According to jim rozen :
In article , Gunner Asch says...

Whats the best way to make a square hole in a round piece of material,
without a broach?

I would start with the aforementioned (in two posts no less!)
'lathe as shaper' approach.

Basically bore a somewhat oversized hole in the round, and then
put the 'corners' in with a shaper-type bit in the toolpost by
stroking with the carriage.

The end result does not have to be truly, completely square, it
only has to have square corners. Boring it oversized to start
reduces the amount of material that has to be 'shaped' off
drastically.

And -- even *with* a broach -- you have to bore slightly
oversized, to match the pilot end.

Enjoy,
DoN.
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