Bed mesh leveling vs tramming: Klipper bed_mesh, Marlin G29 and Prusa's mesh

Mesh compensation and tramming are different jobs

Tramming physically adjusts the bed screws so the bed sits true to the machine frame. Marlin’s G35 tramming assistant exists for exactly this, and takes the screw thread type, for example G35 S40 for M4 clockwise screws ^[10]. Mesh leveling leaves the bed where it is, measures its shape, and moves Z during printing to follow it ^[11].

Klipper’s bed mesh guide is direct about the limits of the second approach: software correction only approximates the bed’s shape, and it cannot compensate for mechanical or electrical problems such as a skewed axis or an inaccurate probe ^[1]. Marlin’s UBL guide likewise says results improve the more physically level and straight the bed is, and asks for physical leveling before the first probe run, especially on printers without a display ^[8].

Tools for the tramming step:

• Klipper BED_SCREWS_ADJUST moves the nozzle over each screw for a paper test, cycling until no screw needs more than a small adjustment ^[4].
• Klipper SCREWS_TILT_CALCULATE probes each screw position after G28 and reports turns as clock-face time relative to the first screw: “CW 01:15” means one and a quarter turns clockwise. Repeat until every adjustment is under about 6 minutes ^[4].
• Bambu Lab X1/P1 beds are trammed at the factory. If shipping disturbs them, Bambu Lab provides a G-code file that parks the nozzle over each knob for 30 seconds, then repeats twice with 15-second pauses for fine adjustment; tightening a knob moves the bed away from the nozzle ^[14].

Adjusting bed tilt invalidates the Z offset of a probe mounted beside the nozzle, so recalibrate the probe after tramming ^{[4, 5]}. The first-layer Z offset guide covers that step.

Klipper bed_mesh

Calibrate the probe’s Z offset before meshing ^[1]. A minimal rectangular configuration needs mesh_min and mesh_max, both given in probe coordinates. Other options and their defaults ^{[2, 1]}:

Klipper’s worked example probes 5 × 3 points and interpolates with mesh_pps: 2, 3 into a 13 × 9 mesh ^[1]. If fade is enabled, Klipper suggests ending it around 10 mm and warns that on a badly warped bed it can stretch or shrink the print in Z ^[1].

Run BED_MESH_CALIBRATE after homing; the mesh is active immediately. With ADAPTIVE=1, only the area under the objects in the current G-code is probed, and that profile should not be saved for reuse ^[3]. Regular meshes go to the default profile and can be saved under a name with BED_MESH_PROFILE SAVE=<name> and SAVE_CONFIG. Klipper no longer loads a profile at startup, so add BED_MESH_PROFILE LOAD=default to the start macro if you rely on a stored mesh ^[1].

Marlin: bilinear and UBL

Marlin supports one leveling type per build ^[6]:

G28 disables leveling by default, so follow it with M420 S1 to re-enable a stored mesh, or build with RESTORE_LEVELING_AFTER_G28 ^{[9, 6]}. Bilinear leveling probes the whole grid with one G29 after homing and is saved with M500; without a probe, PROBE_MANUALLY walks through the points with a paper test ^[7].

UBL splits the job into phases. Marlin’s sample sequence is G29 P1 to probe automatically, G29 P3 to fill points the probe cannot reach, G29 S0 to store the mesh in slot 0, G29 F10 for a 10 mm fade, G29 A to activate, and M500 to save. G26 prints a validation pattern and G29 P4 edits bad areas. In slicer start G-code, G29 L0 followed by G29 J reloads the stored mesh and tilts it to match a quick three-point probe ^[8]. UBL costs about 50 kB of program space against about 5 kB for bilinear, which matters on 128 kB boards ^[8]. Default grids are 3 × 3 for bilinear and 10 × 10 for UBL ^[6].

M420 Z10 sets a fade height so correction tapers to zero by 10 mm ^[9].

Prusa: mesh bed leveling and Bed Level Correction

Prusa printers probe a grid before each print and interpolate a virtual mesh ^[11]. On the MK3 family, Settings → Mesh bed leveling offers:

• Mesh: 3 × 3 by default, 7 × 7 for more detail.
• Z-probe nr.: 1, 3 or 5 readings per point; Prusa suggests 3 or 5, since the result is averaged.
• Magnet comp.: three of the 49 points on a 7 × 7 mesh sit near magnets and can read up to 80 µm off; this option replaces them with values from neighboring points.

The MINI uses a 4 × 4 grid, and LoadCell models (MK4/S, MK3.9/S, XL) probe within the print area ^[11]. If one side still prints thinner, Bed Level Correction offsets the left, right, front and rear by up to ±100 µm, in steps Prusa suggests keeping to about 10 µm ^[12].

Adaptive meshes and probe quality

OrcaSlicer can generate the adaptive mesh commands itself from the objects on the plate, with a default probe spacing of 50 mm. For Marlin it emits G29 with left, right, front and back limits; for Klipper it calls BED_MESH_CALIBRATE with ADAPTIVE=0 ADAPTIVE_MARGIN=0 because OrcaSlicer has already applied its own margin ^[13].

A mesh is only as good as its probe. Klipper treats a PROBE_ACCURACY range above 25 µm as unsuitable for typical bed leveling, recommends probing at a consistent temperature, and requires a new mesh after any probe recalibration ^[5]. If the first layer still varies after meshing, see bed adhesion. Ridges that appear higher up the print are a separate problem covered under Z banding.