What Was Happening
A customer in Clapham SW4 contacted us about their Bambu Lab X1 Carbon, which had been producing prints with visible layer shifting. The problem was intermittent — some prints would finish perfectly, whilst others would develop a sudden horizontal offset partway through. The shift always appeared as a clean jump rather than a gradual drift, and it could happen on the X-axis, the Y-axis, or sometimes both within the same print.
The issue had been occurring for about a month and was getting worse. Initially, it happened perhaps once in every five or six prints. By the time the customer contacted us, nearly every print longer than a few hours was affected. The customer had tried different filaments, different models, and different slicer settings without any improvement. They were primarily printing large ABS enclosure parts for an electronics project, which meant failed prints wasted significant time and material.
Our Diagnosis
Layer shifting on a CoreXY printer like the X1 Carbon has a relatively short list of possible causes: belt tension, pulley connection, motor fault, or — more rarely — a board-level issue. We started with a physical inspection of the belt path.
Both the X and Y belts had adequate tension — firm when plucked, with no visible fraying or wear. However, when we checked the X-axis motor pulley, we found the problem immediately. The grub screw (set screw) that locks the pulley to the motor shaft was loose. It hadn’t fallen out, but it had backed off enough that under high acceleration the pulley could slip on the shaft by a fraction of a millimetre. Each slip translated directly into a layer offset in the print.
We checked the vibration logs in Bambu Studio, which confirmed acceleration spikes coinciding with the shifting events. The customer had also been running a heavily modified speed profile downloaded from an online community — print speeds and acceleration values significantly beyond Bambu Lab’s recommended specifications. Whilst the X1 Carbon is a fast printer, the aggressive profile was multiplying the forces on the loose pulley and triggering the slip more frequently.
The Y-axis pulley grub screw was checked and found to be secure. Frame rigidity was good, and the belt tension on both axes was within specification.
How We Fixed It
We removed the X-axis motor pulley, cleaned the motor shaft and the inside bore of the pulley to remove any debris or oil residue that could prevent a solid grip. The pulley was repositioned on the shaft, aligned correctly with the belt path, and the grub screw was tightened firmly onto the flat of the motor shaft. We applied a small amount of medium-strength thread locker to the grub screw to prevent it from backing off again under vibration. The thread locker we used is removable with hand tools if the pulley ever needs servicing — it prevents loosening but doesn’t permanently bond the screw.
With the mechanical fix in place, we re-ran the printer’s full auto-calibration sequence. This included the vibration compensation routine, which measures the resonant frequencies of the frame and adjusts acceleration profiles to avoid exciting those frequencies during printing. Since the loose pulley had been affecting the vibration characteristics, the old compensation data was no longer accurate.
We also updated the printer’s firmware to the latest stable release, which included refinements to the motion planning algorithm. Finally, we discussed the speed profile with the customer and adjusted the acceleration values back to within Bambu Lab’s recommended range for ABS printing — still fast, but within the mechanical limits of the hardware. We kept the print speed at 200 mm/s for infill and 120 mm/s for outer walls, which is well within the X1 Carbon’s capability.
To verify the repair, we ran a large test print — a tall, multi-feature model that would expose any remaining shifting over a sustained period. The print completed cleanly with no offset on any layer.
The Result
The customer’s 14-hour ABS enclosure print — the same model that had failed repeatedly — printed successfully on the first attempt after the repair. Layer alignment was perfect throughout, with no visible shifting or offset at any point. The entire diagnosis and repair took approximately 90 minutes. The customer has since completed several multi-day print runs without any recurrence.
The customer also resumed the same high-detail models that had been failing previously and confirmed repeatable success across multiple overnight jobs.
Why This Happens
CoreXY printers use a crossed-belt system where both motors work together to move the print head in X and Y. This design is fast and precise, but it means every mechanical connection in the belt path — pulleys, idlers, tensioners — must be completely secure. A loose pulley on either motor shaft introduces backlash into the system, which at low speeds might not be noticeable but at high acceleration produces enough force to slip the belt by one or more steps.
Grub screws loosen over time due to vibration. Every print cycle applies thousands of rapid acceleration and deceleration events to the motor shafts, and the cumulative effect on a grub screw without thread locker is predictable — it gradually backs off. The X1 Carbon prints faster than most consumer printers, which means higher acceleration forces and faster grub screw loosening. Factory assembly typically applies thread locker, but not every unit receives the correct amount, and even properly applied thread locker can fail after months of heavy use.
The intermittent nature of this fault makes it particularly frustrating to diagnose. The shift only occurs when the acceleration is high enough to overcome the remaining friction on the loose pulley — which depends on the specific geometry of the model being printed, the print speed, and even the ambient temperature.
Local Help in Clapham SW4
We cover Clapham SW4 for 3D printer setup, calibration and repair. Whether you have a Bambu Lab, Creality, Prusa or another brand, we can diagnose and fix mechanical and software issues either at your location or at our base in Putney, which is a straightforward trip from Clapham via the A3 or Wandsworth Road.
We also offer periodic preventive checks for high-usage printers to catch pulley and belt issues before they cause failed long prints.
Prevention Tips
- Apply medium-strength thread locker to pulley grub screws during initial setup. This is the single most effective prevention measure — it costs pennies and takes seconds, but prevents the most common cause of layer shifting on CoreXY printers.
- Check pulley grub screws every 500 print hours or every 3 months. With the printer powered off, try to rotate each motor pulley independently of the shaft. If it moves, the grub screw needs tightening.
- Stay within the manufacturer’s recommended speed and acceleration limits. Community speed profiles are often optimised for one specific machine in ideal conditions. Higher acceleration means higher forces on every mechanical joint, which accelerates wear on pulleys, belts, and bearings.
- Monitor Bambu Studio vibration logs after long prints. Unusual spikes in the vibration data can indicate developing mechanical looseness before it produces visible print defects.