7 Essential Laser GRBL Settings to Master Precision Cuts

$30 is the S-value that equals 100% of your laser's rated output. If your G-code asks for S1000 and $30 is 1000, the controller drives the laser at full power. Set $30 lower than the real capability of the tube or diode and every S-value in your job is scaled down; set it higher and the controller keeps asking for current the head cannot deliver. In practice you calibrate $30 once, then do material work with S-values inside LightBurn or LaserGRBL.

Most diode and CO2 builds on modern GRBL 1.1 firmware run $30 = 1000. Older 0.9 builds used 255. Check the firmware version before you copy a number from a forum post, because a value of 255 on a 1.1 board means every job runs at roughly a quarter of the power you asked for.

$31 sets the minimum power the controller will command when the S-value is very low. During engraving and fine marking, the laser spends most of its time at low S-values, and without a floor the diode can drop out of lasing, which shows up as pale or broken lines. Too high a floor and the head keeps burning while it travels between features, which preheats the surface and discolors the edge. On a 1000-scale setup we start $31 around 1 to 2% of $30 and move in small steps.

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  Start with $30 = 1000Confirm the firmware is 1.1 before assuming that scale.
• 2
  Do not raise $30 to fix weak cutsThat hides a focus or lens problem and shortens tube life.
• 3
  Set $31 from a test gridEngrave a ramp of low S-values and look for dropout.

$32 = 1 turns on laser mode. In this mode the controller keeps the steppers energized and modulates the laser while the head is moving, so raster engraving runs continuously instead of stopping at the end of every scan line. With $32 = 0 the machine behaves like a router: the laser switches off between moves and the motors may pause, which leaves burn dots at the start of each line.

Turn laser mode on for anything with blended moves, engraving, or variable power along a path. There is one case where operators keep it off: a simple through-cut job where you want the beam to be strictly on or off and your post-processor already handles that. Even then, check that the acceleration values in $120 and $121 are realistic, because laser mode exposes them. If the head cannot reach the commanded feed in the distance available, the controller reduces speed and your power-per-millimeter changes mid-cut.

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  $32 = 1 for engravingContinuous modulation, no scorch dots between passes.
• 2
  $32 = 0 stops the motorsRaster jobs take far longer and show start marks.
• 3
  Laser mode reveals motion limitsWeak acceleration shows up as uneven cut depth.

$110 and $111 are the maximum feed rates for the X and Y axes. They set the ceiling, not the working speed. A machine that can travel at 5000 mm/min may cut a 3 mm acrylic sheet at 300 mm/min, and that difference is fine. The ceiling matters when you run raster engraving, where the head spends the whole job at max speed with the laser pulsing. Raise $110 and $111 only after the mechanics can take it without losing steps.

$120 and $121 are the accelerations for X and Y, in mm/s². This is the setting most people ignore and then blame on the laser. Low acceleration means the head creeps up to speed and slows down early, so corners receive more energy per millimeter than the straight sections and you get dark corners. High acceleration makes the frame ring, the belt stretch, and the cut line wobble.

Tune acceleration with a square test cut. Cut a 20 mm square at your normal feed and power, then look at the corners under low magnification. Dark, rounded corners usually mean acceleration is too low for that feed. Overshoot, a slight bulge past the corner, means it is too high. Change one axis at a time and re-cut the same square. We keep a small set of reference squares for acrylic, plywood and mild steel sheet and compare them whenever a motion value is touched.

• 1
  Speed ceiling is not cut speedSet $110/$111 from the mechanics, not the job.
• 2
  Corners are the acceleration readoutDark corners: too low. Bulges: too high.
• 3
  One axis at a timeChange $120, cut, inspect, then move to $121.

$0 is the length of the step pulse the controller sends to the stepper drivers, in microseconds. Most modern drivers, including the common step/dir modules used on desktop lasers, latch on the rising edge and need something in the region of 2 to 5 µs. GRBL's default of 10 µs is safe for almost everything and is where we leave it unless a specific problem appears.

The symptom that points at $0 is a motor that stalls or skips only at high speed or high acceleration, with no mechanical binding. If the pulse is too short for the driver, the driver misses steps as the step frequency rises. Raising $0 to 20 or 30 µs can rescue a marginal driver, but it also caps the maximum step rate, so you may have to lower $110 or $111 afterward.

Do not use $0 to fix skipped steps that appear at low speed. At low speed a missed step is nearly always mechanical: a loose pulley, a belt tension problem, or a driver current setting that is too low. Firmware changes cannot compensate for those.

• 1
  10 µs is a safe defaultLeave it unless you have evidence of pulse loss.
• 2
  Raise it only for high-speed stallsThen re-check the max feed rates.
• 3
  Low-speed skipping is mechanicalCheck pulleys and belt tension first.

$22 = 1 enables the homing cycle. With homing on, the machine finds its own zero at startup by driving each axis into a limit switch, which makes the work coordinate system repeatable between sessions. On a laser that cuts parts to fit an assembly, that repeatability matters more than on an engraver making one-off signs.

$27 is the pull-off distance in millimeters: how far the axis backs away from the switch after it triggers. Too small and the switch stays engaged, so the next homing attempt fails or the machine reports an alarm. Too large and you lose usable travel at the corner of the bed. On a machine with a 400 mm bed, a pull-off of 1 to 3 mm is normal.

Two practical points. First, homing speed is set separately from $110 and $111, so you can home slowly and cut fast. Second, if homing direction is wrong, that is a direction-invert setting, not a $22 problem. Fix the direction before you touch the pull-off value, or you will chase an alarm that has nothing to do with $27.

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  $22 = 1 gives repeatable zeroUseful when cut parts must align in an assembly.
• 2
  $27 of 1–3 mm is typicalEnough to release the switch, small enough to keep travel.
• 3
  Wrong homing direction is not $27Correct the axis invert first.

Start with the machine, then the job. Set the homing values and step pulse first, because those decide whether the axes move accurately at all. Then set $110 and $111 to a ceiling the frame can handle, and $120 and $121 from a square test cut. Only after motion is stable do you calibrate $30 and $31 against a real material sample. If you tune power before motion, you will chase edge quality that keeps changing with speed.

Keep a written log per material. Record the S-value, feed, number of passes, focus distance and the resulting edge. A single-page log per material family saves more time than any firmware tweak, because it separates the variables you already solved from the ones you are still testing.

Remember what GRBL is not responsible for. Lens contamination, focus offset, air assist pressure, bed flatness and material batch variation all change the cut more than most firmware values. If a cut that worked yesterday fails today with no settings changed, the cause is almost certainly outside the controller.

For production parts, firmware tuning sets the process window, but the geometry still has to hold. Our own shops run laser work alongside 127 high-precision CNC machines, including 16 simultaneous 5-axis centers, and we hold ±0.005 mm on machined features when a laser-cut blank needs a finished interface. If your part needs both, it is worth defining which features come off the laser and which are machined.

• 1
  Motion first, power secondStable axes make power tests meaningful.
• 2
  Log every materialS-value, feed, passes, focus, edge result.
• 3
  No settings changed but cut failedLook at lens, focus, air assist and material.