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  1. #793
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    Dec 2008
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    21

    Re: Rotary table indexer

    I haven't disconnected it but your suggestion did give me an idea. I loosened all the screws assembling the motor mount/coupling and gradually tightened them evenly while checking the rotation. That cleared the tight(er) spot and it is now even easier to turn by hand with the motor disconnected. So easy, in fact, that I can't believe that any self-respecting motor of that size wouldn't eat it up and spit it out.

    In fact it does turn the table now well enough for me to complete the job I was doing. I only need to turn the table between cuts (not while cutting) and it managed that except for some missed steps at one stage which let me know that it's still marginal.

    I'll swap out the driver when it arrives and try it with this and a replacement motor.

  2. #794
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    Dec 2008
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    21

    Re: Rotary table indexer

    OK, I tried something else. With the indexer driving the motor, I tried to stop the motor by hand (I have a small handle attached to the shaft extension). Not particularly difficult. I then connected another motor (same spec) and tried to stop that - I couldn't.

    Seems the motor that's installed isn't giving anything like its rated torque. Which would explain my problem.

    Another separate (I think) issue. As I said I have a handle attached to the motor shaft extension to help in manually setting up the rotary table. I've noticed that, if I have the motor connected to the indexer but not it's power-supply, when I turn the motor by hand the LCD panel backlight lights up. Presumably the motor is acting as a generator. Is this likely to do any damage? The motor is attached via an easily released connector so it's easy to avoid but I'd like to know anyway.

  3. #795
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    Jan 2007
    Posts
    132

    Re: Rotary table indexer

    The stepper should be already protected, because the stepper creates back EMF, from the magnetic field collapsing every step it takes.

  4. #796
    Gold Member
    Join Date
    Dec 2007
    Posts
    2148

    Re: Rotary table indexer

    Quote Originally Posted by dnaman View Post
    I've noticed that, if I have the motor connected to the indexer but not it's power-supply, when I turn the motor by hand the LCD panel backlight lights up. Presumably the motor is acting as a generator. Is this likely to do any damage? The motor is attached via an easily released connector so it's easy to avoid but I'd like to know anyway.
    Absolutely yes!

    Most motors when manually rotated will generate a voltage that can exceed safe limits of the circuit and backfeed, destroying unprotected devices. With most motors normally a snubber diode would be used to re-circulate the current so it stops backfeed, but this isn't practical in a lot of cnc applications as this would cause current to continue flowing.

    Connect your multimeter to the stepper leads and spin it, you might be surprised by the level of voltage generated.

    cheers, Ian
    It's rumoured that everytime someone buys a TB6560 based board, an engineer cries!

  5. #797
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    Join Date
    May 2009
    Posts
    32

    Re: Rotary table indexer

    I've been using this excellent design from one of the supplied kits for a couple of years now with no problems. But just got a question stimulated by discussion over on the ME forum. When dividing by a number "N" which isn't an integer divisor of the number of steps per rev, how does the controller deal with the resulting error? Does it use something like the Bresenham algorithm to minimise the error.

    Sorry if this has been asked and answered before!

    Thanks, John.

  6. #798
    Registered
    Join Date
    May 2006
    Posts
    178

    Re: Rotary table indexer

    Quote Originally Posted by JohnHaine View Post
    I've been using this excellent design from one of the supplied kits for a couple of years now with no problems. But just got a question stimulated by discussion over on the ME forum. When dividing by a number "N" which isn't an integer divisor of the number of steps per rev, how does the controller deal with the resulting error? Does it use something like the Bresenham algorithm to minimise the error.

    Sorry if this has been asked and answered before!

    Thanks, John.

    Hi John,
    that's exactly how it works.

    So basically internally it multiplies the number of divisions by the actual division its on then divides the result by the number of steps for a full circle - this gives it the nearest number of steps it needs to get to that division (doing the multiply first prevents loss of resolution although it means it needs 32 bit maths which is why the max steps per rev is 16 bit).
    Internally it knows exactly how many steps it's currently taken so simply adds (or subtracts) that from the answer above to get the number of steps to take.

    This means there isn't a constant number of steps since it may need to take an extra step (or lose one to be accurate).

    In practice the accuracy of the maths is half the single step resolution, so a 400 step driver on a 90:1 worm gives you 0.01 degrees per step so the accuracy is 0.005 degrees. (Mechanical accuracy not withstanding).

    Hope this answers your question.

    Cheers,
    Steve.

  7. #799
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    May 2009
    Posts
    32

    Re: Rotary table indexer

    Brilliant, many thanks Steve.

  8. #800
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    Sep 2010
    Posts
    1

    Re: Rotary table indexer

    I know this has been asked before but I couldn't find a good answer what would be a good stepper motor and driver for a 10" rotary table

  9. #801
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    May 2006
    Posts
    178

    Re: Rotary table indexer

    Quote Originally Posted by wiseman9986 View Post
    I know this has been asked before but I couldn't find a good answer what would be a good stepper motor and driver for a 10" rotary table
    For 'normal' use a typical Nema 23 motor of around 2-4A will easily spin a 10" table.

    I use a similar motor but set the current low to just over 1A and via a 90:1 worm that turns a 10 inch swing lathe head (with drive motor still engaged) along with any work in an 8 inch chuck without any problems.

    As the load on the stepper gets higher the torque available falls, so worse case you may need to run it slower but for division speed isn't that much of an issue.
    You can get around this by increasing the supply voltage to the driver (if it's higher than the safe supply voltage for the controller then you would need a separate power supply for the controller - a small 12v wall wart type would be fine).

    So in short, find a Nema 23 motor of around 2-4A and a suitable driver with similar capabilities. (Preferable a driver with half step capability - which should be most).
    The driver doesn't need to be anything clever, just a cheap driver off eBay. They're cheap enough so you can buy a spare or two, although avoid anything that uses the L298 chip and keep the voltage a little lower than the max rated voltage.

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