Changing focus?

[Josh Richard]

I am new to the world of engraving. I am a high school Technology Education Teacher and have learned a lot by reading posts on this forum, thank you. We have used a gravograph LS 100 laser in the classroom for two years now. We make a lot of acrylic key chains and paper cars! We added a rotary attachment this year and now are making glasses and mugs.
One thing that I could not understand is the benefit of moving a table out of focus. What are the benefits and when should I try it?
Thanks for the help
Josh

[Scott Shepherd]

Josh, taking it out of focus will make the beam diameter larger. In some material, especially acrylic, engraving in focus can leave undesirable lines. If you take it out of focus, it makes the beam wider and the lines overlap more, which can help in the appearance.

You're in a classroom, you should experiment with it and use it as a teaching point! You can't hurt anything by running it out of focus. You could also have them do some math, geometry and trig did predict the size of the dot at a given location!

[Dan Hintz]

To expand on what Steve said about running it out of focus on acrylic...

Rastering acrylic can leave you with very fine cut lines in the final image... by defocusing a bit, the melt area is much larger and smooths out the ridges. When vectoring, acrylic acts as a partial waveguide, meaning you can focus into the material partway, which leads to thicker material being successfully cut.

[Josh Richard]

Thanks for the help gentlemen. I will experiment today.
Is there a general distance you more your table +/-?

[Richard Rumancik]

For rastering, it usually does not mattter which direction you go (above material or into material). The beam is hourglass-shaped so there are two points on the hourglass that are equally larger in diameter, one above the FL and one below.

For vectoring I think most people focus into the surface of the material. That puts the focal point (smallest beam diameter) inside the surface. When you focus on the top face, the beam will enlarge as it gets to the exit side. A larger beam = lower energy per unit area. By focusing inside, you reduce the maximum beam diameter that the material sees, increasing the average energy density. That can allow you to penetrate materials that would not cut through when focused on the face.

I would guess that most raster offsets with a 1.5 or 2" lens might be less than say .080 (2mm). For vectoring, people often focus 1/3 to 1/2 way into the material. Just a rough starting point. It is dependent on material, lens FL, thickness of material, use of air assist, etc. It is best determined by experiment.

[Mike Null]

For material up to and including 1/16" thickness I focus on the surface for rastering and cutting. For thicker material I might focus down into the material a bit for cutting only.

[Richard Rumancik]

Mike, when I read my post now it I realize it sounds like I always focus into the material. I didn't mean to imply that - I agree that for thin material it isn't necessary. Even for 1/8" I probably would not bother. It's usually when I have problems cutting through that I tend to start adjusting focus.