What size stepper motor do I-need.

**Neale** · 09-06-2014

I'm trying to use the spreadsheet to check some parameters of my planned new router. This has thrown up a few questions:
1. Acceleration seems to be the largest contribution to torque required, but I can't find where load acceleration is built in. It must be in one of the formulae somewhere but I can't manage to unpick the formulae in the hidden columns to find it.
2. The spreadsheet recommends a motor torque based on an assumed safety margin of 3; if the safety margin isn't high enough with my intended motor, I presume that I could restrict the machine to a lower acceleration? But given that I don't know what acceleration is assumed, it's difficult to see what's happening. I do see that the acceleration torque number is linked to cutting speed.
3. For a twin ballscrew X axis, I assume that I can use the spreadsheet using actual ballscrew dimensions (single ballscrew) to give critical speed, but then use a double-length ballscrew in the spreadsheet to do the torque, etc, calculation.
4. If I go to a twin-motor setup, can I simply double the nominal motor torque? Or is it easier to halve the gantry mass and do the sums for a single motor/ballscrew?
To put some reality into this exercise, what would be sensible numbers to use for cutting speeds and accelerations? Bit of a "how long's a piece of string?" question, but my current router is so far out of the norm I don't feel that I can extrapolate from what I'm doing at the moment, and I don't know what kinds of numbers are reasonable. For starters, I would assume profile cutting, say, 9mm ply with a 6mm cutter in a couple of passes. Currently, with speed limited by machine rigidity, I would cut that at maybe 600mm/min.
Many thanks for any help available!

**JAZZCNC** · 09-06-2014

I'll save you a lot of headache and just say use 3Nm motors run at 65-70Vdc on 75-80vdc drives and you won't have any problems cutting anything.

Cutting speeds will depend on many factors like spindle power and tool material but 4500-5000mm/min wouldn't be out the way with 2.2Kw spindle using carbide tooling.