Extruder: Herringbone gear wobble

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Extruder: Herringbone gear wobble

Postby Pilot-PJ » 2012-Jun-Fri-20-Jun

My extruder is jamming and it seems to be due to an over constrained gear. The center of the bolt shaft isn’t orthogonal to the plain formed by the ‘points’ in the herringbone pattern. I’ve cleaned up all the teeth, and it still jams. When I spin the gear at one point it’s almost touching the extruder body and then 180 degrees (pi) it is fully ¼ inch off the surface.

Any thoughts or experience with alignment?
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Extruder: Herringbone gear wobble

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Re: Extruder: Herringbone gear wobble

Postby drawcut » 2012-Jun-Fri-21-Jun

I would loosen the extruder motor mounting screws and spread the gears farther apart until you can get them to work without binding. You'll have extra backlash but at least you'll be printing. Contact PB HQ and ask for a replacement gear.

I'm not sold on the herringbone gears for this application, but if they are made decently, they will work fine.
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Re: Extruder: Herringbone gear wobble

Postby Dick Brewster » 2012-Jun-Fri-21-Jun

One problem is that the hex head on the bolt is only made to a steel forging level of precision, so if the hex pattern is concentric to the bolt shaft it is only accidental.

When you press the hex head into the hex recess in the gear, it can and most likely will cause the gear to take on an orientation that results in a wobble. What you can to is try all six possible orientations of the bolt hex head vs the hex recess in the large gear, and use the orientation that give the least wobble.

On mine, the gear sat orthoganal to the bolt as long as I didn't shove the bolt head into the hex recess in the gear. You can install the bolt and gear in the extruder bearings and rotate the gear/bolt combo to check before you press the bolt head into the gear. All I had to do was rotate the bolt through the six positions to see when the head lined up with the recess.

This is poor design because it depends on the location of an as forged feature to locate a relatively precision device, a gear.
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Re: Extruder: Herringbone gear wobble

Postby levlandau » 2012-Jun-Sat-11-Jun

Yes, I have noticed these same issues, and it seems when the bolt is pressed all the way into the gear, it does not align correctly. Also, the small herringbone gear that came with my LC was fairly poor quality and mismatches from layer-to-layer could be seen. I am planning on ordering a nylon laser sintered gear from shapeways in order to end up using something that is quality and made correctly.

So does anyone have an idea of how to overcome the poor design related to the bolt??
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Re: Extruder: Herringbone gear wobble

Postby Dick Brewster » 2012-Jun-Sat-12-Jun

levlandau wrote:Yes, I have noticed these same issues, and it seems when the bolt is pressed all the way into the gear, it does not align correctly. Also, the small herringbone gear that came with my LC was fairly poor quality and mismatches from layer-to-layer could be seen. I am planning on ordering a nylon laser sintered gear from shapeways in order to end up using something that is quality and made correctly.

So does anyone have an idea of how to overcome the poor design related to the bolt??


Something that helps is to clean out the hex recess in gear. If none of the 6 possible orientations of the bolt head line up with the hex hole in the gear, then you could grind (Dremel) or cut the hex recess big enough to clear the bolt head so the bolt head doesn't force the gear to wobble. If the gear wobbles while just the bolt shaft is in the gear, before the hex is engaged, then you might be able to orient the hex to one of it's six positions and have it cancel out any wobble due to the fit on the bolt shaft when the bolt head is pressed into the hex recess.

If the extruder never has to run backwards while printing (does it?), the hex recess in the gear could be enlarged to provide a little clearance around the bolt head. It would still need to fit at least as well as a sloppy socket to keep the gear from turning relative to the bolt.

All of the teeth on my small herringbone gear were deformed on the printer bed side of the gear (the side away from the set screw). Rather than manually trim 13 gear teeth, I carefully removed 1 mm from that side of the gear on a belt sander. Now my gears run smoothly. If you use a belt sander on the small gear be careful to not get the plastic hot enough to melt it.

My longer term plan is to print my own helical gears and get away from herringbone gears. I haven't seen a good argument for the helical gears yet, maybe after some PBot experience I will find out what the concern is. Is there a problem with the stepper motor handling the side thrust?
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Re: Extruder: Herringbone gear wobble

Postby drawcut » 2012-Jun-Sat-13-Jun

I wouldn't worry much about the bolt itself. Even though it's a forging, it's still plenty accurate enough here (unless you got a badly out of spec bolt - it happens). Many, many Reprap extruders use this same setup and work fine with forged bolts. The problems are mostly coming from the gears themselves. There have been known QC issues with some of the part runs on the Printrbot, the gears are one of them. PBHQ should take care of you if you let them know your problem. You will still need to do some part cleanup but that's the nature of the printed parts.

