Truing up chucks

[Andrew TinsleyParticipant @andrewtinsley63637]

[Andrew TinsleyParticipant @andrewtinsley63637]

I recall reading recently about getting three jaw chucks to run true, I think it was either George Thomas or professor Chaddock, who advocated that a back plate should be turned under size and the 3 bolt fixing holes to be made somewhat larger than necessary.

This would enable one to set the chuck to have minimum (zero?) run out. I think there was even mention of using 3 screws on the backplate periphery to give a grip true type adjustment. I cannot visualise how this would work in practice.

Now all of this sounds heresy to me! I have always read and been told that the backplate should be turned to exactly a push fit into the chuck. However if such luminaries advocate a poor man's grip true chuck, then I am hardly in a position to complain!

What are peoples views on this heretical claim? More important, how would the 3 adjusting screws be made to work?

Andrew.

[Neil WyattModerator @neilwyatt]

No 3-jaw is perfect, although these days it's not unusual to get one that's very good at a reasonable price.

The policy they advise is just a poor-man's way of emulating a Burnerd grip-tru chuck, it can make any chuck accurate, but obviously the optimum adjustment is really only when setting it up. A decent chuck will repeatably hold the same diameter accurately, but might not show any great benefit at markedly different diameters,. A poor chuck might not even hold teh same workpiece concentrically if it is rechucked!

In my view this is worth doing if you have a good chuck that has significant runout not due to damage to the scroll and that can't be solved by grinding the jaws true.

I would not do it for a decent chuck with a runout of the order of a thou or two.

Neil

[HopperParticipant @hopper]

Yes I have red that somewhere too, possibly one of the GHT books I read in the past year or two, or just as easily Sparey, Bradley or others. The three adjusting screws would be drilled and tapped in from the OD of the chuck and bear on the step in the back plate, but probably not necessary if you were prepared to spend the time doing alignment by bumping with soft hammer and then nipping up the mounting bolts. Have never tried it myself as within a thou or two is close enough for three jaw for me and I use the four jaw for anything finer.

[Andrew TinsleyParticipant @andrewtinsley63637]

Thanks for the replies. For some idiotic reason, I assumed that the three adjusting screws were in the backplate! Obviously they have to be in the chuck body. Talk about having a one track mind!

Interesting that so far, no one thinks this is worth doing. If the run out is a thou or two then its OK. If greater than this, then use the 4 jaw. The inference I get from this is that a Burnered Grip tru is a waste of money! I bet that will upset quite a few people!

I have an old American chuck that isn't too good in the run out stakes, jaws and scroll seem good too, so I may well try this tip out. I don't have anything to lose.

I have a Czech chuck that came with my ML10 lathe This one is a puzzle, Measuring the run out at the work, gives just under a couple of thou. However when running, the periphery of the chuck body has a huge run out! I have not measured it, but I would guess about 20 thou! A neighbour of mine came in while I was using the lathe and the first thing he said was "There is something wrong with that chuck, just look at the run out!". I am now hijacking my own thread!

Andrew.

[MWParticipant @mw27036]

It's pretty much impossible to have absolutely zero runout between different mating parts, however, so long as you turn everything at the same setting, it makes it much easier to ensure consistency.

I presume what lathe manufacturers did (or maybe still do) was to temporarily fix the tailstock and head stock body together and bore/ machine all the mating surfaces at the same time. so that everything lines up on the bed. 

Maybe CNC has made this much easier to achieve consistency but in my experience cnc can still falter especially where many are being made. 

Michael W

Edited By Michael-w on 06/05/2017 12:33:39

[egaParticipant @ega]

Posted by Andrew Tinsley on 06/05/2017 12:25:22:

The inference I get from this is that a Burnered Grip tru is a waste of money! I bet that will upset quite a few people!

It was indeed GHT that passed on the tip given to him by Commander Barker about the poor man's Griptru; that didn't stop GHT making great use of the Griptru in his own creations.

Interestingly, GHT corresponded with Burnerd at some length before they admitted the need to slacken the backplate screws before adjusting.

