VFD Speeds 5 Pro

Hey everybody, new to Shapeoko and the forums figured I needed to make my first post.

Ive Noticed my VFD control display shows my spindle speed is usually 1000rpm below what I set it at in carbide create. Is this something I can fix or should I just add the 1000 to my tool path.


There are many threads on this if you search. The bottom line is while the Carbide video on the VFD states a 5% variance can be expected it can vary close to 10% at a higher speed. It is not linear but consistent meaning the difference changes based on RPM but will always be off the same amount for a specific RPM. As you mentioned you can change in tool path or in the tool file to account for the difference. Silly, I know.

Tim’s right. There are lots of threads. There are a bunch of steps that the hardware goes through between a G-Code request for a particular speed coming from CM entering the controller and a real RPM at the VFD spindle, and lots of places it can get non-linear on the way. I can go through them sometime if necessary.

It is possible to build a calibration curve into the VFD programming to correct (somewhat) for that non-linearity. However, it differs a bit from machine to machine, so it would have to be tweaked individually for each machine-VFD combination and even then, it may not stay perfectly in line under different conditions.

Carbide has left an approximate (and conservative) generic curve in the VFD and then locked its programming to prevent (possibly dangerous) meddling with other more critical programmable parameters. The difference is not a fixed number of RPM from requested, but closer to a fixed percentage of RPM requested, at least on mine.

If you’re really worried, send Gcode commands via the MDI in CM for speeds 8000 - 24000 (e.g.M3S10000 will tell it 10000 RPM, M5 will stop it) and plot the requested vs. displayed on the VFD to build a calibration curve you can apply manually in real projects. There is no need normally to check the actual RPM with a tach - the VFD is pretty accurate as a measure of spindle speed.

Personally, I can tolerate a 5-10% variation once I know that’s as bad as it gets.

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