Understanding chuck test certificates

[Peter FaceyParticipant @peterfacey79131]

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.

[Peter FaceyParticipant @peterfacey79131]

[Jon LawesParticipant @jonlawes51698]

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

[HopperParticipant @hopper]

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

[egaParticipant @ega]

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

[Brian WoodParticipant @brianwood45127]

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 1Participant @tonypratt1]

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

[egaParticipant @ega]

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 RimmerParticipant @peterimmer30576]

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.

[DC31kParticipant @dc31k]

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 3Participant @nickclarke3]

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 WoodingParticipant @garywooding25363]

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.

[HopperParticipant @hopper]

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.

[DC31kParticipant @dc31k]

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.

[SillyOldDufferModerator @sillyoldduffer]

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.

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 WoodingParticipant @garywooding25363]

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

[Oven ManParticipant @ovenman]

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

Peter

[egaParticipant @ega]

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?

[DC31kParticipant @dc31k]

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 GilliganParticipant @michaelgilligan61133]

Hopper’s explanation seems eminently logical to me

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

… perhaps I need help

MichaelG.

[DiogenesIIParticipant @diogenesii]

..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?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by DiogenesII on 18/06/2022 07:12:32:

..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?

.

Not sure if that “… well” was intended for me

… but, if so, it completely misses my point.

MichaelG.

[HopperParticipant @hopper]

On rereading the OP it seems Long V and Short V is an assumption by the OP. The chart itself only says LV and SV, which could be anything, even just randomly assigned model letters to identify different jaws. The OP should call Pratt Burnerd's UK HQ and enquire. But he has made his first and only post here and no return since so can't be too worried about it.

[Chris GunnParticipant @chrisgunn36534]

I know this is a bit off topic, but SOD, if you want to take parts out of the chuck and put them back later with precision, get yourself some soft jaws, very cheap and easy solution.

Chris Gunn

[HopperParticipant @hopper]

Or practice using the quick method of setting up the four jaw chuck. IE set the dial indicator on the back of the job in the horizontal plane. Rotate the job once and set the dial's zero to halfway between the highest and lowest reading on the indicator. Then go around and adjust each pair of jaws until each reads zero. A pair of "twiddlers",short chuck keys, makes this easier. Then go around again and final tighten and adjust that last thou or so. Two to maybe four revolutions of the chuck. It takes about a minute or two with a very little bit of practice.

If you just use the trial and error and hope for the best method, it can take all day.

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