[Andrew TinsleyParticipant @andrewtinsley63637]

Somewhere I have the instructions for my six jaw Burnered grip tru chuck. I will see what that says about adjusting, if anything.

It has at least 5 sets of jaws, some of which I don't know what they are for! "No good giving me something complicated mister"

[LambtonParticipant @lambton]

Hemmingway Kits sell a very simple kit for making any chuck adjustable rather in the manner of a Griptru.

The only downside is a small increase in overhang. I made one from such a kit for a 5C collect chuck and it worked perfectly. I anyone makes one though use 4 adjusting screws rather than the three on the drawing then it is much easier to get the chuck running true using a similar technique to centering work in a 4 jaw chuck.

Edited By Lambton on 06/05/2017 15:23:20

Edited By Lambton on 06/05/2017 15:23:58

[davebParticipant @daveb17630]

I'm fairly sure Hemingways do an adjustable backplate kit, possibly inspired by GHT.

I have a Pratt Burnerd chuck, threaded body, made for Myford which has a wobble on the threaded side but the front and jaws run reasonably true. Monday morning job? Works fine but really annoying.

It may be worth trying to get the best from a new, good quality chuck but a worn one will not hold true at all diameters or even the same diameter twice.

[Roderick JenkinsParticipant @roderickjenkins93242]

I've certainly mentioned on this forum that I have some SC chucks set up like this – the 3 jaw was like it it when I got it with my SH S7. The point, as Neil mentioned, is that most chucks have good repeatability at any given diameter even if there is some runout from the lathe axis. This means that if you have several parts to make of the same diameter the chuck can be set for that particular diameter and subsequent parts can be chucked with minimal run out. There does not seem to be any downside to this, I've never had a chuck move once the bolts have been tightened. I adjust the chuck on the backplate with a hide hammer, the Griptru adjusting screws are a more refined way of doing this. With the easy availability of ER collets there is, perhaps, less reason to use this function.

I think there is a general misunderstanding of the Griptru – it is no more accurate than a standard chuck but can be made so for a particular diameter. If you want guaranteed minimal run out over all diameters in a SC chuck then you need a super precision version. But for one offs it is cheaper and more accurate to use a 4 jaw independent and a DTI, IMHO.

Rod

[Mike PooleParticipant @mikepoole82104]

Much discussion has taken place over 3 jaw chucks and the griptru version and their repeatability and accuracy. If you just regard every 3 jaw as not accurate and use a 4 jaw independent or a collet or turn between centres then life is simple. If you are going to clock every job set up in a 3 jaw to make sure it hasn't picked up a bit of swarf or something then you may as well start with the 4 jaw. I was taught to plan the operation sequence to do all the operations without removing from the chuck if concentricity was important. If not possible the between centres or 4 jaw would have to be used.

Mike

[George BeckerParticipant @georgebecker85540]

Andrew,

Neil is right on the money. There are three problems that loom large with 3 jaw chuck runout. One is amount of runout at any given diameter, two is runout variability over the whole range of diameters, three is repeatability of setting accuracy at any particular diameter. If you are really looking for zero runout you are not likely to achieve it repeatably with the worn American chuck you mentioned. A worn mechanism is not likely to give the same results on retightening if the runout tolerance is really zero. I you are chucking up a single part a rubber or leather mallet with or with out shimming might be the easiest route. Even if rechucking the same or same diameter parts the mallet might still be the easiest. If you are talking about a long production run perhaps a hard look at tolerances is in order or maybe a look at an alternative workholder or both. A chuck that runs to tight runout tolerances consistently, especially over a large potion of its range is to be treasured. A question about you Czech chuck. Running out of true does it cause any vibration problems?

George

[John PurdyParticipant @johnpurdy78347]

Andrew

The advice to convert a normal 3 jaw to a poor man's Griptru was published by Professor Chaddock in Part nine of his Quorn article (ME 3 May '74).

