4th axis build with Harmonic Drive


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Thread: 4th axis build with Harmonic Drive

  1. #1
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    Default 4th axis build with Harmonic Drive

    I'm starting a 4th axis build for my converted X3 size small mill. After reading up on the topic I've concluded that basing the build on a strain wave gearbox (aka Harmonic Drive) will best meet my needs.

    I've sourced a SHG-25-80-2UH Harmonic Drive off e-bay and intend to power it with a stepper motor driven with the unused 4th axis of the mill's G540 stepper motor control box.

    4th axis build with Harmonic Drive-hd-front-jpg

    Now I need to decide whether to mount the stepper in line axially to direct drive the gearbox via. a coupling running through the hollow bore, similar to this build on Youtube

    4th axis build with Harmonic Drive-direct-drive-section-jpg

    or to mount the stepper parallel to the axis of the HD and drive it with a timing belt pulley arrangement similar to this...

    4th axis build with Harmonic Drive-parallel-belt-drive-jpg

    Math check...
    with a step angle of 1.8° I've got 200 steps per revolution.
    The G540 does automatic micro stepping at slow speeds of 10:1, so that now makes 2,000 steps per revolution.
    The HD ratio of 80:1 bumps that up to 160,000 steps per revolution, or 444 steps per degree rotation.

    That's about the extent of my knowledge thus far....
    Is this adequate resolution for engraving work on objects from 1" tto 6" diameter?
    Is this so high that the 4th axis will creep?

    It seems like the direct drive route would be a more straight forward build, but it sacrifices the hollow bore and the ability to work the ends of longer stock (ends of 1" rod).

    Any help with the theory or experience based design decision making is much appreciated.

    For workholding, I intend to use either a 6.5" 4 jaw (independent jaws) lathe chuck for rectangular stock or an ER40 collet chuck for bar stock.

    One additional point, I have a 40" x 24" CNC router that I may also try to use this 4th axis on with a tail stock. The router, however, is driven with the CNC Router Parts control box and the 4h driver is already being used for the slaved Y-axis stepper.... so I'd have to piggy back another driver onto the router to run a 4th axis.

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  2. #2
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    Default Re: 4th axis build with Harmonic Drive

    I'd go with the timing belt system; it seems easier to build, and like you say, it leaves the bore unobstructed. It seems like you should have resolution to spare. It's not going to go very fast, but you're not using it as a lathe. The chuck seems a little large - I'd wait until I'd mounted the 4th axis to see how much clearance there was top and bottom before settling on a chuck. Remember, for 3D carving, the tool has to be able to reach the middle of the rotary axis and also clear the widest workpiece you mount in there, although if you're just engraving on the outside of a cylinder you can use a shorter tool.

    [FONT=Verdana]Andrew Werby[/FONT]
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    Default Re: 4th axis build with Harmonic Drive



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4th axis build with Harmonic Drive

4th axis build with Harmonic Drive