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IndustryArena Forum > MetalWorking Machines > Benchtop Machines > Is a Harmonic Drive best solution for a 4th axis?
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  1. #1
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    Is a Harmonic Drive best solution for a 4th axis?

    I mean, everything looks great about them. Darn near zero backlash, etc., etc. I'm not worried about speed.

    But if I throw a little too much torque at one while cutting some steel, am I going to have a nice little package full of former gear teeth bolted to my table? Are they likely to self-destruct?

    And if I go with one, what is a good ratio? 10:1? 20:1? 50:1? Looks like I'll be bolting on a 570 oz stepper from Automation Technolgies, at least to start...

  2. #2
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    Re: Is a Harmonic Drive best solution for a 4th axis?

    If you need more torque than a harmonic will handle, look at cycloidal drives, which can also have zero-backlash. They're used to power the joints in robot arms, which have to hold absolute position or move strongly against cutting forces. They're more mechanically complex, and hence more expensive, but you might be able to find used ones. Here's an article that explains some more about them: https://www.motioncontroltips.com/cycloidal-reducer/. .
    Andrew Werby
    Website

  3. #3
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    Re: Is a Harmonic Drive best solution for a 4th axis?

    Have a bit of a dig around on cnc zone - there are at least three threads here where the merits and pitfalls of harmonic, hypocycloidal, worm and belt reduction mechanisms have all been argued ad nauseam.

  4. #4
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    Re: Is a Harmonic Drive best solution for a 4th axis?

    i don't know that anything other than a harmonic drive can offer as much torque with as little backlash in the same volume.

    a 570 oz-in stepper coupled to a 50:1 harmonic drive would be overkill unless the harmonic drive is the size of a phone book.

  5. #5

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    Re: Is a Harmonic Drive best solution for a 4th axis?

    I have a 4th axis of sorts for my mill via a rotary table which I had CNC converted (IIRC, I got the cheapest rotary table in the store; at the same time also getting a drill-press vise to use for holding stuff, and made some aluminum smooth jaws for it).

    For this, had used a 69 oz-in NEMA17 connected to the handle assembly via a belt drive.
    Seems fast enough, and seems to still have a fair bit of torque, and have it set to speed of around 720 degrees/minute, with an approx 90:1 reduction (approx 4 degrees per revolution). AFAIK, internally I think it is a worm gear or similar. Main failure modes are generally belt related (like the belt coming off the pulleys or slipping under sufficient load). Had machined a pulley both to go on the handle assembly and also a similar pulley for the motor.

    It is actually capable of going a bit faster than 720/min, but this tends to make it not hold position as well, and makes it more frequent for the drive belt to jump the pulleys. There is an adjustment screw that reduces these issues though (by keeping the motor aligned with the handle pulley). Technically this is using a repurposed vacuum cleaner drive-belt with some strategic use of knurling (a proper timing belt and pulleys would probably have been better).


    I was basically unable to mount hardware directly to the rotary table, as the HSS tools I have were pretty much entirely ineffective against whatever type of cast iron it was made out of. Best I could ultimately do was sand the paint off in that area and then JB-weld on another piece of steel which I had modified into a mounting brace for a NEMA17 motor bracket. Could also have possibly JB-welded on a piece of steel plate and then machine it on the table (to face and drill/tap some mounting holes).

    But, yeah, I suspect a NEMA23 is probably overkill if doing something similar to this (more so, this isn't even the largest size of NEMA17's which exist).

    Overall, it seems to basically work (excluding not having good mounting hardware for mounting stuff to it).

  6. #6
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    Re: Is a Harmonic Drive best solution for a 4th axis?

    I'm not really looking to "high speed machine" with the thing - just worried that if I run a little fast by accident that I'll destroy it. Looking like the harmonic will be in the budget - Eldon, what gear ratio would you suggest?

    I'm also thinking of using the 570s for everything but the Z, mostly to just keep things simple...

  7. #7

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    Re: Is a Harmonic Drive best solution for a 4th axis?

    Did you end up buy an harmonic 4th axis? I am looking to it at the moment. I plan to machine some helical and dual helical gears out of steel. I don't need it to be fast but sturdy and precise. I saw a lot of ebay adds with 1:50 or higher harmonic reduction 4th axis paired with nema 23. They seem right but I am liking for some reviews that I can not find. I am also worried about if they will pair right with the other axis. I read somewhere that this high reduction tends to run really slow.

