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Understanding chuck test certificates

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Peter Facey16/06/2022 11:52:13
1 forum posts

I recently bought a brand new Pratt Burnerd chuck. I am having difficulty understanding its test certificate (see image).

I expected it might refer to internal and external jaws, but not "long vee" and "short vee" jaws. What do they mean?

I can understand the 30 micron runout figures for the diameter and face of the body. For the rest, on the left they seem to be using 4 diferent test bars with the LV jaws, so perhaps LV means internal jaws. It seems they are tightening the chuck onto each test bar three times, using the three pinions, and getting different results each time. I don't really see how these results fit with their claim "Super precision chucks have a repeatability of 0.03mm TIR guaranteed".

On the right we seem to be using rings 1 and 2. So perhaps under "SV jaws" they are testing the external jaws by expanding them onto test rings and measuring the face and diameter runout. Not sure what the "LV jaws" bit under "Rings" is.

Jon Lawes16/06/2022 13:08:50
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984 forum posts

Will the results make a difference to your work or is it just out of interest?

Hopper16/06/2022 13:30:59
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6653 forum posts
347 photos

It looks like the repeatability is within 30 microns of TIR. That is a different thing from the TIR being within 30 microns of zero. For instance, test bar 1 has a TIR of 40, 40 and 30 microns. So the repeatability is within 10 microns of TIR. That means you can rely on that chuck to always run out by that amount (30 to 40 microns) and not sometimes runout by zero and other times runout by 40 microns. Thanks to repeatability the error is consistent (within 10 microns).

That;s the way I read it anyway. Perhaps you could contact Pratt Burnerd and get their take on it? (The price they charge for those things, they should send a young lady around to explain it in person!)

At the end of the day, getting 40 micron TIR out of a three jaw chuck is about as good as it can possibly get, due to the nature of the beast. Most run at double, triple or more than that. If you want better, use a four jaw independent or good quality collets.

Edited By Hopper on 16/06/2022 13:38:08

ega16/06/2022 14:19:59
2565 forum posts
203 photos

Purely by the way, the certificate appears to have been completed by a French or other person of continental origin.

Brian Wood16/06/2022 14:47:52
2579 forum posts
39 photos

Ega,

I have used the continental seven symbol with the horizontal bar through it for the last 43 years, it avoids any ambiguity and I am most certainly not French or continental !

Brian

Tony Pratt 116/06/2022 16:17:02
2023 forum posts
12 photos

Unless our education system has changed which is possible UK students do not use a ‘bar’ on a 7 , but hey just for fun I occasionally do 😉

Tony

ega16/06/2022 16:57:18
2565 forum posts
203 photos

Brian and Tony:

I think I am right in saying that the barred 7 is intended to avoid confusion with the French 1 (as used in the certificate); I don't myself use this rather strange form but have started to use the barred seven in the interest of clarity.

Many years ago when I was working abroad I remember my English 4s causing some confusion.

Finally, some of my best friends are French!

PS I should be surprised to learn that handwriting is taught in any serious way in schools today.

Edited By ega on 16/06/2022 17:00:12

Pete Rimmer16/06/2022 17:04:00
1256 forum posts
69 photos

I use a barred 7. Why? Because when I was in secondary school my coursework had them.

Anyway, the 1 digit is typically continental on that test cert.

DC31k16/06/2022 18:35:09
727 forum posts
2 photos
Posted by Peter Facey on 16/06/2022 11:52:13:

...perhaps LV means internal jaws

If you have a look around, you can find a copy of Indian Standard IS 2876 (1999) online in pdf format. This is a direct parallel of ISO 3089. Perusal of the standard will give you some idea of the tests involved.

It may be that PB use a standard test certificate for every chuck they sell. I would think these days that the number of manually-operated scroll chucks they sell each year is small compared to those used on CNC-type machines, so perhaps the LV and SV makes more sense in that context (where, because of the limited movement of the jaws, they are very often two-piece items).

Nick Clarke 316/06/2022 23:11:17
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1475 forum posts
64 photos
Posted by ega on 16/06/2022 14:19:59:

Purely by the way, the certificate appears to have been completed by a French or other person of continental origin.

I started to use a barred 7 when I began scoring at darts in school to avoid confusion. But then a git of a history teacher (described as the ugliest man in England by an old boy speaker at speech day) corrected each date in a one page summary of the eighteenth century with each one as a spelling mistake - and yes 1777 was three spelling errors!!

Gary Wooding17/06/2022 07:09:12
996 forum posts
255 photos

It's strange how topics drift.

The OP asked the meaning of "long V" and "short V" jaws. Instead of actually answering the question we get a discussion about handwriting ones and sevens.

I would like to know the answer to the original question.

Hopper17/06/2022 08:04:34
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6653 forum posts
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Posted by Gary Wooding on 17/06/2022 07:09:12:

It's strange how topics drift.

The OP asked the meaning of "long V" and "short V" jaws. Instead of actually answering the question we get a discussion about handwriting ones and sevens.

I would like to know the answer to the original question.

Possibly refers to the outside jaws having the long end of the stepped jaws machined to a V whereas the inside jaws have the short end of the stepped jaw machined to a V. Hence Long V and Short V jaws refers to the length of the V.

