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Charles Hawkins
01-09-2017, 9:34 AM
Hello!
I have a 60watt Chinese laser, 700mmx500mm bed size, and also have some mirrors a friend gave me out of projection tv's. These mirrors have a metal coating on one side, (1st mirror side) and also the 2nd mirror side, which is the glass. I would like to etch a Logo on the 2nd side, and was interested in how to do so without harming my laser engraver with the reflection of the Laser beam. Would I coat the mirror with a paper, or tape of some kind, or perhaps with a coating to stop the reflection of the beam? If it was a conventional mirror with the silver paint, it would not be a problem, as the back side of the mirror could be engraved with the paint being consumed. But, these apparently have a metal film, which I don't think my laser would even touch! They are great quality mirrors, very distortion free, and would like to find use from them.
Also, what settings would work with the glass, to produce a nice quality etch without fracturing the glass and ruining the fine details of the etching? Thanks in advance, I am learning a lot from perusing this forum!
Charlie
the Chaplain

Bert McMahan
01-09-2017, 11:16 AM
I'm not 100% sure on how the reflections work in this case, but could you just tilt the mirror slightly? Even a small shim under one side would cause the beam to reflect upwards at an angle rather than all the way back to the optics or tube.

Could be wrong though :/

Charles Hawkins
01-09-2017, 3:20 PM
Hi Bert,
Not sure if that would work either, as then the focus wouldn't be optimal for the area slanted. So, the thickness of the shim would allow the graphic to be unfocused in the area that was higher or lower than the center focus point? I think....
Charlie

Bert Kemp
01-09-2017, 5:21 PM
I'm not really sure you have to worry to much about the reflection. The laser head is in constant motion, by time the beam hits the mirror and reflects back the lens is out of harms way I would think. But if you really wanted to be extra careful you might mask with blue tape or some other masking. I've lasered mirrors with out masking and had no problem.

Glen Monaghan
01-10-2017, 10:17 AM
...by time the beam hits the mirror and reflects back the lens is out of harms way I would think.

And you would think wrong, especially on the slower Chinese gantry systems (although, at the relative speeds we're looking at, having even the fastest gantry system wouldn't matter). C'mon, we're talking about the laser beam propagating at the speed of light vs a gantry and lens module moving at mechanical speeds. The beam would only take less than a nanosecond to leave the lens, hit the substrate, and reflect back to the lens... In comparison to the laser beam, the lens is moving glacially. That said, the reflected light returning to the lens would have exponentially less energy than the originally projected beam.

Charles Hawkins
01-10-2017, 10:54 AM
I'm not really sure you have to worry to much about the reflection. The laser head is in constant motion, by time the beam hits the mirror and reflects back the lens is out of harms way I would think. But if you really wanted to be extra careful you might mask with blue tape or some other masking. I've lasered mirrors with out masking and had no problem.

Hi Glen and Bert, yes the laser beam is instantaneous in it's reflection back into the lens, and my laser engraver has a caution about engraving on reflective surfaces, so there must be something to that. If and when I try it, I will mask it with damp paper, or blue masking tape. I was curious as to whether anyone had actually engraved on the front side of a mirror before. I will try it, and update this when I do. Thanks for all your replies, and that is how we all learn!
Charlie

Bert Kemp
01-10-2017, 11:06 AM
Hey all I can tell ya is I've engraved the front side of mirrors with no ill effects.:rolleyes:


And you would think wrong, especially on the slower Chinese gantry systems (although, at the relative speeds we're looking at, having even the fastest gantry system wouldn't matter). C'mon, we're talking about the laser beam propagating at the speed of light vs a gantry and lens module moving at mechanical speeds. The beam would only take less than a nanosecond to leave the lens, hit the substrate, and reflect back to the lens... In comparison to the laser beam, the lens is moving glacially. That said, the reflected light returning to the lens would have exponentially less energy than the originally projected beam.

Kev Williams
01-10-2017, 12:23 PM
About 3 years ago, just to test this theory, I ran my LS900 at full power across a switchplate sized piece of mirror polished stainless for 15 minutes, very slowly. I was fully prepared to buy a new tube if this theory proved right.

Absolutely nothing happened. After 15 minutes, the lens was cool, all mirrors were cool, nothing was warm to the touch whatsoever except the stainless, which was damn warm.

I'm hardly a laser expert, or an optics expert, but it just seems to me that you simply cannot put a focused laser beam 'back into it's can' by simply reflecting it back from a mirrored surface thru the original optic train. Once the focused beam hits the mirrored surface, the beam is going to scatter. A lot. Some of the light will likely backtrack into the lens, which the lens will refocus. Aside from the fact the amount of light would only be a fraction of the original beam, the lens will focus that light 2" above the lens. Beyond the focal point, the light will scatter out widely, never coming close to backfeeding the laser tube.

I will concede that ANY beam backtracking on something like a 4000w cutting machine may have some serious repercussions. But as for our relatively low powered machines, I couldn't even warm up my lens head even when trying to. So I for one am not afraid to etch mirrors. :)

Charles Hawkins
01-10-2017, 3:45 PM
Cool! hehe sounds like a plan then, I will engrave/etch the front of the glass and see if it will be able to be used for something!!
Charlie


About 3 years ago, just to test this theory, I ran my LS900 at full power across a switchplate sized piece of mirror polished stainless for 15 minutes, very slowly. I was fully prepared to buy a new tube if this theory proved right.

Absolutely nothing happened. After 15 minutes, the lens was cool, all mirrors were cool, nothing was warm to the touch whatsoever except the stainless, which was damn warm.

I'm hardly a laser expert, or an optics expert, but it just seems to me that you simply cannot put a focused laser beam 'back into it's can' by simply reflecting it back from a mirrored surface thru the original optic train. Once the focused beam hits the mirrored surface, the beam is going to scatter. A lot. Some of the light will likely backtrack into the lens, which the lens will refocus. Aside from the fact the amount of light would only be a fraction of the original beam, the lens will focus that light 2" above the lens. Beyond the focal point, the light will scatter out widely, never coming close to backfeeding the laser tube.

I will concede that ANY beam backtracking on something like a 4000w cutting machine may have some serious repercussions. But as for our relatively low powered machines, I couldn't even warm up my lens head even when trying to. So I for one am not afraid to etch mirrors. :)

Dave Sheldrake
01-10-2017, 8:45 PM
It won't damage the tube easily but it can destroy the final lens quite quickly. It's why even lower powered RF lasers used to cut metal (600 watt range) have in effect an offset optic at the resonator end. A burst lens *could* result in a colimated beam going back into the optical train.

That said I've only ever heard of it or seen it personally once in 16 years and that was on a 200 watt machine

Bert Kemp
01-10-2017, 9:31 PM
Well maybe my thinking is wrong but the point I was trying to make was that it would be unlikely to damage your tube or lens engraving a mirror.:D oops I already responded to that


And you would think wrong, especially on the slower Chinese gantry systems (although, at the relative speeds we're looking at, having even the fastest gantry system wouldn't matter). C'mon, we're talking about the laser beam propagating at the speed of light vs a gantry and lens module moving at mechanical speeds. The beam would only take less than a nanosecond to leave the lens, hit the substrate, and reflect back to the lens... In comparison to the laser beam, the lens is moving glacially. That said, the reflected light returning to the lens would have exponentially less energy than the originally projected beam.