1um precision CNC mill

Building a CNC machine with glass scale feedback. The 12 pole brushless motors are attached to a 5mm/rev screw.

  1. What stiffness can I achieve?
  2. How many N force correction with 5um error. (The screw will run ~0.4 poles/mm, so this is 1/2400 of a phase)
  3. Is this a PID regulator that is tunable?

Just some info: 1Nm into a screw with 5mm raise should give 1256N, and for static and linear force, about 400N is needed.

Your precision won’t be limited by the motors or the ballscrews but by the rigidity and stability of the frame. Fyi even DMG Mori doesn’t promise 1 mikron(if that is what you meant)accuracy to its machines. For that you’re going to need linear motors, air bearings and a frame that has weighs several tons. So a Kern.

Where do you have your numbers from? I use steel with elastic modulus of 220GPa. Overhead beam is I beam with h=0.3m, and A=0.05m2, 1.2m long. Posts and rails have similar dimensions. Calculated stiffness (F=kx) in middle of gantry is 220N/um. I just want the motors to be stiffer than this, otherwise a lot of steel is wasted. The gantry beam is 50kg, and the rest of the setup to get this stiffness is 250kg. It is all bolted to cement support.
I have been running the mill with stepper motors and glass scale feedback. Always wanted to upgrade to servo motors, and need some input on this. Using HiWin HG30 bearing with 300N/µm, and 4 bearings per railset give 1.2kN/µm per axis.

Edit: By um I actually mean μm i.e 1E-6m

From running sub-mikron optical tables and using and designing cnc machines, I know something about the subject.

Steel is overall a quite bad material since it rings quite badly and transfers vibrations very well. This is why professional machines are either special cast iron, polymer concrete or uhpc or some combination of the former. In addition stiffness a good vibration damping is also key.

Also the tolerances of ultra precision hgw30 is 0,008 mm, a c0 ballscrew has travel error of about 7 microns. There’s way more error there, especially if you’re not using the most expensive stuff.

Also odrive only has 8192 steps, a stepper with 8x microstepping or more has more and thus is more accurate so if accuracy is what you need, get steppers or proper industrial servos with 39 bit encoders
Also you need scales with sub micron accuracy to get anywhere close to a one micron, even ±1 scales can’t usually do it since the resolution is not fine enough.

I don’t know what you intend to do with your machine but your expectations are a bit optimistic.


8 x 200 = 1600… which is much less than 8192?

Huh, wonder what was I thinking there…

But its 64x microstepping for 1.8deg steppers.