In article ,
Harry Conover wrote:
I recently purchased a used 4-jaw chuck for my Atlas 12" lathe.

The problem is that none of my chuck keys will fit it. The 3/8 is too
large, and the 5/16 marginally too small, so unless it's some *******
metric dimension, an 11/32" inch square chuck key would be perfect.
The problem is that I can't find any on eBay or even in the tool
catalogs.

Who made the chuck? It may well be a metric dimension. My
calculations suggest that it may be 8.75 mm (your 11/32" converts to
8.7313 mm

The question is: "Does anyone know where a can purchase a 11/32"
square chuck key for a lathe 4-jaw chuck?

Do you have a milling machine? If so, get one of the square 5C
collet holders (usually comes in a set with a hex one), add a collet to
match the diameter of the stock (I used drill rod for mine, because that
is what I had handy), turn the end to the diagonal measure of the square
socket, clamp it in the milling vise and mill a bit off one side.
Rotate it 90 degrees and mill again, repeat until you have done all four
sides, then measure the size of the (partial) square you have produced.
Subtract the desired size from it, and divide by two to tell how far to
advance the qull or crank up the knee, and mill all four sides again.
Keeping it in the collet chuck, test the fit in the chuck you are
fitting, and if it right, turn the workpiece around in the collet chuck
and cross-drill the other end for a press fit of the handle. Drill it
to be a slide fit, and knurl the middle of the handle for a press fit.

Enjoy,
DoN.
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