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IndustryArena Forum > Mechanical Engineering > Linear and Rotary Motion > Help! Harmonic Drive 4th Axis Troubles
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  1. #1
    Join Date
    Dec 2010
    Posts
    634

    Help! Harmonic Drive 4th Axis Troubles

    In emergency mode - let's leave out why I'm in emergency mode but lets just say that a few years ago I bought a 4th axis, got it turning and never used it. I'm now working frantically to finish a job which requires 4th axis work which is due on Saturday.

    I'm setting it up for the first time and to align it in the table, I purchased a long piece of 1" steel drill rod. This 4th axis has a 4 jaw chuck so my first step is to align the drill rod in the 4th axis. I found a very quick way to do this on youtube here: https://youtu.be/2KMhx4DbyDg

    In the process of doing this, I saw that depending on the direction the table is rotating, I get different amounts of runout. Here's a quick video showing what I'm getting: https://youtu.be/LYiYb_h7PB4

    I bought the drive several years ago but it looks exactly like this:

    CNC 4th Axis Router A Axis 100mm Chuck Gapless Harmonic Drive Reducing Gear Box | eBay

    I can't understand at all what could be the problem? Could it be the bearings are garbage?

    Any ideas on what to do?

    I'm thinking that perhaps if I can force my cam software to go in only one direction I just might be able to finish the part but I'm not even sure yet if that's possible.

    Any and all ideas are welcome and much appreciated.
    -Andy B.
    http://www.birkonium.com CNC for Luthiers and Industry http://banduramaker.blogspot.com

  2. #2
    Join Date
    Dec 2010
    Posts
    634

    Re: Help! Harmonic Drive 4th Axis Troubles

    P.S. if anyone can think of a relatively inexpensive stepper based 4th axis that I could get overnight, that would be appreciated too. I really don't want to spend more than $500 if at all possible.

    Thanks again!
    -Andy B.
    http://www.birkonium.com CNC for Luthiers and Industry http://banduramaker.blogspot.com

  3. #3
    Join Date
    Apr 2004
    Posts
    5728

    Re: Help! Harmonic Drive 4th Axis Troubles

    That does seem odd. You'd think that if it was concentric going one direction, that shouldn't change if it reverses. It must be the harmonic drive itself that's doing that, since bad bearings would be bad no matter which direction they're turning. I don't know what your CAM software is, but you could download a copy of DeskProto and choose a routine that keeps the 4th axis going in one direction. (The demo will work for a month).

    If you want to try another 4th axis solution, I could sell you a Sherline CNC-ready rotary table and chuck. These are quality tools, made in the USA. It's a planetary drive and not a harmonic one, but I don't think the runout will vary if it changes direction. The rotary table is $320 and the independent 4-jaw chuck is $141.75, so it works with your budget if you can supply your own stepper motor (at least until you add overnight shipment).
    Andrew Werby
    Website

  4. #4
    Join Date
    Dec 2010
    Posts
    634

    Re: Help! Harmonic Drive 4th Axis Troubles

    Hi Andrew.

    Thanks for the offer. Luckily a friend nearby had an F-style rotary axis he's letting me borrow to get this job done. It won't be great as it's belt driven but I'm hoping it does well enough to get this prototype done for the presentation I have Sunday.

    The harmonic drive really has me confounded. I may try to take it apart to see if perhaps the bearings are garbage or something. It was like $350 or so so i'd rather not totally pitch it. If $100 of bearings will fix it, I'd be pretty happy. Unfortunately, the faceplate blocks access to the front housing of the drive and the cap screws have been ground down so I'll need to drill through them to remove it. Cross your fingers!
    -Andy B.
    http://www.birkonium.com CNC for Luthiers and Industry http://banduramaker.blogspot.com

  5. #5
    Join Date
    Jan 2005
    Posts
    15362

    Re: Help! Harmonic Drive 4th Axis Troubles

    Quote Originally Posted by BanduraMaker View Post
    In emergency mode - let's leave out why I'm in emergency mode but lets just say that a few years ago I bought a 4th axis, got it turning and never used it. I'm now working frantically to finish a job which requires 4th axis work which is due on Saturday.

    I'm setting it up for the first time and to align it in the table, I purchased a long piece of 1" steel drill rod. This 4th axis has a 4 jaw chuck so my first step is to align the drill rod in the 4th axis. I found a very quick way to do this on youtube here: https://youtu.be/2KMhx4DbyDg

    In the process of doing this, I saw that depending on the direction the table is rotating, I get different amounts of runout. Here's a quick video showing what I'm getting: https://youtu.be/LYiYb_h7PB4

    I bought the drive several years ago but it looks exactly like this:

    CNC 4th Axis Router A Axis 100mm Chuck Gapless Harmonic Drive Reducing Gear Box | eBay

    I can't understand at all what could be the problem? Could it be the bearings are garbage?