As for improved gears: I'd make a spur set rather than herringbone or helical. There's really no reason spur teeth wouldn't work and they should be more tolerant of misalignment than herringbone or helical.

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Re: Extruder: Herringbone gear wobble

Postby mikemwa53 » 2012-Jun-Sat-13-Jun

Is there a file out there for a spur set to print? I was going to print me another set of the herringbones but I'd like to try the spur set to see how they work.
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Re: Extruder: Herringbone gear wobble

Postby photozz » 2012-Jun-Sat-15-Jun

If the extruder never has to run backwards while printing (does it?)


Yeah, it does. When the head lifts up to go from one part of the print to another, it retracts the filament slightly to reduce stringing.
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Re: Extruder: Herringbone gear wobble

Postby Dick Brewster » 2012-Jun-Sat-17-Jun

drawcut wrote:I wouldn't worry much about the bolt itself. Even though it's a forging, it's still plenty accurate enough here (unless you got a badly out of spec bolt - it happens). Many, many Reprap extruders use this same setup and work fine with forged bolts. The problems are mostly coming from the gears themselves. There have been known QC issues with some of the part runs on the Printrbot, the gears are one of them. PBHQ should take care of you if you let them know your problem. You will still need to do some part cleanup but that's the nature of the printed parts.

As for improved gears: I'd make a spur set rather than herringbone or helical. There's really no reason spur teeth wouldn't work and they should be more tolerant of misalignment than herringbone or helical.

Gear designer in my day job >>


Drawcut, which spec is that? I have never seen a spec that tightly controlled the concentrically of the forged bolt head to the body of the bolt. Also on the hobbed bolt I received from Brook, the head is far enough off center that I can tell it is off without measuring it. I didn't measure it but it appears to be roughly 0.010 inch off which is reasonable for a forging. Doing some basic calculations, 0.010 inch out of concentric at the bolt head will cause about 0.12 TIR wobble on the gear if the bolt head and bolt body are both a press fit into the plastic gear.

I'm with you on the appeal of spur gears. I have heard they make the bot noisy but wonder how noisy slow moving lightly loaded plastic gears would be? What are your thoughts on that?

If I get around to designing my own gears it should be easy to do helical and spur and compare. What pressure angle do you recommend for printed gears? My gut feel is a high pressure angle but I'm not a gear designer although I have consumed a lot of beer with a couple of world class gear designers.
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Re: Extruder: Herringbone gear wobble

Postby CL1 » 2012-Jun-Sat-20-Jun

Dick Brewster wrote:What pressure angle do you recommend for printed gears?

I'd use PA 14-1/2. More tolerant of center-to-center variations. Undercutting and the weaker tooth compared to PA 20 won't be an issue at typical tooth numbers and print fill %s.

FWIW, Brook's new Laser Cut extruder uses plain spur gears. ;)

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Re: Extruder: Herringbone gear wobble

Postby levlandau » 2012-Jun-Sat-23-Jun

Dick Brewster wrote:
You can install the bolt and gear in the extruder bearings and rotate the gear/bolt combo to check before you press the bolt head into the gear. All I had to do was rotate the bolt through the six positions to see when the head lined up with the recess.



This is a good idea, and I plan to try it, but I don't understand why it would work...? Assuming a perfect gear (e.g. one printed by shapeways) and a bolt which is offset from the center, rotating the bolt in each of the six positions should produce a similar wobble. This idea seems like it would only work if both the bolt and the gear are out-of-spec.

Is there a way to modify the bolt or buy one of a higher spec so that we don't have to deal with the wobble?? Wouldn't the wobble issue also pose the same problem for the spur gears??
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Re: Extruder: Herringbone gear wobble

Postby Dick Brewster » 2012-Jul-Sun-00-Jul

levlandau wrote:
Dick Brewster wrote:
You can install the bolt and gear in the extruder bearings and rotate the gear/bolt combo to check before you press the bolt head into the gear. All I had to do was rotate the bolt through the six positions to see when the head lined up with the recess.



This is a good idea, and I plan to try it, but I don't understand why it would work...? Assuming a perfect gear (e.g. one printed by shapeways) and a bolt which is offset from the center, rotating the bolt in each of the six positions should produce a similar wobble. This idea seems like it would only work if both the bolt and the gear are out-of-spec.

Is there a way to modify the bolt or buy one of a higher spec so that we don't have to deal with the wobble?? Wouldn't the wobble issue also pose the same problem for the spur gears??



