Focusing Question for Cutting

[Mark Taylor2]

I'm a bit confused by information out there on the web.... I'm cutting not engraving wood ranging from 1/32" thick to 1/8" thick with occasional forays up to 1/4". If I have, say a lens with a 2" focal point determined by the slope test, where should the focus point be set? At the surface of the wood as in "A" in the drawing? Or halfway through the wood as in "B" in the drawing? I'm new at this and trying to sort things out.

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[Rich Harman]

...where should the focus point be set?

I always focus on the surface - whether it is 1/8" or 3/8" thick and all my cuts are "visually" perpendicular, meaning that you cannot easily detect any angle in the cut. Most of my parts will stand on edge. I also always cut through in one pass. I've never experienced a benefit when focusing down into the material. If you were making multiple passes then it would make sense to lower the focus with each pass - but I don't have any experience with that aside from the occasional recut when I get the speed/power settings wrong.

[Keith Colson]

I always focus at 80% e.g. 10mm acrylic I would focus at 8mm. Doing this gives me a smoother edge on my laser. Parts are easier to remove from the sheet too as the kerf ends up a little more even.. I suggest you try both methods and decide for yourself. Find a material that you have trouble cutting well or cutting smooth and compare the results.

That said, you should make sure that your beam focus point is exactly at the surface if you are calibrating the machine.

Cheers
Keith

[Dan Hintz]

For engraving (with fine detail), focus on the top of the substrate (focusing below/above comes into play when trying to get certain effects and won't be covered here). For cutting, it will depend upon the material and if multiple passes are being cut.

For IR-translucent/clear acrylic, anything greater than about 3/16" will see me focusing about 1/3rd of the way into the material, otherwise it's on the surface. For opaque materials (wood, dyed acrylics, etc.), it's back to surface focus... unless you make multiple passes to cut, in which case your first run is at the surface, followed by incremental increases in focal depth.