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  1. #1
    Playing devils advocate there and like some opinions for and against with the two methods described below.

    Both methods will rely on a tranverse mounted stepper motor at the rear, this will drive two pulleys, one either side via jack shafts.
    All pulleys will be well supported on bearings.

    Two lengths of open belting will pass from these pulleys to twin pulleys on the front of the machine and the open ends will be secured to each side of the two uprights.

    Basically open belt drive to replace two ballscrews and this will be over a 1 metre length at max, probably more like 800mm.

    Now the different design points.

    Plan A will have one end of the belt secured on a fixed bracket and the other on a bracket that can be adjusted on a turn buckle.

    Plan B will have both ends fastened to the same fixed bracket and adjustment will be handled by independently spring loading the front idler sprockets.

    Thoughts on this please ?
    John S -

  2. #2
    Plan A for me John.? Much simpler and just has affective.

    Is this to move a gantry for a router.?

  3. #3
    Yes moving gantry design
    John S -

  4. #4
    I'm just about to try out open belt. I went for Plan B, I pre-tension springs with circlips then run a bolt in to connect the tension roller to the spring.

    The idea is that I can slacken the belts when not in use then set them by taking up the slack plus 2 turns.

    I'm using 16mm T5 steel reinforced belting. I hope to find out how stretchy it is tomorrow, will 2 turns be enough?
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  5. #5
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 5 Hours Ago Has been a member for 9-10 years. Has a total post count of 1,833. Received thanks 192 times, giving thanks to others 5 times.
    I'd say plan A, provided it's adjusted correctly.
    However plan B would allow for a bit of idiot proofness if anybody happens to go tinkering with belt tensions.

  6. #6
    Quote Originally Posted by John S View Post
    Yes moving gantry design
    Well couple years ago built a belt machine for a friend in Ireland have a look here. New Machine Build Belt Driven extrusion framed wee beasty!! - CNCzone.com-The Largest Machinist Community on the net!

    Built from scavenged bits with a few new. Check vids in post #5 and you'll see it wizzing about.!!
    In the Vid with moving gantry it was running 3:1 ratio and that wasn't enough so ended up at 6:1. @ 3:1 the motors weren't happy at low speeds.
    I was restricted with room to use larger pulleys and just happened to have a spare belt and pulleys so thats why it ended up using the setup it did. . . Not ideal but got the job done.!!

    The point telling this was to make you aware of the ratio your likely to need for smooth machine at low feeds.

  7. #7
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 5 Hours Ago Has been a member for 9-10 years. Has a total post count of 1,833. Received thanks 192 times, giving thanks to others 5 times.
    Quote Originally Posted by Robin Hewitt View Post
    The idea is that I can slacken the belts when not in use then set them by taking up the slack plus 2 turns.
    Why?
    Modern belts don't stretch.

  8. #8
    Quote Originally Posted by m_c View Post
    Why?
    Modern belts don't stretch.
    Everything stretches - it's a matter of how much, which in this case depends on things such as the width of the belt and type of tension members in the belt.

    Page 7 of this document has a graph to show how they stretch:

    http://www.gates.com/facts/documents/Gf000289.pdf

    If you're using the belt on a machine with low or zero cutting forces (e.g. laser cutter), then the gantry can be light so the force should be small enough for this not to be an issue. However if it's for a router as the title suggests I would do the calculation to find out how much the belt will stretch, or at the very least use the widest belt you can fit.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  9. #9
    Quote Originally Posted by m_c View Post
    Why?
    Modern belts don't stretch.
    Belts may be modern but I'm not

    If the belt doesn't change it's elongation over time when left in tension that could save me a very inconvenient switch. I was thinking to measure the length of the bed in stepper resolution at start up then scale the G code to match.

    This is kind of crucial because I have designed for a 3/4" aluminium tube inside the gantry which carries the head control wiring to the flexi cable conduit that connects to the head.

    If the belt needs constant calibration this tube goes full length to get a switch at the far end.

    If the belt doesn't need constant calibration the tube only goes half way and the fitting is different.

    It will be a pig to fix if I get it wrong.

    This is the last part before I can fit the long belt, so what do I do? Full width or half way?

  10. #10
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 5 Hours Ago Has been a member for 9-10 years. Has a total post count of 1,833. Received thanks 192 times, giving thanks to others 5 times.
    You don't have to retension timing belts on cars, and the latest ones last 150,000miles or 10 years with no maintenance.
    And given quite a few run with fixed tensioners, stretch isn't a concern.

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