Cutting 3/8 acrylic with explorer

[Dana Florian]

I have a large job requiring a lot of .354 cast cell acrylic. I can accomplish this in one pass with my 30 watt explorer with a standard 2" lense. 100% power 1% speed 1500 ppi. Would a different lens gain me more speed? Appreciate anyones thoughts? Thanks Dana




Explorer II 30w Bob-cad cam
Mitsubishi FA10 Wire EDM
Mitsubishi Fx1 Wire EDM
Mitsubishi DWC 90CR Wire EDM
Mitsubishi DWC 90F1 Wire EDM
Novotec SD-10 Hole Drill EDM

[Joe Pelonio]

For cutting, a 1.5" lens is going to be closer to the material so more likely to flare up, smoke, and will have a finer cut so that it may melt back together.
It might allow you to speed it up, but if you try it watch it closely and use plenty of air assist.

That lens is good for finer detail engraving such as photos, but I'd stick to 2.0 for cutting.

[Rodne Gold]

I have 30 w explorers , as well as 1.5 and 4" lenses , neither will really help much , the 1.5" is useless unless you are cutting REAL thin stuff and the 4" spot size is a lot bigger and thus energy density is much lower , will cut thicker stuff better in terms of sloping sides , but MUCH slower . Pex is actually a sort of waveguide material and this doesnt let the beam diverge that much once it has entered. The 1.5" lens "should" work better if thats the case as its energy density is higher cos of the spot size , but focus is so critical that small deviation in table height will be problematic , but more problematic is them fact that cast acrylic has shocking tolerances in terms of thicknesses , up to + and - 10% thickness variation over a sheet , even 5% deviation either way will throw out the 1.5" lenses focus. The 2" is the best way to go.
You might have some joy with decreasing your PPI and/or setting air assist correctly.
Ideally you need to generate enough heat to liquify the pex to attian a polished edge and enough air pressure and direction to eject the melt thru the kerf (cut) without any resolidification. So direction of air assist is vital , it should be redirected just after the cut and not co-incident with it , ie it should just trail the cut and it should not be so strong as to to cause too much cooling and thus frosted edges or striations. Easiest is to reduce air assist to just on the verge of flaming and then increase it by 20-50%. Rather use pex blocks to support the sheet that you cutting rather than the honeycomb , it is often not flat. A piece of black anodised ally on the table surface will protect it and stop ALL flashback (a problem with the honeycomb)
Cut the pex with the protective coverings on both sides.

[Dan Hintz]

Rodne,

While your post as a whole gets a thumbs up from me, but I have to take issue with one of your statements.

Pex is actually a sort of waveguide material and this doesnt let the beam diverge that much once it has entered.

While acrylics are often used as light guides, this should have little to no bearing on beam divergence. Their use as light guides is due to near-total internal reflection when the angle of light entering the material is close to being parallel with the surrounding edges. Since we're shooting a laser as a near right angle to the workpiece, you should have little internal reflection. It may have a minor effect on beam divergence (a few degrees, maybe, due to some reflection), but other than that minor amount the beam should pass essentially unimpeded (angle-wise, anyway).

[Rodne Gold]

I dunno the technicalites of it all , but its the general conclusion at some laser sites and I can cut acrylic , up to 1/2" without major sloping sides , far less divergence in term of the cut than the the beam should theoretically exhibit?
Same thing with some foams, I can cut like 1.5" with dead straight sides.

From http://www.parallax-tech.com/cutting.htm
. As a note: because acrylic acts as a wave guide to CO2 radiation, a 2.5" lens can be used on 1" thick acrylic with reasonable results. This gives any cutting system a great range on thickness for a single set-up if a perfect cut is not necessary.
(from synrad directly)
http://www.synrad.com/e-newsletters/08_24_06.htm

[Dan Hintz]

I dunno the technicalites of it all , but its the general conclusion at some laser sites and I can cut acrylic , up to 1/2" without major sloping sides , far less divergence in term of the cut than the the beam should theoretically exhibit?
Same thing with some foams, I can cut like 1.5" with dead straight sides.

Let me be a little more specific, and I think you'll see everything is in agreement. The Synrad site (which the other link copies practically verbatim from) says 1" thick material is easily cut with even a 25W tube and a 2.5" FL lens, as long as you're willing to accept a less than perfect edge. As you noted, you personally can get about 0.5" with a good edge... and that's in agreement with what Synrad is saying. 0.5" of any material with a nearly-straight edge using a 2.5" FL lens is good in anyone's book.

So you don't have to wonder anymore "why" it works better with acrylic, I'll give you an easy explanation... index of refraction. The IoR for acrylic is about 1.5 (give or take). This essentially means when your outward-spreading beam hits the acrylic, it reduces the angle on your laser's cone of light (the beam spreads outward at a slower rate). Therefore, you get a beam that is able to cut deeper (and straighter) into the material.

