Undersize thread

I'm trying to do something pretty basic, it isn't working.

The double ended nipple is to replace a safety valve so I can use compressed air to check the operation of the regulator and valve gear on a radio controlled 16mm scale loco. I start with a bit of hex bar and turn both ends to between 5.0 and 5.05 with a chamfer to assist the die starting. The tailstock chuck is used to press the die and holder onto the work and the thread cut with no difficulty. A slight undercut and a clean off the start. Ideally drilling through should complete a simple job, instead the afternoon has been spent in three attempts to make this work. As it is in each case the thread is seriously under size to the extent that the OD is between 4.5 and 4.6. I have used this die before and it has produced usable threads so any clues as to what is or I am doing wrong.

Regards

Martin

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

Just a thought, but I would turn the blank to slightly under nominal size rather than over to reduce the possible risk of tearing the thread.

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

Unless I'm misunderstanding the issue here, you appear to be using a solid die to cut the thread, whereas I would expect in a case like this to use a split die initially and adjust final thread size to fit the piece you are fitting into.

I would expect a non-adjustable die to give you a standard spec final size for your thread. Also, as mentioned by JasonB are you using the correct type of holder - needs to be metric as Imperial size is slightly larger and can cause misalignment.

Apologies if I've got the wrong end of the stick!

regards Mike

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.

With a carbide grit coated hacksaw blade, the die could be made into a split die. Then it could be opened up slightly to produce a larger diameter thread.

Fizzy's leaded bronze sounds a good idea, I have never tried that.

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

Howard it's not my 4.94mm that the OP is worried about it's his 4.50 to 4.60 which is quite a magnitude more!

Don't think you would get it to swell by 20thou even with liquid nitrogen

Edited By JasonB on 05/09/2022 18:51:31

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

Right, another pic of the threads, despite being a crappy pic it seems plain enough that the thread is poorly formed.

The die holder is the correct 20mm one, bought at the same time as the die. I must admit that I did think a solid die was odd, but adequate for the one or two uses.

I think firstly I'll try stock slightly undersize at 4.9 and see where that goes. Secondly I'll split the die, nothing to lose really and cheap enough.

Lastly a proper split die from Horley is only a tenner which is certainly less than the time I've wasted this afternoon. Thanks all for the useful info, I'll let you all know in due course how O get on.

Regards

Martin

Edited By Martin Shaw 1 on 05/09/2022 19:22:38

Edited By Martin Shaw 1 on 05/09/2022 19:23:01

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.

Unlike die nuts these solid dies are meant to be used without a split for cutting threads not just restoring or cleaning them up. Only one screw is needed to prevent them from turning in the die stock.

If you are going to use the diestock shown for split dies then I would put a point on the screws.

Robert Butler

I have checked the die thoroughly, there does appear to be some detritus which possibly makes me wonder about the material. Anything I have bought will be CZ121 but I did acquire a considerable quantity of brass material bequeathed to me, no idea what it actually is although it seems to machine ok. I had no joy with the stock slightly undersize at 4.85, still ending up around 4.5/6. I then converted the die to split, this worked in as far as I could cut a nominal 5mm thread and finish size at 4.95.

This is tight on the flexible pipe coupling and was oversize on the safety valve mounting which kind of makes me question the associated tap as well. Anyway I will order a suitable tap and die from Horley and try it all again. I don't want to excoriate Ian at Arc but at £2 a pop they can't be the highest quality considering Zoro want £35 for a Presto one, essentially the same thing. An annoying day to say the least, tomorrow will be better, repeat as necessary.

Martin

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

I assume that you apply end load to the die holder whilst cutting and not let the newly cut thread drag the die along. And of course, you have the die the right way around, usually the engraved face is the leading face. Once the thread is cut, you can reverse the die to thread right up to a face.

Edited By Speedy Builder5 on 06/09/2022 07:50:42

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's actually 10mm of thread twice the thickness of the die and yes I suppose I could cut the thread on the lathe but really, an M5 fine. There are a number of factors to consider some of which I may well have less than perfect but the woeful finished thread perplexes me. I have also been cutting 8BA threads recently, they work out fine so I would suggest technique is not the major factor here. I have ordered a new split die and when it arrives further attempts will be made.

Regards

Martin

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