Cornering speed limits: classic jerk, junction deviation and square corner velocity

Three ways firmware describes a corner

When a path turns, the toolhead cannot stop dead at every vertex without wasting time and disturbing extrusion, and it cannot take sharp corners at full speed without shaking the frame. Firmware resolves this with a cornering limit, expressed in one of three ways.

Classic jerk (Marlin). Jerk is the largest change in velocity, in mm/s, that may happen instantly at a junction. Marlin explains the trade-off: too low and the nozzle lingers on small segments and corners and can leave blobs; too high and direction changes apply enough torque to cause ringing or skipped steps. With CLASSIC_JERK enabled the defaults are 10 mm/s for X and Y and 0.3 mm/s for Z, and DEFAULT_EJERK is 5 mm/s, which Linear Advance may use ^[1].

Junction deviation (Marlin 2 default). Junction deviation is now Marlin’s default cornering mode, and enabling CLASSIC_JERK overrides it. JUNCTION_DEVIATION_MM defaults to 0.013 mm, smaller values corner more slowly, and Marlin converts from jerk with d = 0.4 × jerk² ÷ printing acceleration ^[1]. With jerk 10 mm/s and 1,000 mm/s², d = 0.4 × 100 ÷ 1,000 = 0.04 mm. At Marlin’s default printing acceleration of 3,000 mm/s², the same jerk gives 0.0133 mm, close to the 0.013 default ^[1].

Square corner velocity (Klipper). Klipper’s square_corner_velocity is the maximum speed through a 90° corner, 5 mm/s by default. Wider angles get higher speeds and sharper ones lower; 0 makes the toolhead stop at every corner ^[4]. Klipper’s look-ahead derives the junction speed for each angle from this one value using approximated centripetal acceleration ^[5]. Change it at run time with SET_VELOCITY_LIMIT SQUARE_CORNER_VELOCITY=<value> ^[6].

Marlin’s run-time values are reported by M503 and saved with M500 when EEPROM storage is enabled ^[2]. RepRapFirmware works in mm/min, so multiply mm/s by 60: 50 mm/s becomes M566 X3000 Y3000 ^[9].

Where the slicer fits in

Slicers can send cornering limits in G-code. PrusaSlicer’s machine limits include Maximum jerk for each axis in mm/s and Maximum junction deviation, which is sent as M205 J and only applies on Marlin when greater than zero ^[14]. OrcaSlicer has the same machine limits under Motion ability → Jerk limitation; for a Marlin 2 printer that uses classic jerk, set Maximum Junction Deviation to 0 ^[11].

OrcaSlicer also sets per-feature jerk: separate values for outer wall, inner wall, infill, top surface, first layer and travel. Its guidance is lower jerk on outer walls and top surfaces for quality and higher on infill and travel for speed, all capped by the printer’s machine limits. On Marlin 2 with junction deviation, OrcaSlicer applies the same 0.4 × jerk² ÷ acceleration relationship ^[10].

OrcaSlicer’s cornering test

OrcaSlicer’s guide suggests skipping this test if corners already look fine at the speeds you want ^[9]. Otherwise it detects the firmware and tests the matching parameter: square_corner_velocity on Klipper; junction deviation on Marlin 2 when Maximum Junction Deviation is above 0 and classic jerk when it is 0; classic jerk on legacy Marlin; and maximum instantaneous speed change on RepRapFirmware. Expect junction deviation between 0.0 and 0.3 mm and jerk from about 1 to 20 mm/s or higher ^[9].

1. On Marlin 2, confirm junction deviation is enabled in the printer’s advanced settings.
2. Set acceleration and speed high enough to provoke ringing, for example 2,000 mm/s² and 100 mm/s, and load an opaque, high-gloss filament.
3. Run the cornering test with a wide start-to-end range. If quality never drops, raise the end value and repeat.
4. Measure the height at which corners start losing sharpness and read the value at that height in OrcaSlicer.
5. Print a second test whose maximum sits near that point; the Ringing Tower model is recommended for seeing the limit.
6. Save the result, then restore normal acceleration and jerk.

Steps follow OrcaSlicer’s guide ^[9]. To save, OrcaSlicer recommends its printer profile (Motion ability → Jerk limitation). In firmware, its examples are SET_VELOCITY_LIMIT SQUARE_CORNER_VELOCITY=5.0 or square_corner_velocity in printer.cfg for Klipper; M205 J0.012 then M500, or #define JUNCTION_DEVIATION_MM 0.012 with CLASSIC_JERK left commented out, for Marlin 2; and M205 X10 Y10 then M500 for classic jerk ^[9].

How cornering interacts with other calibrations

Cornering settings share the corner with pressure advance and input shaping, so the order matters. OrcaSlicer’s guide puts cornering after flow, pressure advance and retraction, and before input shaping ^[12], and its jerk page says jerk works together with pressure advance, adaptive pressure advance and input shaping ^[10].

• Klipper’s pressure advance test temporarily drops SQUARE_CORNER_VELOCITY to 1 so that pressure effects at corners stand out ^[7]. A non-zero value normally reduces extruder flow changes in corners ^[4].
• Klipper’s input shaping guide asks you to put square_corner_velocity back to 5.0 and not to raise it with input shaping, because it adds smoothing; raise acceleration instead ^[8].
• Marlin’s Linear Advance respects the extruder jerk limit and lowers acceleration on a move rather than skip extruder steps ^[3].
• OrcaSlicer lists jerk and junction deviation among contributors to vertical fine artifacts, the ripple its VFA test looks for ^[13].

Reading symptoms

Bulging corners can also come from pressure advance, and shifted layers have causes beyond jerk, covered in layer shifting. Ringing that persists at modest cornering values is a job for input shaping; see ringing and ghosting.