Small Mill very early design stage

[leadinglights]

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.

[HiltonSteve]

Be careful as high accuracy will come with a high price tag, what are you making that needs to be manufactured to that degree of accuracy???

Not so sure about your cooling/heating idea for the aluminium frame, aluminium will expand at twice the amount of steel but for a machine as small as you are describing then not so sure it would be necessary.

For instance, a 100mm length of steel will increase or decrease in length by 0.00126mm for every degree C of temperature change so a temperature change of say 20 degrees C will change the length by 0.0252mm, aluminium will be approx twice this with a change of 0.048mm.

Buy yourself a good heater for the winter and open the doors in the summer then you should not see much difference!