Cutting Thick Glass?

I just tried to cut a 15" wide piece of glass off of a sheet that is 24"x48"x 3/8" thick. This is the first time I've tried to cut glass thicker than 1/4" thick and I did everything but jump on it (I was tempted) and it wouldn't break along my score line. I put a pencil sized dowl underneath the score line and pushed down on the short end and even wacked it a few times (well, maybe more than a few times) with a rubber hammer and it still didn't run along the score. I even tried my running pliers but all I managed to do with those was break off a 10" long sliver from along the side. Even though I was told they were not tempered, I wasn't possitive, but since it didn't shatter when I did that, it really must not be tempered.
I've read about cutting thick glass and even watched a few videos demonstrating it, but either I have some very tough glass or I'm not doing something right. I got a couple of smaller sheets of 1/2" thick glass at the same time as the sheets of 3/8" ones, and if I can't cut the 3/8" I'm not even going to attempt the 1/2" thick ones. Unfortunately, none of them will fit in my kilm if I don't cut them down somehow. Short of trying a chain saw in a water bath, any suggestions?

* YOu could try a straight bit of wood under the end to lift it up. Hold a straight bit of wood with its corner along the score which will be beneath,and hit that with the mallet, sharp tap. Thats how I do 1/4 inch thick,its not something I enjoy.

* Mike
10mm float is my every day glass. I have a wonderful pair of Toyo heavy glass running pliers that have nylon pads and mechanical advantage built in. I can run a perfect score with no flare. 130" x 2.5" with them. I also have a less expensive pair of PPG all metal running pliers. These sometimes take a chunk out of the bottom, so I only use them for cuts thinner than 2"
The #1 rule when cutting float glass is clean the glass before you score, followed by the #2 rule which is use enough cutting oil . I paint my glass with kerosene before cutting. My success rate after first cleaning the glass and painting kerosene is very high. Forget either one and my success rate falls off very quickly. The third trick is to use a Toyo tap wheel cutter. I love the Custom Grip.
When I teach cutting 10mm glass, I place my personal tools in the student's hands. They almost always do it right the first time. These are usually people used to cutting 3mm colored glasses.
Oh yeah, I learned the hard way that if your cutting bench is the least bit concave, running glass will fail every time (unless you mitigate this).

* Thanks for the suggestions, Peter and Bert. I tried both of them. Bert, I tried again on the other end of the glass and I cleaned the area really good and put down a good layer of cutting oil. I use a Toyo TC-600 cutter that has the yellow plastic grip handle for holding oil (I never put oil in it though, I usually just dip the head). I got what sounds (to me) like a good clean cut all the way across.
The last time I tried, I'd put a piece of plywood bigger than the glass on two sawhorses and used that to cut it on. This time I did it on a concrete floor with a thick pad on it. After making the score I tried putting my T-Square underneath to give it a little bit of height and wacked the glass with the palm of my hand. I didn't wack it gently either. Then I tried a thicker board and did the same thing. After nothing gave but my hand, I did try standing on it and hitting it with my foot. The problem though is I couldn't hold it down with one foot heavy enough to get a good stomp on it and I didn't want anyone else to stand on the end in case it shattered instead of broke.
Bert, if I was able to take one of your classes, I'd probably be the first student to not get a passing grade in glass cutting. I could probably take it to the local glass store and have them cut it. Maybe they would let me watch so I could see how it's done in person. I think the cut is ok but I must not be hitting it right or not supported it right or both.
Thanks for the suggestions.

* Bert, do you only paint the small section where you will make the score?

* Bert, do you only paint the small section where you will make the score?
I have a bristle brush about 3/8" wide and I paint next to the straight edge. When I do a circle, I put the brush under the cutting head and run it around the circle. I have never trusted oil cutters to deliver the oil. Where I live, there are pumps all around that sell K-1 heating kerosene for winter heating. It costs around $3 a gallon. A gallon, of course is sort of a lifetime supply. I keep a jug on my bench with no lid and it still lasts for years. The paint brush is pretty large with a round pointed wooden handle. This can be used to peg the glass to break it after scoring.
Mike the big deal is getting the break started. Once it is started, running it is not so difficult.
Another trick I have learned is to cut towards myself. The end of a score is always stronger than the beginning. You might not start exactly at the edge, but you always end at the edge. I also place the glass overhanging my bench, so the cutter does not slam the bench at the end. It us always easier to begin a break starting at the side where you finished the cut. There are many ways to do all these things, but I found this strategy to be sound, and it has really helped me to be a reliable glass cutter. I had to retrain myself to cut towards me, as I originally learned the opposite way. It has been well worth the effort for me. If I am really uncomfortable cutting towards myself, I'll 180 the glass and start the break from the side I ended the cut on.
And Mike, watching the guys at the glass shop is one of the main ways I learned what to do. Especially cutting odd shapes in heavy glass. I watched in 2 different shops and went home and practiced. It didn't hurt that I bought several tons of 1/2" glass from a salvage company for less than $1 a square foot. The big thing I learned from that is that I much prefer 3/8" glass to 1/2", and I never mess around with 3/4". But I can pattern cut in 10mm or 12mm, now that I have the practice under my belt. I have a coffee table to make next week that is a wavy L shape 66" long with a 40" Leg. I hope to get it the first try in 10mm glass. I'll place the glass on my bench and kneel on the glass to cut it, using one score all around. Then I'll flip it over and use the tapping tool to start the break all around and the propane torch to finish the crack, afterwards. I'll remove the inside curves in sections, so at the end, it will be a relatively thin piece coming out last. I'll draw the inside curves slightly more than 90o.

