Help Determining What RPM to Set Bridgeport to with VFD

[truckeeroundhouse]

Hello. I am a founder and teacher at a community makerspace in our small mountain town in the Sierra Nevadas - the Truckee Roundhouse.

We have an old Bridgeport mill in the makerspace metal shop that was donated years ago by a shop that closed down. It is not used very frequently (sadly!).

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?

Here are a few photos. When the VFD is set to "freq" the range is 0-60, presumably Hz.

 

[jwmelvin]

The little handheld techs are like $20 from Amazon. Seems perfect for this.

 

[mksj]

My recommendation is to set the VFD at 60Hz and use the mechanical speed adjust. Using a Reeves drive in one fixed position will cause uneven wear on the drive. With the VFD set at 60Hz, then the Reeves drive speed dial should be reasonable accurate. If the mechanical speed dial is set to say 1000 RPM, then just scale the speed based on the Hz/60 x the mechanical speed indicator, so if the VFD is 30 Hz/60Hz x 1000 = 500 Hz. If you want to occasionally check the speed then use a hand held tach. for $20. Also remember that as you decrease the VFD speed below 60 Hz you loose Hp in a somewhat linear fashion and also the mechanical advantage.

 

[Ulma Doctor]

with 60Hz set on the VFD, the spindle speeds should be accurate enough
when you vary the frequency, the spindle speed is raised/reduced in a linear fashion

 

[markba633csi]

I agree, set vfd to 60 hz then use dial to adjust speed
Motor is most efficient at it's design frequency
-Mark

 

[truckeeroundhouse]

Thank you very much everyone.
If anyone wants to come teach our members (and me!) how to use this amazing machine we are always looking for teachers!