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  #31 (permalink)  
Old 02-23-2012, 03:01 PM
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One problem I see with the method; glass (as with most construction materials we use) needs to have a smooth edge. It is very susceptible to cracks @ stress risers. A stress riser is a notch of any kind. The sharper the corner of the notch, the more likely it is to crack. Think of the problem this way; you put a load on a part say, 10#, and spread that load across the part. If there are not radii or notches, it spreads evenly and the stress is very low. Say the part has a stressed area of 10sqin, the stress value would be 10#/10sqin or 1psi. Anything can stand that low stress. However, put a scratch across the area, then the load is concentrated on that discontinuity. If the scratch is sharp it has 0.0sqin area so the stress in 10#/0.0sqin or infinite! Part is guaranteed to break under any load. Unless you can figure out a way to make those cuts very smoothly, your method will fail in service.

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  #32 (permalink)  
Old 02-23-2012, 03:25 PM
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Quote:
Originally Posted by willys36@aol.com
One problem I see with the method; glass (as with most construction materials we use) needs to have a smooth edge. It is very susceptible to cracks @ stress risers. A stress riser is a notch of any kind. The sharper the corner of the notch, the more likely it is to crack. Think of the problem this way; you put a load on a part say, 10#, and spread that load across the part. If there are not radii or notches, it spreads evenly and the stress is very low. Say the part has a stressed area of 10sqin, the stress value would be 10#/10sqin or 1psi. Anything can stand that low stress. However, put a scratch across the area, then the load is concentrated on that discontinuity. If the scratch is sharp it has 0.0sqin area so the stress in 10#/0.0sqin or infinite! Part is guaranteed to break under any load. Unless you can figure out a way to make those cuts very smoothly, your method will fail in service.
Smooth edges are good for sure.

Laminated glass can be polished after cutting.
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  #33 (permalink)  
Old 02-23-2012, 04:26 PM
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Originally Posted by willys36@aol.com
One problem I see with the method; glass (as with most construction materials we use) needs to have a smooth edge. It is very susceptible to cracks @ stress risers. A stress riser is a notch of any kind. The sharper the corner of the notch, the more likely it is to crack. Think of the problem this way; you put a load on a part say, 10#, and spread that load across the part. If there are not radii or notches, it spreads evenly and the stress is very low. Say the part has a stressed area of 10sqin, the stress value would be 10#/10sqin or 1psi. Anything can stand that low stress. However, put a scratch across the area, then the load is concentrated on that discontinuity. If the scratch is sharp it has 0.0sqin area so the stress in 10#/0.0sqin or infinite! Part is guaranteed to break under any load. Unless you can figure out a way to make those cuts very smoothly, your method will fail in service.