If you have a perfect gear, it won't work. In another thread today CL1 suggested filing or grinding the bolt head so it fit the stock but cleaned up hex recess in the gear. Sounds like a good idea to me.
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Re: Extruder: Herringbone gear wobble

Postby CL1 » 2012-Jul-Sun-00-Jul

levlandau wrote:This idea seems like it would only work if both the bolt and the gear are out-of-spec.
Yes, that is correct. But even tightly spec'd parts still have tolerance. What Dick suggests is *managing* the tolerance by trying to *balance* it between the two parts. USING their out-of-spec'ness to advantage!
Is there a way to modify the bolt or buy one of a higher spec so that we don't have to deal with the wobble??
A file applied with intent and care to the Hex head would be simplest. A commercial alternative might be the arcol hyena hobbed bolt. http://wiki.arcol.hu/arcol-hu-hyena-announcement/ It uses nylock nuts at each end. Presumably the nut hex is more concentric than the forging of a bolt head. Maybe not. But at least it will be easier to try different nuts until you find one which is concentric. (Or well matched to your Gear ;) )
Wouldn't the wobble issue also pose the same problem for the spur gears??
Yes. And perhaps moreso, since they have no self-aligning centering action. On the other hand, they are a far simpler print to see any errors and simpler to fix if necessary.

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Re: Extruder: Herringbone gear wobble

Postby Dick Brewster » 2012-Jul-Sun-02-Jul

I like the way the arcol hyena hobbed bolt http://wiki.arcol.hu/arcol-hu-hyena-announcement/ allows axial adjustment without having to make a special thickness washer like I did.

I'm a little concerned about the long term security of the nylock nut used as the driving element for the hobbed shaft. If that turns out to be a problem, a pair of nuts jammed together with a manly amount of torque on the gear end would fix that.
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Re: Extruder: Herringbone gear wobble

Postby eddiema » 2012-Jul-Sun-07-Jul

CL1 wrote:
levlandau wrote:A file applied with intent and care to the Hex head would be simplest.
CL1

My bolt has been quite straight both on the original gear and my PLA replacement.
But I was thinking it should be fairly simple to make a jig to file or sand the bolts so the heads are straight.
Both my gears have been quite tight and a slightly smaller bolt head would have been a good thing.
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Re: Extruder: Herringbone gear wobble

Postby drawcut » 2012-Jul-Sun-08-Jul

Dick Brewster wrote:
drawcut wrote:I wouldn't worry much about the bolt itself. Even though it's a forging, it's still plenty accurate enough here (unless you got a badly out of spec bolt - it happens). Many, many Reprap extruders use this same setup and work fine with forged bolts. The problems are mostly coming from the gears themselves. There have been known QC issues with some of the part runs on the Printrbot, the gears are one of them. PBHQ should take care of you if you let them know your problem. You will still need to do some part cleanup but that's the nature of the printed parts.

As for improved gears: I'd make a spur set rather than herringbone or helical. There's really no reason spur teeth wouldn't work and they should be more tolerant of misalignment than herringbone or helical.

Gear designer in my day job >>


Drawcut, which spec is that? I have never seen a spec that tightly controlled the concentrically of the forged bolt head to the body of the bolt. Also on the hobbed bolt I received from Brook, the head is far enough off center that I can tell it is off without measuring it. I didn't measure it but it appears to be roughly 0.010 inch off which is reasonable for a forging. Doing some basic calculations, 0.010 inch out of concentric at the bolt head will cause about 0.12 TIR wobble on the gear if the bolt head and bolt body are both a press fit into the plastic gear.

I'm with you on the appeal of spur gears. I have heard they make the bot noisy but wonder how noisy slow moving lightly loaded plastic gears would be? What are your thoughts on that?

If I get around to designing my own gears it should be easy to do helical and spur and compare. What pressure angle do you recommend for printed gears? My gut feel is a high pressure angle but I'm not a gear designer although I have consumed a lot of beer with a couple of world class gear designers.



I'd have to check some refs at work to find the spec for bolt dimensions. But if you can see the hex is that bad on your bolt, I gotta believe you are right on this. (I have been known to be wrong before - once or twice. :lol: )

Spur gear noise: You pretty much hit the nail on that one. These are slow moving, lightly loaded plastic gears. I don't think noise will be a big issue.

I may get around to looking at a spur gear design soon if things slow down a bit at work. I work with steel gears, hobbed or hobbed and ground so plastic will be something new for me. We pretty much stick with 20 degree PAs or 25 if we need to squeeze out some extra strength. All involute gears are relatively tolerance of center distance changes but picking a PA that helps that even more may be a good idea (as CL1 suggested). I really need to stay late at work one day and look at this with all my references and gear software. I'm also thinking that I would like to optimize for printing - maybe try to make the gear teeth tips as wide as practical to help printing. Always trade offs to be made.