    Any ideas on what to do?

    I'm thinking that perhaps if I can force my cam software to go in only one direction I just might be able to finish the part but I'm not even sure yet if that's possible.

    Any and all ideas are welcome and much appreciated.
    By the look in the video, the chuck has been mounted on to the front of harmonic drive, instead there should be support bearings in the front on a spindle that would support the chuck,the harmonic drive would then be driving it just like a normal spindle, this would be no load on the output spindle of the harmonic drive then
    it the only way it is ever going to work
    Mactec54

  6. #6
    Join Date
    Dec 2010
    Posts
    634

    Re: Help! Harmonic Drive 4th Axis Troubles

    Making a new housing for the thing wouldn't be the end of the world. Once I free up a little bit of time I'll get the housing off that thing and see if there's any bearings in there. It didn't dawn on me that they'd do such a thing but I suppose it's possible. The 4th axis my friend loaned me was also complete garbage. The faceplate was all wobbly because there was probably a good .003 to .006 gap between the shaft and the faceplate and the faceplate was actually held on with 2 set screws opposite each other. By adding two set screws I was able to get the run-out down to about .006" (which is crap, I know) but it ended up being good enough to make the parts. This is kind of a proof of concept prototype so it just has to "kind of" work. I can work out the details when it comes time to work these in quantity.

    Hopefully I'll be able to salvage the harmonic drive from my old 4th axis and get some bearings in there.

    I sometimes surprise myself that I'm still surprised when the Chinese junk priced too good to be true ends up not being true and is indeed Chinese junk.

    https://youtu.be/5J6EZmqr5EM
    -Andy B.
    http://www.birkonium.com CNC for Luthiers and Industry http://banduramaker.blogspot.com

  7. #7
    Join Date
    Jan 2005
    Posts
    15362

    Re: Help! Harmonic Drive 4th Axis Troubles

    Quote Originally Posted by BanduraMaker View Post
    Making a new housing for the thing wouldn't be the end of the world. Hopefully I'll be able to salvage the harmonic drive from my old 4th axis and get some bearings in there. I sometimes surprise myself that I'm still surprised when the Chinese junk priced too good to be true ends up not being true and is indeed Chinese junk.
    There are lots of them built this way, they have a surplus of Harmonic Drives and just throw them together, to make a sale, they are used also so they could have quite a bit of wear and tear as well, there are some model Harmonic Drive that you could hang a chuck on them, but most you see on Ebay won't handle a direct mounted chuck, without 2 Bearings to take the load of a chuck
    Mactec54

  8. #8
    Join Date
    Jun 2010
    Posts
    4252

    Re: Help! Harmonic Drive 4th Axis Troubles

    I used a larger HD with a direct-mounted chuck, very successfully. But the crossed roller bearing on it is large and was designed as load-bearing.

    Cheers
    Roger

  9. #9
    Join Date
    Mar 2012
    Posts
    74

    Re: Help! Harmonic Drive 4th Axis Troubles

    Hi,

    I also have this 4th axis, same as yours but is a 4 jaws self-center chuck one. I have not this awful happening so far. In construction, it have two large bearings to support the rotating output shaft, but I don't know how this output shaft are coupled with output shaft of the harmonic drive. I had once disassembled the stepper motor out and saw the harmonic construction only. I only used it a couple of times in doing some small work and can't give you any precision comments of this item.

    Currently, I use it for the build of my 5th axis as for the horizontal axis and is still undergoing,....My painful experience is another purchased piece. It is a second hand/used harmonic drive (SHF 17)that i purchased from Korea on Ebay. I needed to replace the cross roll bearing as it worn and had too much clearance. It was used in the vertical axis of the 5th axis. So, take much care in buying used harmonic drive, or try to chosing new piece...

  10. #10
    Join Date
    Mar 2012
    Posts
    74

    Re: Help! Harmonic Drive 4th Axis Troubles

    Thanks for Roger for the link of his 4th axis building, it is informative...Actually, the outer of ring of cross roller bearing is one piece not two piece, roller pins are inserted to it through an opening insert piece with a screw fixing, for my replacement one, they are inserted through a hole with insert pin....

    I installed the said purchased 4th axis on my trunnion table as shown in below link. Bubble level was installed on the input shaft of harmonic drive of this 4th axis in order to have eye witness or monitoring the status of rotation. The table is firstly leveled, however, I can rotate it a little by hand, the stepper motor is turned on so there is no movement of the input side, this is also checked by bubble level. What this end up is that every time the table cannot return to home horizontal position.

