I recently noticed that when lasing a 3mm hole on 3mm baltic birch that the hole was a smaller diameter on the bottom than it is on the top of the piece. The mirrors are perfectly aligned and the lens is clean.
My laser is a 100 watt Omtech CO2. Is there a place where I can link my laser info to my profile? I’ve looked all over this portal.
Any ideas?
Thanks!
Dale
There is an issue with the Forum software that is not allowing editing those fields. It can only be entered when you first set up your account. I believe they are working gettingthat rectified.
As far as your hole being smaller on one side, the focused beam is an hourglass shape. Depending on which lens you’re using you may need to change your focal distance to get a consistant cut. Run a ramp test and you will see what distance / focal range works best for your lens.
This would be out of focus, it should be more focused on your positon, in other words, I’d expect it small at top and larger at the bottom.
If you’re using a 38.1mm (1.5") lens the depth of focus (dof) is only about 1.54mm. If you focus on top, the bottom is out of focus and will give you a sloping type of cut.
Use a 101.6mm (2") with a dof of about 2.75mm still a bit shy of 3mm but you should end up with a better cut. You still have to take care focusing and use the middle of the material as the focus point.
If your focus is at the surface, only 1/2 of the space below it will be available.
It can also be other issues, such as head alignment.
This makes perfect sense. Thanks Jack!
Dale
Jack,
You are obviously a font of knowledge! Have you considered writing a book?
I don’t know what I don’t know. Just saying
Dale
Most people call it getting old. 
Not for lasers, but I don’t think I’m up to it anymore.
Jack, I’m not sure I understand 101.6mm (2") lens. 2 inches is 50.8mm. I’m currently using this lens. Is the answer to set focus distance to the mid depth of the work piece?
From what source did this and other graphics you’ve shown me come from? I’1d like to learn more about the physics.
Thanks!
Dale
I only have a nominell 60 watt machine and I focus on the surface. It just gives me the cone in the cut surface that I can press the counterpart, cut out with 0.075 mm kerf, with a little violence into the cut. If you want the most straight cut, experiment a little with your focus, go 1-1.5 mm into the material, it should give theoretical a straight cut. If it is not a case, it could be that your laser head is not perpenticulated to the machine bed.
PS. I am using 50.8mm focus distance.
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