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rack& pinion

Discussion in 'General Talk' started by peter28, Dec 25, 2016.

  1. peter28

    peter28 Well-Known
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    Hello,

    i am thinking about building a cnc router but i don't want to use the belt system.
    because i want to go 1500mm x 1000 mm and mayby in the future longer 3000mmx1000 mm
    So i will go with the rack & pinion system.
    Now the questions i have is:

    Can i mount the gears directly to the motor and to the pinion ?
    Do i need to use a reduction?
    i saw that cnc routerparts sels them but i live in euro so importing them can be very expencive, do you know of a nother supplier in the euro or UK that sels something of the same?
     
  2. Jen

    Jen Veteran
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    These guys in Dorset sell rack and pinion gears in the U.K. In length of up to three meters that I am aware of: http://www.cross-morse.co.uk/gears.asp
    As far as the question about gearing directly from your steppers is concerned - without knowing if you are building from scratch I would say yes you can go direct but only if the plates you mount the steppers on are made for the size of the gears you want to employ.

    Merry Christmas!
     
  3. Joe Santarsiero

    Joe Santarsiero OB addict
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    You might get some ideas for the drive system from the following build: Area 51 CNC Parts F-117 CNC Gear Rack Drive System
     
  4. peter28

    peter28 Well-Known
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    He uses no reduction ?
    Strange are the powers of the motirs not to much for the r&p etc?
     
  5. Jen

    Jen Veteran
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    Peter,

    I'm not sure what you mean about a reduction.

    I think you need to think in terms of gear ratio.
    How many teeth on your rack per unit of measure vs how many teeth the gear on the motor has. Next take into consideration how many "steps" per revolution you need, then factor in the microstepping your interface supports and set that to give you the movement rate you desire.

    I would not worry about torque unless you intend to do some super heavy machining requiring deep passes over very dense materials, like castings.

    You do not mention your intended application. I am assuming you are seeking some pretty high precision.

    I recently built a belt driven OX from ooznest and am just finishing the process of setting it up. Like you I was skeptical about the "slop" that may be an issue with belt drive, but it proved not to be much of a factor for my application, which is musical instrument making, ukuleles and guitars, including some pretty intense shell inlay cutting and pocketing.

    My machine is 1000 x 750mm. I have it tweaked so that there is very little backlash at all, well within .20 mm which is a skosh over .007". I think I can even do better once I get everything loosened up.

    I hope this helps,
    Jen
     
  6. peter28

    peter28 Well-Known
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    i am going to build a cnc for cabinets and i will need the machine route melamine MDF corian HPL

    i would also like some speed.


    thats why i would like to go with rack& pinoin.
    like this set very nice but from usa so inporting can be very exprensive i think


    Standard Rack and Pinion Drive, NEMA 23 | CNCRouterParts

    + my shop is now about 5 ┬░ but when i go to work i use the gasheater and heat the shop very fast to about 18 ┬░.
    don't those high difference case the belt to stretch?

    Can i use any time of motor then directly on the rack& pinion or not?
    i was thinking of buying the motors here
    Nema 23 Stepper Motor - OpenBuilds

    are they the right once for a direct gear drive?
     
  7. Jen

    Jen Veteran
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    Hi Peter,

    I think that the NEMA 23's would work fine for you. Those are the same steppers that I am using. They allow me to obtain a very good amount of speed when cutting materials that allow it. The main factors for deterring feed speed is the power of the spindle you use, the diameter of the bit, and the material you are cutting. I am simply using a DeWalt D26200 router for my spindle and can cut the thin hardwoods I use very quickly.

    You are a cabinet maker, so think about how fast you can move a router by hand through the material you have in mind. The speed you can safely get with your CNC router will be similar, allowing for problems with inertia on complex shapes. You determine your feed speeds via the software you use to define your tool paths.

    For my applications I often use very small bits, like 1/2 or 1mm cutting hard abalone shell, so it can be quite slow. When I cut my tone woods for tops and bottoms it will be quite fast. For necks and fretboards it is somewhere in between.

    Remember whatever type of machine you choose you will determine the feed rates via software. For cabinet making hi precision is not necessary, so any type,rack and pinion, ball screw, or belt driven should be fine for you.

    You could employ heavier steppers, but if you should choose to, you would be increasing the cost of hardware like controller boards, power supplies and etc. as well.
    So getting back to your original questions:
    Yes you can mount the motors directly and you can do it without a gear reduction.

    Cheers!
    Jen
     
    GrayUK likes this.
  8. Jen

    Jen Veteran
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    I just noticed you had a question about shop temperature effecting belts. I would think it will have very little effect as the belts are made with high tensile strength fibres that have very little give in them. Something like Kevlar I imagine. Like the stuff timing belts on an automobile are made of.
    They should be able to withstand a very large temperature swing with negligible size variation. I would think that a solid two meter long rack gear would have a larger size change.
    I am in no way saying any one system is superior to another. Remember we are talking about a router to work with wood, not a high precision milling machine that needs to hold tolerances to a couple of tenths of a thousandth of an inch.
    Personally I think a ball screw setup is the most accurate. However hi precision ball screw systems could easily triple the cost of a machine and would not make enough of an impact on your application to justify the additional cost or complexity.
     
  9. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    Where this really becomes critical is when steel rack is rigidly attached to aluminum rail. The coefficient of expansion for aluminum is roughly twice that for steel and across substantial temperature variations could cause bowing in the rails.
     

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