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  1. #1
    I'm about to start a second machine build. A 3m x 2m for general materials, wood, polystyrene and occasional plasma. Sort of multipurpose.

    I'm going to upgrade from T5 belt to rack and pinion but i'm not sure what is the smallest rack size I can get away with is? This machine will get used maybe once a week but for several hours at a time. I'll be gearing between the stepper and pinion as well.

    Any suggestions or examples anyone?

  2. #2
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 7 Hours Ago Has been a member for 9-10 years. Has a total post count of 1,832. Received thanks 192 times, giving thanks to others 5 times.
    Having just looked at the actual size diagrams in my HPC gears catalogue, I'd probably pick something in the 2-3MOD range.

    The smaller you go, the more noticeable any contamination will be, and the larger you go, the more the non-linear motion from straight cut spur gears become.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  3. #3
    Marbles, if you're looking at their catalogue, HPC gears do some nice anti-backlash gears BTW ;-)
    You think that's too expensive? You're not a Model Engineer are you? :D

  4. #4
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 7 Hours Ago Has been a member for 9-10 years. Has a total post count of 1,832. Received thanks 192 times, giving thanks to others 5 times.
    Anti-backlash gears don't solve the non-linear issue though. You'd have to use helical rack/gears for that ;-)
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  5. #5
    Quote Originally Posted by m_c View Post
    Anti-backlash gears don't solve the non-linear issue though. You'd have to use helical rack/gears for that ;-)
    Having recently designed a machine with rack and pinion, where the requirement was speed as opposed to smooth running/accuracy, I would suggest you go with the smallest economical one you can. Nonlinearity will be a real concern with anything larger than 2.

    To avoid contamination, mount the rack, with the teeth pointing down

    For a visual comparison:

    https://emvioeng.com
    Machine tools and 3D printing supplies. Expanding constantly.

  6. #6
    Thanks for all thoughts

    I'm beginning to think that if I was just cutting large blocks of foam or using as plasma i'd stick to a belt drive system because of the speed and accuracy. Not entirely sure why more people don't use belt. I'm guessing its just that its less well known as a tech and maybe is not so good over long distances.

    However as I do wish to cut timber and make up a 3m x axis R&P and has history so is probably the way to go. How about helical rack/gears and probably 2.0 MOD with a gear ratio to suit?

    What ratio required to suit high speed roughing cuts in polystyrene and and also give some repeat accuracy in timber is the next thing to sort out.

  7. #7
    Quote Originally Posted by m_c View Post
    Anti-backlash gears don't solve the non-linear issue though. You'd have to use helical rack/gears for that ;-)
    Yes, it's kind of in the name ;-)
    You think that's too expensive? You're not a Model Engineer are you? :D

  8. #8
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 7 Hours Ago Has been a member for 9-10 years. Has a total post count of 1,832. Received thanks 192 times, giving thanks to others 5 times.
    The issue with belts over longer distance, is you get into resonance/flex issues. You can minimise it by lots of tension, but then the design of what they're tensioned between becomes critical, and you have to make sure everything under tension is strong enough.

    There is that system that is essentially a rack the belt sits/locates in, with the belt looping up and over idler and drive pulleys, which eliminates having a long unsupported belt, however from what I remember it was more expensive than other options.

    It's like most things, you have to weigh up the pros/cons of each method, and decide what's the best compromise for you.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  9. #9
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 7 Hours Ago Has been a member for 9-10 years. Has a total post count of 1,832. Received thanks 192 times, giving thanks to others 5 times.
    Quote Originally Posted by magicniner View Post
    Yes, it's kind of in the name ;-)
    And they're not really a good solution for large CNC machines. Any anti-backlash gears I've looked at, the backlash torque is never high enough that the forces on CNC won't be able to over power it.
    A quick check of the HTC catalogue I've got, lists the torque for a 50t 2 MOD gear at 2.5Nm. I'm pretty sure a decent cut could manage to overcome that, and on high speed machines, I'm sure the accelerations involved could manage to do the same.

    And that's before you consider the additional friction they'd introduce into the system, which depending on the machine, may or may not have any significant effect.

    That's not to say they wouldn't help, but you have to weigh up the pros/cons of using them.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  10. #10
    Quote Originally Posted by komatias View Post
    Having recently designed a machine with rack and pinion, where the requirement was speed as opposed to smooth running/accuracy, I would suggest you go with the smallest economical one you can. Nonlinearity will be a real concern with anything larger than 2.

    To avoid contamination, mount the rack, with the teeth pointing down

    For a visual comparison:



    I'm planing a R&P router myself and would like to ask two things?? What do you mean by contamination? And, When you say mount the rack, do you mean something like this?

    Click image for larger version. 

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    Sorry if the image is small.

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