Project Da-Ta CNC mill

[Da-Ta CNC]

Hello,

Me and my friend are working on CNC machine.

Working area is 650x500mm
Step motors are Nema 34
Frame is made out of 100x100x8 tubes
Worker is 180cm tall for comparison

We have no clue what kind of spindle should we buy for this machine. We are looking at BT30 spindles - ATC.
Can someone with more experience help me with spindle motor?

What do you think about Z axis mount? It is HEB 300mm profile reinforced with 10mm plates. Will be stable enough? Or should I add some supports on the side?
Thank you in advance.