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Hints and tips for cutting small(ish) male threads with a die

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Stuart Bridger08/09/2019 18:10:30
453 forum posts
25 photos

I am fairly sure that the answer here is practice, but would appreciate any advice.

Trying to cut an M3 x 0.5 thread with a split die on mild steel. Two attempts have failed so far with stripped threads. Adjusting the die, I'm either not getting the thread to start or the thread is so fine that it strips very easily.

I am using a lathe with a tailstock holder, by hand.

What major diameter would you recommend starting with? Dead on 3mm?
Possibly the die is not of best quality, would I better off going for a die that isn't split? If I do carry with a split die, what's the best way to set the size?


fizzy08/09/2019 18:21:31
1716 forum posts
116 photos

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.

David George 108/09/2019 18:28:13
1253 forum posts
438 photos

Hi Stuart to use a tailstock die holder there should be play in the holder to allow the die to expand when as you tighten the centre side screw with the two side screws undone. the centre side screw has a point which pushes the slot wider and makes the die cut less of the thread and then you just nip the other screws to lock the die. after you have cut the thread you try a nut on and if it is to tight loosen the centre screw and tighten the die up and try again. make sure you are using a good cutting lubricant at all times.


old mart08/09/2019 18:52:27
1819 forum posts
148 photos

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.

Brian G08/09/2019 19:00:22
705 forum posts
28 photos

I find putting a chamfer on the end (with a single point threading tool - please don't judge me, it saves setting the compound) helps start the thread.


Stuart Bridger08/09/2019 19:04:37
453 forum posts
25 photos

Thanks for the feedback. I must have a very short memory. Last weekend I successfully cut two threads M2.5 on a 3/32 bar. Apologies for the mixed units 2.5mm is 0.098in on 3/32, 0.09375, so about 4 thou under.

So three possibilities, 4-5 thou under is the trick, I got lucky with the die adjustment or the die is a better quality.

I will try again with 4-5 thou under the nominal major and see how I get on.

JasonB08/09/2019 19:09:47
18277 forum posts
2022 photos
1 articles

Make sure you don't have a 25mm OD die in an imperial 1" holder or 20mm die in a 13/16" holder as it will not be central.

I would turn the work to 3mm dead and don't bother with a chamfer just run the lathe at about 75rpm and bring the die holder upto the work by hand (no bar fitted to diestock) and just hold it firm until I have the length needed than let go and turn off. Drop of tapping fluid helps.

Edited By JasonB on 08/09/2019 19:10:33

HOWARDT08/09/2019 19:40:58
567 forum posts
15 photos

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.

Brian Oldford08/09/2019 20:00:57
673 forum posts
18 photos

Make several passes under back-gear and don't spare the Trefolex.

Andrew Johnston08/09/2019 20:29:43
5549 forum posts
650 photos

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.


old mart08/09/2019 20:33:20
1819 forum posts
148 photos

Be careful with the amount of diameter reduction you use, these small size threads don't have much to spare. I use these thread charts on the Motalia website, they show the thread depth.


Edited By old mart on 08/09/2019 20:34:18

IanT08/09/2019 21:25:21
1544 forum posts
144 photos

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...


Stuart Bridger09/09/2019 08:58:48
453 forum posts
25 photos

Thanks for for all the advice, some really useful stuff there. I have today ordered up some trefolex
and a couple of new dies, one split and one solid. I will update further when it arrives and I have had time to experiment.

Howi09/09/2019 09:23:47
276 forum posts
19 photos

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!

JasonB09/09/2019 10:03:40
18277 forum posts
2022 photos
1 articles

Here is the way I do it as described earlier.

First make sure your die is the right size for the die holder metric OD will go into imperial but the screws will push them off ctr. I have an imperial diestock so buy imperial OD dies

Writing facing out, I have just nipped up the 3 screws so not spreading and not closing the die. This is Tap & Die's "HQS" quality carbon steel.

Stock is EN1A 3mm nominal which come in at 2.96mm faced off no chamfer.

In Video, ass a drop of tapping fluid, start lathe at a slow speed and push teh die onto th eend of teh rod and hold it there once it bites until you get to the length required and let go. Stop lathe and reverse if you have it, this can be done at higher speed but did not do it here. Clean swarf and test your nuts or part.

I do it this way upto about M8 coarse and 1/2 x 32 in steel, any bigger and it becomes hard to grip the diestock. For larger I will put the tommy bar into the diestock and just turn by hand. I think winding by hand or taking several cuts would slow my output too muchwink


Edited By JasonB on 09/09/2019 10:05:56

Neil Wyatt09/09/2019 11:07:42
17970 forum posts
709 photos
77 articles
Posted by JasonB on 09/09/2019 10:03:40:
I do it this way upto about M8 coarse and 1/2 x 32 in steel,

Crikey, you must have a grip like Geoff Capes!


Russell Eberhardt09/09/2019 15:33:37
2591 forum posts
85 photos

The major diameter of an M3 screw should be just under 2.9 mm so you could start with that diameter. A few thou can make quite a difference at these small sizes. A good lubricant is Rocol RTD compound. I had difficulty with 12 BA (1.3 mm dia.) threads breaking off in the die until I was given a bit of RTD compound to try.


Stuart Bridger10/09/2019 13:16:07
453 forum posts
25 photos

A few observations, no time for practical tests yet
ID of my dieholder is 20.72 mm (a bit bigger than 13/16 - 20.64mm)

Original die is 20.48mm OD
New split die is 20.49mm OD (Tracy Tools)
Both are a slightly loose fit in the holder, without tightening screws - I will most likely make a new holder a bit closer fit.

New solid die (from Arc Euro) is 19.90mm OD - New hold holder to be made

Screws in the die holder are cup point, not cone point, so they need to be replaced

JasonB10/09/2019 13:22:16
18277 forum posts
2022 photos
1 articles

Tracy's ones sound like nominal 13/16" as does your old one and will be OK in your size holder as you do need a little room should the die need to be opened.

I Know ARC's are Nominal 20mm OD so a new holder will be best for them.

Howard Lewis13/09/2019 23:44:20
3361 forum posts
2 photos

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.


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

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