Are you using a metric 20mm tailstock die holder? an imperial 13/16" one is larger and the die can get pushed to one side which is enough for it to cut too much off.

Just been and measured one cut with the same ARC die that I did last week in Colphos, good enough for me as the 6g tolerence that these solid dies cut to is supposed to be just under nominal (5mm) size

Edited By JasonB on 05/09/2022 16:53:08

Have you checked the die for any rubbish that has jammed in the threads which could reduce the outer diameter. Check you measuring instrument to ensure that this is giving the right dimension and the zero point hasn't moved. Try reducing the diameter to say 4.8mm and try again if OK then die could be trying to remove too much material and partially stripping the thread it has cut and not advancing. Had this happen once when trying to thread some small diameter steel rod that was dead to size.

Martin

I used to encounter this problem often when making fittings for our boilers out of brass – some would be ok but many cut undersize. I wouldnt entertain using a non split die – thought they were just for chasing? The solution, using the exact same split die was to cut everything from Colphos (leaded phosphur bronze). Perfect threads every time.

0.06 mm is only JUST under size, 0.03 mm a side, Probably the truncation and crest rounding that the thread spec requires.

(0.00236" in old money. Half the thickness of an average human hair )

If you are that worried, put the work in the freezer before turning it to 5 mm diameter and cut the thread while it is still cold.When it soaks back to ambient, it will probably be the 5 mm that you seek. Then it might even be oversize!

Howard

Has something nasty happened to the die? Possibly an optical illusion but the section of cutting thread visible In the picture looks gunged solid to me.

Dave

Check the screws in the die holder for alignment with the dimples in the die. If any are offset, it can distort the die, causing ruined threads.

Martin.

Robert Butler

A M5x0.5 thread at 6g tolerance should have an outer diameter of 4.98 mm max (see link).

6g external thread

I only use none adjustable dies. I would turn the bar to 4.95 mm.

This die has a chamfer. Turning also chamfer on the bar will make it more difficult to get the thread starting because more threads are engaged at the same time.
I only chamfer the stock when doing CNC threading to soften the sudden increase of the load when the cutting tool starts threading.

Edited By Huub on 05/09/2022 23:14:28

Cutting what looks like about 3mm of thread, does not, IMO, provide sufficient working space to achieve a good thread.

I would be cutting a longer thread and shortening it to the required length. I might be trying the die on a different material, too, to try to prove it was the die at fault (not expecting that).

I would suggest single pointing the thread and use the die to provide the finished thread form.

Practice, with the die, is likely necessary before trying to thread such a short length of thread like that.

It looks that your die is marked on the wrong side. I have one if I use it on that side it is marked it does not work and do not cut well. After some time I noticed a hard to see mark on the back side " cut from this side" . If i use the back side it is cutting OK. If I compare to others with the cut start related to the lettering M6 and so on ,there is no chamfer only at the backside marked cut from this side. On your part it is good to make a small chamfer for better cut start with the die. This is from my experience.

regards

Werner

I just tried it again on brass turned to 5mm dia and much the same result as the Colphos one I posted a photo of yesterday. Both cut at about 75rpm in a tailstock dieholder with one screw lightly done up just to stop the die rotating.