Two questions:
What rpm where you running?
Are the soft jaws you machined full size blanks?
With the chuck spinning centrifugal force tends to open the jaws and it can greatly reduce the grip when the rpm is above 2000 or so. The effect is increased when full size soft jaw blanks are machined and only the minimum amount of material is taken off; you finish up with a large amount of metal in the jaw spinning at a largish radius.
My policy with soft jaws is the bore them out as close as possible to the inner mounting bolt then chamfer them back and machine away an excess outside the outer mounting bolt. This reduces the amount of metal spinning around and reduces the centrifugal force tending to open the jaws.
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An open mind is a virtue...so long as all the common sense has not leaked out. |