Engraving Mirrors (again)

Hi all, Been on the "Creek" for a while now, but this is my first post. I am still fairly new to the laser, but have managed to come up with some projects that turned out very nicely.....mostly due to the help and advice given in this Forum. I'm getting ready to start playing with glass mirrors. I know that there have been several related posts (I think I've read them all), but what I am looking for is just a starting point for settings. I have an Epilog Mini 18 and plan to engrave from the back side. Any advice on where to start in terms of power/speed/dpi? Thanks!

Hey Bill,

I have a totally different format machine doing huge stuff. However I can give you my .02. I run mirror at Full power and low speed to etch off all that copper and sliver backing. The stuff is nasty! I usually use files 300dpi-600dpi depending on what sort of imaging you are doing. Lettering at 600dpi and 300 for imaging. Here is an example of a huge mirror run at 100P 30S 300DPI.

Attachment 285594

Thanks, Jon. That tiger is IMPRESSIVE!

Bill
I have a 40 watt laser and I run 100% PWR and 10% spd, and even at that I sometimes have to make a second pass. Cheap craft mirrors one pass usually is enough, but better quality mirrors have a thicker or harder back coating, so it takes 2 pass's.

Excellent info, Bert. Any particular dpi? Thanks!

My last settings for a single pass mirror on my Epilog helix 24 (40watt) was 400dpi, 35 speed and 95 power. It got through the backing all right, but I wasn't 100% satisfied with the finish. It didn't look like sand blasted because it had some horizontal streaking appearing in the engraving. Next time I will down the speed to about 25% and see what happens.

Good luck to you!

- Henri

It's all going to depend on your machine and the backing on the mirror.
Some mirrors have a light backing. Some have a heavy coating and
more than one layer.
I did a train image on a large mirror that took 6 passes at low power. The
mirror was very old with a thick coating on the back. Getting through all
of it with one pass would be tricky (and probably not possible with my
machine) but I didn't want to use a power high enough to etch the
glass itself. So many lighter passes worked.
As someone else mentioned.. "sneak up on it" .. start low and gradually
climb up in power till you get through the backing or as far into it as
you want.

I was that careful because the mirror wasn't mine and it was antique.

Quote:

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Originally Posted by Henri Sallinen

It didn't look like sand blasted because it had some horizontal streaking appearing in the engraving.

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Sometimes that streaking comes from a mismatch in the dpi.
If you scan at 300dpi and etch at 400dpi, you're almost guaranteed
to get streaks (referred to in this forum as 'banding' ) and it is a
mathematically predictable pattern.

Personally, I think the manufacturers give us too much leeway in dpi.
The beam has a specific spot size (per lens, in focus) and trying to
increase the resolution beyond that point can cause issues unless you're
careful and know how it will affect the image. It's a combination of the
beam size and the amount of rotation of the steps in the motor.. you can't
get smaller than the beam, nor travel less than one pulse of the motor, no
matter what resolution you choose. But they give us the options as if we
can increase the resolution. Maybe we should call it something else.
Most of our machines have a native resolution of around 200dpi with a 2" lens.

The streaking may have also been caused by the backing in the mirror. I have not engraved mirror in at least 5 years, but for me, the cheaper the mirror the easier the backing was to remove. A more expensive mirror contained some sort of grid/mesh backing inside the paint, so it left a grid-like pattern in my engraved image.

Whoops. Forgot my settings. My notes from years ago show I used 50 speed 80 power but it doesn't indicate if that was the cheap mirror or the one with the grid pattern

Quote:

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Originally Posted by Bill Munroe

Excellent info, Bert. Any particular dpi? Thanks!

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I use 250 DPI, my laser only gives me 3 DPI options 250,500, & 1000

Today I received a job with the mirror larger than my work surface. I will have to divide the engraving in three sections. Since the red dot is a useless tool, any tips on aligning the work piece precisely enough would be highly apreciated.

Yes--something I use in my laser daily just FOR verifying alignment: blue painters tape. At low power the tape will turn white when you etch it. You'll have to experiment with your power settings to verify the laser won't burn THRU the tape.

If you're aligning segments of half-finished graphics together, you might try "storage" tape. Exactly like shipping tape but it's thicker and not quite as sticky as shipping tape. Again, needs experimentation. But since it's clear, it makes it easy to see if you're out of alignment, and easy to measure for your corrections...

Thanks for the tip Kev. I won't be ordering the red dot on the next machine.