O.K., I will rephrase. Accuracy, accuracy, accuracy within limits, within budget and within practicability. My target is 0.001" throughout the range, but I know that this is hard. Hopefully the same will be achievable on a small part. As far as rigidity goes, I am the fortunate owner of an old (1960s)sensitive drill that weighs about 300lbs and teaches you that a big lump will always beat stylish when it comes to accuracy.
For rigidity I am hoping to make the frame and non-moving gantry in 15mm steel plate but may drop back to 20mm aluminium with some form of temperature control - possibly peltier heater/cooler elements in the vertical and cross members.