I recently dusted off my rotary module after receiving my 200W CNC kit. The rotary had always worked great for milling and lasering. When I tried it with the 200W CNC however, I encountered issues. The rotary no longer turned smoothly, but would “stick” at certain rotations, ruining the carving material (hope the GIF conveys the issue):
I checked the forums and mailed support. The consensus from both was: driving belt too tight; loosen the screws a bit and then tighten them again. First attempt and the chuck would no longer rotate, however the stepper motor, driving belt and gear appeared to rotate just fine. I asked support for more specifics like distance between motor and gear but I just got the reply “belt should not be too tight, and not too loose”. Great.
Eventually, after a lot of trial and error, I tightened the belt quite a bit and observed smooth rotation of the chuck. However, when trying to carve I would end up with completely twisted carvings.
This seemed like some sort of over/undershoot per full rotation, so I put a screwdriver in the little ridge on top of the rotary module right above one of the jaws and gave it the command to rotate 360 degrees. Indeed I observed that it would always over/undershoot (depending on the direction) by 3-or-so degrees. To correct this, I played with the M92 command (steps-per-unit) until I saw it rotating 360 degrees, at least by eyeing it. Ended up at M92 B894 instead of the factory default M92 B888.889. This seems to work okay-ish, but I cannot test it because I wasted all of my rotary material troubleshooting the problem.
My question to the community is: has anyone had a similar issue? If so, is there a specific way to calibrate the steps-per-unit for the rotary module other than eyeing it?
Huh, sounds strange… Are you sure that all grub screws or other fittings in the movement path (e.g. that of the belt wheels) are tight and not gone loose? The jerky rotation suggests something slipping IMHO. Also: If you put a bit of resistance to the chuck by e.g. holding it fast with your hand (as much as you dare…): Does it slip/have play?
If I hold it somewhat tightly with my hand it absolutely slips, even now that I have tightened the belt. The grub screws are on the gears that are driven by the belt, not the four screws fastening the stepper motor, correct? How do you suggest I adjust them?
I have never disassembled my rotary, so I am just uttering “educated” guesses. When I said grub screws I assume that the wheels where the belt is running over (one on the stepper, one on the upper axle) are tightened to the individual axles by a grub screw - I may already be wrong here. If I’m right, I perhaps would loosen them and pull the wheel from the axle and see if the grub screw(s) have carved a ridge into the axle - if so, you might try to sligthly displace the wheels and recrew them. If not, I’d tighten the grub screws hand-fast. If the module still slips on load, I’d trace the path further - it is to my understanding relatively unlikely but not impossible that the strain wave gear is broken, but it may also be that the chuck is not tight on the axle that comes from the gear. In other words: Each mechanical joint along the force path may have gone loose and might need retightening.
Take all this with ahuge grain of salt: As I said, just making assumptions, never have dismanteled my rotary.
Perhaps a last question: If the module slips: Do you hear strange sounds? If you hear a loud repetitive sound, the stepper might be skipping steps - again, unlikely, but if so, it may be that the stepper has a problem.
EDIT: Let me know if you are still lost, I might consider dismanteling my module to help you - but I will not be able to do so before the weekend.
Thanks for the comments so far! @Hauke I took apart my module so you don’t have to . The good news is: I found where the issue is. The bad news is: I don’t know how to fix it.
This is the general overview of the internal assembly of the rotary module. I’m not very knowledgeable on mechanical parts so apologies if I use the weird terminology:
Top: strain wave gear “slot”. Should fit the drive belt gear. Lots of lubricant, not sure what type of lubricant.
Bottom: stepper motor + gear.
On the right bottom: drive belt gear itself.
The problem is the interface between the drive belt gear axle and the strain wave gear can slip. When I hold the drive belt gear and manually rotate the chuck, the chuck is able to move regardless. I hear the drive belt gear axle slip inside the strain wave gear slot.
On the drive belt gear axle, I see some machined edges.
Presumably, the strain wave gear slot has/had this shape (“inverted”) to prevent it from slipping. I cannot see this shape through all the lubricant, and I also cannot feel it when I trace the edges of the slot with a small screwdriver. Maybe it has worn off.
The slot is not closed a closed socket; I suspect there is some type of bearing there but I cannot say for sure. I was not able to take it apart far enough (it should be possible but I could not figure out how to take off the chuck itself).
I attempted one fix: fitting the axle of the drive belt gear with a tight piece of heat shrink tube. I figured it would kind of work like putting a rubber band on a worn-out screw, but with the shrink tube the drive belt gear axle was way too big to fit the slot.
Side note: both gears have grub screws, but I’m not sure what their function is. Loosening/tightening doesn’t seem to have an effect.
I hope this information is useful. If anyone has an idea on how to fix this, please let me know!
Very helpful pictures. I’d agree: The machined axle looks very much like it should have a fitting counterpart in the hole it slits into. I guess that with these pictures Snapmaker support should be able to identify the problem. I can see two possible issues: A) the matching slot has indeed worn of, which I doubt, because you’d see lots of black lubricant from the metal shavings (and the other side wold show wear also I suppose, unless they made very bad material choices), or B) the counterpart for the axle somehow slipped deeper into the hole and you might be able to retrieve it.
Is the hole where the axle slits into going through to the other side? Perhaps you can shove the missing part back from the other side?
I have replied to support citing this thread. Hope they can help me. I don’t think there is a counterpart for the axle, but I can only confirm once I remove the chuck and go in from the other side. If they reply I’ll update the thread.
Hello Mayco,
I would guess the machined axle should receive a screw on the chuck’s side to lock the chuck on the correct position. If this screw has losen, maybe it allows the chuck to turn mostly freely?
Hi @MakeItBetter, I wish that was true, would be a nice solution and an easy fix. Unfortunately the axle is solid and there is nowhere to put a screw into.
Update: support got back to me and nailed down the problem! The hole the driving gear axle goes into is called the “harmonic reducer”. This part has two grub screws that grip onto the axle. One of them had come loose almost completely. I tightened them and that solved the issue!
What made is tricky is that the “harmonic reducer” was completely loose from the axle, so it would not come out of the bearing it sits in. I pushed a slightly too big hex screwdriver bit into the hole so that it would get “stuck” in the reducer, and then I was able to pull the reducer out of the bearing.
All in all I’m happy with the support and quite impressed by how repairable some of the hardware is.
. The good news is: I found where the issue is. The bad news is: I don’t know how to fix it.