Edit: I wonder if I can add some crown to a spur pinion to help out any misalignment. We use this at work but it's not part of our 3d model - it gets put in the hobbing or grinding process.
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Last edited by drawcut on 2012-Jul-Sun-09-Jul, edited 1 time in total.
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Re: Extruder: Herringbone gear wobble

Postby drawcut » 2012-Jul-Sun-09-Jul

CL1 wrote:
levlandau wrote:
Wouldn't the wobble issue also pose the same problem for the spur gears??
Yes. And perhaps moreso, since they have no self-aligning centering action. On the other hand, they are a far simpler print to see any errors and simpler to fix if necessary.

CL1


Maybe yes and maybe no. People are reporting that the herringbone gears are binding up from the wobble. The self aligning action will only help if the parts deflect easily enough and the motor has the torque to turn it through. With spur gears there may be less binding in a 'wobble' set since it won't have the same self aligning action. They may still have binding as the angle changes and the teeth try to bottom out in the root of the mating tooth (or the backlash goes to zero) but herringbones have the same issue with the added issue of their axial clearances changing and binding with the wobble.

At work, we have found that spur gears are much more tolerant of misalignment than helical gears. This experience goes back at lot farther than just me. A lot of engineers, machinists and assemblers have all come to this same conclusion. Herringbones are nothing more than two helical gears back to back and opposite hand. They are at least as sensitive to misalignment as a plain helical set and likely even more so. Of course this is experience with steel gears - plastic will have differences but I think most of the same principles will apply.

Totally agree that a spur gear set should be easier to get a quality print. That may end up being the most practical solution for that reason alone.
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Re: Extruder: Herringbone gear wobble

Postby scantrontb » 2012-Jul-Sun-19-Jul

drawcut wrote:Totally agree that a spur gear set should be easier to get a quality print. That may end up being the most practical solution for that reason alone.

personally i think the only reason people went with the herringbone gears were because they "looked cooler" than a plain ole' spur gear. :) ignoring that fact that they are more sensitive to axial and planar misalignment's than the equivalent spur gears for our purposes.
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Re: Extruder: Herringbone gear wobble

Postby levlandau » 2012-Jul-Wed-00-Jul

What about removing the recess from the herringbone gear and attaching the bolt/shaft to the gear by pressing it in so it is a tight fit? Maybe even use splines to grab on to the plastic better?

Would this remove the wobble issue and avoid the problems when using an out-of-spec bolt?
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Re: Extruder: Herringbone gear wobble

Postby pbjbentz » 2012-Jul-Sun-00-Jul

My Herringbone gear was clearly not on straght even before encounterint the head of the bold. The hole was not perpendicular. I drilled out the hole to just barely oversize to allow some gear wobble, and the herringbone automaticlly aligns itself.
Another issue re: the gears not meshing smoothly, the herringbone gear had a bunch of "web strings" from gear to gear from the printing. I guess the print head let the ABS plastic stretch from one location to the other. I had to use a knife to clean out all the deep grooves in the gear, cutting these "webs" and scraping the remaining stubs out. After doing this for each tooth, the gears mesh better. Pretty well infact.
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Re: Extruder: Herringbone gear wobble

Postby scantrontb » 2012-Jul-Wed-21-Jul

pbjbentz wrote:My Herringbone gear was clearly not on straght even before encounterint the head of the bold. The hole was not perpendicular. I drilled out the hole to just barely oversize to allow some gear wobble, and the herringbone automaticlly aligns itself.
Another issue re: the gears not meshing smoothly, the herringbone gear had a bunch of "web strings" from gear to gear from the printing. I guess the print head let the ABS plastic stretch from one location to the other. I had to use a knife to clean out all the deep grooves in the gear, cutting these "webs" and scraping the remaining stubs out. After doing this for each tooth, the gears mesh better. Pretty well infact.


on the end of the small gear, away from the set screw end, is the first layer of the print, and while some of that is definitely the "stick to the bed" squish, not much we can do about that, though if it is excessive amounts of stringing, then you might want to return the gears (check the X,Y Axis drive gears too)... however if it was the black CAST large gear, then you may have gotten one like i did where they slightly over-filled the mold and it spread out over the top of the EDGE of the gear teeth... blech... taking a hobby knife to all those teeth and chipping the web away one-by-one was annoying, but my gears mesh quite nicely right now also.

