Author Topic: Getting better accuracy setting up a 3 jaw chuck.  (Read 699 times)

Offline Joules

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Getting better accuracy setting up a 3 jaw chuck.
« on: March 19, 2018, 11:21:06 AM »
I was having a play setting up some work in the 3 jaw chuck.  I had made this part in the 3 jaw and knew setting it back up would be near impossible.  Having injured my wrist a while back I was reluctant to replace the chuck with the heavy 4 jaw.  I have previously used shim in a 3 jaw to get parts near.

I started by rechucking the work and setting up the dti, sure enought several thou runout...   so went to the high spot and lossened the nearest jaw, only a smidge, just enough for the needle to drop a little and retightened.  I then worked my way round the chuck getting the needle closer and closer to zero movement.  In this case half a thou was more than enough.  The thing is you can use the slack and wear in the scoll to your advantage and get the work pretty dammed close to true in a similar way you would setting up a 4 jaw chuck.  OK the old hands may know this trick already, but it was a new one on me so passing on for the benefit of others.
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline Joules

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Re: Getting better accuracy setting up a 3 jaw chuck.
« Reply #1 on: March 19, 2018, 02:34:10 PM »
Who doesn't like a picture.    Quick example with some extruded alloy bar, so you know the surface isn't great.  Trued rough to less than a thou, half of it turned to clean up.  Flipped round and trued again, then turned.   You can just make out the transition about half way up.  I can barely feel it with a nail and I would guess less than a thou difference.

As long as your chuck isn't bell mouthed you can still do accurate work by teasing the scroll and jaws to run true.  So for now I would say you can reposition work accurately in a 3 jaw chuck just as you would a 4 jaw, but maybe a tad more fiddly.
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline Stilldrillin

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Re: Getting better accuracy setting up a 3 jaw chuck.
« Reply #2 on: March 19, 2018, 04:00:19 PM »
Joules.
 I have bored the chuck's locating recess, some .005 oversize. This allows a small amount of adjusting float, when mounting the chuck.....

I always use the same key hole, in the chuck. When needing a precision hold.

Consequently. I hold a length of 5/8" ground bar. (Old, reamer shank). In the accurate position. Then clock this true, tapping the chuck, with a lead tup. Tighten the chuck retaining nuts.......

This gives me repeatable accuracy, whenever using the same key hole.

So, as not to wear the single key insert. I use the other two positions, whenever possible.
David.

Still drilling holes... Sometimes, in the right place!

Still modifying bits of metal... Occasionally, making an improvement!