LaserCanvas
Focus: the setting that fixes most cutting problems
Focus puts the beam’s narrowest point where you want the work done. Set it with the spacer or gauge that came with the machine, re-set it whenever material thickness changes, and hold the sheet flat — a warped board lifts part of the cut out of focus and that section will not go through.
Why focus is not a fine adjustment
The lens brings the beam to a waist a fraction of a millimetre across. Move away from that waist and the spot widens fast — the same power spread over several times the area. The result is not a slightly weaker cut; it is charring instead of cutting.
That is why "it cut fine last week and now it will not" is a focus question before it is a power question. Nothing about the machine has to change for focus to be wrong — a different sheet thickness is enough.
Setting focus
- Spacer block or gauge. Most machines ship with one. Rest it on the material, lower the head until it touches, lock. Quick and repeatable.
- Ramp test. Cut a line across a board propped at an angle. The narrowest, cleanest part of the line is the focal height — measure there. Slower, but it finds true focus rather than nominal.
- Autofocus, if fitted, probes the surface and does this for you. Confirm it against a known material occasionally; probes drift.
Whichever you use, focus is set relative to the top of the material, not the bed. Put a 6 mm sheet where a 3 mm sheet was and the surface has moved 3 mm closer to the lens.
Where to focus in thick material
| Material thickness | Focus at | Why |
|---|---|---|
| Up to ~3 mm | The surface | The waist covers the whole thickness |
| 3–10 mm | About 1/3 into the material | Spreads the taper; squarer edge, better bottom-side cut |
| Engraving, any thickness | The surface | Only the top matters, and a tight spot gives fine detail |
The reason is the hourglass shape of the beam. Focused at the surface, the bottom of a thick cut sits well below the waist and the kerf narrows to nothing before it gets there. Focused a third of the way down, the waist sits in the middle of the work and both faces are closer to it.
Flat matters as much as focused
Perfect focus at the origin is worthless if the far corner of the sheet sits 2 mm higher. Plywood in particular arrives bowed and takes on moisture unevenly.
- Hold it down. Magnets on a steel bed, honeycomb clamps, or weights at the corners. A bowed sheet flattened at the edges still rises in the middle — weight it there too.
- Check the bed itself. Cut four small squares at the corners of the work area at a setting that just barely cuts through. If one corner fails, the bed is not parallel to the gantry.
- Watch for debris under the sheet. A single offcut lifts a whole corner.
Common questions
How do I know if my laser is out of focus?
The cut is wide, dark and does not go through, and the engraving looks soft rather than crisp. A quick check: engrave a thin line at two heights a millimetre apart. The sharper one is nearer focus. If both look equally soft, you are some way off in the same direction.
Do I need to refocus for every material?
For every material thickness, yes — focus is set from the top surface, so a 6 mm sheet needs the head 3 mm higher than a 3 mm sheet. Same thickness in a different material does not need refocusing, though it will need different power and speed.
Should I focus on the surface or inside thick material?
Surface for anything up to about 3 mm and for all engraving. For thicker stock, focusing roughly a third of the way into the material puts the beam waist in the middle of the cut, which gives a squarer edge and a cleaner exit on the underside.
My cut goes through on one side of the bed but not the other. Why?
The bed is not parallel to the gantry, or the material is not flat. Test by cutting four small squares at the corners of the work area at a setting that only just cuts through — if the same setting succeeds in one corner and fails in another, it is the machine, not the material.