MDI spindle speed is not correct.

[Malachilee]

Hi everyone,

I'm having trouble getting my lathe spindle to work correctly. When I enter "S3000 M3" on the MDI screen, the spindle encoder is showing the spindle motor running at 552 RPMs, which is not the correct speed. I'm wondering if there is a setting that I have missed that might be causing this issue. I have already checked the voltage using the spindle bench test, and it shows the correct 10.14 volts when the spindle is running at its full speed of 3000 RPM. If anyone knows what might be causing this discrepancy, please let me know.

Thank you.
Lee

[cncsnw]

Actual spindle RPM for any given analog voltage depends on how you have your spindle drive (VFD) programmed, and what the gear/belt ratio is between the motor and the spindle.

You have entered 4000 RPM for the maximum spindle (lathe chuck) speed in high gear range. Have you configured your VFD to make the spindle actually go that fast when it is given a 10V analog speed reference?

Also: when the G code says it should be going 3000 RPM, and the spindle encoder says it is going 552 RPM, how fast is it really going according to a trustworthy tachometer?

[Malachilee]

Hello cncsnw,

I apologize for not giving you a clear description of my setup earlier. Currently, the servo motor that controls the spindle is mounted to the machine, but the chuck has no belts connected to it. The encoder is directly attached to the servo motor, so I assumed that the RPM of the servo motor is accurate.

I am facing two issues. Firstly, when I adjust the RPM level on the VCP spindle speed controls, the spindle does not start moving until the RPM meter is at around 80%. When it gets to that point, the spindle jumps to about +485. Also, if I go to full spindle speed on the VCP, I only get to around +910.

Please see the attached images.

This is what made me think I had overlooked a setting. Why does the Acorn not send a signal to power the spindle until the VCP reaches 80%?

The second issue is that I have my MAX speed set to 4000 in the wizard, and the displayed encoder speed is around 910. I also confirmed the speed with a digital tachometer.

Thanks
Lee

[Malachilee]

Sorry, I just double-checked, and the Acorn is putting out voltage at those lower speeds. Does this mean that this could be an issue with my driver settings, then?

[tblough]

For threading, CNC12 requires an encoder connected to the spindle at 1:1. You can only use a motor mounted encoder if it is connected to the spindle at a 1:1 ratio with cogged belts. If there is any slippage of the belt, or the ratio is not exactly 1:1, you will have problems threading.

You definitely have your drive configured incorrectly. Disconnect the Acorn analog output from the drive and connect a 9V battery to the drives analog input. Do you get 90% of the full speed rpm you have configured in the drive?

[cncsnw]

... then connect a 1.5V battery to the analog input. Do you get 15% of the drive's maximum speed?

[Malachilee]

I found an adjustable benchtop power supply and connected it to the analog inputs on my spindle driver. While everything else was still connected to the Acorn, I simulated the analog output 0-10v from the Acorn using this new power supply. The spindle began rotating at 0.2V and seemed to be functioning normally. But when I plug the analog input wires from the driver into the analog outputs on the Acorn, the spindle won't start rotating until around 4.2v, even though the Acorn is putting out the correct voltage at any/all specific RPM. I can not figure out what is causing this. If anyone can think of anything, please let me know.

[tblough]

Assuming your wiring is correct, the only thing I can think of is your VFD is loading the Acorn and drawing more current than it can supply. You might try a signal isolator between the Acorn and your VFD:

https://www.amazon.com/Isolator-Current ... B09JL7DKQR

[Malachilee]

Hi tblough,

I wanted to thank you for suggesting that I add a snubber to my setup. I didn't have one installed, but I happened to have one lying around that was close to the one on the Centroid website. The only difference was that it was a 120-ohm snubber rather than a 100-ohm one, but I'm not sure if this would make any difference. Unfortunately, even after installing the snubber, there was still no change in the spindle's behavior. It still wouldn't turn on until the voltage reached around 4.2V. I don't know what to try next.

Thanks
Lee

[tblough]

I did not recommend a snubber or any solution with a resistive component - that will only increase the loading on the Acorn and make problems worse. You can try a 0.1mF capacitor to block any AC signal components. This sometimes helps with noise, but that does not seem like your problem.

Your problem seems to be that your VFD requires more current on the Analog input than the Acorn can supply. That is why I suggested a signal isolator.