Why Can't Lasers Cut Glass?

[Kanastrous]

I understand that lasers are used to etch glass, and can be employed to heat glass to fracture along desired lines (CO2 lasers seem preferred for the purpose).

What physical constraints prevent lasers from being used to -cut- glass in the same way they cut steel, tungsten, wood, etc?

Is the problem that the proper frequencies to cut glass are tough to achieve with available laser technology? Or is the reason something more fundamental to glass and its interaction with laser light?

[Formless]

The latter. Glass "cutting" isn't about cutting, its about controlled fracturing. Being that glass is amorphous, it won't ever cut cleanly, although you can come very close using the right techniques. In the case of bottles, for instance, you do this by etching or scoring the glass and then using heat to stress the glass along the line until it breaks. The finer and shallower the initial score line, and the gentler the heating and cooling (preferably using hot and cold water, not fire or lasers or anything that intense), the cleaner the cut because the fracture lines won't radiate very far from the score line. This video demonstrates the principle really well.

[Kanastrous]

Thanks!

[Kanastrous]

I guess what I also wonder is if a laser can -etch- glass (vaporize or ablate or whatever effect, removing the volume of material being etched away) why that removal-of-material can't penetrate fully through to the other face of a glass sheet, thereby making a cut.

[aerius]

Laser can cut glass.



Thing is there's a lot of other ways to cut glass which are usually faster & cheaper.

[Kanastrous]

I guess it's that the laser cutters available to us aren't suited to the purpose.

Is it a matter of frequency? Power? Both...?

[Formless]

Well, now that I've seen aerius's video I guess I'm somewhat wrong and lasers can cut glass, but then again what I'm about to say will probably help understand what's going on in it and why laser cutting is so much more expensive than other methods.

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I think its because of the way glass conducts heat and fractures. The more suddenly you heat a glass object, the more likely it is to simply shatter due to heat differential. Glass fractures so fast that its almost impossible to capture the event even with the best slow motion cameras. The Slow Mo Guys did a video where they had to use a camera running at 343,000 fames per second to see glass fracture lines from a heating/cooling event traveling at any kind of perceptible speed. For reference, that turned a 5.1 second shot into 19.5 hours of footage. (Now imagine trying to hunt down the moment when the glass actually shattered during editing!) Objects with similar kinds of hardness as glass and as little ductility tend not to want to cut, they want to break or snap. Diamond cutting is the same-- you break the gem along the sheer line of the carbon matrix rather than actually "cutting" it.

[Sea Skimmer]

I guess what I also wonder is if a laser can -etch- glass (vaporize or ablate or whatever effect, removing the volume of material being etched away) why that removal-of-material can't penetrate fully through to the other face of a glass sheet, thereby making a cut.

Of course a laser can cut glass, have you ever felt glass in a window warm from the sunshine? It would not get warm if it was unaffected by the passage of sunlight energy.

What's really going on is glass is highly transparent to visual wavelengths, but it's not so transparent to all other kinds of wavelengths. In fact primarily the transparency is in the .2 to 3 micron band, with some useful (if your talking optics use) passage up to about 6 microns. That includes all light humans see, and a fair bit of UV and IR light we do not. It does not include the longwave FLIR band used by many military systems, that actually can't see through a glass window, the 'glass' on the FLIR device is not silica glass.

Increasing visual wavelength energy input into a glass cutting laser would be really damn inefficient, effective only because the transparency is not 100% at any band, this would just be a pointless idea.

Just as a note, aterials exist like Germanium that are 0% transparent to visual wavelengths, but allow very high transmission of certain IR bands, good enough to take a detailed IR photo through them. I have an image to illustrate this but noway to put it online at the moment. EDIT: You can find it on page 58 of this PDF.
www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA566304

So what that means is you'd want your cutting laser somewhere not in the .2 to 3 micron range. Outside of that a sustained laser should inflict a lot of damage, it's power output allowing.

But like others are pointing at, glass simply does not deal well with sudden thermal expansion as a material in the first place. You could cut it, but the cut would be a shattering effect if it was done at any industry useful speed. Otherwise you would need to burn off very small amounts of glass with very high power, but low energy short pulses. Basically you'd be etching it thousands of times in one spot,digging out a hole in stages. Glass is also dense and has a high melting point, which doesn't exactly help the situation. Even for metal cutting lasers are typically only used for small thicknesses, plasma cutters are far more effective for stuff that isn't paper thin. Using a continuous wave cutting laser would just be asking for the whole piece to explode. The glass will conduct too much heat away from the cut.

If you preheated the entire pane of glass the situation would be easier, but that's a tremendous energy cost and again kinda just useless for industrial purpose.

And while some people are saying glass is normally fractured, not cut, that's true in general for actual production process, but it's simply because that's easy to do with commonly needed shapes. You certainly can physically cut out glass with mechanical tools and have it not break, drilling holes just needs a sharp bit of your careful and tape the surface. This is done all the time. Once you coat auto glass in plastic you can do all kinds of damn things too it without cracking, but a laser would set the plastic on fire.

[Grumman]

For reference, that turned a 5.1 second shot into 19.5 hours of footage. (Now imagine trying to hunt down the moment when the glass actually shattered during editing!)

That would be easier than it seems: you don't need to watch the whole thing from start to finish because it's immediately apparent looking at any given frame whether it's before, during or after the part you're looking for. So if you've got 20 hours of footage, you just skip to 10:00:00 and check if it's before or after the shatter, then skip to either 5:00:00 or 15:00:00 and check again, and so on. You're halving the search area each time you check, so you'd quickly narrow it down to the part you want.

[Batman]

More quickly than watching the thing in its entirety perhaps, but that's still an awful lot of steps.

[Q99]

Even easier than that- you can have a program detect when there's a change in the image, I believe.

[Beowulf]

More quickly than watching the thing in its entirety perhaps, but that's still an awful lot of steps.

1.7 million frames of footage. That's about 2^21. So 21 steps. It's not too bad. And it's going to be more than one frame, so drop a couple steps out.

[Dass.Kapital]

Just as a side note:

From a vague memory from ages ago (Decades) when I was a Tradesman in glass I seem to remember,

1) A 'specialized' shop that used to cut difficult shapes out of glass using a 'Water knife' with the subject pane also being under water while the 'Knife' made/cut the pattern.

2) Seeing videos of glass being held under water and thence being cut with simple 'Tin snips'.

Does any one have any idea as to the Why's, Where fores or even if my old and fading memory is correct?

[Khaat]

There are some weird things possible with water knives, but the tin snips underwater thing I couldn't explain (apart from "under water, the risk of glass splinters is ... dampened.")
Quick search:

But she's basically shaving it with the scissors, not cutting it cleanly in two useful pieces....

[Sea Skimmer]

IIRC hydrogen embrittlement supposedly causes water to actually accelerate the glass fracture, while the water also suppresses vibration. Which together suppresses the shattering reaction. But you don't get a clean cut from doing this IIRC, so its only good for certain uses. Kind of ranks up with the chemicals that chemically disrupt FIRE in level of strange.

The water knife is extremely high PSI water, 25,000psi and up, with a small amount of abrasive material injected. You can cut anything with such a device and the right abrasive, and on top of that the cut is very narrow with minimal lost material. Because of that it's used all the time for detail shape cutting on steel plates.