1. #1
    while trailing through the difference between servos and steppers.
    I was wondering about the holding of the X and Y axis.

    Does the X and Y have to have the same holding torque?.

  2. Nope, and holding torque has little relevance for CNC, its dynamic torque that's critical as that what prevents loss of position...

  3. #3
    That answer makes me wonder - given that ballscrews have such low friction and a lead that means that a carriage could drive the screw, what happens to the Z axis when the power goes off the stepper and it loses almost all holding torque? With a heavy spindle plus Z mounting plate, etc., I could imagine the spindle dropping under its own weight. Does this happen in practice? Or does, say, a 1605 ballscrew in conjunction with the permanent magnetism in the stepper manage to stay put when power goes off?

    Just curious!

  4. #4
    Quote Originally Posted by Neale View Post
    That answer makes me wonder - given that ballscrews have such low friction and a lead that means that a carriage could drive the screw, what happens to the Z axis when the power goes off the stepper and it loses almost all holding torque? With a heavy spindle plus Z mounting plate, etc., I could imagine the spindle dropping under its own weight. Does this happen in practice? Or does, say, a 1605 ballscrew in conjunction with the permanent magnetism in the stepper manage to stay put when power goes off?

    Just curious!
    Often they stay put but if heavy then yes they can and do drop when powers off.

  5. The Following User Says Thank You to JAZZCNC For This Useful Post:


  6. #5
    So it would help to have a balanced spindle?

  7. Quote Originally Posted by ramsbury View Post
    So it would help to have a balanced spindle?
    Maybe, but be aware of the tradeoffs. Balancing the spindle doubles the inertial load so reducing the acceleration (for the same dynamic torque from the z-axis motor) which may be an issue for fine 3D profiling.

    Reducing the 'lead' on the z-axis screw to increase the force needed to back-drive it is often the best option. It's one reason why 5mm pitch for Z as against 10mm for X & Y is usually recommended.

  8. #7
    Quote Originally Posted by ramsbury View Post
    So it would help to have a balanced spindle?
    Personaly I wouldn't bother unless you know the weight is going to be quite heavy because it's mostly not an issue. The Mechanical advantage of a 5mm pitch screw combined with the bearings and motors phases creating a slight binding create enough resistance to stop it back driving.

    Like Irving says adding a counter balance can be negative and If you have done your job correctly the motor should be strong enough to drive the Z axis in normal use so doesn't need any assistance. So if you do have heavy Z axis which back drives then a better way is a holding brake.

    The only time you'd really need or want to use a counter balance is if the weight is so much the motor size required would be un-realistic or economical to do.
    For most DIY jobs and esp router based machines it's not needed. Often you'll see it done on Mill's etc but usually it's done because they don't need high accel on there mills and prepared to accept the trade off.!!. . . .OR. . . There just tight arse's and won't stump up for the correct motors to do the job..

  9. #8
    Quote Originally Posted by JAZZCNC View Post
    Personaly I wouldn't bother unless you know the weight is going to be quite heavy because it's mostly not an issue. The Mechanical advantage of a 5mm pitch screw combined with the bearings and motors phases creating a slight binding create enough resistance to stop it back driving.

    Like Irving says adding a counter balance can be negative and If you have done your job correctly the motor should be strong enough to drive the Z axis in normal use so doesn't need any assistance. So if you do have heavy Z axis which back drives then a better way is a holding brake.

    The only time you'd really need or want to use a counter balance is if the weight is so much the motor size required would be un-realistic or economical to do.
    For most DIY jobs and esp router based machines it's not needed. Often you'll see it done on Mill's etc but usually it's done because they don't need high accel on there mills and prepared to accept the trade off.!!. . . .OR. . . There just tight arse's and won't stump up for the correct motors to do the job..

    Motor cost is not an issue, I believe they are a major component. such as Rails and drive whether R&P or Screws.
    the drive motors currently used in a similar commercial machine are 820oz steppers but believe their machines to be Built to a higher standard than I can and rated at 250mm/sec. ,,So I wish to use three 1600oz two on the X using R&P 1 on the Y but not sure about the Z or the gearing for the drives yet. which I believe will probably be 820oz in case I wish to put a Larger spindle in the future im currently looking at 2.2 kw water cooled.

    so counterbalance questions have been Answered,, Not needed and get better Calculations for Motor to screw ratio.

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