Pictures of problem engraveing glasses

[Kevin Berkshire]

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In the pictures, you can see the fading on the letters and numbers. I am using a setting of 80 power and 40 speed, 250 DPI and a 2.5 lens. Again this is using a 60watt Accuris Powersharp 46 laser engraver. This is a bitmap file that was used by the previous owners. I have not yet figured out how to set up bitmaps using coreldraw x4 and send them to my engraving software, which is LaserMaster 8.0 V3.
Dan, you mentioned gray tones vs black settings. Should it be a 100% black letterig?
Could this also be caused by out of focus? The glass has a 1/8" differents in the area, but the engraving len shoul handle up to a 1/4". I have 1 inch of area to engrave in, so I took measurements from the top line and then the bottom line and set the laser in the center using the focus gauge. Not really sure of what to try next.
I won't be able to write back until next week ( Monday or Tuesday) working this weekend..... :(
So far, Thanks for the help. Everyone..;)

[Martin Boekers]

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Could this also be caused by out of focus? The glass has a 1/8" differents in the area, but the engraving len shoul handle up to a 1/4". I have 1 inch of area to engrave in, so I took measurements from the top line and then the bottom line and set the laser in the center using the focus gauge. Not really sure of what to try next.



Not quite sure what you mean by this.

Are you using a rotary device?

Depth of field is pretty critical, if you have varience of .125 that could be the issue.

Also may sur the glass is as level as possible. Sometimes I do have what I call a "dropout" of letters.
If you tilt the glass you can see the etch at certain angles, but at others it appears to be transparent.


Marty

[Dan Hintz]

I'm assuming your mention of depth of focus means you're not using a rotary... if that's true, then you should be safe focusing at the midpoint to get +/- 1/8" in depth.

Text should be at something closer to 70% black. And to be honest, I can't see anything in those pictures... way too blurry... trying putting a sock in the glasses to give the camera something to focus on.

[Martin Boekers]

I'm assuming your mention of depth of focus means you're not using a rotary... if that's true, then you should be safe focusing at the midpoint to get +/- 1/8" in depth.

Text should be at something closer to 70% black. And to be honest, I can't see anything in those pictures... way too blurry... trying putting a sock in the glasses to give the camera something to focus on.

A black sock:D

I couldn't see much either.

I wasn't sure if he ment the curve from the width of the glass or from the height
of a more rounded wine glass.

The thing about the lense being able to hold focus (?) for .25 was a little confusing.
I don't think that's possible.

Marty

[Chris DeGerolamo]

Don't rule out the engraving may be inconsistent due to poor glass quality, especially if the issue is happening at random.

[Rodne Gold]

Yeh, I suspect poor quality glass and the wrong power/settings leading to poorly controlled and patchy fracturing

[Michael Hunter]

Telling someone to "put a sock in it" could be considered quite rude ....

On the other hand, those pictures are hardly worth a thousand words.

[John Noell]

The thing about the lense being able to hold focus (?) for .25 was a little confusing.
Marty I think he meant that by using a longer focal length lens (2.5") he has a bit more "acceptable" depth of field. I too use a 2.5 lens for that reason; it helps a lot with coconut shell which is unevenly curved. I get good results as long as I stay within about .25" of exact focus.

[oscar martinez]

I will share my settings I engraved lots of glasses every day.
Spirit gx 60 watts,2.5 lens,I use the gauge tool some times i have to set up at mid point.
63% power,93% speed ,600 DPI. I use this mix instead of wet paper , dish soap25%, water50%,25% black paint(water base washable paint The kind for kinder gardens) I apply with a brush at the area to be engraved .The mix does't have to be exactly
the proportions but to have an idea. I use this settings for glass .
For crystal i change my settings to 55% speed 90% power. 500 DPI.(no need to wait for mixture to dry has to be wet)
Certain materials not suitable for laser engraving(especialy cheap glasses). In that case I tape the area to be engraved with blue masking tape.I proceed to engraved in order to get rid of the masking an the glue is important to make sure is not glue at the small details I sand blaster whit 220 grit and i am ok with it . of course sand blaster tape is much better.
I hope this helps.I have not problems engraving glass or cristal.When i use tape no need of soap on tape.
I use rotary attachment.
Atte Oscar
From Golden State.

[Kevin Berkshire]

Thank you all for your suggestions. I will try some more samples and try to take better pictures if there are more issues. I am confused about the cheap glass concerns, because every artical I have read said to use a lower quaility glass due to less metals in the glass. Right now I am using glass made by Libbey. Again, Thank you your help. Without talking to people like you all I wouldn't have any hair at all......

[Rodne Gold]

The laser works on glass when impurities in the glass expand at different rates to the glass itself , leading to a controlled local fracture. At some stage , the multitude and variety of impurities becomes counterproductive and the fracturing is uncontrolled and leads to splintering etc. Net result , bad engraving

[Dan Hintz]

The laser works on glass when impurities in the glass expand at different rates to the glass itself , leading to a controlled local fracture.
I'll occasionally disagree with Rodney from time to time, and this is one of those times. ;) Glass "engraves" because the laser melts a tiny "pool" at each firing point. As this pool cools, the rate of contraction varies across the pool, which leads to localized stress (which leads to micro cracks).

The result is the same (cracks), but the actual cause is not what has been widely flaunted in the magazines, tutorials, etc. I have had no problems with metal content in glass, and I prefer a high-quality glass when I can get it (bubbles and other impurities in the pour of low-quality glass can make a good engraving look poor in comparison).

[Mike Null]

Dan

I don't quite agree with your description. I believe the glass expands rapidly under the heat of the laser and thus fractures.

[Dan Hintz]

Dan

I don't quite agree with your description. I believe the glass expands rapidly under the heat of the laser and thus fractures.
Mike,

If I can find the last paper I read on this, I'll forward it to you. This is a known phenomenon.

EDIT: Found the paper... published in 1980. In the conclusion of the paper:

Glass fails by surface fracture caused by residual, tensile stress induced during cooling after irradiation by CO2 laser pulses... At irradiation levels near the threshold the residual stress is confined to a layer about 20% of the absorption depth. I grabbed it from work off of one of our pay-per-view tech journal sites, so I don't think I can send it to you. I can probably provide more detail from the paper, though, if you want it.

[Dan Hintz]

Here's a plot of the stress profile in typical soda-lime glass. It's a 1D profile as the laser radiation is absorbed in a much shorter distance than the dimension of the beam spot itself.
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EDIT: Note near the end of the description:

The high compressive stresses... produced during the heating phase do not cause failure since the compressive yield stress for glasses[sic] in considerably higher than the tensile yield stress.In other words, it's not the heating, it's the cooling.

EDIT 2: The depth of absorption is on the order of 5 um, so at the 20% mentioned earlier the stress is mostly in the top 1 um.

[Mike Null]

Dan

I'm from Missouri. Try heating a piece of glass on the electric stovetop--leave part of it off the element.

[Martin Boekers]

Dan

I'm from Missouri.


Now Mike,

Is that from Missoureee or Missourahhh. :D

[Dan Hintz]

Dan

I'm from Missouri.
:D I know the cause has been questioned quite a bit in years past, and although I've made mention of the actual cause a few times in the past year or so, I thought some technical background on it would help clear up the misconception. Or maybe not ;)

Even now, magazine articles (A&E, for example) still quote the cause as being exploding impurities in the glass. I thought my prior mentions of engraving high-quality glass would lean people away from believing that, but it keeps popping up. Maybe I'll have to do a little brow-beating :p