Hi, by a pyramid design I didn't actually mean a pointy column as such, but you could see a truncated cone as a pyramid with the top cut off.

My preference for a column is one that has the sides sloping so that lateral forces that attempt to sway the column left to right or forward and back are resisted by the sloping sides being in tension and compression alternately.

A long straight column that is as wide at the bottom as the top has no bracing integrity to flexing and will bend like a spring, even if ever so slightly, which will manifest itself when it vibrates due to the weight of the head being half way up the column and acting like an upside down pendulum.

Torsional twisting is not such a problem as it only needs thicker material for the column sides, and eventually with enough material in the column sides the anticipated strength condition that normal CNC milling needs can be met.

If a round tube is used it would have to be quite a large diam and with a thick wall, and this makes attaching any slides a difficult process unless a large amount of metal work is attached by welding to make a flat area to bolt linear slides on and clear the bulge of the round tube.

A large square tube can be achieved by welding two heavy section channel irons together with a thick steel plate between them to give an internal longtitudenal rib effect.

I think it would be simpler to fabricate from the word go a column to a suitable design by dedicated separate pieces of steel plate as opposed to making a pattern and having the same cast in one piece from iron.

This would also allow you to get away from the constraints of aspect ration as the column attains it's ideal height and base area.

But as a simple plan is mostly a simple exercise to achieve, using off the shelf heavy sections of square steel tubing will eventuate in a design that is quick and easy to build without huge amounts of welding as David DeC showed in the UMC-10 build.

My opinion is that if you skimp on the tubing wall thickness for economy, you will end up with flexing problems later on.

Only for the fact that it is difficult to machine, a column and base in one piece is better than a bolted assembly any day.
Ian.