According to jim rozen :
In article rs.com, DoN.
Nichols says...

I recommended the 5C collets because that is the only size in
which the collet blocks are made which you would need to accomplish
milling a square on a workpiece without an index head.

As an aside, the following is a very nice project for new machine
owners: 3C collet blocks.

That sounds like an excellent project. And maybe 3C would even
fit his lathe (with a proper spindle nose adaptor). But, IIRC, 3C maxes
out at 1/2" maximum grip diameter, which is not large enough for the
workpiece, IIRC.

Starting with hex, or square CRS stock, one bores out for a 3C collet
to fit, including the taper on the working end. Press in a small pin,
and fabricate the nut and pin wrench to tighten things up.

Of course they should be heat-treated and ground, but for most hobby
work simple CRS stock as supplied works just fine. The essence of the
project BTW (aside from producing usefull tooling) is learning to
indicate and bore the hole and taper accurately with respect to the
outer dimensions of the stock.

I've still got mine, from my 9" SB model A days.

Nice!

Thanks,
DoN.

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