A good glass cutter for cars, WILL always take the glass to a belt sander and grind [very smooth grit] the complete edge of the glass all the way around the piece. till it HAS NO NOTCHES or small chips in the edge. I've watched a lot of them. Sandblasting leaves no rough edges at all.There fore still the best way to do this, trouble with this is finding a blaster who will take on the job, Surely one would have to leave a deposit on his glass.Theres just no absolute garantees on "curved glass, Flat glass is awhole diffferent world hth
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  #34 (permalink)  
Old 02-23-2012, 06:43 PM
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Originally Posted by bsa_bob
A good glass cutter for cars, WILL always take the glass to a belt sander and grind [very smooth grit] the complete edge of the glass all the way around the piece. till it HAS NO NOTCHES or small chips in the edge. I've watched a lot of them. Sandblasting leaves no rough edges at all.There fore still the best way to do this, trouble with this is finding a blaster who will take on the job, Surely one would have to leave a deposit on his glass.Theres just no absolute garantees on "curved glass, Flat glass is awhole diffferent world hth
I understand at least the basics of load spreading and cracks(I grew up fishing on some sketchy Wisconsin ice ) thats why I'm am going to double tape the window cut. One line of tape will be the actual outline. The second will be further out about 1/4inch or so. If you look back at the pictures I posted, the best way to cut this with the rotozip is at an angle, so obviously you can't just cut to size or you'll have a bevel, which is not conducive to load spreading the purpose of the double tape is twofold, one to be able to deal with the bevel, and two to get the edge sanded down straight and smooth. So for the last 1/4 inch, I will be taking the belt sander with very fine grit and bringing it down to the tape of the actual outline. I'll have smooth lines, and be able to massage the windshield down to the correct size of the frame. We all know that two chops are not alike and that they are not always dead even. So I'll be having a friend helping me put it up to the frame, and the grinding a little more until its perfect. Should slide right in like a.... uhhhh.....fill in your own blank.
Zak
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  #35 (permalink)  
Old 02-23-2012, 06:49 PM
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Quote:
Originally Posted by JMSTOY
I understand at least the basics of load spreading and cracks(I grew up fishing on some sketchy Wisconsin ice ) thats why I'm am going to double tape the window cut. One line of tape will be the actual outline. The second will be further out about 1/4inch or so. If you look back at the pictures I posted, the best way to cut this with the rotozip is at an angle, so obviously you can't just cut to size or you'll have a bevel, which is not conducive to load spreading the purpose of the double tape is twofold, one to be able to deal with the bevel, and two to get the edge sanded down straight and smooth. So for the last 1/4 inch, I will be taking the belt sander with very fine grit and bringing it down to the tape of the actual outline. I'll have smooth lines, and be able to massage the windshield down to the correct size of the frame. We all know that two chops are not alike and that they are not always dead even. So I'll be having a friend helping me put it up to the frame, and the grinding a little more until its perfect. Should slide right in like a.... uhhhh.....fill in your own blank.
Zak
Hey zak i just wanted to wish you good luck on a tough job,be patient and who cares if it ain't cut exactly like "boyd coddingtons glass" if it works and don't crack you'll be the happiest man alive for awhile thinking "i beat the pros at their own game" You can do it!!!! i can tell by how you describe your well thought out plans take care man!!! bob s
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  #36 (permalink)  
Old 02-23-2012, 06:54 PM
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Quote:
Originally Posted by bsa_bob
Hey zak i just wanted to wish you good luck on a tough job,be patient and who cares if it ain't cut exactly like "boyd coddingtons glass" if it works and don't crack you'll be the happiest man alive for awhile thinking "i beat the pros at their own game" You can do it!!!! i can tell by how you describe your well thought out plans take care man!!! bob s
Thanks bob! Good weather this weekend here, I'll be cutting down my "good" practice windshield hopefully Saturday! I'll post pics! And you're right, I will be the happiest man alive!
Zak
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  #37 (permalink)  
Old 02-24-2012, 04:22 PM
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My first post/reply. From Sweden europe, so we have a language problem to. Im also on Chevy talk as Sweed 62. Lets see if i find out how Hotrodders forum and pictures work.
I chopped a German Ford -71, 25 years ago, will add some pictures when i find out how.
I will try to share my experiance at cutting glass.
Being a dental technichian at that time, i used hand held electric motor like the dremel type engine, but weaker. 2. 35mm shaft mandrel, at a diamond disk from Horico, i do believe it was a Horico SH 394 c 190. Diameter 19mm, thickness 0.19 mm. It was a sintered diamond anyway and i think its important because not sintered diamond discs just worn out at this application . It is my experiance that one or at most two of these sintered diamonds cutted thru 8 meters of laminated glass, there was no worn, but the disk was kind of brittle, unbeliveable performance, check Horico live support, if they have some, for the right type.
I used cooling water in the tub, this is low tech, no cracks before finished, but later. did not chamfer edges because i didnt know, crack could have been caused by me gluening glass instead of og rubber .
My edge cutting disk made a straight cut but, a very major problem for me was the centric plastic sheet, that acted very strange, even thou i cut thru it it acts like glued together again, makes me forced to cut the glass from both sides, and then cut the plastic shet with thin blade.
Another cooling agent solving that problem, somebody?
Svenne

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  #38 (permalink)  
Old 02-24-2012, 06:34 PM
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I'm an international star! Lol Ok maybe not.
Svenne, that center sheet is doing its job as safety laminate. Possibly the heat melted it back together. I've heard of two ways to fix this, either cut it with a razor, or my personal favorite, pour lighter fluid on it and light it(carefully). It will flash flame, and melt it, without getting it hot enough to crack the glass.
Zak
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  #39 (permalink)  
Old 02-25-2012, 05:24 PM
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Eat your heart out expensive water jets!

