This question is about multiple passes in laser cutting/etching. For Leeloo Dallas information, see Fifth Element.
As a laser-noob, I’m trying to understand when I will want to use multiple passes. Is it a matter of tough materials to cut/etch? Is it a matter of what will cut/etch “nicely?” Is there a best practice for multiple passes? Looking for some general insight here.
Thanks!
I have also had questions about multiple passes too. Does this cause excessive scorching at the cut?
Thanks. 
The natural rubber I have from Laserbits is pretty tough. I’ve gone to taking two 80% power passes at 20 in/min and that seems to cut through it without leaving too big a kerf. I’ve engraved the rubber to get a relief for the stamps but then cut the stamps out of the rubber. The other time I I have done two passes is with some solid walnut that it a little over 1/4" I did two passes and that cut through this particular wood without widening the kerf much.
But this is a good question I’ve been thinking about. I’ve read about it in the past year but it’s getting to be quite a jumble with all the info in my brain.
Thanks!
Forgive my ignorance… Is that fast, slow, somewhere in between. I don’t know what the scale looks like.
That sounds like a decent amount of power to me. (Again, I could be completely wrong about that.) So I’m guessing that if you go with more power or slower speed you get too wide a kerf?
The proofgrade (1/4) acrylic is set to cut in 2 passes. The first pass will make it through most of the time, but the second pass smooths it out and makes the cut straight vertically so it doesnt flare at the bottoms…
second passes for engraves will obviously give you more depth or darkness
Slow.
I work in mm/sec on my lasers. Slow for me is 10mm/sec (20in/min is 8.5mm/sec). I do a lot of work at 500-1000mm/sec for engraves and 10-30mm/sec for cuts. Depends on materials and power settings of course. But for cuts I’m almost always running 95% power.
Interesting. Thanks!
Not obvious at all to me. Are you saying that all 3D engraves will require multiple passes?
Ah sorry! Yes 3D engraves are just regular engraves at super low speeds and high power settings. The more passes you run the deeper the dark parts go.
You can already see a LOT of relief in regular dark engraves. Running them twice will increase that even more. Might try a manual 3D engrave soon.
No worries!
So does the 
software do that automatically? I mean, it’ll know where the deepest/darkest part is and do that section as many passes as needed to achieve the goal?
I would have not thought that it would take 2 passes to cut through proof grade material. I guess the polished edges make up for the extra cutting pass. 
Ok to clarify a bit here, its on the 1/4" acrylic that it takes 2 passes. It isnt using full power or going as slow as it could, so theres definitely room for a one pass take on it. I like the really clean results from the second pass though… better than ive ever gotten on my k40 by miles. It would take my k40 3-6 passes on 1/4"
Thank you for the clarification. I am new to lasers so I didn’t know how the k40 did. 
Thanks for clarifying. I was wondering about that. I only had the 1/8 inch proofgrade acrylic I have some 1/4 and 1/2 in now to try.
It already engraves depth/power by shade of gray. There is a 3d engrave option, but it’s not available yet. I’m sure they will have some already calibrated settings for those materials. Thats really the great thing about proofgrade. Saves so much trouble
I’m hoping to try a manual 3D engrave soon when I get a little time. That gf medallion just looked too cool. Just had to send my laptop with all my adobe stuff off to service though, so gonna be a few days where I’m graphically challenged.
The answers here cover most of it - it’s a technique that’s handy for some materials and approaches; we’ll use it automatically for Proofgrade materials when it’s useful.
I only came here for Leeloo 
Interesting point. Proofgrade™ will take the guesswork out.
Looking forward to that!
How does the 3D engrave option differ from “engrave depth/power by shade of gray”, both in terms of what the hardware does and what the engraving looks like?
Presumably it also focuses at different levels rather than just changing the power.
I cant answer that question with full certainty, because I dont have 3d engrave available in my software right now, but essentially its just a grayscale engrave that theyve already figured out the settings for you.
The glowforge 3d engraved token they posted took 2 passes to get to the wanted depth. Im assuming if you set the power max very high, and the speed very low using manual settings it would achieve the same result. They might be varying power and speed based on which pass it is (and whether or not you have a pro, and what material), so I cant tell you what that might be, but you can tell it to do passes at whatever settings you want.
I think I mentioned this before, but it might be possible to do a lower power fast engrave as the last one in the series to just clear off the surface of charred material. @karaelena sent me a video recently of a wood test from another laser manufacturer and they said they use this technique to clean things up at the end. Ill see if I can find it and post it.