Hello Zone

Long time lurker but first time poster. A friend and I are about to dive into our first CNC-mill build. The design is pretty much sketched out, but we would like some opinions and thoughts, before we rush out to build moulds and buy steppers :-) The machine is intended for precision machining of both aluminum and steel, so we would like to keep tolerances tight. It is not your usual C-frame-design, but it seems to have some merits and some flaws. We will probably build the spindle ourself, but this has not been drawn yet. Some of the facts and thoughts about the design:


  • Work envelope is ~400mm*250mm*270mm
  • Epoxy granite base, column and ram. The moving beam has internal steel bracing. Rebar is embedded in the base and column.
  • Steel inserts for all mating/mounting surfaces
  • Work envelope relative to total machine size is larger than common C-frame mills, but smaller than gantry-style machines.
  • Worst case deflection of structure is 3mu for a radial cutting force of 1kN, but rigidity increases closer to work surface. Most deflection is due to twisting of the column. This is ignoring compression of the ball screws.
  • The spindle is able to move completely clear of the table. This provides good clearance for planned ATC and toolsetter
  • All 3 axes are moving on 20mm rails.
  • Ballswcrews are 1604 with 0-backlash nuts. C7 precision with screw mapping for compensation
  • X- and Y-axis driven by 2Nm closed loop stepper motors(china). Should provide plenty of torque, inertial ratio of about 1.5. Z-axis possibly 3Nm since the moving beam is heavy(~70kg). It is however also supported by a gas spring.
  • A concern is the moment due to the offset spindle compared to z-axis rails, which would seek to twist the moving beam. However, most cutting forces are horizontal, and thus the relevant moment arm is only ~40mm(vertical distance between rails and spindle nose)
  • Way covers for Y-axis are problematic