Thanks for the comments Jonathan,

Yes, I did incorrectly refer to mass as weight which, while not mathematically the correct term is the common usage (e.g. My weight is 60 kg).

I used 95% as the efficiency for ballscrews and that gives the 0.15 N.m I quoted. The 0.5 (50%) figure was for an acme screw which gives the 0.11 N.m figure.

I agree that the force due to acceleration is not significant in such a small machine.

Yes, it's a bit difficult to work out the friction effect. I don't yet have a machine to measure but I would guess that the length of the dovetail on the head is about the same as the distance from the column to the C of G of the head. The head will tend to rotate about the lower contact region on the head. We can simplify the calculation by assuming a small contact area at the base and a small contact area at the top of the dovetail slide (I don't think this is an unreasonable simplification). So, at the top of the slide there will be a force of about 18 kg (176 N) pulling the head from the column. With the 60 degree dovetail the force normal to the surface will be 18/sin(30) = 36 kg. With your figure of 0.2 for the coefficient of friction this will increase the torque required to lift the head by 40% - hardly a show stopper. I wonder if some flexing of the components gives rise to a stick slip type of friction effect which makes things worse?

Russell.