thanks for pointing out the hole alignment issue, i haven't gotten to that part of the build yet, now i'll keep an eye out for it.
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Re: Extruder: Herringbone gear wobble

Postby Trentent » 2012-Aug-Thu-15-Aug

My Herringbone gear wobbled like a mofo and I couldn't get it to sit straight. I had to have my stepper motor screws "loose" so that the motor would "flex" when it encountered the uneven portion of the black herringbone gear. This meant I could only have two of the screws installed on the stepper. If I had all 3 then the motor was too close to the black gear on the uneven part and the motor gears would just lockup against each other. By looking at my black gear I can easily see where the stepper motor gear was rubbing against it as there is a white "dust" from the wear. Once I was able to get some prints out and calibrated my Printrbot to some precision I printed out the spur gears. It worked flawlessly the first time. I would highly recommend it to anyone who has issues with the Herringbone.
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Extruder: Herringbone gear wobble

Postby zachnfine » 2012-Aug-Thu-17-Aug

My herringbone gear seemed to bind whe I turned it against the smaller gear by hand, but in operation it's been fine so far (as far as I can tell). I may try the spur gear on my next extruder.
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Re: Extruder: Herringbone gear wobble

Postby eddiema » 2012-Aug-Fri-20-Aug

zachnfine wrote:My herringbone gear seemed to bind whe I turned it against the smaller gear by hand, but in operation it's been fine so far (as far as I can tell). I may try the spur gear on my next extruder.

If you print your own herringbone it will probably work fine. I've had no trouble with the PBHQ gears but printed a new ones for fun and they work perfectly.
I printed it at .2mm and it was a bit of a fiddle getting it to slice.
There has been some early talk about the accuracy of the bolt heads and that is certainly worth a look.
I understand people have had much frustration with PBHQ gears but when printed well they work well.
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Re: Extruder: Herringbone gear wobble

Postby CL1 » 2012-Aug-Sun-06-Aug

I think the early comments about the printed herringbone gears working well are still true. However, the cast gears do not seem to be as tolerant of error and alignment, so we see many posts about how the herringbone gears don't work. This goes against the early comments about them and seems to bring some confusion. I still find the printed herringbone gears to work fine (even with wildly off-axis initial alignment) and suggest that owners with a cast set print replacements. YMMV.

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Re: Extruder: Herringbone gear wobble

Postby zachnfine » 2012-Aug-Wed-16-Aug

I realized the other day that, in order to keep the herringbone gears from binding, I'd attached the extruder motor just barely less than absolutely tight to its mount. So when the gears even think of binding the motor moves slightly, and the rotation goes very smoothly. That's probably why I've had no issues, I've compensated for them. I'll probably print new gears, and if they work better, I'll attach that motor more tightly.
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Re: Extruder: Herringbone gear wobble

Postby holmes4 » 2012-Sep-Sun-19-Sep

I too had a lot of trouble with my cast herringbone gear wobbling - it didn't help that the small printed gear was so awful that it was as if it had no teeth at all in some spots. A bit of Dremel work and some judicious placement got me something that worked.

No matter what I did, I could not get the bolt to sit straight in the cast gear. I ended up Dremeling a little bit from the socket on one side and got it to sit reasonably straight, though I can see that the gear itself is uneven. Once I get my prints looking ok, I will print my own set of gears and swap them out.
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Re: Extruder: Herringbone gear wobble

Postby scantrontb » 2012-Sep-Wed-21-Sep

holmes4 wrote:I too had a lot of trouble with my cast herringbone gear wobbling - it didn't help that the small printed gear was so awful that it was as if it had no teeth at all in some spots. A bit of Dremel work and some judicious placement got me something that worked.

No matter what I did, I could not get the bolt to sit straight in the cast gear. I ended up Dremeling a little bit from the socket on one side and got it to sit reasonably straight, though I can see that the gear itself is uneven. Once I get my prints looking ok, I will print my own set of gears and swap them out.


as talked about in some of the other posts/threads about the herringbone gears, you may want to print out some straight teeth gear pairs, they will allow more play in the various different directions without binding (as badly). personally i think the only reason so many extruder setups have the herringbone gears is because they "look cooler" than plain straight gears, even though they are not as good for the function requirements as straight ones are.
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Re: Extruder: Herringbone gear wobble

Postby jheacock » 2012-Sep-Sat-14-Sep

I found a small amount of excess plastic at the bottom of the hex hole on my cast gear (hard to see black plastic in a dark hole),
After that was cleaned out using a dremel bit held in a drill press to keep depth of cut constant. The bolt now seats properly in the hex hole. I still have a problem with a few poor teeth that I need to cleanup - but that is another job.
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