Adjustable 3-jaw chuck designs

Can anyone remember if there are any published designs for making or adapting adjustable 3 jaws similar to the old 'grip-tru'....I have vague hopes of adapting a Chinese cheapy but can't find any drawings online (probably looking in the wrong byte!) One can still buy adjustable (for run-out) modern chucks but they appear to be eye wateringly expensive.

I do have quite nice 3 & 4 jaws which came with my WM180 but a bugger to change and I just want to knock out my clock bits with a touch of centreing to keep things good......and yes, I do know about ER collets!

........It saddens me to say that I'm not really interested in replies from the 'sneering group', so they need not bother!

There is the adjustable backplate sold in kit form by Hemingway.

Not sure if it was G H Thomas or Tubal Cain. It was recommended that the backplate was skimmed on the diameter and the mounting bolt holes were enlarged, giving adjustability.

I believe that in one of the three volumes by "Duplex", a similar set up was recommended plus three grub screws to adjust the centre. I will try to find the "Duplex" drawing if possible.

The Hemmingway kit is maybe the best bet. I made one and I find it is easier to adjust than a Burnerd Griptru.

Andrew.

I have four chucks, two 5" Pratt Burnerd and a good Chinese 125mm and a reasonable Chinese 160mm modified to enable fine adjustment. The Pratts, unusually, have front and rear mounting holes/ threads in them, so I use six SHCS screws in them. The 125mm is rear mounting, originally three bolts, but there is plenty of space to drill and tap a second set. The 160mm has six 6mm SHCS screw in the rear because the backplate was slightly under 160mm and the original 8mm's would have been too close to the edge.

The registers in each backplate are about 0.010" undersize and the bolt holes are loose to match. Just slackening the screws a little enables the workpiece in the jaws to be set dead on with a DTI, by gently tapping the body of the chuck with a copper hammer and rechecked when the screws are retightened.

The chuck manufacturers would not recommend this as they want to sell set tru types at double the price.

Of course this only applies to scroll chucks, not four jaw independents.

Edited By old mart on 05/08/2019 16:50:07

Was it not Commander Barker who recommended the undersized register and oversized bolt holes? He maintained that he had never had a chuck move in use while using this technique. I would be slightly wary using a heavy interrupted cut but typical model engineer operations would probably have no problems.

Mike

Thanks so much for these excellent replies!

...........I did toy with the idea of aquiring another 3 jaw an dskiming out the register for the 'copper mallet' method but the WM180 has chuck fitting which is the Chinese 'bolts for little fingers' method of fixing and I'm not keen to open up the bolt holes in the spindle plate. The chucks are puposely tight on the register enough that i have to use a brass plate against the headstock to 'jack' the chucks off using the fixing nuts as jacks......

Hello Mike,

It was indeed Commander Barker! Come to think, I believe it was Professor Chaddock that recounted the story.

Andrew.

I think your idea has merit and could be done with no alteration to the machine if you had a sub-plate between chuck and spindle register.

The thinnest arrangement would be with a front-bolted chuck to sub-plate and a chuck register larger than the spindle register (so they telescope slightly).

A rear-fixed chuck would need a thicker sub-plate and maybe bolts with holes drilled in the sides for a tommy bar (look up 'capstan nut' for the concept).

My everyday 8 inch chuck is held on 3 bolts on the backplate with no register. Hasn't shifted since I fitted it and it's done a lot of turning of all kinds.

I have a cheap 3 jaw chuck that is quite accurate on some diameters of stock and much less on others, the accuracy varying between 0.001" and 0.005" T.I.R. Having checked the parallelism between register, back plate and chuck faces I come to the conclusion that the scroll is likely to be the cause of the variation so it seems that for one-offs I might as well use an independent four jaw chuck as attempt to adjust the self centering one...……I am hoping to improve things now that I have discovered small burrs on the scroll Colin.

The scroll needs to be checked for radial slackness.

Edited By colin hawes on 06/08/2019 10:33:04

Those people who are worried about the possibility of the chuck moving when only the bolts are holding it to the backplate are rarely concerned when using a milling machine with no register at all and 100% interrupted cuts.

I second that.

The main reason for a close fitting register is to ensure the chuck body is concentric. If the chuck is a high quality one then the concentricity of what it is gripping will be as the manufacturer specified and stated on its test certificate.

Having the body concentric also ensures it will be in balance which matters especially at high spindle speeds.

With a slack register and oversize boltholes, one could set up the chuck holding a test peice and indicator, at that same diameter even a bad chuck might grip more accurately than a good one on a tight register, but there is no saying that it will be as good at other gripped diameters.

If the bolts holding the chuck to the backplate are correctly tight then I doubt and cutting force it will encounter would cause and movement.

Ian P

I last set the OD of the Pratt chucks to run dead on was when I gave the inside of the jaws a light grind to remove the slight bellmouthing. I used a solid carbide drill to provide tensioning holes through the case hardened jaws. The Pratt chucks have good tight scrolls accurately ground and centre within 0.003" at all diameters. I rarely need to fine adjust the chucks, but the option is always there.

Edited By old mart on 06/08/2019 16:45:24