Why does my laser burn darker at corners and at the ends of lines? (Laser mode and dynamic power)

Why it happens

A machine slows down for corners and for the turnaround at the end of each raster line. If laser power stays the same while the head slows, that spot gets more energy and burns darker. The fix is in the controller and the post-processor, plus one setting on the toolpath form.

Controller requirements (per the Grbl 1.1 documentation)

1. Laser mode on: $32=1. With laser mode off, Grbl stops at every spindle speed change to let a spindle reach speed, which shows up as stutter and burn dots. With it on, motion runs continuously through power changes.
2. M4 dynamic power: power is scaled to the actual speed, so corners and acceleration zones get the same energy as straight runs. The laser also switches off whenever the machine is not moving.
3. M3 constant power: power stays as programmed regardless of speed. Corners burn darker unless you add lead-in and lead-out moves.
4. Rapid (G0) moves switch the beam off automatically in laser mode.

The Grbl post-processor supplied with the software uses M4 to turn the laser on and scales the 1 to 100% power value by ten to match Grbl's default maximum of 1000. J Tech Photonics also lists a Grbl 1.1 dynamic M4 post and states it needs $32=1.

The toolpath side: Overscan

On Laser Cut - Fill and Laser Picture, Overscan lets the head travel past the edge of the burned area with the laser off, so it decelerates outside the work. The official documentation says the right amount depends on the machine and feed rate, so run test pieces to find it.

Other start and stop marks

• A dot at the start of each segment points to the laser firing during a Z move. See the entry on focus height and Z handling.
• On Mach3, J Tech says to set the spindle delay to zero, because that delay leaves burn marks at starts and stops.

Pro tips

• The Grbl documentation warns that dynamic power may not suit every material, since some cut poorly at reduced power. Test before a real job.
• Turn laser mode off again before running router toolpaths. The Grbl documentation calls this out for spindle safety.