Adjustable endstops for Prusa XL

This is a simple mod that assists in keeping your corexy gantry square.

NOTICE

Installation of this mod requires that you re-tune your coreXY belts.  If you are not comfortable with this process then DO NOT PROCEED.

I recommend that you familiarize yourself with the Prusa article on belt tensioning before proceeding.

I also recommend reading this entire instruction set before you remove any parts.  The parts take 4 hours to print, spend it reading below!  The squaring process involves printing my Vernier Skew Test, it's good practice to print one ahead of time to know where you started.  If it reads 0, then you can just stop here your printer is square!

Required hardware

You will need two M5 x 25mm grub screws with flat ends.  I used M5x25mm extended tip screws but any long M5 grub screw should work as long as it has a flat tip.  You don't want a pointed or cup point screw as these could damage the carriages.

If you cannot recover them from the original parts, you will also need 4 M3s square nuts.

Installation

Loosen and remove both belt tensioners at the front of the printer.  Release the two M3 and one M8 screw holding the corexy-front-left and corexy-front-right parts to the printer.  Load the m3s nuts into the new parts and install them back on the front of the machine.

Re-install the belt tensioners loosely on both sides using the M3x22 screw removed from the old corexy-front parts

Squaring your Prusa XL

A note about the Prusa belt tuner: In my experience it performs very well if you are in a quiet room and NOT using a modern cell phone.  The fancy background noise cancellation algorithms on these devices seem to interfere with the tuner app.  If you can use an older device or even better a computer with an external microphone then you will likely have better results.

Start off by getting your belts to about the same tension and at about 80Hz.  Tightening the right belt will cause the left to tighten a little, so there may be some back and forth, that's ok just get them close.  Remember to move the gantry around a lot while tensioning to spread the tension across the system.

Print my Vernier Skew Test part.  Once that cools take a reading from the scales.

From my limited testing, a 1mm reading of the scales means 3 turns of a tensioning screw.  So for example:

• Scale reads +0.5mm, right belt was too tight per tuner:  Loosen the right tensioner screw 1 full turn and tighten the left tensioner screw ½ a turn.
• Scale reads -0.9, belts were even per tuner:  Tighten the right tensioner 1.5 full turns and loosen the left tensioner 1.5 turns.

You may have to go back and forth a couple of times to find 0.  I recommend keeping a log of skew measurements and what you changed after each.

Now, once you are at or close to zero, adjust the endstops so that when you pull the gantry forward there is no play on the left or right.  This replaces the Alignment step of Prusa's official docs.

Feedback

I want to know how it went!  Please comment with details of how the process went.  Did this help?  How much did you have to adjust the tensioners for a given skew offset?  I only have two printers to try this on so I want to hear how it goes for others.