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Three Jaw Chucks

Repeatability

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Peter Simpson 2	29/05/2011 19:08:38
28 forum posts 1 photos	I have been experimenting with my Burnerd 3 Jaw chuck. After skimming a 1" diameter piece of BMS I reversed this in the chuck and did the same to the other end. I put DI on the free end an measured 0 run out. I then removed the metal from the chuck to simulate another activity that may have been required. The metal was then rechucked and the DI was set back up again. The best repeatability was approx .005" to the worst being .020" I removed the metal several times, used all three chuck key positions to nip up the metal. The repeatability was poor to say the least. Could one expect to get a repeatability of .002" every time you carry out this experement. Is the chuck worn out or strained by it's previous owner. Could I expect better results if I purchased a chuck from the likes of RDG or Arc Euro. Many thanks for any info supplies
mgj	29/05/2011 22:05:16
1017 forum posts 14 photos	I did have the figures, but if you look at Workholding in the lathe by Tubal Cain, he goes into the errors in 3 jaws. I think, for a standard quality 3 jaw, using the preferred pinion, about 5 thou is all that they guarantee new, at a distance form hte face. See the appropriate DIN standard which many work to.. If you were .020 out then either the chuck is not set properly in its backplate (unlikely in view of its other results) or possibly you had a flake of swarf in somewhere - possibly even internally. Or maybe you didn't use the preferred key. Could one expect repeatability of .002 every time (using any key?) No. (unless you were lucky) I have a brand new Pratt Super Precision chuck. The error on that varies between 1.27 thou to about. .0004" using the preferred pinion on bars. About 1/2 thou on 100 mm rings on the same preferred pinion. Thats a new instrument of a grade somewhat higher than the standard chuck (and it had to be reworked twice to get it into the SP tolerance so it took some weeks to be delivered) Would you do better with a new chuck from RDG and ARc euro. Possilby - BUT, what are you to gain? No one in their right mind takes a bar and puts it into a 3 jaw and regards it as true. If you need a bore concentric with a circumfenrence, you either turn them at the same setting, or you reset in a Griptru, collet or 4 jaw. For rough work, yes you can turn end for end or reset, but if it matters, as in an axle tube with bearings at each end and you want to be able to slide and axle through, then no you most certainly cannot, not even with an SP chuck, unless you are very lucky, because the error is greater than the reaming allowance oversize. Edited By mgj on 29/05/2011 22:08:22
John Stevenson	29/05/2011 22:15:07
5068 forum posts 3 photos	What many people don't realise about chucks is that they take into account worn jaws etc but no one thinks about the scroll. This piece does more work than anything in the chuck and it is located on a central boss. Once this scroll or usually the softer boss wears it depends in which direction it is pulled to determine where the jaws are. It's rather obvious when you stop and think. I have seen many articles on grinding jaws but never one article of repairing a scroll or boss. It is this wear that will be causing Peters errors of 5 thou to 20 thou. John S.
ady	29/05/2011 22:23:23
612 forum posts 50 photos	Collets are the way to go for concentricity and repeatability, which is why they are so popular in industry. There's a high quality 6inch 3 jaw here , a steal at 2 grand a pop.
John Stevenson	29/05/2011 22:30:00
5068 forum posts 3 photos	Posted by ady on 29/05/2011 22:23:23: There's a high quality 6inch 3 jaw here , a steal at 2 grand a pop. Report back on it after you have bought it. John S.
mgj	29/05/2011 22:59:46
1017 forum posts 14 photos	Or it could have been stripped down for cleaning and not put back in the right order with the right pinions in the right places. However, unless the jaws are allowing material to wobble- causing chatter etc, then it probably doesn't matter, because one shouldn't be relying on a 3 jaw for repeatability. Nor collets if you want real precision, because even they have an error. A 4 jaw is the only way to reset properly (or a Griptru, or slacken chuck backplate and clock) because with a decent DTI you can reset to the limits of surface finish. The chosen route all depends on what the job has to do when complete.
Chris Trice	29/05/2011 23:04:01
1376 forum posts 10 photos	Posted by John Stevenson on 29/05/2011 22:15:07: What many people don't realise about chucks is that they take into account worn jaws etc but no one thinks about the scroll. This piece does more work than anything in the chuck and it is located on a central boss. Once this scroll or usually the softer boss wears it depends in which direction it is pulled to determine where the jaws are. It's rather obvious when you stop and think. I have seen many articles on grinding jaws but never one article of repairing a scroll or boss. It is this wear that will be causing Peters errors of 5 thou to 20 thou. John S. What John said.
chris stephens	29/05/2011 23:38:07
1049 forum posts 1 photos	Hi Guys, What he said, and let us not forget that jaws can get bell mouthed which causes all sorts of problems especially when parting off. If you wobble the stock in the chuck as you tighten the jaws you can sometimes improve the run out. chriStephens PS for those who don't know which is the preferred key hole, it is either marked with an "O" or is the one nearest the label on the chucks face.
Chris Trice	30/05/2011 01:20:04
1376 forum posts 10 photos	As a chuck wears, the original nominated pinion can give a worse setting than one of the others. It's worth trying each one and checking run out with a DTI to see if one is better than the others although in this case it sounds like the chuck is well past its best. If the scroll is a loose fit on the central boss, you're never going to get a good or consistent result.
