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  1. #1
    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.

  2. #2
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 2 Hours Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 2,502. Received thanks 292 times, giving thanks to others 7 times.
    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).
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  3. #3
    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.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  4. #4
    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 itís been fine for two years

    Cheers. Paul


    Sent from my iPhone using Tapatalk

  5. #5
    Quote Originally Posted by depronman View Post
    Seconded on the 3 phase motor and vfd
    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.

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