Hi guys, I'm the president of a student organization and we are in the process of designing a fixed gantry styled CNC milling machine to donate to our University. I was hoping yall might be able to help me to understand what I don't know (which is a lot).

First questions are about the entire milling head area. Does a spindle motor capable of ATC need to be a servo motor with an absolute encoder? If I want it to be capable of everything from wood to mild steel (or maybe tool steel with conservative speeds and feeds and proper rigidity [which i will get to in a moment]) would 500-6k rpm be okay or should I try to incorporate a gearing system to increase spindle speed for those softer materials? How would I go about designing a gearing system? What taper would you recommend for light duty steel cutting with lots of aluminum and wood/plastic cutting? would 8 ft-lbs of torque be enough for machining mild steel with conservative speeds and feeds using that taper?

rigidity questions: of course I recognize there isn't much replacement for pure mass in the rigidity question, however we intend to create a dual fixed gantry build (X and Z axis sandwiched between two fixed gantries). do you think that this, coupled with otherwise proper design might make for a setup that is notably more rigid than its single gantry counterparts? what other design considerations should I be thinking of for proper rigid, low vibration design? we intend to design the frame with various sized structural tubing and filling any section not filled with cables with sand or non shrinking grout to increase mass and reduce vibrations.

Thank you in advance for your responses, I am sure I'll have more questions as this build progresses.