Thanks guys,

I've been going over some of the numbers, I'm thinking the torsional load on the gantry is the weak point.

Using this calculator
http://www.kennametal.com/en/resourc...and-power.html a 16mm 4 flute endmill in steel with recommended chip load feeds speeds I get a tangential load of 1000n / 100kg.

Assuming a Z travel of 1m, 1000nm torque on gantry.

Using HIWIN HG25 linear rails http://cnc4you.co.uk/resources/Hiwin%20General.pdf
basic dynamic load rating (lowest rating)
26kn / 2600kg. At a separation distance of 300mm and 1m Z that would be 780kg or 7800nm (but there is 2 of them so this times two really). Clearly the linear guides are adequate.

So then there is the torsional deflection of the gantry beam.
θ = TL/GJ
θ radians deflected
T torque = 1000nm
L length = 1m
G Shear modulus = 80 steel
T Torsional Constant = 180 m4
200x100x8 box
section http://www.roymech.co.uk/Useful_Tabl...ns/RHS_cf.html

= 0.006944444 radians / or 0.12114 mm at 1m Z travel

This is the calculation of a 1m gantry supported at only one end, not sure of the calculation for a torque at the mid-point. I assume it would be less. I assume it would be at least halved with 2 box sections welded together.

So I'm assuming it would be in the range of 0.05 which is acceptable I feel. It most likely wouldn't be enough to matter in most cases and even if it did you could work around it.

What do you think? Have I made a math error?

Thanks Rufe0