* I'm not a thick glass cutting expert by any means, but the thing I've most learned is that if I can't SEE the break running down through the glass and across the entire score line before I make the final separation, I'm not going to get a clean break (or sometimes any break at all). If I can do it with strong breaking pliers after a great score, wonderful. If I need to help the score along by whacking the underside of the score a few times with a hammer, or hammer and chisel, fine. (I don't like to do that, it makes for uneven fractures)
Usually I need to flip the glass over on a padded surface and help the score along by pressing or tapping the line and watching for the break to travel; the extra lubricant on the score line seems to prevent skips in the score line so that it's nice and even. I don't try to fully separate the glass until I've watched the break physically travel through and across the glass. It's not something you have to think about with 3mm glass because the break has such a short distance to travel anyway and it will generally travel right through a skip on the score line. But that won't happen with thick glass, so you have to pay more attention to those things.
Don't know if that helps or not, Mike. But you've not only got to break the surface of the glass with the score, you've also got to watch the break penetrate the entire cutting line.

* Oiling the glass is an interesting issue. If you cut float glass dry, and wait a few seconds, you see little white shards come out of the score. This is the score closing up. After this happens, you may or may not get a clean break along the score. It is much harder to get a good break after the white stuff shows up. Oil going in to the score stops the white shard thing from happening, increasing your chances for a good break. Being an essentially lazy person, I have been known to skip a step here and there. Skipping the oil step (or the cleaning step) on float glass is really costly for me. I may be lazy but I am not stupid...
When I did copper foiling of stained glass, cutting dry was very important to me. It made the process go much faster, and it worked fine. I can't tell you why the rolled glasses are less sensitive to the oil/white shard thing, but they are.
The other step I do not skip is sanding the sharp edges, after cutting, and before cleaning. This step does 2 things. First it cuts way down on finger cuts while cleaning the glass. It also removes microstarts. In the tempering industry, at it's inception, they discovered that glass that had been cut but not seamed, tended to break during the heatup, considerably more than glass that had the sharp edges removed by a belt sander (called seaming). Seaming can be accomplished with a dry belt, a wet belt, or a diamond hand pad. I usually use a dry belt sander. On smaller pieces, I might use a hand pad. It doesn't take much effort to remove the sharp edge. I use Glass Plus as my cleaner after doing the edges, and it removes the kerosene, with no more effort than cleaning a dry piece of glass.

* Cynthia, I thought you were on to something there. I've never turned the glass over and started a run that way, so I tried it with this sheet. I pressed on it like you said, then I tapped it with my rubber hammer, then I stomped on it then I tried a flat head steel hammer. I hit it pretty hard with the rubber hammer but didn't use as much force with the steel hammer. I did this at the edges and then worked my way across and back. <sigh> No change. I then turned it back over, put a wide board under neath it so it would support the glass from the score line back from the piece I'm trying to cut off. I stood on the glass supported by the board and then stomped down with one foot. Still nothing. I considered your chisle comment for a few moments and decided if that glass is this hard for me to break it along the score line, I probably should re-think wanting to use it. Even if I got it to break, this was only the first cut of several that I was going to have to do to get a piece that will fit in my kiln (I can take a 22" round shelf and I measured that a 15x16 rectangle piece could fit on the shelf).
So for the time being, I'm going to leave them sitting in my garage. I'll check to see how much it will cost to have the local glass place cut it down to size for me. If nothing else, I'll make a bullet proof green house (you never can tell when someone will use their 9mm to break into your green house and steal your tomatoes ) or just free cycle them to someone local that knows what to do with thick glass. Luckily I got the whole stack for very little, so I'm not out a lot. I don't want to keep trying until I shatter one of them as we have a bunch of dogs and I don't have any place I can work on these large sheets that they don't go or wouldn't be a real pain to make sure I got cleaned up completely.
Bert, I've followed everything you have said on Warm Glass about working with thick float and even though I know it's not as easy as you make it sound, you really do make it sound easy. But then if it really was easy, anyone could do it.
Thanks for all the suggestions.