Stub Mandrel	14/06/2011 21:45:20
4318 forum posts 291 photos 1 articles	If you have a decent chuck, its worth usinga dial guage/dti and rechucking (and cleaning jaws and work) a few times to get a decent result. I have a video of a rechucked axle with various diameters and I swear theres barely a flicker in the DTI 4" from the chuck! lucky perhaps... Neil
Roderick Jenkins	15/06/2011 08:50:50
2376 forum posts 800 photos	When I got my S/H S7 a couple of decades ago I discovered that the 3 jaw chuck had been mounted on a loose spigot with oversize holes in the back plate for the mounting screws. This means that there is about 10 thou movement available to true up the chuck by loosening the screws, centring the job with a DTI and then nipping up the screws. This system works very well and I have never had any issues with slippage, even after a floor shaking dig in.. Rod
Peter Simpson 2	15/06/2011 11:19:14
28 forum posts 1 photos	Roderick, I would investigate the possibility of that solution if the error on rechucking was similar every time, but the results vary every time I check the run out. Looks like a new chuck is the only solution. Thanks Peter
Versaboss	15/06/2011 11:23:39
512 forum posts 77 photos	Rod, although this method goes against all the textbooks, it was advised to me by a chuck/workholding specialist some years ago. I just used this method when I mounted a very cheap Chinese chuck. With a tight fitting spigot this chuck was not very impressive, but after reducing the spigot diameter say 0.15 mm and adjusting the chuck as you described, I got a runout of less than 0.02 mm on a 10 mm dowel and about 20-25 mm in front of the chuck. I stopped the adjusting process then... Maybe it's only good for that diameter, but could always be repeated if the need arises. Greeting. Hansrudolf
Metalhacker	15/06/2011 12:51:00
82 forum posts	As a variation on Rod's idea, I modified my taiwanese chuck on my 9x20, by using the over size backplate holes, and tapping the chuck in 4 places like a 4 jaw so some 3mm grub screws bear on the undersized spigot in the backplate. Works great and I can get about 0.01 mm accuracy relatively easily. Just use the 3 jaw as you normally would but when more accuracy required reset the backplate. Much quicker than the 4 jaw. probably not as accurate or as strong a hold but quite convenient most of the time. Best Wishes, Andries
Roderick Jenkins	15/06/2011 14:08:11
2376 forum posts 800 photos	I use my 3 jaw exactly as Andries says, I should have explained it more clearly. If you want true concentricity then nothing beats using a 4 jaw and a DTI. Even collets have some run-out. cheers, Rod
Chris Trice	15/06/2011 22:36:04
1376 forum posts 10 photos	That'll get the chuck body running truly but if the scroll inside is worn and there's any excessive clearance between the scroll centre and the central boss it rotates around, you won't get consistent repeatibility even if the body of the chuck runs true. Judging by the figures originally quoted, this chuck sounds knackered and due for replacement.
KWIL	15/06/2011 23:21:15
3681 forum posts 70 photos	There remains just another variation, that is with diameter of chucked piece. No scroll is perfect even when new and you may well find variation from this even when you have carefully set up with one diameter and them move on to another.
Chris Trice	15/06/2011 23:51:51
1376 forum posts 10 photos	The original posting states there's a variation even when rechucking the same diameter. It's a turned piece so it's presumably perfectly circular. The other aspect not mentioned is worn jaws as in they are belled out by being overstrained and only gripping on the heel of each jaw inside the chuck bore. This seems the most likely fault as it'll allow the chucked workpiece to "wobble" at the outer end.This would account for the large variance in readings and is easy (relatively) to rectify by re-grinding the jaws. Edited By Chris Trice on 15/06/2011 23:55:29
Pat	16/06/2011 01:44:54
94 forum posts 1 photos	Many years ago I was taught to tighten self centring chucks on big lathes - 6ft Colchester - in the following sequence making sure the power was off and the back gear released it was then possible to turn the chuck by hand. 1. - Clean chuck jaws and work - suds was piped to a hose with a brush in the end so this was easy if a bit messy! 2. - Prepare by adjust the jaws to be slightly wide - the chucks had been marked with concentric groves to ensure this was easy to do 3. -Lift the work to be on center height - often requiring the hoist due to the weight of the part 4 - Swing the job between the chuck jaws 5 - Starting with the jaw marked one LIGHTLY nip the work so that it was just held readjusting the sling height if necessary 6 - Rotate by hand and nip up the other jaws lightly in numerical order 7 - Repeat step 6 taking another circuit of tightening this time with a bit more force 8 - Release the sling support 9 - Only now tighten using all key positions in stages if necessary to achieve full grip Failure to follow this procedure which appears long winded was quickly shown as the work would be a long way off centre no matter how many jaws the self centering chuck had. I also believe it reduces the wear on the scroll and jaw slots as well as giving better grip with better centring as a bonus. If you use a four jaw self centring chuck you can feel the difference on even a small lathe using a 3" chuck.. Regards - Pat
Nicholas Farr	16/06/2011 07:30:16
3988 forum posts 1799 photos	Hi Pat, the procedure you have described is one that I've always used, even on keyed drill chucks, and very rarely do I have a drill spin, even in old worn drill chucks. I have had criticism for this as being a bit slow, but when using a pistol drill for instance, I've drilled my hole while they are still faffing about trying to get through with their drill picking up and spinning, causing a stop/start approach. Regards Nick.

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