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Geared Nema 17 vs Nema 23

Discussion in 'CNC Mills/Routers' started by runninfarmer, Feb 26, 2015.

  1. runninfarmer

    runninfarmer Veteran
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    I've been building and designing 3D printers for a while and now I'd like to dabble in CNC milling. The electronics options for Nema 17's are much more afforable with RAMPS etc, and was wondering if a geared Nema17 (5:1 ratio or higher) would perform just as well as a Nema23 with similar torque rating? Maybe I'm missing something with CNC electronics but I like the economy of using USB controlled steppers with lower current Pololu drivers. Thanks for any info!
     
  2. Tweakie

    Tweakie OpenBuilds Team
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    Interesting you should mention this as I would have been convinced that it should work absolutely perfectly until...

    A couple of weeks ago I had occasion to drive a single axis using two stepper motors operating through a differential gearing arrangement where one MEMA 23 did all the work and a NEMA 17 (with 60:1 reduction gearing) did a positional correction. For some reason the NEMA 17 failed miserably in this task but once replaced with another NEMA 23 all was well. I have not had time to investigate just why this was a problem (as the initial calculations showed it should work) but it perhaps suggests that you may well have to try your reduction gearing arrangement to see just what torque is available before committing yourself to the design.

    Tweakie.
     
  3. runninfarmer

    runninfarmer Veteran
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    Interesting info, I wonder if acceleration and max speed come into play? I'm glad you have experimented with this. I'm looking at a 14:1 geared Nema17 on Phidgets.com and it's specs are 30k g-cm (416 oz-in) of continuous torque with max speed of 295 RPM and acceleration of 1E+6 1/16 steps/sec^2. Link is here: http://www.phidgets.com/products.php?category=23&product_id=3326_0

    Is that speed to low for a CNC milling application? I know mills/routers are running at much lower speed than a 3D printer, but I'm not sure if that speed would be too low? Also I wonder if the constant directional and speed changes would be too hard on the gearbox, but maybe they're designed to handle that?
     
  4. Tweakie

    Tweakie OpenBuilds Team
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    I don’t know if this helps but just as a guide…
    On my machines the stepper motors are usually run at between 60 and 200 rpm when machining (G01,G02 feed-rate moves) and 900 rpm during the rapid (G00) moves.

    Tweakie.
     
  5. Tweakie

    Tweakie OpenBuilds Team
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    Oops, forgot to mention - I am driving ball-screws where 1 motor revolution = 5mm machine travel.

    Tweakie.
     
  6. Florian Bauereisen

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    Hi,

    First off:
    Stepper motors and servo motors are completly different and not to be mixed up.

    Stepper motors are designed to run quite low rpm.
    Looking at their torque curve, you will find that all of them will have their max torque between 0 ( holding torque - the one actually quoted in the kataloge) and aprox 250 rpm with "enough" or usable torque maybe till 800 rpm. Depending on your needs.

    Turning them faster will find you having steeply declining torque, that is the reason most cnc have a max speed and theirafter the axis simply stalles ( steppers do not even deliver enough to move without even a load on the cnc).
    This can be countered quite a bit by using a higher voltage - you d have to look into your specs to find out.

    So gearing is usually used to match the inertia of the ball screw to the inertia o f the motor to give a responsive / dynamic machine.
    You will notice that most cnc machines using 20mm or upward ballscrews will be geared. To regain speed again these big screws usually have 10 mm or more - whtats it called? turns? (movement per rpm).
    16mm ballscrews normally come with 5mm per turn ... see above.

    Opposite to common asumption a long thin motor ( two stack or three stack ) matched to your ballscrew ( Nema 23 fits 16mm ballscrews in general) will give you a more dynamic machine than using a Nema 34 siced motor on the same ballscrew.
    Of course the later one woud have more torque but acceleration will be lame compared and your job might/ will take longer even though you think you have more "uumph".

    Of course a slight gearing can be used to add torque if you have "speed to spare" especially like on a belt driven cnc where one rpm will give you a movement of several mm ( 2D x pi).
    Thats the reason most cnc will use geaing below say 4:1 - you still need some speed left for working with it.

    So, if you want to try, find out if you could run higher voltage, and find a gear matching your speed vs. torque requirements.
    I personally guess that you will end up with a small machine using dreml type routers capable of engraving pcb boards or cutting balsa.
    If that´s what your after it might be worth investigating.

    good luck

    greets

    flo
     
  7. runninfarmer

    runninfarmer Veteran
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    I never mentioned servos, so I'm not sure where that came from? I'm after milling aluminum, but if a geared nema17 can offer 400 oz-in of torque, I thought it'd be worth it to consider. I know I read the F117 build is using 570 oz-in motors so I figured the 14:1 nema 17 might be able to be comparable.
     
  8. Florian Bauereisen

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    NO no, of course you didn`t mention servos.
    Didn`t try to offend you.
    I merely wanted to mention the difference.
    Its just not as steppers could be viewed as"electric motors" in a common sense, while servos can.
    With a servo one could use the torque all along the speed curve, but not with a stepper. thats all.
    400 oz should be enough torque to mill aluminum, but is 300 rpm enough for your needs? that is for you to decide.
    greets

    flo
     
  9. Tweakie

    Tweakie OpenBuilds Team
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    I think the Phidgets geared motors will only provide their advertised spec. when driven with the Phidgets HC driver board ??.

    Tweakie.
     
  10. runninfarmer

    runninfarmer Veteran
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    No problem, sorry I misunderstood. I've never used servos so they might be worth investigating. Are they more or less expensive? That's one of this issues, the max 300 RPM will probably hold me back from pulling the trigger. May have to just bite the bullet on the more expensive electronics options. Thanks for info
     
  11. Florian Bauereisen

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    unfortunately more expensive...
     

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