Laser focus height and Z on a router: what does the software control and what do I set at the machine?

Who controls what

Focus is a hardware distance between the lens and the work. The software has no focus field. You set the focus distance at the machine, zero Z there, and the software only moves Z relative to that zero.

What the software controls

• Cut Depth on Laser Cut - Fill. When you want several passes that step the head down to keep the beam focused in the cut, enter a Cut Depth. The number of passes is Cut Depth divided by the laser tool's Pass Depth.
• Safe Z and Home Position, both taken from Material Setup.
• Project toolpath onto 3D Model, which drops the path onto a modeled surface.
• The order of laser on and Z moves, through the post-processor. The official guide says the laser must switch on only after the head reaches its Z level, otherwise each segment starts with an overburn mark.

What you set at the machine

1. Bring the head to its focus distance using the maker's gauge. Examples: Onefinity's J Tech guide uses a 1/4 inch gauge that just slides under the laser shroud. A machine maker's page for the 15 W Opt Lasers head gives 3 mm above the stock. Sienci's LaserBeam uses a focus finder placed on scrap of the same thickness as the job.
2. Zero Z at that height. Sienci's guide says to zero Z after focusing so the focus height is saved for the job.
3. For engraving, J Tech says to focus on the top surface. For cutting, focus at the middle or bottom of the material: at 1/8 inch for 1/8 inch stock, and nearly touching the surface for 1/4 inch or thicker.

If Z moves leave burn dots

J Tech warns that the laser burns a dot wherever it pauses for a Z move, and offers post-processors with no Z output for that reason. Keep Safe Z moves small, or use a laser post that removes Z.

Pro tips

• Because Z zero is the focus height and not the bit tip, re-zero Z every time you switch between the router bit and the laser.
• Use multi-pass Cut Depth only when the head can travel down into the cut. See the entry on laser engraving over a 3D carve for following a curved surface.