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  1. #1
    Just wondering if anyone has thought about spinning the Ball screw nut, keeping the Screw itself immobile? You wouldn't get the Screw whipping etc. There has to be a downside but I cannot see it ...
    Tim G-C

    I disapprove of what you say, but I will defend to the death your right to say it.

    (attrib. Voltaire but written by Evelyn Beatrice Hall "The Friends of Voltaire" 1906)

  2. Quote Originally Posted by Wobblybootie View Post
    Just wondering if anyone has thought about spinning the Ball screw nut, keeping the Screw itself immobile? You wouldn't get the Screw whipping etc. There has to be a downside but I cannot see it ...
    Yes its been done before and it works well.. and the downsides are:
    1/ the motor driving the ballnut is on the moving assembly so adds to the weight needed to be moved
    2/ the ballnut has to be driven via a timing belt or similar arrangement which just makes things more complex...
    Last edited by Lee Roberts; 28-12-2009 at 01:28 AM. Reason: Spelling der !!!

  3. #3
    While it will work, it actually adds a lot of cost to it.
    As mentioned you have a larger mass to move due to mtor motor moving with the nut, you also have to design the gear mechanism with a timing belt and pulleys and mounting for the motor.
    When you could just buy a thicker crew with a larger lead, or use rack.

  4. #4
    3/ Wiring would also need to be considered for a moving assembly, making things a little more complex as well (snagging).

  5. #5
    OK Thanks ... It was just a thought ... I will keep it in mind though, not sure why but just store it away for future reference.
    Tim G-C

    I disapprove of what you say, but I will defend to the death your right to say it.

    (attrib. Voltaire but written by Evelyn Beatrice Hall "The Friends of Voltaire" 1906)

  6. #6
    Modern lasers do this, they call it spindle drive.
    Large commercial lasers moved away from ballscrews some while ago and went on to precision rack and pinion because of the whipping over long lengths and by precision I mean $$$$$
    Usually two large pancake servo motors driving split loaded pinions and complex electronics to stop them crabbing.

    Now they have moved to spindle drive where they have two massive end castings / fabrications where a large , typically 70 to 90mm ball screw is held stationary in tension of about 20 tonnes and the ball but is driven by a 7 hp servo motor.

    Because laser cutting is non contact they can reach insane speeds.

    It's not a new idea, the old Bridgeport BOSS series mills that appear on Ebay with the big finned stepper motors and squarish table did the same on the X axis. Nut is in bearings on the knee driven by belt from the stepper, it's only an inverse Z axis after all.

    .
    John S -

  7. #7
    I would think you could get away with smaller ballscrews as they will be in tension and no whip plusthe ball nut will be smaller as well.

    Not sure where the weight part comes in as most of the weight will be on the track bearings.

    peter

  8. #8
    I can see the hangups with over complications but as for the extra weight I would have thought in a way that could be a plus point after all weight = mass and mass is good ... right??
    Tim G-C

    I disapprove of what you say, but I will defend to the death your right to say it.

    (attrib. Voltaire but written by Evelyn Beatrice Hall "The Friends of Voltaire" 1906)

  9. #9
    Fixed mass is good, heavy bed and rails etc, moving mass is bad, takes force to stop it.

    .
    John S -

  10. Quote Originally Posted by Wobblybootie View Post
    I can see the hangups with over complications but as for the extra weight I would have thought in a way that could be a plus point after all weight = mass and mass is good ... right??
    not when you have to accelerate it... inertia plays a part.. however there are 2 sides to the story...

    If this was the X-axis the motor has been added to the gantry mass, but since we are not having to accelerate the inertia of a big screw, just the nut the motor can be smaller. Since on bigger machines the inertia of the large screw rather than the gantry is often the overriding factor in deciding motor torque there may be net benefit in terms of a smaller motor requirement.

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