I quote

"The answer is simple if you take the advice given to me many years ago by no less of an authority than the late Cmdr. Barker. That is to take the chuck off it's backplate and turn 1/32" off the register. Open out the holes in the backplate by a similar amount and put it all back again. You now have a chuck, like the Griptru, which can be set to run true holding work of any diameter regardless of wear in the jaws or scroll. This outrageous advice, which the editor declined to print on a previous occasion, but I hope he will on this, really does work. The work can be set to run really true to collet chuck limits, and contrary to all belief the body does not slip on the backplate." End of quote.

I have a 4" Griptru which I purchased with my Myford Super 7B in 1977. I find it invaluable. It takes less than a minute to set work to run to 1/2 a thou or less TIR using a DTI on a magnetic stand on the cross slide. The DTI lives permanently at the back of the lathe bench, within a short reach. Usually I find that the limiting factor is the actual concentricity of the work itself.

John

[Andrew TinsleyParticipant @andrewtinsley63637]

Thank you John,

That is exactly where I read about it! Thanks for reminding me, at least I got that bit correct! George, there isn't much in the way of vibration. It is a TOS chuck, which I always thought to be pretty good. There seems to be no difference in noise or vibration when it is changed for another chuck. So quite where the excessive run out on the outside diameter of the chuck comes from, I do not know. it came with the ML10 I purchased as a stopgap. The jaws are firm and the backplate has about the same degree of run out as the work, I just checked it at 1.2 thou. The run out of a 1 inch diameter bar of silver steel is 2.2 thou The outer body of the chuck is out 32 thou, no wonder it looks a bit wobbly! All very strange, maybe it is a Friday afternoon job. The chuck is good enough for me!

As to using my old Cushman chuck to carry out the modification recommended by Prof Chaddock. I can see no reason why I should not experiment. I fully understand the various comments which imply it is a waste of time. I suspect that I can get the average errors to be less using the device than it is as present. If so, then time well spent for the small amount of effort. I am not under the impression that I can get zero run out. I am not that wet behind the ears. A halving of the current run out would make me quite happy.

Thanks everyone for their input, much appreciated,

Andrew.

[HopperParticipant @hopper]

If your old Cushman chuck is worn like most old chucks, the jaws will be bellmouthed so no amount of improvised grip-tru mounting will counteract that. Best to regrind the gripping surfaces of the jaws first, to make sure they are parallel. It can be done with a dremel or similar mounted on toolpost and with suitable packing pieces between the chuck jaws to push them outwards while being ground. The packing pieces go between the bevelled edges of teh jaws. You will find too that the old chuck's concentricity is greatly improved by this too. My 1937 Crown chuck now holds true within about a thou and a half across most diameters, ie good enough for a three jaw.

[HopperParticipant @hopper]

Pic of the rig to grind chuck jaws per the above post

See the whole article here, scroll down the page a bit when you get there **LINK**

[egaParticipant @ega]

John Purdy:

Thanks for the correction – DHC not GHT. On reflection, GHT, having a Griptru, would not have needed to follow that "outrageous advice".

[Barry Taylor 3Participant @barrytaylor3]

I have a zip file of old Popular Science articles that has a great easy to make tool for grinding internal grip faces of a chuck and the external. Jaws under compression both ways. Let me know if you want it.

[not done it yetParticipant @notdoneityet]

take the chuck off it's backplate and turn 1/32" off the register. Open out the holes in the backplate by a similar amount and put it all back again.

Please tell me the point of any spigot at all, if it is not actually doing aanything? If it is not touching anything (which is very likely) one might just as well cut a recess in the back plate – it would be just as useless as a spigot not touching anything!

Maybe the spigot is there to minimise the swarf which can collect in the otherwise gap? Again, the chuck manufacturers would not need to make their chucks with a gap behind that cover plate if that were the ca

se!

So, go on someone explain the above idiotic suggestion. Why 1/32nd? Pointless carp, if you were to ask me! Just a means of bodging a poor job (in the first place).

The only possible reason for a smaller, non-fitting, spigot is to prevent the chuck flying off instantaneously (or destroying the bed) if some or all the securing bolts were to come loose/break. If that is the case, clearly there must be the possibility of that happening -not what some are suggesting above.

