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IndustryArena Forum > MetalWorking Machines > Benchtop Machines > PM728-VT conversion. And question.
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  1. #1
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    PM728-VT conversion. And question.

    I have the 728-VT and have done the conversion. The Ultra Precision conversion was a snap. Had it all completed in one long afternoon.
    It is truly a bolt on conversion. All the parts are heavy cast iron and well finished.
    I highly recommend the Centroid controller with Clearpath servos. If you really
    want the flexibility and precision.
    My question is... I need a good way to mount an encoder so I can have spindle control.
    My current bad idea is to, go VFD drive. I could then lose the bottom spindle pully to make room
    for about a 2 1/2" timing pully. I would mount the encoder inside the head (plenty of room) with the shaft
    protruding through the top deck also with a 2 1/2" pully. This will allow the encoder to be 1:1
    with the spindle.

    Any ideas? All comments appreciated.

  2. #2

    Re: PM728-VT conversion. And question.

    On my G0704 I did it how you suggested. 1:1 pulley and everything. I just used a small V belt. I have a VFD/BLDC and control it via PWM.

  3. #3
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    Re: PM728-VT conversion. And question.

    What do you plan to use the encoder for?

    A VFD should give good speed control. But for that I don't think you need an encoder for feedback. A PWM (pulse width Modulated) control should also work. I think this is what's originally on the PM728, variable speed.

    But for some operations, like rigid tapping, you need position feedback. An encoder gives this. I believe the position, in real time, is fed back to the CNC controller. The controller matches the motion/position of the axis, like Z, to the position of the spindle. So you don't need precise control of the spindle, you just need to know precisely the position of the spindle in real time. I've done threading on a Sherline using an encoder but no CNC control of spindle speed.

    So, what is the purpose for adding an encoder? Do you really need one?

    Thanks

    Hugh

    Quote Originally Posted by rlaury View Post
    My question is... I need a good way to mount an encoder so I can have spindle control.
    My current bad idea is to, go VFD drive.

  4. #4
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    Re: PM728-VT conversion. And question.

    You are confusing me. How do you combine VFD and PWM control? I've thought they were two different control schemes. VFD varying the drive frequency to change the speed of an AC (induction) motor. The PWM varies the pulses of the drive voltage between 0 and Vmax very quickly. The drive voltage is the average of time on and time off. How to you combine these?

    Won't a V-belt tend to drift in orientation over time? If it scootches the reading from the encoder will drift off. I'd would think you need a positive drive like timing belts or gears. (for an encoder gears have a lot of backlash) How did you get around this drift with a V-belt?

    The 1:1 drive is critical.

    Quote Originally Posted by CL_MotoTech View Post
    On my G0704 I did it how you suggested. 1:1 pulley and everything. I just used a small V belt. I have a VFD/BLDC and control it via PWM.

  5. #5

    Re: PM728-VT conversion. And question.

    Yes, you are correct. Sorry, I was rather tired when I responded, it was a late night being the Easter bunny. I use the 0-5v output from my BOB to control speed at my VFD.

    I haven’t had any issues with the v belt drifting. There’s basically no load, that’s my only explanation because I otherwise agree with you. I also drive my spindle with a V belt. Slipping doesn’t seem to be a problem, and belts last years assuming no other failures.

  6. #6
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    65

    Re: PM728-VT conversion. And question.

    Thanks. That makes sense wiring in the VFD. I just though I was missing some critical knowledge.

    If you're not depending on the spindle position a little "slip" won't matter. And you're right, it'll be small with no load. I'd be interested to hear how well it works for rigid tapping and similar operations.

    Thanks for the response. All is well.

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