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  1. #1
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    Question Belt driven screw ?

    Hello everyone,
    My name is Tom and this is my first post, I've been reading and
    looking for a couple months now, this site is awsome I've learned
    so much here and been inspired to build my own machine!
    The z axis and gantry are nearing completion, being built of steel
    and aluminum with many cast parts quite heavy, I'm guessing 125lbs.
    Using 640oz motors from campbell designs all axis will be belt driven,
    the drive screws are 1/2 8 double start acmes. HERE'S my question>
    Should I use a 1 to 1 pulley config or 1 to 2 small pulley on motor
    larger on screw or other I'm STUCK here! ( gecko 202s & mach 3)
    Any help here would be greatly apreciated !!!!!
    Sincerely Tom

  2. #2
    Community Moderator ger21's Avatar
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    I'd start at 1:1. If you need more speed or power, then adjust accordingly.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

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

  3. #3
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    If precision of any importance then calculate the best gear ration for your table.

    Konstantin.

  4. #4
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    Like Ger said, start with 1:1.

    What you end up with will depend on the friction in your linear motion guides, rigidity and accurate setup of all the machine components including linear motion guides, screw end bearings, motors, and pulleys (to prevent binding), and the voltage of your power supply.

    125lbs is not that much weight. I've managed to get a tiny 250-oz NEMA23 stepper to move that much weight in a horizontal orientation at 550 IPM with a 5/8"x0.200 ballscrew, THK SR20 linear rails, 48v power supply, and Gecko G201 (1:2 pulley ratio). I probably could've gone faster but I was already peaking the parallel port output at 45kHz, and the G201 is fixed at 10 microsteps. Will resume tests later when I get a G100.

  5. #5
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    Do you really need that much speed???? If 1:1 will work ok at lower speeds (if you need low speed) Great. The less moving parts the better for adjustment & maintenance. At 8 TPI on your lead screw 1:1 is going to make for a near light speed movement at top speeds.

    Assuming you are using steppers They will need to ramp up & down slowly for rapids or possibly loose steps getting there.

    It all boils down to what you want/need from your system.
    If it works.....Don't fix it!

  6. #6
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    Light speed? Not quite. 1:1 with an 8tpi screw, Gecko G202 10 microstep drive, and Mach 3 -->

    60 seconds * 45000hz / (8tpi * 10 usteps * 200 steps/rev) = 168.75ipm

    The power supply is important. A 24v power supply may result in stalling at high speed, whereas a 48v power supply will allow the gantry to muscle through anything and everything in its path.

  7. #7
    Community Moderator ger21's Avatar
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    It's a 2 start screw, so double that speed.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

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

  8. #8
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    Ah yes, you're right.

    Still slow though.

  9. #9
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    HI Guys, I've gut a cnc, the same - made of steel and aluminium having a gantry weighing 30kg (I don't know how that adds up in lbs sorry) and each axis driven with a 400oz/in servo motor and pulley ratio 1:5. In my opinion thing that it can be set to 1:3 the least, naturally depending on the motor rpm.
    Martin G

  10. #10
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    Question 4 turns to move an inch

    Thanks for the input so far its nice to be able to share my
    hobby with interested parties!

    By my calculations it will take roughly 32 turns of the screw
    to move the gantry 130 inches, leadscrew whip should never be an
    issue, at 1to1 1 motor rev would move things 1/4 inch 1/2 a
    motor rev would move things 1/8 and so on.
    So for any detail would i be asking to much of the motors at
    1to1 no accel or decell room, thats why i'm wondering if it would
    be better to have the motors spin faster than the screw, smaller
    pulley on the motor larger on the screw @ 1to2 the motor would
    spin 8 times to move an inch and one motor rev to move 1/8 ?
    I don't have experience with these 640oz motors nor any other
    steppers, don't know what to expect from them or how they'll
    perform best.
    Just for clarification, many agonizing hrs spent so far setting up
    and machining these parts to assure paralellism, so far it's paid off
    everything moves freely without any deviations ...whew!
    Don't mind the lathe but changing setups on the mill is like pulling
    teeth!!! thanks again Tom...

  11. #11
    Community Moderator ger21's Avatar
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    By spinning the motor twice as fast, it'll probably have about half as much power at the higher rpm. With a 1:1 ratio, you'll get a resolution of .00125 per step, which should be plenty fine for a router.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

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

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