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Old 10-09-2005, 08:59 AM
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Cutting glass by sandblasting, Urban Myth?

Cutting tempered glass by sandblasting, urban myth?

Over the years I’ve heard that you could cut tempered glass by sandblasting it. I have heard this over and over, but never have I actually talked or read about anyone who has actually done it. I tried it years ago and failed so figured it was just a dream. Well, every time I shoot someone down for suggesting it, I have this nagging feeling that they are right. I mean, after all, glass will get “etched” by a sand storm right? Back in the tacky seventies guys use to have flowers and crap like that etched in to their glass, this was done by sandblasting through a stencil. So I know the sandblasting will etch the glass, so why can’t you just keep “etching” it until you go through?

Yesterday at work I replaced a door glass in a Toyota Prius and instead of tossing the old one in the trash that I would perform a little experiment. Today I covered the glass with duct tape to protect it and contain the glass if I should fail and it breaks into a million pieces. I left one strip of exposed glass about 3/8” all the way across. I also put a piece of sheet metal above and below the exposed glass to shield the glass and tape next to the strip from the sand.



I used a regular old small siphon sandblaster and “lapis luster” filtered sand. I didn’t check the pressure but it was about 80lbs I believe.



After about 30 lbs of sand and twenty minutes I was making headway. I went to the house for a potty break and returned to find the glass had broke! Darn it, it was my fault. I had leaned the glass up against a wheel-barrel and it was hitting at the top above the cut. I knew this was wrong as I was doing it but forgot with all the sand hitting me in the face.



The thing is, I was actually doing it. I think if I had supported it properly, it would have worked. I plan on getting another window this week and giving it another try. Here is the kicker though, I used 30 lbs of a 100 lb bag of sand and only cut about 1mm tops. This glass is real thin being it is a late model gas mileage car. It is only about 3.5 mm thick. My “cut” was between ½ and 1mm (it varied) so to cut all the way through is going to be a full 100 lb bag or more. And this is with this late model paper-thin glass, and old car with ¼”, you are looking at a number of bags and a few hours per window! It would still need to be wet belted or something to fine tune the edge. I will be giving it another try in a few days and I’ll let you all know.

Right now, I say this myth is


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Old 10-09-2005, 09:43 AM
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You are on to something

I think this will work..I do mean that this is the way real hotrodders have learned what we have learned and tha tis by doing..

Good Show

OMT
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Old 10-09-2005, 10:31 AM
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Good luck... I hope it works

You'll just have to watch the heat it generates.

My friend ran a glass shop and for my youth I hung around there. He used to cut a lot of safety for the rod community.

I busted a few tempered pieces, helping him do auto glass.

He told me to think of glass as a very slow moving liquid (hence old glass from houses is thicker at the bottom than the top)

And to think of tempered glass as being "under pressure"
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Old 10-09-2005, 10:38 AM
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Interesting info Ripped, the "under pressure" point is well taken and I have been thinking that may be the reason it can't be done. But the "heat" thing is a myth. Sandblasting doesn't create any more heat than a sunny day. It is the "compresive mechanism" of the granules of sand hitting the surface like millions of little hammers that cause the warpage in sheet metal.

Brian
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Old 10-09-2005, 11:27 AM
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Quote:
Originally Posted by MARTINSR
Interesting info Ripped, the "under pressure" point is well taken and I have been thinking that may be the reason it can't be done. But the "heat" thing is a myth. Sandblasting doesn't create any more heat than a sunny day. It is the "compresive mechanism" of the granules of sand hitting the surface like millions of little hammers that cause the warpage in sheet metal.

Brian
Haha yup.

I had a guy telling me that if you sandblast at night, you can see the panels getting "cherry red".
...
"Oh really...?" was my response.

I have seen occcasional tiny little sparks, but nothing more than that.
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Old 10-09-2005, 01:35 PM
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I thought for years as most of us have that the friction of the sand hitting the surface created heat that caused warping. That was until a fellow forum member (on another forum) a few years ago explained it to me. Yep, since then I have studied it more and sure enough, there isn't enough heat to do diddly. The sand particles act as micro hammers putting little dents and pushing the metal molecules around. This movement pushes them from a "Face-centered" cubic crystal of mild steel into a "hexagonal close-packed" crystal of a harder metal like Titianium. Just as heating and quenching does. You are hardening the metal.

