RE:
same problem here with my Kit Core One received sept 15 2025 : random calibrating/homing/bumping XY to front corner .... some times it bounce 5 times ... sometime endless bumping
--Gantry is perfectly align -- done belt tension adjustment multiple time = OK -- still have random calibration/endless bumping -- extended homing also create bed over heat when you ask chamber temperature to reach higher setpoint : initial bed set point is 115degC when chamber need to pre heat
Open a prusa chat for help , keep telling me to adjust belts ! Belt adjustment should not be a science project !!! the tolerance for this initial X-Y homing/calibration is simply too tight ! all my prints are OK so this initial calibration/homing have impossible tolerance to achieve ... Prusa is not interested to change this in the FW so i'm stuck with my expensive unrialable printer because return shipping would cost too much 😡
Printing since 2019 , Ex MK3s owner , current Core One Kit owner
RE:
same problem here with my Kit Core One received sept 15 2025 : random calibrating/homing/bumping XY to front corner .... some times it bounce 5 times ... sometime endless bumping
--Gantry is perfectly align -- done belt tension adjustment multiple time = OK -- still have random calibration/endless bumping -- extended homing also create bed over heat when you ask chamber temperature to reach higher setpoint : initial bed set point is 115degC when chamber need to pre heat
A mistake I kept making in the first couple of weeks was not fully understanding out how belt tensioning affects gantry squareness. Prusa need to do a better job of explaining this because I feel it is a source of a lot of frustration. To start with, the gantry needs to be square with the belts completely loose. Next as the belts are progressively and evenly tightened, squareness needs to be periodically checked. After completing the tensioning process it needs to be checked again. My view is that Prusa should redesign how the gantry is attached to the y-axis smooth rod bearings.
RE: Failed Y-calibration of a new CoreOne
My endless bumping/XY calibration still present in random condition , even after multiple belts tunning , gantry alignement , pullys alignement...
i'v been working as automation/electronic specialist into a fast packaging plant for 35 years , and 3D printing since 2019 : please consider my knowledge …
i 3D print in my garage , including winter low room temperature ( 15degC) , i dont want to mess with belt tunning and failed prints because of XY calibraion failed.
CoreOne FACT :
--MANY MANY users have this XY calibration/CornerBumping problem
5 pages thread link here
18 pages thread link here
-- the abnormal/extended 5 minutes calibration CornerCollision/bumping « multiple retry» will eventually dammage/misalign the printer top structure + Axis parts . Even the current « normal » amount of bumping are excessive .
--Most user who still encounter failed XY calibration have good prints thus confirm calibration tolerance being too tight : they still perform good prints
--High speed corner collision doesnt help stable calibration readings? Why not go slower?
--Some initial VERY cold chamber (low rooms temperature) doesnt seem to help
--My first print attempt often fail at XY calibraion , after a reset and AutoHome request , it usually pass and work fine
Core One firmware change suggestion :
--Slow down that corner hit/collision test (the one happend at each prints starts)
--do only 1 or 2 pairs of bumping calibration and simply trigger this message if calibration fail :
« XY calibration does not meet specification : abort print or continue anyway ? »
--Change the initial bed 115degC setpoint when chamber pre heat are requested : this cause bed over heat and can lead to dangerous over heat/bed dammage/fire :
Make the bed initial temperature setpoint same as print bed setpoint until the chamber reach its setpoint
Printing since 2019 , Ex MK3s owner , current Core One Kit owner
RE: Failed Y-calibration of a new CoreOne
Core One firmware change suggestion :
--Slow down that corner hit/collision test (the one happend at each prints starts)
--do only 1 or 2 pairs of bumping calibration and simply trigger this message if calibration fail :
« XY calibration does not meet specification : abort print or continue anyway ? »
--Change the initial bed 115degC setpoint when chamber pre heat are requested : this cause bed over heat and can lead to dangerous over heat/bed dammage/fire :
Make the bed initial temperature setpoint same as print bed setpoint until the chamber reach its setpoint
This reads like the release notes of the 6.4.0-RC firmware. The homing has slowed down and it's much gentler. It still has a few bugs but so far, after 2 weeks it stable enough to use until the actual release drops.
RE: Failed Y-calibration of a new CoreOne
The RC wrecked my XL but was fine on the Core Ones. Definitely more lenient when it comes down to home calibration.
Formerly known on this forum as @fuchsr -- https://foxrun3d.com/
RE: Failed Y-calibration of a new CoreOne
The RC wrecked my XL but was fine on the Core Ones. Definitely more lenient when it comes down to home calibration.
Read somewhere that on XL it was crashing the toolhead into the dock?
RE: Failed Y-calibration of a new CoreOne
thanks Hyiger ... i'l wait for the official release ... dont wanna compile/code now
https://github.com/prusa3d/Prusa-Firmware-Buddy/releases 
______________________________________________________
Lazier homing, more cautious calibration
Homing behavior on the XL and CORE One has been refined to avoid unnecessary movements. In cases such as filament load or unload, the printer now performs only basic homing, skipping the extra diagonal movements. If a precise position is required later, only the refinement step is executed. For unload operations with auto-retraction, homing is still performed to ensure the correct filament removal angle.
