Core One Chopstick Mod (Umbilical / X motor jamming)  

Core One Chopstick Mod (Umbilical / X motor jamming)

When printing to the back left corner of Prusa Core One, the umbilical (cable between printhead and frame) may sag, and eventually get stuck between the nearby stepper motor and the printhead. This is observed only when using the back left corner of the print bed and causes diagonal layer shifts.

An installed MMU makes matters worse as it removes the elevated 90-degree PTFE tube mount on the printhead so the tube does not contribute any upwards force on the umbilical.

Core one is a fairly new product, and this seems like a design oversight. The problem is widely recognized online, still I didn't immediately find a convincing solution (e.g. a full-blown drag chain adds unwanted mass to the print head). Oh well - the experience of putting in the finishing engineering touches myself is an experience money cannot buy (not that I didn't pay enough for a "complete" product). But, I digress.

So here is my solution, "chopstick mod". No disassembly required other than two swing arm screws and the top cover.

I'm documenting this while implementing so it's not yet long-term tested. And it's a hack so take it mostly as inspiration, and absolutely not a "step-by-step guide with success guaranteed by following steps religiously".

The printer is now running happily, including a test rectangle to advertised specs (250x220 minus margin for tool movement) and it seems to work as intended. Long term degradation, let's see. I don't fully trust the material.

Parts are under permanent movement with aggressive acceleration, for many hours possibly at elevated temperature. Therefore I'm preferring wire wrap to cable ties (uncoated plain steel wire would be even better) but using cable ties would also seem logical, possibly even an improvement.

Note, pictures need clicking. Also editing is not possible so please check any follow-up discussion.

So, here it comes: "Chopstick mod".

• Build with continuous movement in mind, with acceleration spikes, temperature cycles, and mechanical shocks when the umbilical collides with other parts (the job of the mod is just to prevent anything getting stuck). This needs to be robust or things may get ugly (spaghetti monster vs blob-of-doom...)

The horizontal angle can be adjusted within some limits when attaching the arm. The picture shows "all the way up" but this will probably cause noisy collisions with the ceiling. In my printer, the resulting clearance umbilical / X-stepper is about 15 mm so there is no need to push it all the way as shown.

Two large cable ties make perfect guide rings for the umbilical (letting it slide and bend almost freely, just providing vertical support.
Note, the umbilical will rest on the flat side. On the chopstick, pull them "as tight as possible" but the position along the chopstick axis needs to be adjusted later so the printhead can move everywhere.

The existing Velcro ties find a new home on the chopstick:

The loose end of the cable ties gets attached to the chopstick, forming guidance rings around the umbilical (hint, don't forget to insert the umbilical...)

The connection should be tight but still allowing adjustment.

Here is the expected clearance between umbilical and X-motor:

From the top, it looks as follows BUT...

... we need to cut the chopstick to avoid binding:

So, that's it: Note that operation is not necessarily noise-free (the swing arm may hit the back wall, probably I'll glue some felt pad on the spot) but the head should not get stuck anymore.

I put the .step file here: https://www.printables.com/model/1467315-core-one-xy-area-test-umbilical-getting-stuck. It exercises only slow, extruding movements, take it as a start. Putting multiple small objects into the corners is another possible test that results in faster "travel" movements.

Happy printing, comments are welcome!