    I don't know if this is caused by the backlash, clearance or hysteresis of the harmonic drive? In my thinking now, a new and genuine harmonic drive costs a lot. Pound for pound? , they cannot give a high degree of precision...

    https://www.youtube.com/watch?v=l7VZSNC1_ao

  11. #11
    Join Date
    Mar 2012
    Posts
    74

    Re: Help! Harmonic Drive 4th Axis Troubles

    Oh, somehow the video don't show all in small screen. Need to push the 'full screen button' in order to see the deviation on dial indicator when rotating by hand....

  12. #12
    Join Date
    Jun 2010
    Posts
    4252

    Re: Help! Harmonic Drive 4th Axis Troubles

    I can rotate it a little by hand, the stepper motor is turned on so there is no movement of the input side,
    Late to the party, sorry.
    You can rotate the output BY HAND???
    In that case the spline must be totally shot. To be confirmed of course, but if true that is really, really bad!
    I think you may be able to buy replacement bits however, either through HD or via eBay. I have seen ads for the kits.

    Cheers
    Roger

  13. #13
    Join Date
    Aug 2014
    Posts
    889

    Re: Help! Harmonic Drive 4th Axis Troubles

    You can rotate the output BY HAND???
    Very odd and uncharacteristic of a Harmonic drive. I think you have problem there.

  14. #14
    Join Date
    May 2015
    Posts
    1422

    Re: Help! Harmonic Drive 4th Axis Troubles

    Depends. I just had a job where we were using robotic camera heads where pan and tilt were each driven by a nema 17 stepper into an 80:1 harmonic drive. At about $10k per head, these parts were all new. But we could drive the stepper by pushing in the camera mount platform. Slowly, but no worries or effort required.

  15. #15
    Join Date
    Jun 2010
    Posts
    4252

    Re: Help! Harmonic Drive 4th Axis Troubles

    But we could drive the stepper by pushing in the camera mount platform.
    Let me guess: the motors were not powered up and you had a fair bit of leverage via the camera mount?
    You can NOT back-drive an 80:1 worm and wheel (too much friction), but at least in principle you can back-drive an 80:1 gear train. An HD is the same as a gear train.

    Cheers
    Roger

  16. #16
    Join Date
    May 2015
    Posts
    1422

    Re: Help! Harmonic Drive 4th Axis Troubles

    Correctamundo. As soon as you waved power even vaguely in the direction of the stepper motor the mount may as well have been welded to the head. And it wasn't free spinning even with the power off, it was a lot like those smooth opening hinges you get in cars sometimes - the ones, coincidentally, that use a high gear ratio and friction to damp the motion. Doesn't take a lot of effort to get moving at a certain speed but the force required increases exponentially as you try to go faster.

  17. #17
    Join Date
    Jan 2007
    Posts
    1795

    Re: Help! Harmonic Drive 4th Axis Troubles

    I wouldn't give up on this drive..

    practically 3 type can be found,

    the beltdrive what I s good for wood, with a 1:6 ratio

    another one the wormgear driven... they also good and they have a decent torque they can transmit.. but they have backlash

    and this is the 3rd type.. I think this is the best option, due it has a great ratio also almost backlash free,..

    I think if it has 2 bearing on the front then a new set of bearing should solve it..
    if it has only one bearing, what looking from the short length, then need an additional bearing on the front..

    either way, you wouldn't find better deal.. only for very expenses with ""no backlash""
    ================================================== ===========

    if it shows different in one direction, I think the most possible the two end has alignment issue..
    and it don't need a lot misalignment.. inside a couple of 100th of mm, on the from can shows 0.1 mm difference..

  18. #18
    Join Date
    Aug 2012
    Posts
    181

    Re: Help! Harmonic Drive 4th Axis Troubles

    I have the same chinese 50:1 harmonic drive 4th axis.
    It was to be a replacement for an 8:1 belt driven 4th axis to improve holding torque and backlash.

    Thanks for pointing this out.
    I should meassure the runout in both directions and if it's not acceptable unscrew the chuck and add a bearing on the output shaft.
    Have you taken apart yours in the last month?
    Did it help?

    PS:
    The chuck would be a 3 jaw 100mm K11A? Any idea where to get different jaws for it or CAD models or technical drawings to machine my own jaws?
    Any photos or videos on taking it apart that may give me a better idea of what to expect?

    I am planning to later add a larger chinese harmonic drive between Z-axis and spindle mount to convert my heavily modifed (chinese) 4 axis Yoocnc 6040 into a 5 axis machine to work in a +-60° angle on the surface of long rods.
    The stiffest material to work with will probably be POM (also soft wood and machinable wax). So I don't expect excessive forces.

  19. #19
    Join Date
    Jun 2010
    Posts
    4252

    Re: Help! Harmonic Drive 4th Axis Troubles

    Re harmonic Drive for 4th axis: see this thread:
    http://www.cnczone.com/forums/linear...gineering.html

    In short: I used a 50:1 HD to build a 4th axis unit, and this is the full story.

    Cheers
    Roger

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