firmware & settings guide

Why Laser Engraving Corners Are Too Dark — and How M4 Dynamic Power Fixes It

Your laser engraving corners look darker — sometimes much darker — than the straight sections in between. This isn't a power or focus problem. It's a physics problem: the laser head has to slow nearly to a stop at every corner, but in the default mode the laser keeps firing at full commanded power. When speed drops to 10% of normal, the energy delivered per millimetre shoots up by 10×. The fix is two steps: enable GRBL laser mode ($32=1) and switch your engraving layers to M4 dynamic power, which automatically reduces laser output proportionally as the head decelerates — keeping energy density consistent across the whole path.

• Root cause: laser head slows at corners; power stays full = energy pile-up
• Energy spike: at 10% of feed rate, energy per mm is 10× higher with M3
• Fix, step 1: enable laser mode — send $32=1 in the Console
• Fix, step 2: uncheck "Constant Power Mode" per engraving layer in LightBurn (= M4)
• LaserGRBL: enable Overscan as the equivalent compensation
• Cutting layers: keep "Constant Power Mode" checked (M3) — corners matter less at cutting speeds

Without M4 dynamic power, a diode laser delivers up to 10× more energy per mm at corners than on straight sections — making them visibly darker regardless of your power and speed settings. — Laser Tinkerer, 2026

Why do laser engraving corners get darker?

The answer is in how energy density works. The energy deposited per millimetre of travel depends on two things: how powerful the laser is, and how fast the head is moving.

Energy per mm = Power (W) ÷ Speed (mm/s)

On a straight section at full speed, this ratio is constant and predictable. At a corner, the laser head has to decelerate, reverse direction, and accelerate again — the mechanics of the gantry physically cannot change direction instantly. During that deceleration, speed drops toward zero. If power stays constant, energy per mm rises toward infinity.

In practice, the peak is capped by the machine's minimum feed rate, but even a momentary slow-down from 3,000 mm/min to 300 mm/min means ten times more energy in that zone — enough to make corners noticeably darker, blurry, or slightly charred on wood.

This effect is worst on:

• Vector outline engraving — the head constantly accelerates and decelerates on curves and corners
• Raster scan line endpoints — the head is still decelerating as it exits and accelerating as it enters each scan line
• Small text and tight curves — the head never reaches full speed, so the average feed rate is well below the commanded value

Three GRBL operating modes — and which one you want for engraving

Your laser controller (GRBL firmware) has three relevant states. Which one you are in depends on the $32 parameter and the M-code sent by your software.

The short version: for engraving, you want M4. For cutting, you can use M3. LightBurn lets you set this per layer, so you can have M4 engraving layers and M3 cutting layers in the same job.

How to fix dark corners in LightBurn — step by step

There are two separate things to configure: the firmware parameter (done once) and the per-layer setting (done in every job file).

Step 1: Enable laser mode in firmware — send $32=1

Open LightBurn. Go to Edit → Machine Settings, or open the Console tab (Window → Console). Type the following and press Enter:

$32=1

Verify it saved: type $$ and press Enter. Find the line that reads $32=1 (Laser mode, bool). If it still shows $32=0, the command may not have sent — try again or check your firmware version (requires GRBL 1.1f or later).

While you are in the Console, also confirm that $30 (spindle max RPM, which GRBL maps to max laser power) matches the S-Value Max in LightBurn's Device Settings. Both should be 1000. A mismatch causes incorrect power output regardless of M3/M4 mode.

Step 2: Set engraving layers to M4 in LightBurn

$32=1 enables laser mode, which is required for M4 to work — but it does not automatically switch to M4. You still need to check the layer setting.

In LightBurn, open the Cuts/Layers panel (usually on the right side). Click the layer you use for engraving. In the layer properties at the bottom, look for the Constant Power Mode checkbox:

For engraving layers, leave it unchecked. That is all.