* have you checked the Bullseye site re cutting glass they mention the angle of cutting wheel that is needed for various thicknesses of glass like billet cutting. Also I have found cold or older glass harder to cut.

* I did see a 155 degree wheel that Fusion HQ's sales that is suppose to be for thicker glass. Something about it makes a wider scrore line. I've not checked with Bullseye (every time I go in there, I'm a glass magnet and find at least two armloads of frit and glass go out with me ) but now that you meantion it, I do remember reading something about cutting their billets.
The glass isn't cold, it's about 67 degrees in the garage, but I don't know what it's age is. The person that had it had a bunch of it that she was getting rid of that I think was used for shelves. At least some of the stuff she had she said could go because it went to a shelf unit. The glass is clean and a dark green color, but who knows how long it's been around.

* In general thicker glass needs a duller wheel. However 134o works fine for me on all glasses between 3mm and 12mm. I prefer the tap wheel to the traditional wheel.
I cut extremely cold float glass without problems. If it is too cold to clean the ice off, I'm in trouble, but as long as the glass is clean, I can cut it. I know this because I store my case of 10mm float outdoors in NH. The sheets are 48" x 130" and I always make the first cut, upright in the case. This is tricky as hell, but temperature is not the factor that matters for me. Cleanliness and support are the 2 factors that do matter. Many a winter day, I shovel snow, clean off the glass and cut. I use a drywall square, clean the glass, paint on the kerosene, score, and peg it out using the paint brush handle. Push and I have 2 pieces of glass. If the supports on the bottom are in the right spots, I have 2 rectangles of glass. I have to shovel the path for the sheet dolly. I have been known to slide the glass along the ice using suction cups. The sheet of glass can weigh 150 lb, and I can get it to the bench and on the bench, myself. Usually, the glass is sitting on the sheet dolly, leaning on the edge of the bench. I place the suction cup towards the bottom center, and tilt the glass on to the bench.
I know that many people here are convinced they can cut their glass better when it is warm, and this may well be true. I just know that temperature is not a factor I care about with my glass.
Old glass can indeed be hard to cut. I think this has to do with poor annealing more than anything.

* Ah Hell when it refuses to cut for me, It's to the wet saw. I bought a cheap one for a couple hundred. I love it for cutting just about anything. It may be messy but It has cut down on my cursing. I hate thick glass too. I love being able to cut finish fues projects as well. Another fusing after cutting and the ugly edge vanishes.Lia
This is just my two cents worth.Lia

* The old glass I am referring to was 1.125" thick, and rolled. It was a glass floor in an old library. This was a bitch to cut. I wish I had some to try now that my skill level is better than when I first got some of this back in the late 80's. I have a feeling that it wouldn't cut any easier now than it did then. I have a piece I enameled and fired back then and it is still intact, so I managed to anneal it. The lead bearing Thompson Enamels for float glass were compatible too.

* @ Bert: What do you mean when you use the term "peg" in the context of cutting glass?

* I place the paint brush handle underneath the glass, right under score, giving me a peg to push against. This can be done with a dowel or any object that will concentrate the pressure beneath the score. This is the same force set up by running pliers. I use the paint brush handle because I just used the paint brush a moment earlier to paint kerosene on the glass where I plan to score.

* Mike, are you sure it isn't tempered? If you have lots, I would break one with a sledge hammer under a sheet or something, just to see if they are tempered.

* You should be able to detect tempered glass by looking with a polarized filter. You will see a grid of white dots if it is tempered.

* Jolly, although I'm not 100% positive, the woman I got them from said no they weren't tempored the four times I asked. The other reason I don't think so is that one of them does have a chip out of the side of one corner and when I tried to break these I broke off a 10" long by about 1/8" wide sliver of glass from the side (my running pliers broke it off) and the glass didn't shatter on me. I haven't seen the white dots that you have mentioned either, Bert. In any event, I do put a blanket over it when I'm wacking on it... just in case it decides to go to pieces.
Since using the Bert Weiss approch hasn't worked very well for me, I'm thinking about changing my approch and try the Chuck Norris way of perswasion. I don't have cowboy boots, but I do have a good pair of combat boots. Well, one good combat boot, one of our dogs ate the other one, but I only need one.

* Mike
I have explained many times here, how to make a tool for starting a run in the middle of a score. Essentially I take a cheap slotted screw driver and heat it red hot. This makes it loose some hardness. Then I grid the flat slot so it is arced. Now I make the score, flip the glass upside down, and place the tool directly over the score. Then I hit it with a hammer hard enough to start the break. The idea here is that the tool allows me to direct the force to the exact right spot when I hammer it.
If you do this near the edge, and get it started, you could flip the glass back over, place a peg beneath the start and then push on both sides of the peg.
Or find somewhere to borrow some lovely Toyo heavy glass running pliers.

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