Focusing on surface of maerial you are cutting.

[Joe Hillmann]

Has anyone tested if it is better to focus on top of the material you are cutting or half way through it? Normally I focus half way through what I am cutting but last night I focused on top of a piece of 1/8 plywood using the same setting I normally use when I focus halfway through it and it appears as thought I get better cuts by focusing on the top surface rather than halfway through. I know many people here suggest focusing at the halfway point but I am wondering if anyone has tried it both ways to see which works better?

[Larry Bratton]

I always focus on top. I have never focused 1/2 way through except on rare occasions. I normally get good clean cuts.

[Lee DeRaud]

I'm in the focus-on-top camp. My theory, based on no scientific justification whatsoever, is that if the beam is focused at the halfway point, you've got an out-of-focus beam trying to bull its way through the top half before it gets to the point where it really works, whereas the top-focus beam opens up a channel for the out-of-focus portion to work on the remaining material. That's my story and I'm sticking to it, although I might think differently if I could cut through more than the 1/4" limit of my wimpy 25W machine.

[Gary Hair]

whereas the top-focus beam opens up a channel for the out-of-focus portion to work on the remaining material

I agree completely!

[Mike Null]

For 1/8" ply I use top focus. If I'm trying to cut something like 3/8 or 1/2" I might focus into the material on the third or fourth pass but I try to avoid even thinking about cutting material that thick.

[Dan Hintz]

It depends upon the substrate. For substrates that act as a waveguide (e.g., acrylic), focusing into the material can be significant. For non-waveguide materials (e.g., wood), focusing on the surface is more practical.

That said, focusing to something slightly below the surface (say 1/3rd) will often help cutting thicker material. If you focus strictly on the surface, the beam loses power density by the time it is supposed to reach the bottom. If you focus into the material a bit, the power density 1/16" above the focal point is the same as that 1/16" below the focal point... but you're not wasting the power in the beam above the focal point.

[Dave Sheldrake]

Depends on material as Dan said but also on focal length of the lens, the DOF for a given focal length is an arbitrary figure of 1.4x spot size above and below centre of field. (assuming Meniscus or Plano Convex lens's and 1064 wavelength) Aspheric lens's are a little different as are lasers that use Phase Retarders, Lens's of larger size on higher powered machines should be of a size according to (1/e2) to prevent spherical aberrations due to heat distortion. Lens material will also affect the location of cutting focal point, the (k) values for different materials and different lens's vary by quite a bit between GaAs, ZnSe,Ge and CdTe so that's another variable.

The short version is the power density varies depending on material and while logic suggests it will be the same at the same distance above and below the absolute focal point the reality is it varies by quite a bit. (in a vaccum it will be the same but carbon formation in cutting reduces potential power by quite a lot)
In pretty much most non metallics the optimum focal position is 25%/75% material thickness, so for a 4mm thick sheet you would focus 1mm under the surface, that said for the lower power systems(sub 500 watts) in non gas assisted lasers a focal point at the surface will give better results (depending on nozzle location relative to work and air pressure). As far as possible an exit hole size in the air nozzle should be no more than 2x the distance between the nozzle and the workpiece for pressures under 4.5 Bar to get the best possible results. A bad optical train will make much more difference to cutting that any other single reason.

The above assumes you are using a single element system like pretty much all Chinese and many Western lasers (IE: No beam profilers or Phase Retarders)

best wishes

Dave