Temperature / thermistor sensor zip-tie rubbing on X-axis belt
I'm working on assembling a Core One Kit. I just finished the Core XY Assembly, Step 81, Page 123. (in file prusa-core-one-kit-assembly_2251_en_2025-06-06.pdf)
Looking over the assembly, I noticed that the upper belt (X-axis belt?) is rubbing on the zip tie for the chamber thermistor sensor. I also looks very close to the lower screws holding on the fan grid assembly.
I have found several forum posts where people mention they had the same issue with the belt rubbing, but no one has mentioned what caused it.
I have looked over the manual and my machine matches all of the photos.
The fan, fan grid assembly, and thermistor zip tie location look exactly like the photos. The X and Y motor mount blocks have tabs that locate them in the CoreXY plate. The back of the CoreXY plate is (later) screwed directly to the back plate. I don't see how I could possibly move or adjust anything that would then move the belt away from the fan grid assembly.
Is there a part that is not manufactured correctly? It seems like it would have to be a tolerance issue with the CoreXY plate dimensions of drill hole locations? Or the printed X and Y motor mount blocks that hold the pulleys?
RE: Temperature / thermistor sensor zip-tie rubbing on X-axis belt
Are your belts tensioned yet? It is an uncomfortably close tolerance but should clear after tensioning.
RE: Temperature / thermistor sensor zip-tie rubbing on X-axis belt
The zip tie rubbing is a regular thing. During assembly, I made sure to position the zip tie in a way, that it protude as little as possible from the fan protection and tightened with tweezers. Carefully though, since the fan protection isn't very tough.
I also tightened the screws of the fan protector quite a lot, slightly bending the outer corners of the fans, to make sure the fan protector is as far at the back as possible.
RE: Temperature / thermistor sensor zip-tie rubbing on X-axis belt
I had this also. I had to snug up the zip-tie more to get it to clear the x-axis belt.
RE: Temperature / thermistor sensor zip-tie rubbing on X-axis belt
As mentioned above - snug it up and angle it to the side slightly. I haven't noticed any rubbing at all TBH - maybe I just got lucky during my build!
RE: Temperature / thermistor sensor zip-tie rubbing on X-axis belt
Looking over the assembly, I noticed that the upper belt (X-axis belt?) is rubbing on the zip tie for the chamber thermistor sensor. [It] also looks very close to the lower screws holding on the fan grid assembly.
I had the same concern about the screws on the bottom of the Fan Grid — probably without cause, but I removed the longer bottom screws from the Fan Grid and installed shorter screws (under the Fan Grid) to secure the Fans, instead.
RE: Temperature / thermistor sensor zip-tie rubbing on X-axis belt
Are your belts tensioned yet?
Tensioning the belt just made it press more firmly on the zip-tie. That did not help.
I had to snug up the zip-tie more to get it to clear the x-axis belt.
The zip-tie just wasn't going to get much closer to the fan grid assembly. The plastic just doesn't really bend that way. Plus, the curve of my zip-tie looked exactly like the photos in the assembly manual, for example page 120 "STEP 76 Guiding the cooling fan cables".
I also tightened the screws of the fan protector quite a lot, slightly bending the outer corners of the fans, to make sure the fan protector is as far at the back as possible.
The fan grid assembly definitely wasn't sitting flat. It warps a bit as the screws are tightened. I loosened the fan screws and moved the warp around so that the zip-tie section was as close to the fan (as far away from the belt) as possible. But this did not solve the problem.
Here are some other things that I noticed that also tended to move the X-axis belt closer to the zip-tie.
- At the end of the CoreXY assembly (page 123) process, the Core XY Assembly was not screwed to the back plate. It was only tied to the back plate with the single screw connecting the bed-stop part to the back plate. I installed the screws between the back plate and the Core XY Assembly (page 159). This raised the back of the Core XY Assembly a bit.
- Later, when I installed the front vertical frame pieces ("front right profile" and "front left profile") and top profiles, this helped to square up the back plate with the base assembly and Core XY Assembly.
- Also, when installing the front profile pieces, I realized the vertical smooth rods (page 118-119) were not fully seated into the holders on the Core XY Assembly. I pushed this fully into place.
All of these small things were causing the Core XY Assembly to be higher in the front than it should have been, which would tend to move the X-axis belt closer to the zip-tie.
But in the end, the zip-tie was still touching the X-axis belt. So I just cut and removed the zip-tie.
The way the thermistor is threaded through fan grid assembly, I'm not worried about it moving out of place. And the wires can't be pinched now that the fans and fan grid are all screwed together. Removing the zip-tie is a fine solution, but what I don't understand is how this can only be an issue on some assemblies, but not all assemblies.
It looks to me like a design flaw. There is an engineered interference between these two parts. If that's not the case, then there is a fairly substantial tolerance issue with some of the parts I was sent. These are the only two explanations I can think of. Does anyone else have an alternate explanation?
>>I had the same concern about the screws on the bottom of the Fan Grid .... I removed the longer bottom screws from the Fan Grid and installed shorter screws
On a positive note, this is not an issue for me. I am not worried about the X-axis belt hitting the fan grid assembly screws. It looks like there is plenty of clearance between these.
Since my last post, I finished building the printer and successfully printed my first model. So far so good.
RE: Temperature / thermistor sensor zip-tie rubbing on X-axis belt
I forced/shaped the zip tie to look like a staple. This made the side where the belt passes fairly flat. No rubbing/contact.