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Split die cutting undersize

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Baldric09/01/2019 12:45:36
134 forum posts
10 photos

Recently I was using a split dies to cut 1/2" BSW thread, however the thread seems to end up undersize as a purchased nut is a rattle fit, the same nut is a good fit on a new bolt.

I put the die in the holder, did up the centre screw (which does have a pointed end) and then lightly nipped the outer screws but not enough to do any squeezing. I expected I would then need to look to adjust the dies to cut the thread to the correct size. I did try the bolt in the die and it went in without trouble so seems the die is set larger than the bolt.

The die is from a second hand set purchased from eBay that seems to be in a good quality old box, marked Marconi Radar so I believe was of reasonable quality.

Any thoughts on what could cause this?

Baldric

Neil Wyatt09/01/2019 13:35:40
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Possibly a combination of wear and the fact that split dies have greater tolerance as they can be adjusted.

It's always best to experiment with expanding/closing the die to get a good fit, which is why many authors (GHT for one) liked to make a set of dedicated holders so once a die is set, it can be left.

Neil

Derek Lane 209/01/2019 13:48:38
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148 forum posts
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Have you loosened the outer screws and just tighten up the centre this will open the die slightly then screw in the two outer to just contact the die as before. You may need to do this a few times until you get the right fit

Trevorh09/01/2019 14:13:25
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241 forum posts
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Hi

The whole point of a split die is so it can be adjusted to suit the thread or application, we were taught as apprentices that place the die into the holder and lightly screw in the centre screw until it touches the sides of the die, now screw in the 2 side screws until tight (not overly tight) take a sample cut and try and you will find it will be a nice fit

if your not happy then adjust to suit

you can always go in the other direction and start by having the centre screw in as tight as it will go and take a cut

then back it off until you get the fit your after

regards trevor

Baldric09/01/2019 14:13:38
134 forum posts
10 photos
Posted by Derek Lane 2 on 09/01/2019 13:48:38:

Have you loosened the outer screws and just tighten up the centre this will open the die slightly then screw in the two outer to just contact the die as before. You may need to do this a few times until you get the right fit

This is exactly what I did, the outer ones just nipped after the middle one done up.

Trevorh09/01/2019 14:22:41
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241 forum posts
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then certainly start with just the mid screw in tight both the outer screws backed off

This should give you a tight thread, then half a turn out on the mid screw until it a nice thread

once you get a feel for it its quite a quick process to repeat

The only other way which is slightly cheating is to screw the die and holder onto the bolt you have and set the screws once its already on the thread then unscrew the holder and it should be very close to the correct size

 

enjoy

regards trevor

Edited By Trevorh on 09/01/2019 14:25:49

Mick B109/01/2019 14:27:28
912 forum posts
55 photos
Posted by Baldric on 09/01/2019 14:13:38:
Posted by Derek Lane 2 on 09/01/2019 13:48:38:

Have you loosened the outer screws and just tighten up the centre this will open the die slightly then screw in the two outer to just contact the die as before. You may need to do this a few times until you get the right fit

This is exactly what I did, the outer ones just nipped after the middle one done up.

Hmmm... I sometimes find I need to feel and see the opening of the split before bringing in the outer screws to contact. The adjustment can be quite sensitive - a few degrees of turn in any of the screws can make more difference than you expect - and getting an 'ideal' fit can be elusive.

fishy-steve09/01/2019 15:27:00
103 forum posts
30 photos

Probably a silly question but what diameter is the material your threading. You didn't say you have turned it. Is it possible you've picked up 12mm BDS instead of 1/2" by any chance?

Steve.

Tim Stevens09/01/2019 15:34:23
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983 forum posts

You need to check that your centre screw has a decent point to match the notch in the die. I find that the screws in recent, or cheap, or older die holders can be soft, or worn, or quite wrong for the job.

Cheers, Tim

Mick B109/01/2019 16:51:32
912 forum posts
55 photos
Posted by Tim Stevens on 09/01/2019 15:34:23:

You need to check that your centre screw has a decent point to match the notch in the die. I find that the screws in recent, or cheap, or older die holders can be soft, or worn, or quite wrong for the job.

Cheers, Tim

That's true - I replaced the centre screws supplied in my dieholders with capscrews turned to a 30 deg included cone point.

jimmy b09/01/2019 17:02:05
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434 forum posts
27 photos

Have you checked the die against the bolt?

Don't try opening up or closing the die too much, they can split, don't ask how I know....

Jim

Speedy Builder509/01/2019 17:33:31
1666 forum posts
113 photos

for 1/2" and upwards, I would probably screwcut to near enough and use the die to clean up / get the correct root / crest forms.

Bazyle09/01/2019 17:38:29
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4388 forum posts
184 photos

It could be that a low quality die holder has a hole that is not big enough to allow the die to open.

David George 109/01/2019 17:42:51
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656 forum posts
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You could be using an imperial die in a metric holder with insufficient space to allow expansion of the die.

david

JasonB09/01/2019 17:44:36
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Posted by Bazyle on 09/01/2019 17:38:29:

It could be that a low quality die holder has a hole that is not big enough to allow the die to open.

Or a high quality 25mm die stock with a high quality 1" die being used by a low quality userwink 2

Maurice09/01/2019 18:45:05
414 forum posts
50 photos

I had this problem a short time ago and eventually found that the bore of the tailstock die holder was undersize, preventing the die from fully expanding! Put it in the lathe and increased the diameter of the bore by 10 thou. No more problems.

Maurice

Mike Poole09/01/2019 18:55:08
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1753 forum posts
44 photos

It would be interesting to know what the optimum size for a die holder should be, if you manage to avoid mixing up 25mm and 1” holders and 20mm and 13/16 you can still get the holder that is a bit tight but going too big will soon result in a broken die. Measuring my collection did not reach a firm conclusion.

Mike

Edited By Mike Poole on 09/01/2019 18:55:37

Mike Poole09/01/2019 19:13:53
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1753 forum posts
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Just had the brainwave to measure some Arrand  tailstock holders that I have and they seem to be 3 to 5 thou up on the nominal size.

Mike

Edited By Mike Poole on 09/01/2019 19:14:13

Vic09/01/2019 20:13:55
1965 forum posts
10 photos
Posted by Maurice on 09/01/2019 18:45:05:

I had this problem a short time ago and eventually found that the bore of the tailstock die holder was undersize, preventing the die from fully expanding! Put it in the lathe and increased the diameter of the bore by 10 thou. No more problems.

Maurice

I've not had Baldrics problem to date but it's a good point Maurice. wink

Baldric10/01/2019 08:32:15
134 forum posts
10 photos

I had turned the bar to 1/2" so don't believe that was an issue, and the die holder seems OK with the 3/8" BSW die, I will double check the screw points and holder to make sure that they are all OK, I had made sure they were clean, so no swarf trapped, also I suspect it is the correct holder as the die went in easily, I did not have to push or compress it.

While it could easily be operator error I have used a dies to thread many parts before, adjusting to get a good fit, hence the question. Would a blunt die dig-in and cut under-size?

Seems the bathroom extractor has packed up so bang goes my time in the workshop this tomorrow

I will update on my findings when I have any.

Baldric.

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