[HopperParticipant @hopper]

Maybe just to minimize machining time by simply removing a small amount of the existing spigot to allow a small amount of movement? To maintain maximum thickness of the backplate in the spigot area in order to maintain maximum stiffness?

[not done it yetParticipant @notdoneityet]

Sorry, Hopper, but neither hold water!

No point it being there if not for registration. No point if it ddoesn't actually touch anything in this respect.

Backplate could be left that little bit thicker if strength was required in that area. Chuck could be slightly thinner if the "'spigot space' was not put there in the first place.

A further point is that the bolt holes in the back plate are always left larger than the fixings – the whole reason why the spigot is the register for the chuck. They are not "fitted" bolts by any stroke of the imagination.

[SillyOldDufferModerator @sillyoldduffer]

One thing to remember about the excellent George Thomas is that he wrote at a time when many model engineers did not have access to affordable kit in the way we do. There was a great deal of 'making do' with worn-out industrial seconds, innovative approaches, and cheap and nasty accessories aimed at the hobby market. Quite a few hobbyists worked only with a 3-jaw, between centres, a backplate, and no DTI. George Thomas wrote a very good series on improving accuracy using limited equipment: he may have invented the plasticine wobbler.

Lot's of value in his writings, bul I'm not sure it's always the best way forward today. My 3-jaw is 'good enough' for most of my requirements. When I need better adjustment, I use a 4-jaw. Back in the 'good old-days', not everyone had such an extravagant luxury in their workshop. I'd rather buy one and get on with the job rather than mess with correcting an old 3-jaw.

Dave

[Andrew TinsleyParticipant @andrewtinsley63637]

Ha,

The above is an interesting way of thinking about ME. I started off building steam locos and the kit was merely an ends to a means. Now however, I am more interested in making kit not models! There is always the opposite viewpoint.

My old Cushman has had its jaws ground via a dremel and the improvement was well, not a lot. As I said before, if I can half the average run out, then I would be a happy man. Some simple turning and 4 holes drilled and tapped, sounds a very small price to pay, if it works. I am just about to start doing the job so I can report back tomorrow.

The Cushman chuck is definitely not a worn out heap of manure, in fact it isn't in bad nick as far as I can tell, The jaws are a good fit with no appreciable rock detectable and the scroll looks to be in good condition. Well worth an hour or twos punt, to see if it can be improved.

I have an excellent 3 jaw chuck (Polish) and I prefer to keep that for relatively precise work. The Cushman is used for any old rubbish job, when I don't wan to wear out my good chuck. OK for real precision, a 4 jaw and DTI are the way to go. Changing chucks relatively frequently, as I do, I never have problems with chucks sticking!

Andrew.

[Neil LickfoldParticipant @neillickfold44316]

The truing up of the jaws is a good thing when they have worn a little on the front. I have a couple of Grip True chucks. The older one of the 2 has little bellville washers under the back plate cap screw heads. The newer one does not. Only had ordinary washers under the cap screw heads. Does anyone know if the bellville washers are original or an upgrade from the previous owner.

On the original 3jaw chuck that came with the lathe, I trimmed some off the diameter of the back plate spigot, and then brought some bellville washers. Made the od smaller to fit into the counter bored holes of the chuck. So when I needed to get it close to 0, I can just loosen off the cap screws a little, the pressure from the washers help to hold it in place when when making the small adjustments.

The grip true is quicker than the std chuck and the 4 jaw to dialling in. But a 4 jaw with 2 keys is fairly quick, but not as fast as the Grip True.

With chucks, the 1st thing to assess is if the run out is due to the jaws, or if it is a swash problem. The swash problem is easily solved by turning a bar, closing the chuck onto it and then skimming the back registry face. To assess the swash runout, just close down on a few different sized bars. Then when indicated near the chuck jaws, and then at some amount depending on the length of the bars, like 50mm or so, see if they are in the same direction relative to any reference on the chuck. If it shows a correlation then it is a swash error with that chuck. Some more things to look for anyways.

Neil

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