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CNC Conversion Pt1: The motor?

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  • CNC Conversion Pt1: The motor?

    As some of you may recall, I was starting to work on a mill-drill CNC conversion. Well, the machine itself has been partially serviced, and I've collected a great deal of information, plus contacting a local DIY/CNC enthusiast.

    Anyway, I'm now at the actually buying parts stage, and I'm starting with the motor. I plan on getting a motor and speed control fitted, get the pulleys and belt(s) sorted out, and the spindle rebuilt, so I can test it as a manual mill before starting the ballscrew install.

    I believe I've decided to go with a DC motor and speed control, with custom pullies to give me at least two general speed ranges in addition to what the motor/speed control provides.

    So, unless someone has a better source or supplier or suggestion, I'm currently looking at these motors from Surplus Center:

    1) 1.5HP, 180vDC, 3100 rpm ($109.95)

    2) 1.5HP, 120vDC, 4,800 rpm (though "1 ft/lb torque"?) ($29.95)

    3) 2HP, 90vDC, 2,500 rpm ($159.99)

    There are others in that ballpark, but those are the top three that seemed, with my limited information, to fit my application. (Stock mill motor was 1.5hp 1750rpm with a max of 1100 rpm at the spindle, I'd like to see closer to 2500 to 3K rpm spindle speed.)

    And with one of the above motors (or other, I'm still open to suggestions) they also have these DC speed controllers:

    A) Up to 2HP at 180vDC (with optional heat sink. Also has optional braking/reversing switch, fuses, etc.)

    B) Up to 180vDC at 10 amp (Less info on this one, not as complete as above.)

    I think I would like to go with (1) and (A), which seems like a nice matched set. I have 240vAC available, and with a 1:1 drive I'd have 3K rpm at the spindle (hopefully.)

    Prices seem fairly reasonable, and even with the extra heat sink and reverse switches, is still less than a 3/ph and a quality VFD.

    So, will this work? Anything I'm overlooking? The mill will be pure CNC (manual work will be done on the larger mill) and used almost exclusively for aluminum.
    Doc's Machine. (Probably not what you expect.)

  • #2

    One thing I don't see on those motor controllers is a way to vary the speed by a signal from the CNC controller. Typically that might be done with an analog 0-10V signal.

    I suggest you go with a three phase motor/VFD. All the VFD's I've worked with have connections for remote on/off and speed setting from a device such as your controller. VFD's have many other features such as programmable accel/de-accel, current/voltage overload, etc that may not be present on a low end DC motor driver. Three phase motors are also essentially maintenance free which can not be said of most DC motors.

    If you're new to CNC machining you might think programmable spindle control isn't a necessary feature, but it is. It's a real pain to have to manually turn the spindle on and set the speed prior to the tool contacting the work. Spend a few bucks more and do it right.


    • #3
      Yes, computer control of the speed was something I had in mind, or at least control of the start/stop/brake.

      However, I'm on something of a budget here. $250 to $300 for a motor and controller is about all I can spare, especially considering I'd like to send a little extra on the steppers and ballscrews to get some quality units.

      The one well-featured VFD I looked at was in the $700 range all by itself. Is there some reason a speed-controlled 3/ph motor will be "better" (or worse) than a speed-controlled DC motor? (Physical aspects of the motor itself notwithstanding- I see some of these in the catalog as "intermittent duty" and have bushings, not bearings.)

      Doc's Machine. (Probably not what you expect.)


      • #4
        Doc, I don't know of any modern VFD's that can't accept a remote signal to vary speed.
        Usually it's a 0 to 5volt or 0 to 10 volt signal into the VFD that controls speed.

        Stop start is handled by the parallel port on the PC directed by the conroller program and feeding a current into a relay to energise the spindle.

        Some DC controllers can also accept this same remote signal to control speed but you will have to check on this.

        John S.

        Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


        • #5

          A surplus three phase motor in the 1 to 2 hp range should be nearly free. A VFD can be had for somewhare around $200. So I bet the total would be about equal or less than the DC combination. I think Dealers Electric or a name close to that would be a good place to shop, they're on the web.


          • #6

            Doc.. join the Mach2 group on yahoo..

            Mach2 has a spindle speed control.

            I bought the license. $149.. you can use paypal.
            It looks like everything I have tried to write on my own.
            You can download a trial version.. it must run on windows 2000 or XP.. I bought Xp off ebay. John and others tell me the 2000 is slightly better on a machine.