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  1. #1
    I’m putting together my first R&P drive machine and i’ve got a couple of questions about the spur gear size and type I’m using. As the little 15T spur gears I have on at the moment have to do so much work and i’m unfamiliar with R&P seems logical to ask you guys if this is correct.

    Q1. I am wondering if 15 tooth is a bit small and whether to optimise the set up with something larger. Should I be use a larger size, say 24t - 30t?

    Q2: I’ve just noticed that spur gears can be stated as things like ‘Straight Teeth Positive’ or ‘right teeth positive’. Obviously my machine runs back and forth, which should I be using?

    Q3: Perhaps I am supposed to use a particular type of pinion with the rack I have, which is not the chinese ones I currently have on?

    Q4: Does anybody know what spring tension I should be applying to hold the pinion in the rack?

    Any advice would be most useful. Thanks

    CNC working area 1800 x 1500 x 500mm
    4nm steppers with 4:1 gearing, 48v psu
    1/10th microstepping
    10mm drive shafts
    Rack: SR10/15/1 1 mod, C45 Soft Long Steel EN8 Gear Racking, Quality 9
    Pinion: MOD1 15T
    build log

    stepper gearing pics
    Click image for larger version. 

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    Click image for larger version. 

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    Last edited by marbles; 09-09-2018 at 06:05 PM.

  2. #2
    1) The larger the pinion, the smoother the motion. But as pinion size increases, you're torque (and force) will decrease. Most people use a pinion diameter of about 25mm, but bigger is actually better.

    2&3 ) I've never heard of those descriptions.

    4) Pinions are not really supposed to be spring loaded into the rack. It's done this way on CNC machines because it's easy, and it removes most backlash. Ideally, the rack should be mounted to a precision machined surface, parallel to the linear rails, and the pinion should be rigidly mounted at the proper depth of engagement.

    If you are going to spring load it, then it should be just enough pressure to keep the pinion from jumping out of the rack. And more will just increase the rate of wear.
    UCCNC 2022 Screenset

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    JointCAM - CAM for Woodworking Joints

  3. #3
    Thanks Gerry,

    I think i’ll increase that pinion size from 15t to something like 30t. I would have really liked to have just mounted the pinion at the proper depth of engagement but my machining skills and equipment aren’t up to that yet. Luckily there are plenty of examples of folk using spring tensioners. I’ll just need to experiment until they seem right.

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