    Sent from my MI 5 using Tapatalk

  8. #8
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    Hello all, I'm new to this forum and just to say Harmonic Drive is a Company not just a type of reduction, they make several styles of Cycloidal and Planetary reductions with very minimal if any backlash.
    I thought I'd give my experience with a China 5 axis HY6040 CNC mill- junk.
    My system had 8:1 belt drives and NEMA 23 on both the "A" axis saddle and the "B" axis rotary table. Both could not hold a feather and were useless trying to set up since if you touched it, it moved.
    I purchased two used Harmonic Drive Planetary reduction units, 22:1 for the "A" axis, 5:1 plus the original 8:1 belt for the "B" axis. Custom machined many parts and now they are backlash free, can hold 25 lbs. static and stronger than the rest of the mill.

  9. #9

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    Re: Is a Harmonic Drive best solution for a 4th axis?

    Hi!
    At the end I purchased this type of harmonic drive. 1:50 reduction (https://www.befr.ebay.be/itm/CNC-4th...MAAOSwT7Bc8NFC). It's claimed backlash free but actually I do not know if it is the harmonic or a not preloaded bearing but while not moving I can easily move right and left the chuck for let say plus minus 3-4, like 4mm circular movement. Does someone know why and where to look to solve it?

  10. #10
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    Re: Is a Harmonic Drive best solution for a 4th axis?

    Quote Originally Posted by rey8801 View Post
    I do not know if it is the harmonic or a not preloaded bearing but while not moving I can easily move right and left the chuck for let say plus minus 3-4, like 4mm circular movement. Does someone know why and where to look to solve it?
    A harmonic drive is a flex spline inside an outer spline. There is no adjustment and it does not require angular or preloaded bearings.

    BTW a few years ago I simply mounted a motor on a BS-0 dividing head. It's a worm drive, and it's never moved... ever.

    https://youtu.be/-jcxVS41aTw

  11. #11

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    Re: Is a Harmonic Drive best solution for a 4th axis?

    Quote Originally Posted by FastFarmer View Post
    A harmonic drive is a flex spline inside an outer spline. There is no adjustment and it does not require angular or preloaded bearings.

    BTW a few years ago I simply mounted a motor on a BS-0 dividing head. It's a worm drive, and it's never moved... ever.

    https://youtu.be/-jcxVS41aTw
    Yeh I know it should have almost no backlash but then i do not know what is the problem. Must have something wrong in the mechanical connection with the harmonic drive inside. I really want to use it and spent the weekend to make the control box to then find this issue... I think I will need to open to see it

    - - - Updated - - -

    Quote Originally Posted by FastFarmer View Post
    A harmonic drive is a flex spline inside an outer spline. There is no adjustment and it does not require angular or preloaded bearings.

    BTW a few years ago I simply mounted a motor on a BS-0 dividing head. It's a worm drive, and it's never moved... ever.

    https://youtu.be/-jcxVS41aTw
    really nice 4 th axis btw. which mill is that one?

  12. #12
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    Re: Is a Harmonic Drive best solution for a 4th axis?

    Hi,
    I have low backlash (<2 arc min) 10:1 planetary gearboxes on my steppers. They have been great. According to the Vexta website these gearboxes/steppers
    should produce a stall torque of 6Nm. In practice they do way WAY better than that, approaching 25Nm. They are in danger of stripping gears at that level.
    As it turns out the aluminum spiral cut shaft couplers shear off before that. It was not my intention that they would do so, but they act like a mechanical
    fuse, they shear off rather than wreck the gearbox.

    Low lash planetary gearboxes aren't cheap......the more lash the cheaper they get. A single stage planetary is limited to about 10:1 reduction. Higher
    reductions, eg 30:1 are done in two stages with greater lash.

    Harmonic drives start at 50:1 with 100:1 being common. The flex gear is reasonably tender, if you stall the output shaft you could strip teeth off the flex gear,
    an expensive failure. I would stick very close to the manufacturers spec with regards max torque and have a torque limited slip-clutch between the output
    of the harmonic drive and the load if a stall is likely.

    Craig

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