[matthew798]

I am attempting to validate my choice of stepper motor for my hobby cnc project. It's a vertical CNC able to handle 4X8 panels of plywood. I'm looking for what a woodworker would consider excellent precision (1/64 or better).

I've put together a spreadsheet to determine if my choice of stepper motor is adequate for the machine. I am only trying to calculate the acceleration values for rapids. At this point, I'm convinced my math is incorrect. I am trying to calculate the X axis torque requirements (assuming only 1 stepper, although it will be driven by 2 on either side of the gantry. I think I can simply divide the torque requirements by 2). It's giving me 14170 in/s2 which is clearly insane.

I've attached a spreadsheet with my math. I can't for the life of me figure out where I went wrong but admittedly there are some confusing conversions between metric and imperial, a consequence of my brain melting and buying stuff from china.

I wonder if someone would be able to take a quick look and point out my errors.

The machine is driven by a stepper, through a 3:1 gearbox, then to a rack and pinion (1.5 module, pinion pitch diameter of 30mm at 20 teeth)

Essentially I've picked a desired rapid speed by educated guess (same speed as the AVID CNC pro line with same size steppers) and am trying to determine the maximum acceleration I can achieve with a given stepper holding torque. There is a big assumption made about the stepper torque curve, as they are cheap Chinese steppers with no manual available.

Thanks!

Attachment 502118

[peteeng]

Hi Matthew - Its hard to check a spreadsheet someone else has made. I'm building a R&P machine at the moment so this is good practice. I have neglected the inertia of the crown wheel. if you know its size you can put it in. Short story - motors are technically good to 25m/min... Accel will be 0.5m/s/s possibly 0.1g. Check my numbers did it quite quick... The numbers worked out that the gantry will get the 2.4m in about 3secs. This is unlikely maybe 6secs if that's fast enough for your rapids... You've got about a 2x safety factor for torque at 800rpm so that's good. Heaps of torque for cutting... Good Luck - Peter

edit - this sort of accel comes down to the quality of your drivers to get through the mid band resonance, plus if you haven't bought drivers yet use high voltage drivers say 60V or 80V they will cut through the back emf quick and run really smooth. For instance 24V drivers will be slow especially if the motors are cheap Chinese with high inductance.. Run hard, run fast, run high voltage!

[peteeng]

Hi Matthew - I think the gear ratio calc maybe wrong. Is this the gearing you have? What's the overall reduction? Peter