While the 🥰 CORE One is an excellent printer with a well-designed stock system, this modification offers mechanical improvements for enthusiasts seeking enhanced precision and reliability. The Pins Mod inspired by the Voron community upgrades the XY gantry system by replacing the standard M3 bolts used for idler mounting with 3mm steel axles (pins).

I hesitated for quite some time whether to publish this model at all - not because it doesn't work, but because the build process requires time, patience, and the willingness to understand what you're doing rather than just following pictures. It's not overly difficult, but it is possible to mess it up!

Personally, I would never build a CORE One without this modification, so I can only encourage you to consider going this route.

I'll do my best to describe every detail of what you need to do and help you understand the reasoning behind each step. This will be somewhat lengthy, but it will help you get it done successfully.

• If at any point you find a way that works better for you - do it your way!

• If you want to spend the time designing tools to help with idler alignment, please do so and share it with us! I've provided all the CAD resources to give you a quick start.

That being said, here's a time estimate: Plan around 90 minutes for reading, understanding, and building.

You'll be rewarded with a beautifully play-free gantry system at virtually no extra cost - it's truly worth the effort 😎.

CORE One L user?

The XY Motor Mounts for the Core One L require slightly different parts. Find these adapted mounts in the companion model: Core One L Pins Mod. All other parts and instructions are exactly the same, just print those two replacement parts.

XY Motor Mounts
Modified to accept steel pins instead of M3 bolts for idler mounting, providing enhanced precision and reduced play.

XY Carriage
Updated design to work with the new pin-based idler system. Available in two versions:

• Standard version: Compatible with 9mm smooth idlers (stock configuration)

• Enhanced version: Designed to accommodate 10mm toothed idlers for superior belt engagement

Belt Tensioner Pulley
Redesigned to utilize steel pins with two variants available:

• Standard version using M3 square nut

• Enhanced version with heat set insert for improved reliability

Press-In Tools
Small tools to help you press in the pins without damaging the bearings or the printed parts. The tools are provided in 2 variants:

• Smooth: Match the diameter of ths stock smooth idlers

• Toothed: A variant that match the toothed idlers

Hole Preparation

1. The 3mm pins require a press-fit into the 2.9mm sized holes

2. To ease insertion and reduce stress on printed parts:

   • Use an M3 bolt with washer to carefully enlarge the holes

   • The M3 bolt acts as a drill to create a smooth surface that is also perfectly dimensioned to

     • Hold the pin securely in place

     • … without putting to much stress on the printed parts

   • For the pins that go all the way through the printed part:
     Perform this process for both holes

   • For the pins that go in from the one side but do not go all the way through (there is only an smaller opening on the opposite side to allow to insert a tool to remove the pin)

     • First do this process as described for the hole on the outside of the part (the one where you will insert the pin; this is the one easily accessible)

     • Then use a longer M3 bolt, enter it through the hole you just prepared, and enlarge the inner hole exactly the same way (sorry, no photo for that)

Make sure you prepare all holes as described. Otherwise you will risk to split the parts if there is too much stress especially along the layers.

Heat Set Insert Installation

• If using the enhanced belt tensioner variant: Install heat set inserts

After having assembled the parts, I think the decision to have this 1mm endstop just made the process more complicated with no real benefit. But I did not want to change the design but get the project finished. And you do have the option to just drill open the last 1mm as described with no downside. 

⚠ Critical Safety Note: Never force pins through bearings - any stress on the inner bearing race can cause permanent damage

Initial Preparation

1. Ensure pins have slight chamfers on both ends

2. Test-fit each pin through idler bearings completely

Sizing Process (if pins are oversized)

1. Secure pin in cordless drill chuck

2. Use 180-grit sandpaper to uniformly reduce diameter

3. Rotate drill while applying light, even pressure with sandpaper

4. Work the entire shaft length for consistent sizing

5. Reverse the pin the the drill and repeat the steps on the now accessible areas

6. Clean pin thoroughly and retest bearing fit

7. Repeat process until pin slides freely through bearings

   • Typical sizing time: 1-2 minutes per pin

Quality Check: Final pins should insert smoothly into bearings with no resistance or binding

Required Tool: Parallel-press tool or vise to ensure straight pin insertion for all components.

Required Pin Length:

• Belt Tensioner: 18mm

• XY Carriage: 2x 20mm

• Motor Mount Right: 18mm and 30mm

• Motor Mount Left: 20mm and 30mm

Initial Pin Insertion

• Insert pin from the side where you would insert the bolt on the original part 

• Press pin until it slightly protrudes on the interior side

  • E.g. use provided spacer tools for this step

  • The pin must protrude approx. 0.2mm to help you with alignment while still allowing you to insert the idler into the part

• You can use the provided press-in spacer tools for this step

Idler Installation

• For single idler: Align appropriate idler (9mm smooth or 10mm toothed) onto protruding pin; use e.g. a small Allen key entered from the opposite site to help align the idler

• For idler stack: Use Prusa-provided idler stack installation tool

• ⚠ Make sure the idler is aligned perfectly. Then carefully press in the pin in only half a mm, ensuring the idler is still aligned correctly.

  • If you feel any resistance, stop pressing and check the alignment.

  • If you feel the part bending under the pressure (e.g. for the belt tensioner part), use the provided spacer tools already for this step. That will make alignment a bit more tricky but it's doable.

⚠ If the alignment is not correct and you apply too much force you will push the bearing into the idler. This must be avoided. If it happens, replace the idler and if you are like me: Just order a spare one right upfront and chances are good that you won't mess it up.

• Check again the alignment and that you did not press in the bearing into the housing of the idler.

• When you are sure everything is aligned, install spacers to create proper fit and prevent part flexing in the next steps

Final Pin Installation

• Press pin completely through the assembly

• Critical: Maintain parallel pressure throughout insertion and keep the alignment of the idler

• Avoid any side forces that could damage bearings or deform parts

For all pins:

• You inserted the pin from the side where the bolt goes in in the original part

• Pres the pin in until it is flush to the surface where you entered the pin

Be aware:

• Some pins will be flush on both sides of the part.

• Some pins will have an end-stop of 1mm to the outer surface

• Some pins will end inside the part

Verify that the pins are flush with the outer surface.

Confirm idlers rotate freely without binding. Check for proper alignment and smooth operation.

Check that the bearings are flush with the idlers. They must not be pushed inward.

Make sure the layers haven't separated. There should be no visible stress marks on the printed parts.