Spindle speed control issue with spindle upgrade

I am upgrading a 48x96 Pro machine running Mach 3 to a new CNC Depot spindle (RM40) with the automatic drawbar. I have the new spindle installed and running but am not able to get full rpm’s (18,000) out of it. The VFD running the spindle is set up to look for a 10 volt speed command voltage to run at max RPM (18,000 - 300 Hz). I am only getting 5.6V out of the break out board (CRP850-00E Rev C-02) no matter what I do with the settings in Mach 3.

My next steps will be to hook a scope up to the output of the ESS to see what duty cycle it’s putting out on the spindle PWM output. If it’s 100% duty cycle, then the issue must be with the BOB. If it’s not 100%, then I must have a setting incorrect in Mach 3.

Has anyone done a CNC Depot spindle upgrade with Mach 3? The Mach 3 spindle settings are not clear for me.

Thanks in advance


I appear to be making progress on this issue. The Durapulse VFD appears to be dragging down the spindle signal from the breakout board. With the signal wire disconnected from the VFD, the signal is 10v just as it should be. When it is connected to the VFD, it gets dragged down to 5.6-5.7 volts. The input impedance on the VFD is stated and measured at 20k ohms.

Does anyone know if there is a new version of the breakout board that can handle the 20k ohm impedance of the VFD? or is there something wrong with my current breakout board?

Thanks in advance for your help.


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In december I bought the rm40c and had all kinds of trouble with it, it would spin up but had no torque, as soon as it contacted any material I was cutting it would bog down and stall. After a good bit of trouble shooting with cnc depot they had me disconnect the orange wire from the vfd which as I understand it is the thermal protection switch in the spindle. It has been working as expected since then. I’m running mach4.

So, your spindle will no longer shut down to guard against over heating?

That’s right, Alex said they never had a problem with overheating. I asked the warranty and he assured me it would be warranted for the normal time period.

I know it doesn’t leave you with that warm fuzzy feeling does it? I am still baffled as to how that one wire was causing my issues, and was it the spindle or the VFD…


I am posting here with the hope that this info may be helpful to others.

We solved our issue with the speed control of the new spindle. We added a isolation amplifier between the Avid breakout board and the VFD. The input of the isolation amp has a very high impedance and did not drag down the signal from the breakout board. The output of the Iso amp was run to the VFD.

Here is picture of our solution. Maybe not elegant, but it works great.