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Thread: Spur Gear / Gear Rack

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    Default Spur Gear / Gear Rack

    I'm going to put together a CNC plasma soon using 80/20 parts and I was wondering if anyone has used a spur gear/rack setup to create movment? If so is the accuracy anywhere close to the accuracy of a lead screw? I don't need perfect accuracy but it needs to be close enough.

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    Registered massajamesb's Avatar
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    I feel the accuracy of a lead screw, depending on pitch, is superior to rack and spur gear,IMO. The thing is, the accuracy obtainable by rack and spur gear is greater than the accuracy of the plasma itself.
    I started out with roller chain and sprocket, and decided to go spur gear later on. I am quite happy with the accuracy I get, whether I am using oxyfuel or plasma.
    I am using 20 pitch, 20 degree pressure angle.
    Another side of it, is that you will get faster speeds with spur gear and rack, over most lead screws. You should gear down the motors to obtain your torque, whether with gearboxes or timing belts, etc. and then drive your spur gear from there.
    Plasma needs a moderate amount of torque and fairly high (200-350 IPM) speeds, at least for thinner materials. IMO, leadscrews wouldn't work to well at those speeds.

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

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    Ahh gearing down the motors. That always confused me because I could never find a correct answer on how to calculate the ratios based on the pulley sizes.



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    Registered massajamesb's Avatar
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    Well, for instance, I am running a 15 tooth pulley on the motor, and a 40 tooth pulley on the spur gear shaft, which in essence means that for every rotation of the spur gear ( 20 tooth spur gear), the motor has turned 2.66 times. Thus, a 1:2.66 ratio. It would be easier if you had a (for instance) 10 tooth motor pulley, and a 30,40, or 50 tooth pulley on the spur gear shaft.
    You now have more torque, and better resolution.
    I might add that you should not, IMO, run less than 18 teeth on the spur gear, to prevent undercutting of the gear, and to keep a certain number of teeth in full contact with the rack.

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

    "If the women don't find you handsome, they should at least find you handy"
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    If you know the diametric pitch of the spur gear that engages the rack, then this is the distance travelled by the gear in one revolution, if your motor was direct drive then obviously this is the distance/rev, which in 99.9% cases requires gear redection to enable the motor to be sized economically.
    The gear or reduction ratio will then easily give you the distance/rev of the motor for calibration purposes.
    I would advise you use one of the free sizing programs like Kollmorgen etc to size you motors according to your Speeds/feedrates etc.
    Al.

    CNC, Mechatronics Integration and Custom Machine Design (Skype Avail).

    “Logic will get you from A to B. Imagination will take you everywhere.”
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    Registered massajamesb's Avatar
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    Hey, I should have mentioned that.
    Thanks Al!
    -hi3-jpg

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

    "If the women don't find you handsome, they should at least find you handy"
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    Most commercial burning tables are rack and pinion. Much cheaper and scalable. Usually a spring tension system keep the motor engage to the rack. On a Burny I worked on they had the motor belt driven to the pinion and then for feedback a few inches down the rack was another pinion connected to an encoder.



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    Another Question about R&P. My X axis is 72" with 200 oz/in steppers nema 23. Someone give me a size of a rack and pinion setup to get started with. I am not sure where to look. Id like to run the pinion direct from the stepper if possible to eliminate another design for the stepper mount. Or another way maby Belt and pully? How about this option and where would I start?
    Thanks Steve



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    Registered massajamesb's Avatar
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    IMHO, I do not suggest direct driving your stepper motors. Steppers seem to work best when run at least 1:3 drive ratio.
    Check with www.Mcmaster.com or Brent Hubbard on Ebay for timing belts and pulleys
    http://stores.ebay.com/HUBBARD-CNC-C...QQftidZ2QQtZkm
    There are several other places that market things of that nature, but I would check with Brent first. Email him and let him know what you are doing, he will get you going.
    Just a spur gear attached to the stepper will only give you the resolution that the stepper and spur gear have. Depending on the diametric pitch of the gear, that determines how far each step moves the machine when direct driven. If you have a 1:3 pulley reduction, then you have much more resolution than before, if the same spur gear is used.
    You will also now have more torque, which you will need when slinging your gantry around. If you make a rapid move and lose steps, where does the torch end up, and how much material does it ruin?

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

    "If the women don't find you handsome, they should at least find you handy"
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    Massajamesb
    I have only seen rack and pinion drives direct with the spring loading.Belt drive reduction seems to be difficult.Do you have any links to pictures of such a drive?
    Thanks Larry



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    Registered massajamesb's Avatar
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    I have a few off my machine and others, but will take a bit to load them

    Last edited by massajamesb; 12-07-2006 at 12:08 AM. Reason: bad link
    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

    "If the women don't find you handsome, they should at least find you handy"
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    Nice setup!

    If you have trouble with the link (err 404) try this:
    http://www.torchmate.com/advantage/construction.htm

    Pres



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