Options for replacement lathe spindle motor

Full disclosure - away from home and with the wife, so (a) bored not being able to tinker and (b) unable to check the machine to see what I have...

Anyway.

I'm converting a Myford ML7 to CNC operation, and got as far as glaring at the original single phase AC motor with the pulley reduction and 3-position cone pulley gear reduction system and wondering, in the case of automated speed control that I must be able to do something better. "Better" starts with removing the single-phase AC motor and buying something else to go in its place.

Basic numbers, original motor somewhere around 1/2 HP, 1450 rpm. The existing belt gearing taking this to around 200-700rpm, depending on belt position on the cone assy. Ideally I'd like to up the rpm a tadge to around 1000rpm max (it's a build option with a different spindle motor pulley choice).

Also, I'd like options where I don't have to fiddle (and crush fingers) with belt changing on the cone assy. LinuxCNC/Mesa look to support both 0-10V and step/dir options for spindle control. Of course in an ideal world I'd like to have the option of managing constant surface speed through the control software. Somewhere in the mix is a spindle encoder.

So, it feels like I probably want something capable of driving 0-3000rpm, with constant power (more torque at lower speed) - to a sensible limit of course (I do have the back gear option for heavy cutting/threading).

In the past my experience is largely around the Chinese water cooled spindles, but these are far too high revving with poor performance below 1/3rd rated speed. A general eBay trawl shows a form/fit/function replacement of 4 pole 3-phase motors with VFDs, but I'm a little concerned that these will have similar poor performance at 1/3 rated speeds - necessitating the use of the cone pulleys.

Is there something better that I should be looking at? I know I can get a stepper with a similar electrical power rating but conventional wisdom seems to be that these are poor choices for a lathe spindle drive (even though I'd be going through several feet of rubber drive belts). Are servos better? (and the whole murky world of servos is something that I've never delved into).

Cost is a (not major) consideration - though not a Yorkshireman I like to think growing up in Lancashire that I share some of our cousin's frugal nature. Shiny is not a consideration (I don't need to buy new if it works). I get a bit sweaty thinking beyond around £300 for a replacement.

Constant power and electric motors doesn't exist, unless you introduce gearing, as you can't cheat the laws of physics.

Options are either a three phase motor with VFD, or go all out with a servo.

I'd probably go for around a 1HP 4 pole motor, gear it so at rated speed the spindle is running at 500 (about 3:1 ratio), then overspeed the motor via the VFD to get the desired 1000rpm (where you do actually enter a constant power zone of the motor, as torque starts to drop of above the rated speed).

Yes VFD and 3 phase motor. My lathe came with one fitted and there is a potentiometer on the front panel which controls speeds.
There is a belt change option on the drive train to expand the speed range but in all the years I’ve had it I’ve never used it.
My friend has the same model but single speed belt drive. I can’t imagine changing belts to change speed !
You can get DC motors and variable supply controller but I have heard of issues on the controllers so VFD all the way for me.

Costs:
VFD £80
3phase motor £120

You would ideally make a control box for start stop etc and connect to the control terminals or cheat and get a VFD with pot on a removable control panel and mount it somewhere nearby.

Motors come with face mount, flange mount, side mount (at several positions) so check what will retro fit.

ps chuckled at the ‘out with the Wife and away from workshop intro’. Just keep smiling and agreeing and you’ll be back before you know it.

Seconded on the 3 phase motor and vfd
This is the setup I use on my boxford BUD cnc conversion
A one horse power motor is more than enough
I still have the pulley options but rarely change it from the middle pulley
The motor is set up for 50% over speed
So I can get upto 2000 rpm at the headstock
And back gear is available if I need silly amounts of torque
Motor came with the lathe and is a brooks motor. I changed the jumpers to delta wind
The vfd is a £50 Chinese affair but other than the dinglish manual it’s been fine for two years

Cheers. Paul


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Originally Posted by depronman

Seconded on the 3 phase motor and vfd

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Holy thread revival, Batman! :-)

You've described pretty much what I ended up doing, and it's worked... to a fashion. My biggest problem resulting from the above is getting closed-loop control back into LinuxCNC, and (recognising after the event) tuning the PID in LinuxCNC with the VFD PID. I always ended up with either oscillation or slow ramp rate when changing spindle speed that took seconds (or tens of seconds) to stabilise. I did try tweaking many times but never got to where I wanted it. Coupled with the clumsiness of the countershaft etc, and my desire to remove that, and the third-party spindle encoder has sent me down a different path with light-at-the-end-of-the-tunnel, which involves removing much of the original lathe and replacing the spindle motor with a 2kW servo. That's working quite well at the moment but this weekend's job is re-boring a new timing pulley to fit the servo shaft (the reamer went for a wander on the first one). Pics may follow.

