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

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.

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.

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.

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.

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.

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

Ah, so wind the head up to say 3000rpm and leave it there then use the VFD to give say 30% to 105% control ?

See how it goes but if the motor runs smoothly on a VFD I think 120% should be do-able, do some test cuts to determine the bottom limit,
Regards,
Nick

Eventful morning....

Did some paying jobs, then started messing with the BP, it turns out I have the 1.5Hp / 1450rpm motor :( and top speed is 3500rpm although the vari-speed won't turn past 3000 or below 500 so I'm guessing something is wrong inside the head for a start.

I tweaked the VFD a bit, luckily the front panel pot was still programmed as speed source - it was just set at 50Hz.

After a few tests I finally set max to 100Hz and min to 15Hz, it was on default 25-75Hz.

The head was then wound to 1450rpm so it was at 1:1 ratio. Using the VFD this gave me a range of 430-2860rpm in high and 52-346rpm in low.

I made a test cut on one of the batch parts I was making earlier - these are moly steel and use I use a 1/2" HSS tool at 450rpm, I'd made loads so I knew what the machine sounded like when running them and there was no noticeable difference. With the tacho on the spindle while cutting the speed dropped from 450 to 420rpm but the chances are it did that anyway.

There are some issues up top as the sheaves are flapping like hell at speed.

I think a fixed conversion at 1:1 might be worth doing rather than repair the sheave bushes etc. At least that will give me the most of my meagre 1.5Hp at the spindle rather than lost in the pulleys.

So, improvements... Is a servo motor torque really linear from zero to rated speed?? I.e. will a 1.5kw AC servo really give me 1.5kw or 2Hp at 100rpm AND at 3000rpm or have i read it wrong??


Would I therefore be better off just fitting a 1.8kw AC servo motor for the drive and connecting at 1:1 ratio??


If it was correct then I would have full power from 1 to 3000rpm at the spindle - sounds ideal.


Would this play well with my upcoming CSMIO/A conversion - would Mach3 give speed control and correct rpm readout??