This is why I thoroughly enjoyed building my Prusa as a kit. You see little tricks like this, including this exact one. Or how they embed threaded nuts, or how the print is shaped and oriented to be made without supports, etc. Building the prusa saved a bit of money, but was also a crash course in how to design.
I agree that it looks fragile, but my experience with a similar kind of design in practice has shown that it’s not actually as big of an issue as it seems it would be.
A few other things that can help:
- for the “it can work itself out” problem, I’ve had pretty good luck on other projects by making a standardized “dual dovetail” (bowtie) male/female slot with clearance tuned so that the male piece fits tight into the slot. Printing the male part flat on the bed means that the filament is oriented correctly for strength
- integrated zip tie slots. One design I worked on that was similar to what OP did but needed better grip… I made the slot where the folded over timing belt meshes longer, removed the printed centre pin, and put two integrated zip-tie slots: one near where the centre pin was, so I can use the zip-tie ratchet to actually pull the belt backwards and put tension on it; the other was near the mouth of the jaws, so that after tensioning it I could use a second zip-tie to “lock” the jaws onto the belt
- a third one: you make a little block with dovetails that looks like the “circle plus mouth” piece from this post, but also include a pocket for a nylok that you can use to add tension to the belt after getting the belt secured inside the clamp.
I have tried printing teeth that would mesh with the belt but didn’t get particularly good results from that. Possible that newer printers have better resolution and could print the teeth better, but my test with that just ended up with a little bit of a rougher surface and the teeth couldn’t actually mesh.
Awesome! I’ll have to try it again. Last time I tried was on some Wanhao i3 clone like a decade ago and it was not great, but the printer itself was not great all around other than the price :).
If you printed teeth, you'd have to make sure the teeth in the top loop are precisely aligned in relation to the teeth in the bottom loop based on the length of the belt, right? That seems like it could be limiting
Printed teeth are weaker than the belt's own teeth.
Conversely, with this design the center pin could break loose and the belt would still hold, because the pin just has to stop the loop from fitting through the slot.
That’s exactly it… if you tolerance the mouth reasonably tight, the centre pin doesn’t actually carry much, if any, of the load. Friction in the mouth carries a bunch of it, especially if you add the zip-tie clamp from my sibling post.
Timing belts are crazy strong. From my FRC work the amount of abuse and weight they will deal with is handy.
Belts will burn through polycarb if it isn't tracking properly. They are strong enough to deform any pulley mount on a tight fit.
Only minor complaint is that the teeth will slip if you don't have a ton of them engaged (or they are excessively loose) but otherwise they are amazingly useful.
Author appears to have concerns about the belts working out of the top of the housing, but is seemingly ignoring the four existing screws that could be used to fasten a top plate to prevent this?
This is why I thoroughly enjoyed building my Prusa as a kit. You see little tricks like this, including this exact one. Or how they embed threaded nuts, or how the print is shaped and oriented to be made without supports, etc. Building the prusa saved a bit of money, but was also a crash course in how to design.
That center pin looks fragile, especially if it's printed along Z axis. How about printing teeth that'd mesh with the belt?
I agree that it looks fragile, but my experience with a similar kind of design in practice has shown that it’s not actually as big of an issue as it seems it would be.
A few other things that can help:
- for the “it can work itself out” problem, I’ve had pretty good luck on other projects by making a standardized “dual dovetail” (bowtie) male/female slot with clearance tuned so that the male piece fits tight into the slot. Printing the male part flat on the bed means that the filament is oriented correctly for strength
- integrated zip tie slots. One design I worked on that was similar to what OP did but needed better grip… I made the slot where the folded over timing belt meshes longer, removed the printed centre pin, and put two integrated zip-tie slots: one near where the centre pin was, so I can use the zip-tie ratchet to actually pull the belt backwards and put tension on it; the other was near the mouth of the jaws, so that after tensioning it I could use a second zip-tie to “lock” the jaws onto the belt
- a third one: you make a little block with dovetails that looks like the “circle plus mouth” piece from this post, but also include a pocket for a nylok that you can use to add tension to the belt after getting the belt secured inside the clamp.
I have tried printing teeth that would mesh with the belt but didn’t get particularly good results from that. Possible that newer printers have better resolution and could print the teeth better, but my test with that just ended up with a little bit of a rougher surface and the teeth couldn’t actually mesh.
I have printed GT2 teeth on a prusa mk3s+ that mesh perfectly
Awesome! I’ll have to try it again. Last time I tried was on some Wanhao i3 clone like a decade ago and it was not great, but the printer itself was not great all around other than the price :).
If you printed teeth, you'd have to make sure the teeth in the top loop are precisely aligned in relation to the teeth in the bottom loop based on the length of the belt, right? That seems like it could be limiting
Printed teeth are weaker than the belt's own teeth.
Conversely, with this design the center pin could break loose and the belt would still hold, because the pin just has to stop the loop from fitting through the slot.
That’s exactly it… if you tolerance the mouth reasonably tight, the centre pin doesn’t actually carry much, if any, of the load. Friction in the mouth carries a bunch of it, especially if you add the zip-tie clamp from my sibling post.
Timing belts are crazy strong. From my FRC work the amount of abuse and weight they will deal with is handy.
Belts will burn through polycarb if it isn't tracking properly. They are strong enough to deform any pulley mount on a tight fit.
Only minor complaint is that the teeth will slip if you don't have a ton of them engaged (or they are excessively loose) but otherwise they are amazingly useful.
It looks like the pin doesn't take much load. It looks like the pin holds the teeth open, while the load is on the shoulders of the entryway.
Also works well with a zip tie or 2. The belt holds itself in place as long as the tension is there.
Most of my old printers still use this method on them. [1] You can also use another ziptie as a tensioner.
[1] Very old example. https://flic.kr/p/db51Ue
This is indeed a very elegant fastening method.
I wouldn't be surprised if the designer of that CNC machine borrowed it from the popular Prusa MK2 3D printer, which uses a very similar design for its Y-axis belt: https://help.prusa3d.com/guide/2-y-axis-assembly_79992#81120
Prusa Core One uses that technique, or I miss something.
https://help.prusa3d.com/guide/5-corexy-assembly_835522#8648...
Author appears to have concerns about the belts working out of the top of the housing, but is seemingly ignoring the four existing screws that could be used to fasten a top plate to prevent this?
or, if you upped the pin diameter, you could thread a hole through the pin and throw a washer on top
Or just forego the pin and put a countersunk machine screw, washer, and nylock nut in it's place.