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Author Topic: Bridgeport Knee Mill Conversion?  (Read 159183 times)

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Offline Davek0974

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Re: Bridgeport Knee Mill Conversion?
« Reply #250 on: June 27, 2016, 05:36:25 AM »
Doubts creeping back in again on the spindle motor - 1.8kw servo seems to be the biggest i can get that runs at 3000rpm, but is it big enough? The 1.5Hp motor is 1.2kW so I was guessing at its minimum mechanical speed of 2:1 on the reeves drive it would be equivalent to 2.4kW as its about a 2:1 or 1:2 speed range on the 1440rpm motor.

However, the drive itself takes a fair lump of that power as they are not  that efficient.

So the options are still the same - build around the std motor / vfd and have possibly very vague speed settings, or throw in the 1.8kW servo and suck-it-and-see.

I guess a servo just bogs down until the drive trips on over-current if the spindle is pushed too hard????
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline Hood

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Re: Bridgeport Knee Mill Conversion?
« Reply #251 on: June 27, 2016, 05:52:30 AM »
Yes , assuming you have the fault signal going to the IP-A, either directly of via and E-Stop string or whatever, then the motion would stop when that happens.

Think probably I would go with the standard motor for now and see how things are. Keep an eye out on eBay for a bigger drive/motor later if you need to.
BTW I forgot you do not have 3 phase so the one I  mentioned wouldn't do anyway.

There are often drives on that would drive that motor I linked to but prices vary and are often crazy which makes the Chinese ones look good.
Sad thing about the Chinese motors/drives though, is as you have found out, the limited top speed.

Hood

Offline Davek0974

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Re: Bridgeport Knee Mill Conversion?
« Reply #252 on: June 27, 2016, 05:53:18 AM »
From what data I can find, a dual-cone Reeves drive is about 77% efficient so I only really have about 1.15Hp anyway which is 0.86kW, so at min speed its around 1.72kW and at top speed around 0.43kW, this is ignoring all other losses of course - spindle bearings etc.

So, maybe a 1.8kW servo would be pretty well suited to this job after all??
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline Hood

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Re: Bridgeport Knee Mill Conversion?
« Reply #253 on: June 27, 2016, 05:56:07 AM »
Not sure where you are getting the change in KW from, if the original motor via the pulleys then it will be constant power no matter the spindle rpm as the motor is always at rated speed and as such KW is always at rated speed.
The Torque however will vary with the gearing.

Hood

Offline Davek0974

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Re: Bridgeport Knee Mill Conversion?
« Reply #254 on: June 27, 2016, 06:48:54 AM »
Ah bum, the dreaded torque/Hp confusion again :(

So I have 1.5Hp regardless (fixed speed motor), at 1:1 speed setting the spindle will only see 1.15Hp though due to the transmission efficiency?

Thats how i figured it - i'm still wrong though?
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline Hood

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Re: Bridgeport Knee Mill Conversion?
« Reply #255 on: June 27, 2016, 07:07:48 AM »
Aye, used to confuse me as well, took awhile to get through the bald heid :)

Hood

Offline Davek0974

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Re: Bridgeport Knee Mill Conversion?
« Reply #256 on: June 27, 2016, 07:43:16 AM »
So, what i'm really saying or asking is:-

I have a 1.2kW fixed speed motor connected to a transmission of 77% efficiency with a ratio range of 2:1 to 1:2

Can I replace that with a variable speed 1.8kW AC servo and high efficiency drive (toothed belt is around 98%) ??

And of course, will it work??

I will be keeping the back-gear for low speed / high torque stuff in the 40-400rpm range as now.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline Hood

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Re: Bridgeport Knee Mill Conversion?
« Reply #257 on: June 27, 2016, 01:43:03 PM »
Well the induction motor would have (basing torque on 1.2Kw)

2880RPM  0.8kW and  3.94 Nm

1440RPM   0.8Kw and  7.9 Nm

720 RPM    0.8Kw and  15.78Nm

The Servo would have

3000RPM  1.8kW and  5.7Nm

1500RPM   0.9Kw and  5.7 Nm

750 RPM    0.45Kw and 5.7Nm

So I suppose the way to see would be to look up some charts to see what the torque and hp requirements would be for the type of tools and materials you would be using.
 Personally I think the power drop off of the 1.8Kw servo may just be too much.


Hood
« Last Edit: June 27, 2016, 01:44:40 PM by Hood »

Offline Davek0974

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Re: Bridgeport Knee Mill Conversion?
« Reply #258 on: June 27, 2016, 01:56:59 PM »
Thanks Hood, that explains it nicely, seems I would need to go 3.8kW / 15Nm to get equivalent performance - that is not only too expensive but a lot more power needed PLUS its only 2500rpm.

That has made my mind up to refit the existing motor and run until it fails totally, speed control will likely be a bit hoo-ha but I doubt it will hurt.

The encoder module would have been a ball-ache to fit anyway, I wrongly presumed it uses the motor encoder signals but fitting a shaft encoder would be pretty hard on a Bridgeport.

Enough of this nonsense :)

One day the damn replacement Y axis screw will turn up and i'll be able to start moving ahead again, it's stuck in the post somewhere :(
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline Hood

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Re: Bridgeport Knee Mill Conversion?
« Reply #259 on: June 27, 2016, 02:04:47 PM »
I just use the motors encoder, well actually the interpolated/simulated output from the drive as the encoder itself is sine/cosine. As long as it is 1:1 it is fine. Not sure about the Chinese drives but most I have used have the option to divide or multiply the physical encoders counts for the drives outputs, I did that on the wee lathe and threading worked fine, never tried rigid tapping on it though I think it would have been fine as well.
All I do is have the encoder input on the IP-A jumpered across to the Enc module.

Hood