Step 3: Test on a scrap piece

Burn a simple square outline plus a small filled rectangle on a piece of scrap basswood. With M4 correctly configured, the corners of the square should match the mid-edge brightness. If corners are still darker, double-check that $32 really saved to firmware (send $$ and read the output).

How to fix dark corners in LaserGRBL

LaserGRBL has a different (but equivalent) solution: Overscan.

Overscan extends each scan line beyond the left and right edges of the image. The laser head starts moving before the laser turns on, so by the time it reaches the first pixel of the image, the head is already up to commanded speed. The laser turns off before the end of the line, and the head decelerates beyond the image boundary. The acceleration and deceleration happen outside the image area — so the engraved portion only ever sees the head at full commanded speed.

To enable it: when you import an image into LaserGRBL (File → Open), the import dialog has an Overscan field (in mm or as a percentage of image width). Set it to at least 5% of the image width, or about 3–5 mm as an absolute value. Larger overscan values reduce speed artifacts on slower scan rates.

LaserGRBL also benefits from having $32=1 set in firmware — open the Grbl Configuration panel (Tools → Grbl Configuration) and set $32 to 1.

When is M3 (constant power) the right choice?

M3 is appropriate for cutting, not engraving. The reason is slightly counterintuitive: for most cutting paths, the laser moves at a constant feed rate for most of the cut length. The head only decelerates significantly at corners if the cut path has sharp direction changes — and even then, you typically want full power through the corner to ensure a clean cut, rather than less power that might leave the kerf uncut.

The typical recommendation:

• Engraving layers — M4 (Constant Power Mode unchecked in LightBurn)
• Cutting layers — M3 (Constant Power Mode checked in LightBurn)
• Vector outline engraving — M4 gives better results than M3 on curves

One exception: if you are cutting a design with many tight curves or intricate shapes (like stencils or detailed decorations), switching to M4 for the cutting layer too can actually improve edge quality, because M4 reduces power on the micro-deceleration at each curve — producing a slightly cleaner cut edge on some materials. Test on scrap before committing to a full job.

What $32=1 does — and what it does not do

There is frequent confusion about $32=1 because fixing it often makes a dramatic improvement — but people attribute the full improvement to laser mode alone, when it is actually the combination of laser mode plus M4 that solves corner darkness specifically.

Here is what each fix addresses:

How to test whether M4 is working correctly

Run this test on a piece of scrap basswood or birch plywood — light, even-grained wood shows energy differences clearly.

1. In LightBurn, create a square outline (30×30 mm) and a small filled square (20×20 mm) on the same layer.
2. Set the layer to your normal engraving power and speed (e.g. 60% power, 3,000 mm/min).
3. Burn the first test with Constant Power Mode checked (M3). Note the corner brightness.
4. Burn a second test with Constant Power Mode unchecked (M4). Compare corners to edges.

With M3, you will typically see noticeably darker marks at each of the four corners of the square outline. With M4, the corners should match the straight edges — same tone, same depth. The filled square is less sensitive to this effect but may show brighter edges on entry/exit points with M3.

If M4 makes no visible difference, check that $32=1 is actually saved in firmware (not just sent once without being persisted). Some machines have write-protected firmware parameters — you may need to verify via the GRBL Console.

Wear your OD7+ laser safety glasses during all test burns.

How this relates to the material test grid generator

The Laser Tinkerer material test grid generator defaults to M4 for all GRBL engraving profiles. The reason is exactly this: test grids contain many small cells with frequent direction changes. Running a test grid with M3 would make the corners of each cell darker than the centres, masking the true relationship between power, speed, and engraving depth — the whole thing you are trying to measure.

When you download a G-code test grid from the generator, the header includes:

$32=1 ; enable laser mode
M4 ; dynamic power — scales with feed rate

If your machine does not respond correctly to M4 (some older or non-standard GRBL forks), use the SVG download from the generator and run it through LightBurn directly — you can then control M3/M4 via the per-layer setting.

Frequently asked questions

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