January 14, 2026
Description
This mod is EXPERIMENTAL!. I have installed it on my machine, but you should consider carefully if this is something you want to change on your machine! I will not guarantee that this will not break other parts of your printer!
Damage to your printer is possible with this upgrade!! Proceed at your own risk!
Like many Prusa customers, getting the CORE One meant I needed to learn about Core XY motion system while building my machine. When I was converting my MK4S to the CORE One, I ended up breaking the belt tensioning pulley bracket during the tensioning process. I did not understand what I was doing. When looking at the printed part, I realized that even when printed in PCCF, the part was a weak point. I am not sure if this was intentional or not.
Regardless, I needed to order a replacement. Prusa sent it out quickly and I was able to get it installed a few days later. I got my printer up and running. It ran beautifully for a few months, and then the CORE One+ upgrade came out, as well as a new method to properly tension the belts using the LED strobe capability the printer has. One small problem.....
I broke it again while re-tensioning the belts. I was getting frustrated.
Prusa just recently released all the CAD parts under the new OCL! I had a wicked idea.... Could I modify the part and have it machined? (Note: my Fusion 360 skills are meager at best)
The part, when viewed in Fusion, has quite a few features that enhance printability, but would make machining the part very difficult. I played around in fusion for a few hours, and I was able to remove unnecessary bevels, print enhancing facets, lettering, and a bunch of other features that did not affect the operation of the part.
I was able to come up with this:
Dimensionally, the part is the same. The bolt holes have been sized to 2.5mm in order to allow for proper tapping to accept the standard machine screws that Prusa uses. I have removed every feature that was not 100% required for the part to perform its function. The basic idea was to reduce the number of CNC operations to the absolute minimum.
My wife, smart woman that she is, suggested I print out the designed part just to make sure everything fit properly. The left above was printed in PLA. Everything looked like it was going to work fine.
I sent the STEP file that I exported from Fusion to PCBWay for manufacturing. The part took a few days to work through the process and then a few more days to ship back to me.
I selected Stainless steel for the parts (I ordered two). Why? I'm not sure. I was looking for something reasonably strong that would not rust. I probably should have done it in aluminum, but that isn't what I chose. I have a habit of overdoing things from time to time, so this was on par for me.
Note: the parts are milled in China. This means that if you order them from the US, you will need to pay import duties for the part since you are the importer. Other countries will have to deal with their own duties as well.
In the end, PCBWay did a very good job with the machining, and the part is dimensionally accurate.
I chose not to have PCBWay tap the bolt holes. Again, that was just a choice I made. I don't know how much that might add to the cost, if at all.
Tapping is accomplished by using a M3 x 0.5mm tap. I used this one from Amazon:
I realized too late that the tap was not long enough to go through one of the "ears'" holes and reach the second, so my hope for having threads on both ears dashed. I decided to tap one side, and then drill out the other side to 3.5mm to allow the bolt to pass through and engage the threads on the opposite side. It does not matter which side you drill, since there is enough clearance in the the printer for the head of the machine screw to be either up or down.
You can see in this picture that the machine screw is oriented upside down. There's plenty of room under the tensioner for the screw head.
Note: if you manufacture this part yourself, and you make two to install, and want the machine screw to be right side up on both sides, you will need to drill opposing sides of the bracket, since they are opposite when installed int he printer.
The "bottom" screw hole presented another challenge for me since I discovered the tap was too short. It also turns out the shaft of the tap is a little larger than the cutting flutes. My solution to that was to drill out part of the bottom hole on the tensioner part way to 3.5mm, and then tap the remainder of the hole.
The rule of thumb when it comes to bolt strength is two threads... as long as you have at least two full revolutions of threads making contact, you will have the full strength of the bolt at play. So, drilling part of the hole and tapping the remaining half will not cause a loss of strength.
I don't know. I know it fixed the problem of breakage for me. The new belt tuning wizard on the printer means that you can easily tension the belt with this tensioner without fear of ripping other parts of the machine apart.
I might have the screw holes tapped during manufacturing to have an installation-ready part.
I could size the holes to be drilled at the factory to accommodate the size of the machine screws eliminating the step before installation.
I might choose aluminum instead... it's probably cheaper. Strength would probably be just fine as well.
Because this is an OCL-licensed design, I'm throwing it back out there for some to use if they choose. I'm also including the Fusion 360 file to make further changes to the design.
I may revise the design slightly in the future to make it a little more refined for machining. It depends if there is enough demand for this in the future. Nothing is stopping you from doing it, though!
1.0 - Initial release.
License:
Open Community License v1
7
ross549
