Manky External Threads

Advert

New Replies

Report Bug

Help/FAQs

Manky External Threads

This topic has 14 replies, 12 voices, and was last updated 26 January 2012 at 21:48 by Alan Worland 1.

Viewing 15 posts - 1 through 15 (of 15 total)

Author

Posts
25 January 2012 at 21:00 #15556
Alan Worland 1
Participant
@alanworland1
Advert

25 January 2012 at 21:00 #82983
Alan Worland 1
Participant
@alanworland1
This afternoon I decided to make some 0ba studs for my new project – I thought it shouldn’t take too long – wrong!

I had a rummage through my material stock and found some nice .25 inch stainless rod, faced and turned the end down for a length of 5/8 (it turned lovely)

Put the die in a holder and brought up the face of the tailstock chuck to keep it square. I locked the tailstock to the bed and pulled the work round in the headstock by hand and as the die fed on I kept it square with the tailstock handwheel.

I dont think I have ever seen such a manky thread! (I used a light oil while cutting and relieved the chips)

I turned it all off and tried again, nope!

I tried various mild steel rods (I dont know what spec they are) most of which seem to turn ok.

The best thread I cut is probably just about useable but shows a poor/shallow thread at the start ending in a full thread at the finish ie the entrance to the die. It’s as though the thread is being destroyed by the rest of the die, which has had little use and looks fine?

I have found another die and will have another go but does what I am doing sound ok? Its worked ok before!

Alan
25 January 2012 at 21:15 #82986
jason udall
Participant
@jasonudall57142
Had simular .

maybe over size “rod”. m6 od is not 6.0 for example….
25 January 2012 at 21:16 #82987
Jeff Dayman
Participant
@jeffdayman43397
Hi Alan,

a) you need a good die for stainless, buy a new HSS one if yours is old. Carbon steel will not stand up to stainless for long.

b) make the starting chamfer 20 to 30 degrees angle per side on the stock, and the end of the stock slightly smaller than the tapping size for the thread.

c) give the die/dieholder a really good steady push toward the chuck when starting.

d) use a good quality cutting oil, not diluted

Good luck, JD
25 January 2012 at 21:26 #82990
Alan Worland 1
Participant
@alanworland1
Thanks for the guidance!

Jason, I have been turning the rod to the correct OD so the die is not removing excess material.

Jeff, I am beginning to think my die is dull, it is only carbon steel (I think my other one is as well) and despite putting a good chamfer on the lead is does take a bit of ‘getting going’

Will get a rplacement and see if things improve!

Alan
26 January 2012 at 10:06 #83032
methusala
Participant
@methusala
Hi Alan, Have you tried reducing the o.d of the rod by two or three thou,

as this can help in getting a decent thread in harder materials.

It wont lead to a sloppy thread, as the flanks on the thread will still be

be in contact, but should ease the pressure on the die.

Hope this helps.

Colin.
26 January 2012 at 11:51 #83046
Ian S C
Participant
@iansc
I find that I get one shot a cutting a thread with a die on stainless, it work hardens, so you can’t thighten up and skim a thou or two off to finish the thread as you can in other metals. It does play merry hell with both taps and dies, just don’t try for high percentage fits. Ian S C
26 January 2012 at 12:25 #83047
Martin W
Participant
@martinw
Ian

If Allen has already turned the rod down to size will this not have started to work harden the surface of the SS. If so would there be any benefit in trying to anneal the turned down section prior to trying to thread it.

I ask this out of complete ignorance re the work hardening of stainless materials as my only, cautious, attempt resulted in the tool bouncing off the surface of a piece of SS. That said I had no idea of the type of SS and my approach was tentative so the surface had ample opportunity to harden.

Cheers

Martin
26 January 2012 at 13:15 #83049
AndyP
Participant
@andyp13730
It’s as though the thread is being destroyed by the rest of the die, which has had little use and looks fine?

Alan,

I have an ME die that does exactly the same, works fine t’other way round though, ie with the face of the die without any markings on it presented to the job – worth a try?

Cheers

Andy
26 January 2012 at 13:22 #83052
MICHAEL WILLIAMS
Participant
@michaelwilliams41215
Hi Alan ,

Its true that your die may be blunt or poor quality but what you describe as happening is primarily due to the die’s cutting diameter being wrongly set .

Many dies and especially carbon steel dies do not cut the right diameter . 9 times out of 10 the split in the die has closed up and the die is trying to cut to a smaller od than your work actually has . This always results in stripped threads .

What I would do would be would be to open the die out quite a bit and do a trial thread and then progressively close down until you get the clean accurate thread needed .

On difficult materials or in any case if the die is a bit dodgy it is best to cut the thread in two passes anyway – think of it as one roughing cut and one finishing cut .

MW
26 January 2012 at 17:39 #83066
Roger Mawby
Participant
@rogermawby76093
Hi Alan,

I had a similar problem which I tracked down to axial distortion of the split die in the die stock caused by the screw opening up the die and twising each half in a different diirection. A bit like a spring washer.

It was a poor quality ‘cast’ die stock witha poor seat for the die and rough ends on the adjusting screws.

A decent stock resolved the issue.

I have also found, like AndyP that some dies do seem to cut better threads when turned round!

Mellie
26 January 2012 at 17:50 #83068
Richard Parsons
Participant
@richardparsons61721
Hi Allan cutting an external thread in the lathe with a die can and is often fraught. I always (where I can) set up the gear box and turn the thread down to at least 50% of the finished diameter of the thread and finish it with at least two passes with a die. In the first pass I set the split die open (that is what the ‘centre’ screw is for). I then try it with the nut (or whatever). I then let the die close a bit –may be a turn of the centre screw or less- and repeat the process. A bit of a fag, but it is worth it.

By the way 0BA is 6mm diameter by 1mm pitch. At a pinch you can get away with 25 tpi and by using an ISO metric tool to cut the 50% thread depth.

Rdgs

Dick
26 January 2012 at 18:00 #83071
colin hawes
Participant
@colinhawes85982
A blunt die can sometimes be sharpened with a Dremmel type grinder if it has not been used for too long in a blunt state.

Colin
26 January 2012 at 19:46 #83079
chris stephens
Participant
@chrisstephens63393
After you tightened the screws did you tap the die home in the holder and did you clean any crud out first?

There is no point in holding the die holder square if the die is on the p*ss. Do you have another holder you could try, as it might not be the die that is wrong.

chriStephens
26 January 2012 at 21:48 #83101
Alan Worland 1
Participant
@alanworland1
Had another go today with another die, not new but it looked ok, result! and far less effort required.

Not perfect but a million times better – I have ordered myself a new HSS die. I might try them round the other way to see if things improve.

I used to use the tailstock die holder but found that the cutting centre of the die didn’t always match the centre of the holder so I now generally use a die stock with the tailstock brought up to keep it square and I always clean the holder so it sits properly.

I usually open the die out fairly fully to cut, then try the fit and adjust for a nice close fit with the mating part. I have had bad experiences of die cast holders (with only one screw!) so last summer I managed to obtain some ‘proper’ ones from Boot Sales, they can be picked up quite cheaply.

I did consider screwcutting the thread but the pitch put me off! having an imperial lathe with an imperial gearbox?

I dont know what grade of stailess it was but it turned lovely – take an extra .001 off with no probs, anyway thanks for all your advice and suggestions – I got there in the end!

Alan
Author

Posts