Sorry if any of you own a waterjet!
Well, I did what I set out to do, I cut off about 8 inches of glass on a practice windshield! Here's the scoop.
First, I laid out the tools and secured the power cord up off the ground (saftey first!). Then I secured the hose to the windshield with a small rubber tipped clamp. I had a problem with it sliding around with the vibrations from the rotozip last time. Then I taped off a fake pattern on the edge, about 8 inches from the edge. As I stated on one of the last posts, I taped off the "Real" edge (where it will actually end up). Then I taped off about 1/2 inch farther out. I learned from this experiment that trying to sand through 1/2 of glass is WAY too much, so probably tape off only 1/8 - 1/4 inch. The extra space is now 3-fold. It gives you a little wiggle room if it makes a big chip or something, it lets you get rid of the bevel from the cut, and it lets you "fit the glass". By Fit the glass I mean not every chop is perfectly even or symmetrical, so it gives you room to hold the windshield up to the frame and see your progress. You can sand down to shape.
So this first pic, I cut into the glass about 6-8 inches, and then used a rubber tipped clamp to hold the beginning of the cut, alleviating some pressure. here you can also see how I clamped the hose down.


Then about 1/3 of the way up the side, I cut over to take off that piece. Not too hard, just awkward changing directions.

Rinse and repeat. Here is the 2nd 1/3 of the side gone.

And the last piece.

Here is where it got fun. I removed the out layer of tape, so I could sand down the edge. I used a belt sander with 120 and then 80 grit. 80 grit wasn't too rough, but I wouldn't go any rougher than that I think. Didn't have any 40 grit or I would have just to see. I went with the edge, lengthwise. Most of the belt was in contact with the edge the whole time, and that seemed to spread the heat out quite well without needing water for cooling. Now, you need to keep the belt moving back and forth, or you will get too much heat buildup. To dramatize this, I tilted the sander up so only the leading edge was in contact. You can see below the bubbles that formed inside the laminate. This was between the glass panes, so there is no smoothing this out!

Here is a small area I was very nice to, so it smoothed out great!

Hard to see the edge, but here it is. In this pictures, you can see the edge, I was sanding with the belt moving from left to right, NOT top to bottom! (that's the next picture).

Here is what happens sanding the wrong way, from top to bottom.

And lastly, here is what the rough edge looks like befroe it is cleaned up.

So there you have it, cut and everything. I'm ordering the glass today!
A few side notes: First, get comfortable. I did this bent over standing the entire time. My hamstrings and back is killing me! Get the glass up to a comfortable working height!
Second, if at all, wear a full face shield, the glass chip fly! I wore just glasses, and I look like a 13 year old shaving for the first time! And it kinda feels like i dried my face with fiberglass. So yeah, face shield recommended. Have fun!
Zak
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  #40 (permalink)  
Old 02-25-2012, 07:23 PM
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Great tutorial Zak. Your photos and explanations are terrific. And glad you have including your "mistakes" as well as your successes. That saves the rest of us a lot of headaches. Can't wait to see you final run at the finished glass.
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  #41 (permalink)  
Old 02-26-2012, 10:14 AM
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Good work. Are those shoes OSHA approved? LOL Stu
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  #42 (permalink)  
Old 02-27-2012, 06:14 PM
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Good work. Are those shoes OSHA approved? LOL Stu
Yeah, they're the steel toed.....crocs....
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