Brian
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Old 10-09-2005, 03:09 PM
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Martinsr, A few months ago when this sandblast/heat subject came up here I mentioned what I had learned about it during one of our lunch time b/s sessions at the shop and it started quite an argument so we did a little experiment-We blasted a thin plate with sand while I held my bare hand on the back(I know ) and it truly seemed to actually get cooler from the air flow. I have sandblasted some machinery parts in near dark conditions and there are a lot of Sparks flying off the surface but I don't believe there is any heat build-up at all.
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Old 10-09-2005, 06:23 PM
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Tempered glass is hardened only on the surface. Once you break through that, the ordinary glass in the middle is going to shatter from the pressure release. That's probably why your window broke rather than from lack of support. Think about it -- you didn't cut deep enough into the glass to weaken it that much.
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Old 10-09-2005, 10:13 PM
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Working in construction for 40 some years, doing storefronts and commercial buildings I know that tempered glass is like it is in a picture frame,, when its tempered, the glass forms a 'Frame [shrinkage] around the perimater of the glass, thats why it won't shatter as easy when hit in the middle of the pane,, but just barely crack the edge and it shatters,, stand back and look at a large piece of tempered glass, you will notice it is concaved, but windshields are laminated safety glass,, they don't shatter,,they stay contained,,so its possable that one could be cut with a sandblaster..
I have heard that most glass is cut by grinding it , slowly with water running on it for cooling,,
I don't think sand blasting causes heat,, you can touch the metal and its not even warm,, I believe the warpage is caused by the pressure of the sand hitting the metal,like Mr Martin mentioned ,

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Old 10-09-2005, 10:23 PM
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Bill, I am thinking more and more that you are right. If I can successfully cut the tempered glass it will be ridiculously fragile and break anyway. I will go ahead and try, what have I got to loose? I have a lot of "test panels" at work.

Brian
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Old 10-09-2005, 10:44 PM
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Sorry, I should have posted a reference.

http://www.alumaxbath.com/tech/tgp.htm
Quote:
The basic principle employed in the heat treating process is to create an initial condition of surface and edge compression. This condition is achieved by first heating the glass, then cooling the surfaces rapidly. This leaves the center glass thickness relatively hot compared to the surfaces. As the center thickness then cools, it forces the surfaces and edges into compression. Wind pressure, missile impact, thermal stresses or other applied loads must first overcome this compression before there is any possibility of fracture.

[...]

Tempered glass should receive the same care as annealed glass. Unfortunately, familiarity with the greatly improved strength of tempered glass may mislead people to exert less care in handling it. Careless handling and improper installation sometimes produce edge damage. Delayed breakage can ensue when edge-damaged tempered glass is subjected to a moderate thermal of mechanical stress. Full penetration of the compression layer will likely produce instantaneous total fragmentation of tempered glass. Hence, tempered glass cannot be cut or modified following heat treatment.
That last paragraph pretty much matches the experience Bill Parten relates and what MARTINSR experienced with the sandblasted window. The way it's manufactured causes only the surface to be under compression or "hardened". All that center glass is just waiting to get loose.

http://www.alumaxbath.com/tech/tgb.htm
Quote:
6. Another characteristic of tempered glass is that occasionally a light will not release immediately at the time of damage, but at sometime, perhaps many weeks, later. This adds to the surprise and amazement of by-standers since no apparent cause is immediately evident. This type of behavior is one of the factors leading to the so called "spontaneous or delayed breakage" of tempered glass.
I'd like to see a photo of the glass under that tape and metal.
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Old 10-10-2005, 08:09 AM
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Honestly, I think this experiment is complete. The fact is, this stuff is so unpredictable that you could spend a weekend and ten bags of sand cutting all your glass only to have it "explode" all over your new paint at your first show!

There is no surprise or secret, the glass under the tape was in a million pieces.

Brian
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Old 10-10-2005, 11:39 AM
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[/QUOTE] I will go ahead and try, what have I got to loose? I have a lot of "test panels" at work.


Brian,, why don't you see if you can find a windshield,, and try cutting that,and when you get to the center plastic insert, spray some lighter fluid in the cut line and set it on fire,, it just burns for a second and softens the plastic so it can be cut with a sharp blade, this is the way laminated plate is cut,,it might work on TEMPERED safety plate,,, like you say,,what have you got to lose, just watch for flying glass,, probably don't need to mention safety glasses and gloves,,long sleve shirt and a good suit of armor,,
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Old 10-10-2005, 01:31 PM
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Well, if that is "the way" safety plate is cut, there is no reason for me to do the "experiment". I want to learn something, not that I need any tempered glass cut right now. And honestly, again, I have heard of it, you are saying this is "the way" it is done, but have you SEEN someone do it? Or even told you how THEY did it? Is that a myth as well?

I have SEEN a guy cut a curved windshield, with a regular old glass cutter. He cut 1/2" or so strips off this 55 Chevy truck windshield. He kept cutting these strips off until the four inches or so was gone. There was no burning the plastic, he just cut it with a razor blade, done deal.

Like I said, the fact that I may pull off cutting this glass, what point is it if I have removed the top of the "frame" that is holding all this glass under pressure? So, I cut it off and stick it in my car and it explodes the first time a rain drop hits in on a warm day, what good is that?

It sounds as if cutting it, is only one problem, re-tempering it is another issue.

Brian
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Old 10-11-2005, 08:50 AM
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cutting a windshield

But can you cut a curved windshield with a sandblaster? Was that proved or dis proved? That would be nice to know as most 50's cars have flat glass on the side windows. Then when it's chopped if we can cut the windshield our selves with a sandblaster only the back glass would have to be dropped into the trunk area.
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