On 2008-10-01, Michael Koblic wrote:

"DoN. Nichols" wrote in message
...
On 2008-10-01, Michael Koblic wrote:

big snip

Good! :-)

But you probably don't really need to do this. Just learn to
live with your 3-jaw chuck, or get a 4-jaw and learn to set it properly
when (and only when) you need the accuracy.


Thank you Don and Jim.

In my case it is more about understanding the concepts than about immediate
practical application. I am getting a better grasp.

O.K.

One final question: Grinding - does it damage a chuck or is it mainly the
ways and the bed you have to be careful about? I was thinking it was almost
inevitable for the grinding dust to get inside the chuck and causing
mischief there. I am talking about grinding things clamped in the chuck
rather than doing the chuck itself as Don suggested.

Yes -- the grit can damage the chuck too. If you can, wrap
newspaper (or plastic sheeting) around it and tape to the workpiece and
the chuck body behind the jaws (and ideally covering the sockets for the
chuck key as well. Better, if you can, is to turn between centers for
the grinding, as it allows greater precision of removing and replacing
the workpiece. Since you would normally be grinding something which has
been hardened after turning, you will need to remove and replace the
workpiece. (And turn it a little oversized, as it can grow slightly in
the hardening -- maybe 0.001" per inch of diameter.)

If you have been truing the hardened (not soft) chuck jaws by
grinding, plan to strip the chuck down completely, wash it out with
kerosene or some similar solvent, blow out with compressed air (if you
have that available), and then relube as you reassemble. If you leave
the grit in there, it will both grind the teeth (interrupted threads) in
the jaws, the threads in the scroll plate, and the fit of the jaws to
the body, resulting in looser jaws which will introduce greater error
when chucking.

Probably the more significant reason to grind true the chuck
jaws is for jaws which have developed "bellmouth" -- that is they grip
firmly at the back, but are worn near the front allowing the workpiece
to tilt in the chuck under turning forces. This is typically only found
in very well used chucks.

As has already been mentioned, usually a little runout is no
problem, because you are turning off the skin of the original stock
anyway so everything turned in a single setup should be concentric. If
you expect to reverse the workpiece in the chuck after this stage,
perhaps you want soft jaws turned to precisely receive the reversed
piece, which should give you better concentricity.

Enjoy,
DoN.

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