This change applies only to specific situations—G-code G28 without parameters, which is placed at the start of every print, will still trigger a full homing sequence. We are also considering updating slicer profiles in the future to allow optional homing.
In addition, the long and noisy precise homing calibration is no longer run automatically. Instead, the printer now checks reliability and suggests the calibration only when needed, and you can always choose to skip it. If required, the precise homing calibration can be started manually at any time from the Calibrations menu.
Printing since 2019 , Ex MK3s owner , current Core One Kit owner
RE: Failed Y-calibration of a new CoreOne
Read somewhere that on XL it was crashing the toolhead into the dock?
Yes. Brutal. After three or four times it ripped the belts out of their holders
Formerly known on this forum as @fuchsr -- https://foxrun3d.com/
RE: Failed Y-calibration of a new CoreOne
dont wanna compile/code now
No need You can just download the precompiled code from their GitHub site (under Releases)
Formerly known on this forum as @fuchsr -- https://foxrun3d.com/
RE: Failed Y-calibration of a new CoreOne
thanks , not familiar with github , i'm no fan of beta release but ...i done it ...
it give me hope in prusa .... Auto Home become simpler .... time will tell
Printing since 2019 , Ex MK3s owner , current Core One Kit owner
RE:
Just to add another voices here really - all the above same for me.
Mk4S upgrade to Core One. Pulleys installed correctly etc. Homing and calibration hasn't really worked from the start - I get a single print then it starts bumping and eventually refused to print. Firmware 6.4.0-RC was advised by support - this seems (so far) to be an improvement (over it just not working).
Things I've tried:
- Re-attaching the belts (via Nextruder reassembly) - they seem very short for the job and I had real trouble getting the teeth clamp near-even; dont know why we couldnt have had 10mm more belt to make it easier? Either way this made no difference.
- Many belt tightening sessions - now using the 6.4.0-RC firmware and the light-based method for tightening. This seems alot better than the app (measurements vary widely with the web-app); however I cant do more than a couple of prints without the belts loosening and needing re-tightening. I've started just ignoring the warnings and printing anyway - at least I can print like that now.
- Gantry bending - not so sure about this one really. Feels like a route to breakage.
- VFA test prints etc are actually fine. Also other prints I've done tend to be fine aswell - so it does feel like the tolerances are too tight or something (as mentioned above). I just hope it doesnt destroy itself with all the homing attempts.
Either way - given the upgrade was 3 months late and I've also had to solve the grounding-reset bug, its all a bit of a shame really - I believed in Prusa and their approach. The Mk4S was awesome - built the kit and it just worked from day one. I cant be downgrading and sending anything back so I just hope they move to improve the situation and look after their customers.
For now though - if you have a MK4S - I wouldnt advise upgrading until things here have stabilised. Feels like theres a few design flaws with the CoreOne that need ironing out.
RE: Failed Y-calibration of a new CoreOne
Gantry bending: may take some time to get used to the idea but to me it's plausible and necessary. Only, this should have been done (for the kit) during assembly, not on the complete printer. But, it is as it is... at least the correction does not require any excessive use of force, and the parts are easily accessible.
RE:
My Core One has been running flawlessly since my kit build up and the painful few weeks of tweaking pertaining to this issue. The latest Prusaslicer version that was just released with the "balanced" mode is working well along with the new belt tuning method. For those of you still having excessive "knocking" calibration, stick with it and you'll get there. Just make sure (with the belts fully loosened) to manually bend you X gantry till both sides touch your front frame at the same time. Yes, I had seen how then belt tightening can reverse this (gantry out of square) but some repeat steps can get it there with fully tightened belts.
There still seems to be some conflicting information from Prusa on what is the exact tension value for X and Y (seems that they keep change the recommendation) but just went with their newest tuner.
RE: Failed Y-calibration of a new CoreOne
Good to hear and thanks for the nudge on gantry bend - ill give it a try.
With assembled printer is it a matter of pulling one side of gantry forward (Y direction) while holding the other static? Was a bit unsure of which specific area needed bending from the instructions - my bad clearly but added to the reticence.
RE: Failed Y-calibration of a new CoreOne
Yes but I believe one needs to loosen the belts first or they'll get stressed badly (risk of ripping out the belt tensioning screws?)
RE:
Yes, completely loosen the belts before attempting to bend the gantry. Move the gantry to the front and observe which side needs to be bent forward, wedge one of the universal wrenches on the opposite side between the gantry and the stop, then slightly bend the other side with light pressure. Check if square, then repeat if necessary. When tightening the belts make sure the screws are turned evenly on both sides.
RE: Failed Y-calibration of a new CoreOne
Yes, completely loosen the belts ( I did not loosen all the way the first time and broke the idler arm on X - good thing it is a 3D part, just printed a new one on my Mk3)
RE: Failed Y-calibration of a new CoreOne
It's almost as if on the upgrade kits, Prusa should supply enough PC-CF that you can preprint the replacements on your MK4 before converting it into a Core One.