Servo motor as lathe spindle motor

[dmbgo]

I am currently setting up my lathe to use the Acorn controller.
I have asked a lot of questions and have been very happy with the level of support and help that I have been given so far.
I am currently waiting for some parts to arrive, which I expect next week, and while I am waiting might I test your patience and ask another question?

My lathe currently is using a Huanyang VFD and 3hp motor, which I was going to use with the 0 - 10V control on the Acorn.

I do however have a 2.6KW servo motor and drive laying around which will accept a step and direction input. I was wondering if I should use it on the spindle of my lathe? It would be fairly straight forward to install, but a previous servo with Mach3 had issues with the run signal being cut off too soon, and used to "clunk" alarmingly when stopping.


Cheers

Dave

[jhelgesen1]

Interested also, I am looking at a similar servo as a spindle motor for my G0704 mill conversion.

[martyscncgarage]

I like a good 3 phase motor (inverter duty preferred) and a good VFD (Automationdirect.com GS3 preferred)
Also a standard 5VDC Line Driver/Differential encoder, about 1000 line, belted to the spindle at 1:1 with a timing belt.
Take the time to set things up correctly, you should have good luck.

I don't believe the Servo motors have the inertia that an Induction motor has. I tinkered with the idea and got lack luster results. Others are tinkering with it as well, I think some have it going, but I have yet to see a video of a machine with a servo motor as a spindle motor taking a decent cut.
Servos have to be tuned, and can be fidgity.

My .02
Love to see a servo driven spindle working on someone's machine taking good, reasonable cuts, and also doing single point threading and rigid tapping.

Marty

[Sportbikeryder]

If a servo and encoder are used, I assume with some extra PLC work and a good post processor, a C axis could also be added to the lathe to do milling.

[PeterC]

Looks like a Grizzly lathe. Just cnc'd a G4002 used a Hitachi VFD, Leeson 2HP motor, RLS encoder. Considered using a AC servo but the VFD & 3 phase motor seemed easier.
I also have a mill that is ready to be converted to Acorn from Mach 3. Been using a AB AC servo (think it is 1.8HP) as the spindle motor for about 8 years using a Vital Systems DSPMC controller (analog out, encoder input). Works good for rigged tapping and single point threading.

[dmbgo]

Hmmm, I would quote some the comments here, but I think they are all valid, albeit in different ways.
Based on this discussion I will leave things as they stand with an encoder on the spindle and a 3 phase motor and VFD to drive the spindle.
I can change / experiment with the servo as a spindle motor again later if I want to.
Ironically, when I had the servo driven spindle (the clunky one) on the lathe before, I was encouraged that this was the right way to go, with promises of the fantastic things I would be able to do with Mach3. This was possibly correct, but was beyond the investment of time, effort and learning that I was prepared to put in.
I was hoping that someone would say that the servo driven spindle is an accepted, common practise with Acorn, and definitely the way to go, but it sounds like (as I expected), that I'd really be moving onto unstable ground to embark on this in the early stages of getting things sorted with Acorn.

[PeterC]

Hmmm, I would quote some the comments here, but I think they are all valid, albeit in different ways.
Based on this discussion I will leave things as they stand with an encoder on the spindle and a 3 phase motor and VFD to drive the spindle.
I can change / experiment with the servo as a spindle motor again later if I want to.
Ironically, when I had the servo driven spindle (the clunky one) on the lathe before, I was encouraged that this was the right way to go, with promises of the fantastic things I would be able to do with Mach3. This was possibly correct, but was beyond the investment of time, effort and learning that I was prepared to put in.
I was hoping that someone would say that the servo driven spindle is an accepted, common practise with Acorn, and definitely the way to go, but it sounds like (as I expected), that I'd really be moving onto unstable ground to embark on this in the early stages of getting things sorted with Acorn.

If you did not have the lathe gear box to deal with and could connect the servo 1:1 to the spindle then the servo would work. I looked at removing the gear box to run a servo 1:1 but it would have been too much work.

[dmbgo]

Hmmm, I would quote some the comments here, but I think they are all valid, albeit in different ways.
Based on this discussion I will leave things as they stand with an encoder on the spindle and a 3 phase motor and VFD to drive the spindle.
I can change / experiment with the servo as a spindle motor again later if I want to.
Ironically, when I had the servo driven spindle (the clunky one) on the lathe before, I was encouraged that this was the right way to go, with promises of the fantastic things I would be able to do with Mach3. This was possibly correct, but was beyond the investment of time, effort and learning that I was prepared to put in.
I was hoping that someone would say that the servo driven spindle is an accepted, common practise with Acorn, and definitely the way to go, but it sounds like (as I expected), that I'd really be moving onto unstable ground to embark on this in the early stages of getting things sorted with Acorn.

If you did not have the lathe gear box to deal with and could connect the servo 1:1 to the spindle then the servo would work. I looked at removing the gear box to run a servo 1:1 but it would have been too much work.

The servo that I had setup previously was driving the spindle directly via timing gears and a toothed belt, so the way it was setup bypassed the lathe's gearbox - it was 1:1. The servo I used, apart from the clunks that I previously mentioned, suffered from a lack of power at low RPM, so the one that I photographed at the start of this post, is bigger and probably would handle low RPM better. That being said, I think I'll still proceed with the VFD, encoder and 3 Phase motor for the time being at least.

[dmbgo]

[Muzzer]

There seems to be a notion here and elsewhere that a servo motor and drive would somehow deliver a better "performance" than a VFD and induction motor. The only significant difference between them is that a servo system is primarily designed for position control whereas a VFD is designed for speed control. Conversely, you can easily implement speed control with almost any servo drive but most industrial VFDs don't offer position control.

Once you have a motor envelope of a given size, it makes not a great deal of difference whether it's driven by a servo or a VFD. You might point out that a servo motor is usually a permanent magnet (PM / synchronous) machine whereas a VFD will generally (but not always) be driving an induction machine (IM / asynchronous). But ultimately, torque is generated by a magnetic shear stress (torque per square area) operating across the active area of the motor. That's a cylindrical surface for a radial flux machine. And the maximum magnetic flux density you can generate is limited by the saturation flux of the steel laminations, not the presence or absence of magnets, no matter how powerful. Yes, an induction machine IS slightly larger than an equivalent PM machine but not significantly so - and that's not the usual justification for changing. Once you move beyond the basic magnetic limitations of the electric machine, the servo / VFD controller can't magic up a load of extra torque from nowhere.

So the bottom line is that a servo driven spindle has the native capability to achieve positional control (for rigid tapping). Er, that's about it really. If what you actually want is to have a fixed, single 1:1 drive ratio between the motor and the spindle, you need a motor that can generate back-busting torque at low speeds as well as high power at high speeds. The name for that is "a big motor", whether or not it's a PM or IM or any other sort of motor, driven by a servo or a VFD. The laws of physics continue to apply.

You will notice rigid tapping can be done easily enough using a VFD and a spindle encoder. The CNC controller is closing the loop here and doesn't require a servo drive to do it. There may be something to learn there....