To Z or not to Z

Having built a second CNC machine, my 500mm X-Carve has been sitting on a shelf waiting to become a dedicated laser cutter.

Right now I am in the reading phase. Trying to get educated as much as possible before making decisions. From hardware setup to software choices. From workflow to safety.

At some point I need to make up my mind about whether to use a Z axis or not. Again, this machine will not be doing any milling. It will be carrying a laser head only.

Advantages of having a Z:

  • fine focus ability
  • focus once and forget (focus on machine bed and then lift Z to match material width)

Advantages of not having a Z:

  • lighter, less inertia (can increase acceleration)
  • one less motor (less power, fewer cables, cleaner setup)

On one hand I am thinking if I bought a commercial laser machine, it would have no Z. Once I get used to focusing, it will take next to nothing. And speed is something serious to consider. On the other hand, I already have a Z.

Opinions, experiences, comments are welcome.

Use the Z. You can always remove it later. Continual focusing the lens will lead to " lens shake" problems. This can result in zero shifts enough to create lost work. My 2 cents.

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I guess another advantage of having a Z that I did not mention is multipass cutting. As long as it’s supported by the laser “CAM” software (or added with a script afterwards).

Which now makes me wonder of a couple of things.

What is the (order of magnitude of) the depth of field of a laser diode’s focused beam? Or should I say depth of focus, since this is “behind” the lens. E.g. a jtech’s 2.8W with standard lens focused @50mm. The test image here shows a couple of mm each side of the clear line @26mm.

And how do Z-less machines deal with multipass? I guess a higher power laser will eliminate the need for multipass, but assuming they need to cut something thicker than their single pass capacity. Do they autofocus?

Ive been looking at standalone CO2 machines. Quite a few have the capability for the bed to be adjusted. I believe the cutter head is fixed but the table will move to allow for thicker materials or auto focusing. So there’s technically a Z axis but it’s a fixed Z laser head.

It’s better to have and not use, than to need and not have.

I use a “Z-less” laser machine, and routinely cut 1/8 inch Cherry wood at one focus height (3 inch). It will “cut though” usually in two or three passes, but always needs 6 to 7 passes @ about 12ipm to assure clean cuts in the harder grain areas. The higher focus height increases depth of field at focus, AND assures the lens stays freer of smoke contamination Good air flow at the wood surface is needed, and an air assist nozzle can help a lot by clearing the cut path of burnt wood. Cutting with a slower feed rate seems to hinder cutting due to more charring of the wood. I am using a higher power LD operating at 3 amps of current. YMMV.


What’s interesting in a case like this is that having a Z would be helpful to increase the DoF …to an extent that would eliminate the need to adjust the Z later while running a multipass cut!

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The more I think about this, the more it makes sense to keep the Z, if your machine has one.

You could get repeatability with a Z homing switch (already there) and even use a probe and a G38.2 macro to accommodate for different stock thickness. For engraving this would make for a truly focus once setup.

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A focus once setup would also require a way to fix the focus ring from accidentally coming out of focus.

Old school photojournalists would strap a fixed focal length lens on a designated body in the beginning of an assignment, manual focus to hyperfocal for a set aperture and then fix the focus ring with tape for the rest of the day.