathe bed.

+1 for rotating with a mandrel handle, it allows "feel" which may save damage!

Lubrication with Trefolex , Rocol RTD or something similar is taken as read.

Howard

Edited By Howard Lewis on 13/09/2019 23:46:20

Having had a 40 year gap I am still using tallow (not the same batch), and I probably have enough to last me out. Is it worth me switching to one of the newer compounds or tapping fluid?

Brian

Not if tallow works for you. Downsides of tallow are it can rot in the workshop – don't get used tallow in a wound – and it's not the most suitable compound available.

Although tallow is a reasonable lubricant and dirt cheap, it's out-performed by newer compounds containing extreme pressure additives that don't go off and work more reliably. The 'newer compounds' are hardly revolutionary modern ideas – they date back at least a century. Using a solid lubricant whilst tapping might help eject swarf, but I'm not sure tallow is stiff enough to be forced out and take swarf with it; I feel something more waxy is needed.

Deciding to replace a modern cutting mixture with tallow in hope a traditional material might be better would be a mistake – tallow isn't as effective. Depends on who you are and what you're doing. Taps and dies in my clumsy hands need all the help they can get! That said, faced with an empty spray can, I've successfully tapped threads with paraffin wax and Castrol grease – both worked.

Dave