Advice on conversion

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Thread: Advice on conversion

  1. #1
    Member robbycar's Avatar
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    Default Advice on conversion

    I do benchrest gunsmithing, mainly for myself.

    I have a CNC mill and a CNC mini lathe that I built.

    Both use steppers.

    I want to convert my 1m bed European toolroom lathe to CNC.

    Not sure how large ballscrews I need (I am thinking 20mm)?

    Should I use a 1700oz/in stepper for Z axis or should I go with a servo?

    I would appreciate any advice you guys may be able to offer.

    Rob.

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    Member hanermo's Avatar
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    Default Re: Advice on conversion

    Absolutely, totally, always, use a servo for this application.
    You dont need lots of torque, at all.

    You do need rigidity and accuracy and high acceleration.
    Big steppers dont accelerate well.
    Alll steppers are pretty inaccurate, especially direct-coupled (no belt transmission).

    I am using a 32 mm screw on z axis for my 12x24 lathe,
    10.000 count servos, with 1:2 belt drive.
    This is an optimal solution.

    The 220V ac brushless servo, with controllers and encoders and cables, costs 690€ in the EU, taxes paid (22% VAT).
    These are 7.1 Nm peak, 10.000 count, 220V AC, brushless servos, 750 W. No power supply needed, ac from the wall socket.
    The belt drive, costs about 130€ / axis.
    HTD 8/30 mm, 24 teeth to 48 teeth.

    This delivers 0.2 micron "step size", and better than 1 micron actual resolution.
    You will need a hardware pulse engine, ie a motion controller.
    I use a CSMIO-IPS, from cslabs.

    3000 rpm (servo speed) x 10.000 counts = 300 khz.
    So you need a 300 kHz controller, minimum.
    The CSMIO-IPS is 4 Mhz speed controller, industrial, excellent.

    If you dont think you need the ultimate precision;
    A 400W AC brushless servo drive kit is 290 €, all included.
    It has a 5000 count encoder (half the resolution/accuracy).

    The smaller servo has 10x the acceleration of the big stepper.
    Over 10x the accuracy at 1:2 belt drive.
    Over 4-5x the top speed (the big stepper wont run more than 300-600 rpm).
    The smaller servo needs a 48-60V PSU.



  3. #3
    Member robbycar's Avatar
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    Default Re: Advice on conversion

    Thanks for your excellent response, it seems like servos are the way to go!

    Do I really need the belt reduction or can I go direct? I do not need endless power as I only take small cuts on barrels so as not to induce stress.

    I use 2:1 belt on my mill and this is the source of backlash. But, I guess backlash is much less important on a lathe.

    Rob.



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