Help Determining Accurate Rpm On Bridgeport With Vfd

We have an old Bridgeport mill in the makerspace metal shop that I help set up and teach at (Truckee Roundhouse).

Since the building does not have 3 phase 220V power, at some point the Bridgeport had a VFD added to it. As such the mechanical dial on the mill does not solely control the speed. One must turn on the VFD to "run" and then while the spindle is spinning, dial the mill to a certain RPM value and then use the potentiometer on the VFD to set it to the digitally displayed RPM.

The person that set this all up added a sticker to the mill that said "KEEP MECHANICAL DIAL SET TO XXXX RPM" at some point, but the sticker has disappeared. It was my understanding that in order for the VFD digital dial to read accurately, the machine speed dial had to be set to a certain RPM and stay there.

How can I go about figuring out what this should be? Is there a way to do it mathematically via the power, data plate on the Bridgeport, and VFD manual, or do I need to rent a tachometer from O'Reilly and get some reflective tape and run some experiments?

 

Thanks Howard. I don't think the guy that donated the mill changed the pulleys.

I will post some pics and video a little later when I am over there at the shop.

While I have you here Howard - do you have a good reference link you could share to cutting speeds for 2 and 4 flute HSS end mills in mild steel? I never know what to trust when I google these, and I'd like to print and laminate a reliable chart and post by the machine.

 

Here is the VFD and the mill data plate. When the VFD display is set to "freq" to range is 0-60.