A bit of experimentation here…

It is not possible to cut soda-lime glass with a CO2 laser but, some while back, I read a couple of patents which cover a ‘fracture technique’ where the laser is used to heat the glass (causing compressive stress) and then the glass is cooled (causing tensile stress) which in turn creates a fracture along the defined toolpath.
Obviously I have much to learn here but needing 2 x 20mm diameter, opaque glass, discs I thought this process may be worth a try.
I found that straight line fractures are relatively easy to achieve but circular fractures needed a bit more thought.
On my first attempt I found it impossible to remove the discs so using slightly more power on the second run I created radial fractures in all directions and I now have the two required discs which fit the intended application perfectly. The edge quality is not that good (but that is unimportant for my application) although I think refining the technique will lead to better quality results.

Just thought I would share the info for any others that may be interested in experimenting with laser glass cutting.

Tweakie.

Tweaky:

Excellent. I had been discussing the thought of that a few months back with Bob.. SO you cut small lines like normals coming out from the circle?
How'd you cool it? Air?

Art

Very Cool Tweakie! I had to learn how to cut glass when working on a Sunburst light/window over a door casing that was built sometime before 1865. They were shaped like wedges and I wasted a lot of glass on the ends that came to a point before I figured it out. I have thought of a piece a local glass shop had displayed on their wall since I first saw it (younger than 10) and even more since working on the window. It was a mirror that was shaped like 3 doughnuts stacked to form a triangle. The entire perimeter was a series of arcs with round holes in the center. A few times when that magic mirror crossed my mind I have almost researched it to figure out how it was done. I have come to the conclusion I really don't want to find out, not like that. That mirror has become my Santa clause. I'm pretty sure I know a few gifts I will see in the bag if I peek. I just caught a glimpse of you putting presents under the tree.

Brett  

Thanks for the encouragement guys, it is much appreciated.

If only I knew the secrets of shaped glass cutting it would all have been a lot easier insofar as selecting the correct toolpath for the job. In my ignorance I just used a standard toolpath (as shown in the top of the picture) and although the glass cracked, with that layout it is not going to come apart any time soon. Brute force (more power) worked but it is not really the right way to go about it.

To cool the glass I used cold air (passed through half a dozen turns of tubing submersed in an ice bucket) with the glass resting on a milling machine parallel which had previously been placed in the deep freeze for a few hours.

There has to be a better way – it’s just a matter of dreaming it up.

Tweakie.

>>There has to be a better way –

 There almost always is.

Art

I think you may want to play with the focal length. I studied 3d glass engraving for a while. The secret was to have the focal length set to the INSIDE  of the glass. FOr cutting/scribing I would think the Focal length needs to be either on the top surface of the glass OR the bottom surface.

In manual glass cutting a bottle circumference you scratch the outside surface then run it under boiling hot water a little at a time and you can get a perfect seperation 99% of the time.  So well defined that you can reset the two pieces back together and it will not leak water

From Cold to HOT (;-) the glass expands and fractures at the Scribe line.  From Hot to cold it tries to contract and most times cannot contract enough to do the fracturing due to compression.

Just a thought, (;-) TP

Hi Terry,

Many thanks for the suggestions, they are gratefully received.

You are, of course, absolutely right about behaviour with a scribed line ( I have seen your excellent demonstrations with bottle cutting) but, unfortunately, CO2 lasers don’t produce the right kind of line. Because of the wavelength the CO2 beam is absorbed into the material causing some surface micro fracturing and some melting with the bottom of the kerf being randomly rounded and this prevents the glass from breaking along the line (many before me have tried this method without success).

The patented idea of ‘tensile stress cracking’ turns the tables, so to speak and relies on the laser beam being out of focus to heat the glass rather than mark it and it is usual to cool the glass some 10 or so mm behind the passage of the beam. For me, this is OK for straight lines but the toolpath starts to get extremely complicated for shaped working (where tangential control may be necessary for the air/water mist cooling nozzle).

With my experiment I attempted to create as much thermal shock within the glass as I could manage and it sort of worked (at least I got the discs I needed) but I found that it was necessary to create a fracture at an edge for it to follow the toolpath line and there is a technique to doing this. Perhaps, when I get more time to spare, I will investigate this further and hopefully be able to produce better quality results.

It is certainly a subject for thought. 

Tweakie.

I'm wondering if increasing the temperature gradient in both stages would help.

e.g. put the glass in the freezer prior to lasering and then warming it in the oven/hot water whatever, before hitting it with a freezer spray.

remember the safety glasses - it'll probably explode... 

You could also try increasing the number of radial lines maybe.

Ian

Hi Ian,
 
Pre-heating or pre-cooling the glass is something I will have to try.
Perhaps tangential lines may make it easier to remove the parts - I don't know but I suspect that the fracture produced in radial lines may continue in a straight line rather than following the shape - something else for me to try.
Thanks for the suggestions.

Tweakie.


(There are more details of the tensile stress cracking technique in this document  http://www.jklasers.com/images/ZeroWidthGlassCutting.pdf )

Tweakie:

 In my imaginings, I had planned on drilling a small hole in a peltier element I have here. They are semiconductor slabs really so
drilling a hole shouldnt hurt them. then if the gantry pulled the peltier along while burning the glass though the peltier, I might
be able to get a very large thermal gradient. Ive never gotten to test that theory yet,
 but I have a 2" square peltier laying here.... someday soon maybe. Im think at that point motion speed is then a gradient control
instead of just heat control, it also would give me a pot to turn to increase gradient or lower it. ( Things always work better when you have
a pot to turn.. lol )..seriously though .. a peltier with a hole in it could give a very large gradient , these things get very very cold when you have an
appropriate heat sing and fan on them, and it should be possible to right through a hole in the heatsink and a hole in the peltier.. I think
dragging one across glass may work...

Art