Bridgeport mills, VFD's and the vari-speed head...

[Davek0974]

I have a vfd on my BP but since fitting it's been left set at 50Hz so just a phase converter really.

I am now converting the old girl to CNC and trying to figure out what to do with the speed control.

Is it workable to set the vari-speed knob in the middle at say 1500rpm, mark the position, and then calibrate the VFD to control from say 500 to 3000rpm?

I know back-gear will still be needed for torque/low speed but that could be treated the same maybe?

What's a good way of treating this system?

Fitting a plain pulley head is not really an option as it means buying another machine and butchering it.

[magicniner]

The belt must be up to the speed and power so I'd be inclined to give it a go.
With any luck a Bridgeport expert will chime in with specific info.

[Davek0974]

I have one reply on a US site where the guy did exactly this but he ramped it up to 5000rpm and left the vari-speed at 3000rpm setting so it can be done which is good news.

[komatias]

I use the VFD as a single to three phase converter. This is because the original motor are not really meant to give good torque with varying frequency. I do sometimes drop the speed using the vfd but it mostly stays on 10/10. As you are doing the tool changes manually anyway, you can set the speed manually too right? Now there is the possibility of running the motor to 60hz as opposed to 50 and that will give give you a few more rpm.

[Davek0974]

The idea is not for more speed but for speed control via Mach3/CAM - the idea was to leave the head set at say 1500rpm then use the VFD for speed range and the back-gear for low range.

[magicniner]

George has made a very good point, 3 phase motors tend to produce better torque with reduced speeds than with increased speeds, it might be better to set your vari-drive nearer to top speed and cover more range with reduction whilst still giving you some increase on the top end,
Regards,
Nick