Focusing for cutting

[David Sharp02]

Is it possible to cut thicker materials by moving the focal point below the surface of the material being cut? From what I've read, it sounds like the focal point is typically on the surface of the material being cut. So I would assume that the limitations on the thickness of material that can be cut depends on how quickly the beam diverges after passing the focal point. However, wouldn't the beam be symmetrical, diverging the same amount both above and below the focal point?

So if I can cut 1/4" wood focusing on the surface, would I perhaps be able to cut 1/2" by placing the focal point at 1/4" into the material? Since the beam is strong enough to still cut the material 1/4" below the focal point, it would seem that beam would be strong enough to cut the material 1/4" above the focal point.

I'm really hoping to be able to cut 1/2" MDF but my laser is only 35w.

Thanks,
Dave

[David Somers]

David,

That is a fairly common approach so to answer your question, yes, you can focus in the middle of a material to cut something thicker than you might ordinarily.
You might also try doing multiple passes. You may get less burning by doing that and find this approach more effective. Again, focus the second pass further down into the material to get the most out of this multipass process.

[Bert Kemp]

1/2 inch with a 35 watt is going to take a few passes no matter how you focus it. I would try a surface focus then move the table up a little and do another pass and move table again for a 3rd pass. But the only way to tell for sure is to try different approaches.

[Kev Williams]

De-focusing absorbs the beam's energy...

I drew this to scale-- I've been told an 80w incident beam is around 1/4 to 3/8" inch in diameter, I drew this at .300"--
The lens is 3/4" diameter, it's exactly 2" from the 'wood', which is 1" thick.
On the left shows the beam hitting the wood in perfect focus. At this point the beam spot is at its highest energy,
and will burn thru easiliy. The purple shows the beams actual divergence, but the lack of energy as the beam expands
won't make the kerf this angle, it'll just cut straight down until it 'runs out of gas'...
focus1.jpg
On the right shows the focus point 1/4" lower into the wood...
focus2.jpg
-this pic is a close-up...

At first glance, it looks like a good idea, to get the focused beam spot down into the wood...
However, according to Corel, the beam width at minus 1/4" is almost .050", 10x wider than the focused spot..
I have no idea of the power loss at that beam width (Dave?), but I'd guess it's upwards of 70%...
So, there's not enough power in that area of the beam to do squat, so the wood just absorbs it,
and what's left of the beam that does fit down the kerf likely won't have enough energy to
cut any deeper...

But if you enter the original kerf with a focused spot, the focused beam will make it much farther
down into the kerf before the useful beam energy is lost on the sides of the kerf...

It's okay to de-focus a little, say .040 (a millimeter) or so to help with the second cut, but any
more than that probably won't help much, if at all...

[Ian Stewart-Koster]

Thanks, Kev- that's a terrific explanation... about why NOT to defocus it too much, and why a 2nd run on the same path is better...

But defocusing it say 2mm BEFORE the first run might be in order, might it not?

I've found that a bit better with thick acrylic.

[Bill George]

Cutting thin wood and thin 3 mm plywood is certainly doable with a good 35 watt tube. But cutting the glued together heavier MDF and of course plywood is going to be almost impossible, at least without taking all day. The more passes you need the more burnt the edges.... at least that has been my experience.

[Dave Sheldrake]

Kevs explanation is pretty much perfect There is a long complex explanation but to be honest it would just boil down to the exact same answer. A Laser isn't like a physical knife blade, if you drop the focal point the beam still has to get through the material above the focal point and will be trying to do so with a far lower power density.

1/2" MDF with a 35 watt laser (even with a 100 watt laser) is going to look dreadful, burn like crazy and take a week to cut.
The upper limit for a non industrial laser is 3/8th MDF and even then it looks aweful, I rarely if ever cut any MDF over 1/4 inch with a laser, anything thicker gets done with a router

[Ian Stewart-Koster]

Mathematically, power drops off at the square of the distance...
so if you doubled the distance, your power loss is 1/4 ( 1 over 2 squared).
If you extend the distance by 4, the power is reduced to 1/16... etc.

[David Sharp02]

So from the sounds of it, 1/4" is the best I'm going to be able to do reasonably. I would have just tried cutting a piece of scrap, but my exhaust fan has not arrived yet. I may end up just buying some 1/4" MDF, and gluing the pieces together after cutting them.

[Dave Sheldrake]

If it's an RF machine like an Epilog or ULs, Treotec etc 35 watts isn't too bad.If it's a chinese 35 watt that's going to be closer to 28 to 30 watts and will be a pain in the rear.

[Lee DeRaud]

So from the sounds of it, 1/4" is the best I'm going to be able to do reasonably. I would have just tried cutting a piece of scrap, but my exhaust fan has not arrived yet. I may end up just buying some 1/4" MDF, and gluing the pieces together after cutting them.

I've done a lot of that over the years, as my 25W ULS maxes out cutting 1/4". If you do it right, three 1/4" pieces glued-up is stronger than a 3/4" piece anyway: because of how it's made, MDF is denser at the surfaces than in the middle.

A couple of 1/8" holes and roll pins for alignment make the whole process a lot less work than it sounds.

[Lee DeRaud]

...I rarely if ever cut any MDF over 1/4 inch with a laser, anything thicker gets done with a router

That too: even if you don't have a CNC router, you can cut 1/4" templates to use with a pattern-following bit.