4 Jaw chuck runout

This topic has 5 replies, 4 voices, and was last updated 4 September 2015 at 11:21 by Ian P.

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3 September 2015 at 22:02 #203096
Chris Craven
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@chriscraven20363
Apologies for long post but I'm trying to detail all that has been done.

Having had my lathe for a couple of months, I've just started to use the 200mm 4 jaw chuck that came with the lathe. The 1st chuck I had had 2 jaws that were the opposite of bell mouthed (bell throated?) which meant that after clocking in a piece any pressure on the end would cause it to pivot in the jaws, not a lot but you could feel it as well as seeing the gap at the back end of the jaws. The lathe supplier changed the chuck.

Before fitting the chuck I cleaned the back, then skimmed the and indicated the backplate to check it was running true. I then fitted the chuck to the backplate. and inserted some 50mm dia by 200mm long stock.

With the indicator at about 120mm from the chuck the piece was centred to about 0.0005" (sorry for mixed measurements but dial indicator is imperial test indicator is metric). Moving the carriage back towards the chuck the runout increases until at a distance of about 10mm it rises to +/- 0.0035".

Thinking that I possibly had something caught between the chuck and backplate I separated them, gave them a good clean and reassembled with the same results. I have separated the chuck from the backplate again and with the jaws removed placed the chuck face down on a surface plate and indicated the registering surface and only found a tiny variation (0.02mm [metric test ind] at a radius of 95mm from chuck centre). This does not seem to be enough to cause the centring problem I'm experiencing.

Any thoughts suggestions would be appreciated.
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3 September 2015 at 22:02 #24088
Chris Craven
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@chriscraven20363
4 September 2015 at 05:55 #203106
Thor 🇳🇴
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@thor
Hi Chris,

I don't have any self centering 4-jaws, only 3-jaws. Do you know if the manufacturer of your chuck guarantees a certain runout? Have you measured the runout on the outer surface of your 4-jaw. If there is much difference in the runout close to the chuck compared with 120mm from the chuck, the jaws may not be parallel to to lathe centreline.

My Chinese SC 125mm 3-jaw has a TIR of about 0.08mm, very close to what is stated by the manufacturer.

Thor
4 September 2015 at 07:30 #203113
Chris Craven
Participant
@chriscraven20363
Forgot to say, It's a standard 4 jaw chuck, not a self centring one. The runout on the outer surface is just under 0.001".
4 September 2015 at 07:52 #203115
I.M. OUTAHERE
Participant
@i-m-outahere
Hi Chris,

Is the stock you used round and parralell ?

It may be a high spot on the stock so i would turn a peice up and use a tailstock centre for some extra support.

Once turned round and parralell mount the peice up in the 4 jaw on this newly turned surface and center it up as close to the chuck as possible then repeat your runout test, also put a texta mark on the chuck where the worst runout occurs then loosen two jaws and rotate the test peice 90 deg and see if the runout follows the test peice.

If it does the the test peice is at fault ,if not then you have just located the approx area on the chuck that is causing the runout and you can now focus on that area, if it lnes up directly with one jaw there can be issues with how it was machined etc.

If you still have problems you could use some engineers blue on the jaws then very lightly mount the peice up again on that turned section and give it a twist so it leaves some blue on the test peice to see if you have a high spot on the inside of the jaws that will show up as a shiny spot.

If you do have a high spot there are ways of fixing using a die grinder mounted up in the toolpost but we will deal with that after you check everything else.as there are other things that can cause this problem you are having some can be fixed some can't

Ian
4 September 2015 at 11:21 #203150
Ian P
Participant
@ianp
If I understand your problem it boils down to the gripping edge of the jaw/s not being parallel to the lathe axis?

On an independant jaw chuck its quite an easy problem to solve by dealing with each jaw individually. To check the parallelism of the jaws grip a washer (or even a bit of rod for two opposing jaws) at the back of the jaws. With a finger type DTI mounted on the saddle you can rack in and out of the chuck jaw to measure the slope. With a bit of juggling the same test can be done with the very front of the jaws gripping.

Once you know how out of square the jaw is you can correct it by stoning (diamond type is best), measuring as you proceed and returning it to the chuck to retest.

I assume the chuck body and therefore the jaw guide slots are at right angle to the lathe axis. You can check each of the slots with a DTI on the cross slide. You are only looking for consistency not a precise 90 degrees, even if they are all as 85 degrees (very unlikely) it would not matter.

Ian P
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