Ignore the letters R and L in the pic, just some random pic off the net and not relevant. But Long V on the left, Short V on the right.

jaws.jpg

DC31k17/06/2022 09:02:34
727 forum posts
2 photos
Posted by Hopper on 17/06/2022 08:04:34:
Ignore the letters R and L in the pic, just some random pic off the net and not relevant. But Long V on the left, Short V on the right.

That is good. It bypasses the confusion involved in calling them internal and external or outside and inside jaws. 'Long' and 'short' translate into probably every language in an intuitive way.

In the ones labelled 'R', the inside of the jaw grips the outside of the part. The outside (step) of the jaw grips the inside of the part [and that is the only one of the four cases where the force on the scroll is reversed].

In the ones labelled 'L', the inside of the jaw grips the outside of the part. The inside (step) of the jaw also grips the outside of the part.

SillyOldDuffer17/06/2022 09:46:56
Moderator
8883 forum posts
1998 photos
Posted by Hopper on 17/06/2022 08:04:34:
Posted by Gary Wooding on 17/06/2022 07:09:12:

It's strange how topics drift.

The OP asked the meaning of "long V" and "short V" jaws. Instead of actually answering the question we get a discussion about handwriting ones and sevens.

I would like to know the answer to the original question.

Possibly refers to the outside jaws having the long end of the stepped jaws machined to a V whereas the inside jaws have the short end of the stepped jaw machined to a V. Hence Long V and Short V jaws refers to the length of the V.

...

We'd all like to know the answer, but it seems no-one does! Hopper's is best suggestion so far, but it's too subtle for my taste. Not saying it's wrong, just not obviously right.

I spent yesterday's coffee-break touring a couple of jaw catalogues hoping to find an answer. No luck. Schunk  list about 40 different styles of jaw, but no sign of long or short V jaws. Nothing in Pratt-Burnerd's web info either.

Chasing the best is a hard sport. Having made sure the lathe spindle is good, one buys a chuck with a proper specification. Specifications are good, but worrying unless they're understood! Figures don't mean much unless the way they were measured is known. And having understood the spec, some forum smart-a*se (me) points out luxury chuck owners also have to choose the most suitable jaws for the job from a long list!

My take is the main advantage of posh tooling is saving time. For example, my ordinary 3-jaw is good enough for most turning unless the job has to be taken out of the chuck and replaced, perhaps after a bit of milling. Taking work out of my 3-jaw is a bad move, because chuck doesn't allow accurate resetting, although putting it back exactly as it came out and tweaking with a DTI helps. Trouble is tweaking takes time and doesn't always work because the chuck doesn't have high repeatability.

One answer is to buy a better made 3-jaw chuck, but even these struggle to reset work when high accuracy is required. Better for amateur purposes I think to fit a 4-jaw when reset is needed, because even an ordinary one will give a top-end 3-jaw a run for it's money! But 4-jaws have a major disadvantage when it matters, which is the time taken to adjust them. Not a problem in my slow-moving tinkering workshop, but slow working methods are bad news when a pro machinist has to work against the clock. Collets are often a good answer, but they have other limitations. Jigs and fixtures outperform ordinary workshop methods, but are only worthwhile for mass-production.

I suspect the best way for an amateur to do high-precision work is by sending CAD files to a professional CNC shop. There's no need to have an expensively equipped workshop at all.

devil

Not seriously suggesting we should do everything from comfy chairs! I enjoy my workshop, hands-on making things is interesting fun.

Dave

 

Edited By SillyOldDuffer on 17/06/2022 09:51:53

Gary Wooding17/06/2022 10:10:22
996 forum posts
255 photos

I'm quite happy to accept Hopper's suggestion - it seems to fit very well.

Oven Man17/06/2022 11:14:47
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185 forum posts
34 photos

Can I also second Hopper's suggestion. I came up with the same conclusion but couldn't think how best to describe it.

Peter

ega17/06/2022 14:29:31
2565 forum posts
203 photos

I, too, found Hopper's suggestion helpful.

Where I still struggle is understanding why the inside/outside jaws are so named; the practice seems inherently ambiguous. Perhaps that's why PB resorted to LV and SV?

DC31k17/06/2022 19:38:50
727 forum posts
2 photos
Posted by ega on 17/06/2022 14:29:31:

Where I still struggle is understanding why the inside/outside jaws are so named;

That's easy. The inside jaws are the ones you crash into with your left hand turning tool and the outside ones you crash into with the right hand turning tool. The ambiguity in the jaw and tool nomenclature is a form of magnetism - the more there is, the stronger the attraction.

Michael Gilligan18/06/2022 07:02:07
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20289 forum posts
1064 photos

Hopper’s explanation seems eminently logical to me yes

My only slight misgiving is that I would find it difficult to describe those areas as V-shaped

… perhaps I need help

MichaelG.

DiogenesII18/06/2022 07:12:32
589 forum posts
234 photos

..well, the ticket clearly has spaces below the 'Body No.' serial box for the recording of 'LV' & 'SV' Jaw Set serial numbers - what else might they refer to other than the sets of external & internal jaws supplied with the chuck?

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