Set up a 60mm cube of HE30 aluminium in the four jaw so the a) the hole is where I want it to be AND square with the front face?
The hole will be 50mm and I can set the face square with the DTI, but getting the hole on centre at the same time is driving me nuts!
I need to leave a clearance between the back face of the cube and the front face of the chuck so I don't hit the chuck with the boring bar and to allow the chips to fall away.
First attempt was square but 0.5mm off which is not good enough. For personal satisfaction I'd like to get better than 0.1mm and as close to bob on as possible.
If the jaws are good and the cube is true then it should be held square anyway regardless of how well it's centred. However, chuck it a few mm further forward than you want it to end up. Set the dial gauge on the sides of the cube and adjust until it's centred but still held fairly lightly. What you're looking for is the lowest reading on the gauge on each face which will be in the centre of each face as you swing the chuck. You do opposite sides in turn until it's central in one plane and then the other. Should only take a minute or two with practice.
Then bring the tailstock ram up and press the cube back a couple of mm against the jaw pressure. This should true the front face of the cube to the face of the ram which ought to be dead square. Then nip each jaw in turn a fraction at a time working round and round the chuck until it's held tight. Then a final check that it hasn't moved and you should be good to go.
If you're still having problems then it's likely the cube isn't quite true and will need a skim across the front and rear face anyway. Check this out is against an angle plate or with the cube on a machine bed and run the DTI across each top face in turn. In fact with a good 4 jaw chuck there's nothing much you can do to alter the squareness of the front face once the jaws are tight so any major error tells you the cube is distorted.
-- Dave Baker
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"Why," said Ford squatting down beside him and shivering, "are you lying face down in the dust?" "It's a very effective way of being wretched," said Marvin.
To amplify on Dave's post... Mount the DTI on the cross slide of the lathe. This means that you can get a reading on one face of the cube, move the saddle to the right, rotate the chuck half a turn and move the saddle left again to get a comparative reading on that face of the cube.
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