You may get 1/8" thickness cut in an opaque material, but 1/2" in acrylic. When they say "waveguide", they simply mean the material allows the wavelength of light to partially pass through. Granite would not be considered a waveguide

[Rodne Gold]

Perhaps you should e-mail Synrad and tell them they are wrong and why.
What did I say that was incorrect in the context of the original post.
Do you feel the 1.5 or 4" lens would perform better?
I actually do have those lenses and the same machines as he has and the first priority in lasering for us is to cut cycle times while maintaining acceptable quality and thru experimentation I can tell you that neither will do it faster.

[Dan Hintz]

Perhaps you should e-mail Synrad and tell them they are wrong and why.
What did I say that was incorrect in the context of the original post.
Do you feel the 1.5 or 4" lens would perform better?
I actually do have those lenses and the same machines as he has and the first priority in lasering for us is to cut cycle times while maintaining acceptable quality and thru experimentation I can tell you that neither will do it faster.

Rodney, I believe you misunderstand my post... I'm agreeing with Synrad. My point was simply that the material being used as a waveguide for lighting (LEDs being the chief use) is somewhat irrelevant, I'm not saying anything about the lens FL choice. Crown glass has the same index of refraction and will work equally well as a waveguide, but I'm sure you'd agree it doesn't cut as well as acrylic. I'm merely discussing a minor point, that being a waveguide is superfluous info... not really sure why Synrad lists it, but as the other site you linked to shows, the info just gets copied from one location to another without reason. Kind of like a news story saying a car blew up that had air in the tires... it probably had nothing to do with the car blowing up, but if one person "in the know" stated that fact, it would probably be repeated with every iteration of the story.

[Gary Hair]

even 5% deviation either way will throw out the 1.5" lenses focus.

If I am cutting 1/2" material and it deviates 5%, that would be .025", between 1/32 and 1/64. Unless I have been told incorrectly, I have about 1/8" of focal range, so .025" would not affect it that much. Even 10%, .05", is less than 1/16". I don't know if this is different on a 1.5" vs. 2" lens, but I wouldn't think 1/16" would be too much out of focus.

Gary

[Rodne Gold]

DOF of a 1.5" lens is 1/2 of a 2" lens , dof is a function of the sqare of (focal length/entry beam diameter) so it doesnt reduce proportionally with the focal length.
If you use a beam expander DOF gets even worse , the explorers use expanders in line so the entry beams diameter is expanded and is bigger.
There is a formula of 0.027 x ((f/d) squared) to work out DOF

If beam diameter is 7 mm with the expander than f/d ^2 for a 50mm lens is 51 x 0.027 = 1mm either side of the focal point
for a 37.5mm lens its 26 x 0.027 = .5mm either side of the focal point or a dof of 1mm total , thats less than 1/16th
Add in the fact that low power lasers have low energy density in the first place and the OP has a 30w laser , even being within DOF (defined as the point where the spot size is no bigger than 1.4x the spot size at perfect focus) might not allow cutting as the energy density at the upper and lower limits of DOF is around 1/2 of perfect focus.

[Gary Hair]

Rodney,
Being the analytical person that I am, I put your formula in a spreadsheet. Since I don't speak "metric", I converted the result into inches

Here are the numbers I come up with using your formula:
Lens lengthBeam Diameterf/dDOF mmDOF in
50.8 7 7.257142857 1.4219853060.056
38.1 7 5.4428571430 .7998667350 .0315
50.8 3 16.93333333 7.741920 .3019
38.1 3 12.74 .354830 .1698
(sorry about the formatting, my spreadsheet didn't paste in with formatting)

I don't think the beam diameter on my Explorer is 7mm, it is more like 3mm. I did an alignment right after I got it and the burn mark from the beam coming right out of the tube was nowhere near 7mm, that's over 1/4", it was closer to 1/8" or smaller. When I do the calculations at 3mm it seems to come out closer to what I thought my dof was supposed to be. If the beam is expanded to 7mm then the dof is almost unusable - .056 or .0315 is not much to work with.

Do you know for sure if all Explorers have the expander?

Gary

[Rodne Gold]

Mine didn't initially, but we retrofitted them as we had problems with the lens at the top of the firing tube. (they were impossible or very difficult to clean and shattered)
7mm was just an arb no , I think its closer to 5mm on our side , straight from the tube its about 3mm.
The retrofit involved fitting quite a substantial plate and assembly, this was about 2-3 yrs ago and I cant remember exactly where they went in the machine. Evidedntly Explorer II's have erm fitted.
Didnt make much of a difference in terms of its ability tho. We have had a lot of problems with our explorers , especially with tubes blowing. We have the coherent tubes in the explorers and , imho , the far more reliable synrads in the Spirits. The synrad/spirits put out less power than the Explorers and are rated a lot slower , yet they outperform em by quite a big margin in just about all repects.

[Larry Bratton]

Whewww..that's all to high tech for my feeble mind. I liken my knowledge of lasers to my mechanical knowledge of an automobile. When I turn on the switch, she's supposed to start up and take me where I want to go. If she don't, I call somebody.
My compliments to you gentlemen that have this depth of understanding on this subject.