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  1. #11
    Looks like your motors are rated for 400W @ 3000rpm, so 1.27Nm. If you calculate for 15T pulleys on the motors and 30T on the ballscrews using the following specifications:

    Mass of axis in kg: [x y z]=[25 25 20] (X-axis mass halved as two motors)
    Cutting force in N: [x y z]=[50 50 250]
    Acceleration in m/s^2: [x y z]=[2 2 2] (changing this makes a big difference)
    Ballscrews: [x y z]= RM2010
    Ballscrew length in m: [x y z]=[1.5 1 0.35]

    Yields the following:

    Feedrate in m/min: [x y z]=[15.0 15.0 7.50]
    Inertia in g-m^2: [x y z]=[0.12 0.11 0.07]
    Torque in Nm: [x y z]=[0.38 0.34 0.29] (X torque 0.80Nm if gantry mass=0)
    Inertia ratios: [x y z]=[3.65 3.18 2.00]

    If you instead used 20:60T pulleys on all axes:

    Feedrate in m/min: [x y z]=[10.0 10.0 5.00]
    Inertia in g-m^2: [x y z]=[0.24 0.23 0.22]
    Torque in Nm: [x y z]=[1.03 1.00 0.97]
    Inertia ratios: [x y z]=[7.24 7.03 6.50]

    So as you can see, even though at a glance you may think changing the ratio would put less load on the motors, you'd actually more than double the torque requirement by using such large pulleys and at the same time get a lower feedrate. This is because most of the torque goes into accelerating the pulleys and ballscrews, so just to illustrate the point about the gantry mass, here's what happens if you use 15:30T and increase the mass of the gantry to something silly like 1000kg:

    Mass of axis in kg: [x y z]=[500 250 250] (X-axis mass halved as two motors)

    Feedrate in m/min: [x y z]=[15.0 15.0 7.50]
    Inertia in g-m^2: [x y z]=[0.42 0.22 0.09]
    Torque in Nm: [x y z]=[1.23 0.65 0.37]
    Inertia ratios: [x y z]=[12.73 6.53 2.86]


    etc...

    In other words, your motors are a jot bigger than necessary!

    [References:
    http://www.nskamericas.com/cps/rde/x...ive_Torque.pdf
    ttp://www.hiwin.com/pdf/bs/ballscrews.pdf]
    Last edited by Jonathan; 31-03-2014 at 01:46 AM. Reason: Don't trust my numbers after midnight.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

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