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(Harry Conover)

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,

Another alternative is to chuck a length (5/8 diam., perhaps) of round stock in
your three jaw (use O1, if you want to harden and temper it), and turn an
appropriate length down to the width of the diagonal of an 11/32 square. Then
file the four flats by hand right in the lathe.

You don't have to mechanically index it. You can get it square with a flat
file and a small square to check the right angles; just keep the 11/32
diagonal diameter at the corners. Don't file the flats in sequence, but after
the first, file one next to that, then go back to the other side of the first
one and file that for the third.The first try may be a goof, but by the second
or third, it will come together.

Turn a short concavity or chamfer to blend the key back into the main piece.

Part it at 4-5 inches. For a handle, drill the back end radially for a
1/4-5/15 rod. You can use a crotch center in the tailstock for that (with the
drill held by the chuck), or the drill press. There's a number of ways to
secure the handle from slipping, if that's wanted (one variation uses a notch
around the center of the handle and a ball bearing held by a coil spring).

Frank Morrison