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IndustryArena Forum > WoodWorking Machines > DIY CNC Router Table Machines > Anyone here tried the cheap Chinese harmonic drive 4th and 5th axis off ebay?
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  1. #1
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    Anyone here tried the cheap Chinese harmonic drive 4th and 5th axis off ebay?

    Last time I asked about cheap Chinese rotary axis for diy builds there wasn't anything decent on the market in that "affordable for hobby" category. Recently I've noticed some new options that utilize a harmonic drive.

    For example:

    https://www.ebay.com/itm/19289023686...cAAOSwGJ5cigkc

    And

    https://www.ebay.com/itm/22451853762...oAAOSw-iNg3sbw

    And

    https://www.ebay.com/itm/15440777564...0AAOSw67hgcA4C

    In theory, using a 50:1 harmonic gear solves the issues found on previous belt drive and worm gear versions. I'm wondering how they are in practice?

    I.e. Do they operate backlash free and with enough torque and precision for continuous 4 axis or even 3+1 aluminum milling?

  2. #2
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    Re: Anyone here tried the cheap Chinese harmonic drive 4th and 5th axis off ebay?

    I have the harmonic 4th. Not the 5th. It would great. Amazing holding power and I've never had an issue with losing steps. I added an endstop too. It's a great addition

    Sent from my SM-N960U using Tapatalk

  3. #3
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    Re: Anyone here tried the cheap Chinese harmonic drive 4th and 5th axis off ebay?

    Hi,
    the only problem with harmonic drives is the deep reduction, 50:1 is the typical minimum reduction, great for high torque but SLOW as a wet week.
    Cycloidal gear reducers work well at ratios around 20:1.

    When I built my mini-mill seven years ago I use low lash (<2 arcmin) planetary gearboxes of 10:1. They have been superb. They allow very modest steppers to have
    GREAT torque, but only at the expense of speed. To be honest I've never been able to measure or even notice the lash. So I would say that any of the cheap Chinese
    made planetary gearboxes of less than 15 arcmin lash would be fine for hobby purposes, and they are available at ratios of 3:1, 5:1, 10:1 etc. Over 10:1 then your into
    a two stage reduction and increase in lash.

    Craig

  4. #4
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    Re: Anyone here tried the cheap Chinese harmonic drive 4th and 5th axis off ebay?

    The feedrate is reduced and if too many steps per min are sent it will not move correctly. Off hand, I cannot remember the actual said mine moves but, I can still cut at around 200ipm. Depending on stock used, obviously.
    I'm not using turning tools, that's impossible. But, as an accurate and strong indexer, it's great. Continuous machining is possible using fusion cam now too.

    Here is some trim I made for a kitchen window. I first used aspire and realized it's no good unless doing very simple rotary stuff. It so has to be wrapped if you do, so no pockets come out good. You could make multiple files to accomplish it.
    In fusion you just create a new sub setup and change the axis to reflect what side you're using. Then you can travel in xyz to do pockets and clearing that don't end up using the side of your tool and leave a bunch of ridges everywhere.

    One picture is showing that the left side one was done with aspire. The other with fusion. The aspire one looks like garbage.

    Sent from my SM-N960U using Tapatalk

  5. #5
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    Re: Anyone here tried the cheap Chinese harmonic drive 4th and 5th axis off ebay?

    Quote Originally Posted by ubergeekseven View Post
    I have the harmonic 4th. Not the 5th. It would great. Amazing holding power and I've never had an issue with losing steps. I added an endstop too. It's a great addition

    Sent from my SM-N960U using Tapatalk

    What size motor are you using with the harmonic drive? And have you used it for any aluminum parts?

    I'm interested in where the limitations are re part size when working with aluminum. Could you, for example, use your 4th axis to make a 2" x 6" x 20" part?

  6. #6
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    Re: Anyone here tried the cheap Chinese harmonic drive 4th and 5th axis off ebay?

    Quote Originally Posted by joeavaerage View Post
    Hi,
    the only problem with harmonic drives is the deep reduction, 50:1 is the typical minimum reduction, great for high torque but SLOW as a wet week.
    Cycloidal gear reducers work well at ratios around 20:1.

    When I built my mini-mill seven years ago I use low lash (<2 arcmin) planetary gearboxes of 10:1. They have been superb. They allow very modest steppers to have
    GREAT torque, but only at the expense of speed. To be honest I've never been able to measure or even notice the lash. So I would say that any of the cheap Chinese
    made planetary gearboxes of less than 15 arcmin lash would be fine for hobby purposes, and they are available at ratios of 3:1, 5:1, 10:1 etc. Over 10:1 then your into
    a two stage reduction and increase in lash.

    Craig
    I have a large precision cycloidal gear from a Fanuc robot arm. It has a 10" turning face weighs a ton. These can also have higher ratios like harmonic drives -especially for the larger ones. Mine is 121:1.

    My one is rated at something like 1 arcmin backlash with is negligible for a 100+:1 gear. I think I was just too optimistic about using it with a 3.5a stepper. The input gear shaft is 1.5" diameter so they don't even make the right coupling for a 1/2" stepper shaft.

    On the speed issues, are you confident your cam set-up isn't the issue? I watched a tutorial on YouTube explaining how speed issues with a 4th axis can be related to how the cnc software interprets degrees as minutes (or something).

    I haven't tried it yet but the tutorial apparently offered a fix.

  7. #7
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    Re: Anyone here tried the cheap Chinese harmonic drive 4th and 5th axis off ebay?

    Quote Originally Posted by Goemon View Post
    What size motor are you using with the harmonic drive? And have you used it for any aluminum parts?

    I'm interested in where the limitations are re part size when working with aluminum. Could you, for example, use your 4th axis to make a 2" x 6" x 20" part?
    I'm using a nema23 stepper. In the way I set it up, the maximum diameter I can use is 8 inches. The length is the width of my table at 48 inches. It could be mounted in the y direction for longer. Steady rests needed at that length for sure I made an entirely removable setup for my table top though. I've thought about building the same setup into a purpose built cnc that can travel the bounds of the build area in xyz. Maybe someday.
    If I did build it into my table itself, between extrusions, I could get a 16 diameter or somewhat bigger.

    I have not yet done aluminum on it. Only wood and plastic. I'm sure I could do it. I'm not sure the speeds and feeds I could use without moving something with the force.

    Sent from my SM-G996U using Tapatalk

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    Re: Anyone here tried the cheap Chinese harmonic drive 4th and 5th axis off ebay?

    I should also say that the trim vertical pieces I made are around 30 inches in length

    Sent from my SM-G996U using Tapatalk

  9. #9
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    Re: Anyone here tried the cheap Chinese harmonic drive 4th and 5th axis off ebay?

    Quote Originally Posted by ubergeekseven View Post
    I should also say that the trim vertical pieces I made are around 30 inches in length

    Sent from my SM-G996U using Tapatalk
    Here is the way I have mine setup. Aluminum plate and extrusions.

    Sent from my SM-G996U using Tapatalk

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