Hi Doddy,

Vaguely considering the (Chinese) servo spindle option for my Boxford 240TCL after reading about a similar conversion on a TCL125. My machine is currently fitted with the Gemini / Lenze combo. I too am working to use LinuxCNC so any further info on how you are getting on would be very much appreciated.

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Originally Posted by clutchslip

Hi Doddy,

Vaguely considering the (Chinese) servo spindle option for my Boxford 240TCL after reading about a similar conversion on a TCL125. My machine is currently fitted with the Gemini / Lenze combo. I too am working to use LinuxCNC so any further info on how you are getting on would be very much appreciated.

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How I'm getting on?, it's cold, damp and miserable in the shed, so not very far/fast, and other more interesting things to do. But, the lathe is in one piece and I'm perfectly happy with the performance of the lathe/servo. I do need to "press the button" to prove threading but I don't think there's anything standing in my way. I've a weeks holiday looming so will probably hanker down to it then.

There again... I have a spare saddle in the post for a complete mod to the X axis to replace the screw... or there's the casual enclosure for the mill... or any number of roundtuit jobs to be done...

I'd have gone for a 2 pole 3-phase induction motor with a decent quality VFD. That will run at close to 3000rpm at base speed. Go for an oversized machine like 2-3kW or so, so you don't need to change pulleys etc.

I don't agree with this current trend for using servos on spindles. Servos are designed for position control and VFDs for speed control. You can use a VFD for closed loop position control but servos are not so suited to speed control. As for the size of the motors, you can get induction motors in the same package as servo motors. While they are slightly larger, it's not a massive difference but a larger "conventional" TEFC industrial motor is considerably cheaper.

I'd have gone for a 2 pole 3-phase induction motor with a decent quality VFD. That will run at close to 3000rpm at base speed. Go for an oversized machine like 2-3kW or so, so you don't need to change pulleys etc.

I don't agree with this current trend for using servos on spindles. Servos are designed for position control and VFDs for speed control. You can use a VFD for closed loop position control but servos are not so suited to speed control. As for the size of the motors, you can get induction motors in the same package as servo motors. While they are slightly larger, it's not a massive difference but a larger "conventional" TEFC industrial motor is considerably cheaper.

Ah, Muzzer, ask why though. It was to have an accuracy in positional control on the spindle position for threading. Oversized to avoid pulleys - yup, that's the rationale behind replacing a 350W motor with a 2kW motor. The servo is driver by step/dir exactly for positional control, via a 1:1 pulley direct to the spindle.

Cost, actually, not substantially different to the 3 phase 4 pole motor with VFD that I replaced with less than 12 months on it, to be honest. And I don't have the lag through the VFD for spindle speed.

Oh, and another reason - because it's an interesting experiment if nothing else.

...though replacing the BLDC motor on the mill with a servo was precisely because it fitted the same highly restricted space envelope, for the same power, for less than the price of the replacement BLDC control board which I'd blown twice (okay, factor in the new pulley and belt and it was probably a bit more total cost), but it removed all the analogue uncertainty of spindle speed control or any third-party interfaces (that spindle, also step/dir control). Plus (and some might cringe at this) - the option of a spindle lock

Servos have very good speed control. AC servos pack a huge amount of power into their small size.

Downsides of servos
Need some tuning - pretty easy with a decent brand
Cost - may or may not be more than a non servo setup depending on brands and a bunch of factors.

Hey there, how did you get on with the AC servo spindle on the myford?
I am considering it on a BUD Boxford conversion.

It's fine - I've no problem with the servo as a solution. It did require, of course, a special-to-type cradle to mount it on the ML7 but nothing that 10 minutes with a TIG, and 20 minutes with a grinder can't fix.