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  1. #11
    Quote Originally Posted by silyavski View Post
    Dean,
    i have always wanted to ask that question, hope you know the answer- if the motors that drive the gantry don't drive it directly but with pulleys 1:1 ,if the gantry is racked by chance, wouldn't the belt give away and just start skipping instead of destructing something?
    Well yes and no dependant on motor type used.?
    With steppers In reality what happens if one motor stalls is the other keeps going until the other motor stalls, by which time a number of things can happen depending on speed it was travelling at time. Often motors usually stall at higher feeds has that's where torque is lowest and machine incorrectly tuned for high velocity and high Acceleration just to compound the matter.!

    Now often when first motor stalls due to low torque the other motor isn't far behind on available torque so it doesn't take much to bring both to a halt but that's not what causes the damage.? . . . . It's the 50-60Kg gantry travelling at 150mm/sec (9mtr/min Av rapid speed) just at one side which does the damage.!! . . . Doesn't matter how fast you are at reacting to seeing the motor stall you can't stop the machine fast enough and in the few seconds it takes to hit the "Oh shite" button before one side has travelled 300+mm further than the other side.!! . . . I'll leave the rest to your imagination has to what can be damaged or screwed up.

    Now if you were using Servo motors then yes possibility it would strip the belt teeth but chances are it wouldn't stall either and the drive would detect any following error and E-stop anyway.

    So the moral of the story is when slaving either be conservative on the tuning, use drives that detect stalling or use a different approach.?
    I've done all 3 and my preference is single motor with belts connecting screws but I'm also ok with slaving you just have to be aware of the limits and this is why I like drives that help and take some of the risk away.!
    Last edited by JAZZCNC; 05-11-2013 at 07:06 PM.

  2. #12
    Hi everyone,
    I'm back after some time...
    First of all let me wish you and you beloved relatives all the best possible for this new starting year. May 2014 be placed under new creative spirit, and wonderfull designs (as usual !).

    I have taken some time to digest all the valuable informations posted on this thread and finally started the construction after some revisions on my design.
    The parts of the main gantry beam (supporting Y and Z axis) have been welded together and the tube sections filled with vibration damping material. It has been coated with epoxy paint for durability and ease of cleaning and it is now ready for machining to generate the reference planes and associated straight edges. The overal weight is now 17Kg.
    Some pictures:
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    As you can see, I've finally chosen to drive the X axis with its 2 ballscrew sets with one single stepper plus HTD 5M-25mm timing belt and 24 tooth pulleys. The belt tension device is not yet defined.
    The beams will also be filled with antivibration material compound.
    The rails supporting X axis will be leveled using epoxy technique.

    There will be a cover for the whole Z axis assembly and an other one for the front belt drive system.

    Need your advice for the axis end sensors: Do I need 2 sensors for each axis or only one sensor activated by 2 separate fingers (one at each end) ? What are you using on your own machines ?

    Looking for your comments.

    Philippe
    Last edited by idefix; 04-01-2014 at 09:08 AM. Reason: addition

  3. #13
    Nice development.

    To be honest i have similar size machine and though i have bought the sensors/the smallest mechanical ones, that people around forums say work fine/, i have never mounted them. May be others will disagree but for this size machine, driven by steppers i don't see a reason to put sensors at all. I use the soft limits and with some care am ok. The worst that could happen is for the motor to stall at one end of the axis. Yeah, it happens some times and nothing breaks.

  4. #14
    Quote Originally Posted by idefix View Post
    Need your advice for the axis end sensors: Do I need 2 sensors for each axis or only one sensor activated by 2 separate fingers (one at each end) ? What are you using on your own machines ?

    Looking for your comments.

    Philippe
    For the limit switches I use 1 x Proximity switch per axis that travels with the axis and is triggered at each end with adustable target. The switches are PNP NO(normally open) and wired in parallel sharing just one input.

    Home switches are Fixed and Use proximity.

  5. #15
    The CAD above is beautiful.

  6. The Following User Says Thank You to TonyD For This Useful Post:


  7. #16
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 3 Hours Ago Has been a member for 5-6 years. Has a total post count of 1,168. Received thanks 212 times, giving thanks to others 5 times.
    Philippe,

    I'm designing a machine which is similar to yours, although slightly larger capacity, and trying to use as many ideas discussed in this forum as possible.

    I am working on the Z carriage design and, as you said earlier, trying to keep the moving and fixed plates as close as possible. I cannot see from your CAD drawings how you have managed to fit the ballscrew nut in between the plates. The nut is wider than the profile rail + bearing height - have you put packing under the bearings to increase the space? I can see that you have machined steps into the plates and a recess where the ballscrew nut is fixed.

  8. #17
    Hi Neale,
    I'm happy to share related information back with you.
    I have broken the design so that you can see the area as detailled as possible. Let me know if this is what you are looking for.
    As a comment note that for the Z axis my rails are type HGR15 with Hiwin HGW15CA blocks and the screw is a 1605.

    Distance from Gantry plate to Z plate (plane supporting the blocks) is 24mm

    Click image for larger version. 

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    Last edited by idefix; 09-01-2014 at 08:18 AM. Reason: add picture

  9. #18
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 3 Hours Ago Has been a member for 5-6 years. Has a total post count of 1,168. Received thanks 212 times, giving thanks to others 5 times.
    Many thanks for the details. I had started with 12mm plates but I think that you are using 20mm which allows you to machine recesses for extra clearance, and there are also packing pieces under the rails. Back to the drawing board!

    Are you using something like Solidworks for the drawings? The drawings of slides, ballnuts and so on are very detailed and look as if they might have come from the manufacturer!

  10. #19
    Yes Neal, using SW for the drawings with some parts already loaded from the web such as rails, blocks and M4 and M6 screws plus the timing belt pulleys. All the rest is own design.

    I do confirm that base material thickness for the plates is 20mm (initialy for ridgidity purpose) but helps making the cavities...

  11. #20
    For a small gain in strength and machining time, you should be able to make the recesses in the Z-axis not cover the whole length, as the ballnut doesn't traverse the whole rail.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

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