Cutting Windshields
This is a long story on how not to and how to cut windshields for a roof chop. First off, it's easy to chop a roof but very difficult to chop one right if you have a one piece curved windshield like on my 54 Pontiac. I found out that mine was not chopped properly after spending 7 years working on it and finally trying to install a windshield. More on this later when I get some pictures.
In Dec 2006, I tried cutting a windshield using a dremel tool and a diamond saw blade. It looked very promising as I test cut an old windshield without any problems (see below). Unfortunately, the 54 windshield cracked in the corners...
Feb 2007, After spending a month to track down a clear no shading windshield and $500 later, I starting cutting windshield number 2 using a sandblaster. Five hours and 200 lb of sand to cut through both sides! The cut is about 1/4" wide and the two pieces were held together by the inner plastic layer. A cut with a sharp razor knife and they separated. Final touch-up with 40 grit sandpaper and she's done! By the way you can hand sand the glass edge with 40 grit sandpaper to fit.
May 2007 Boo-hoo, I opened my tool box drawer which knocked a box of bolts which bounced off the shop vac and smack into the middle of windshield number 2 and cracked it down the center! Aaaaagh! After refusing to go in the garage for 3 days, I cleaned up the mess and ordered another windshield. It's now costing me $1100 for the 3 windshields!
June 2007, I practiced installing the cracked windshield and found out that the roof chop is done incorrectly and it wouldn't fit anyways with the standard rubber channel. So on to plan C, cut the windshield oversized and silicon it to the frame like they do to modern vehicles - SUCCESSFUL!
With a bit of fixing on the windshield frame - bending the flange, the new windshield fits pretty nice. I'll be making another webpage with details on installing the new windshield as this one is getting a tad long.
Tips on cutting windshields
Make a Template!
With windshield number 3, the plan is to not use the rubber channel to hold the windshield in place, imstead I'll glue the windshield to the windshield frame flanges. There's two methods of gluing the windshield in place: butyl rubber tape and urethane adhesive. Butyl rubber tape was used in the older vehicles and looks like a roll of black rope. Urethane adhesive is used in modern vehicles. It is ultraviolet sensitive and will deteoriate with sunlight and all new windshields have a black coating on the edge to protect the adhesive.
The first step is to make a template of the opening - it should allow a top and bottom clearance of roughly 1/8" (1/4" overall). I used the bottom half of the windshield as a guide and then fitted the top half to the windshield opening. If the windshield is cut properly, the top will now be a couple of inches wider than original. Two pieces of poster board taped together was just enough to cover the complete windshield. You can see the outline of the top of the windshield number 1 and dimensions that I had measured to add to it on the following picture.
Template in position
The windshield is curved and there is a significant difference in making a template based on the windshield's outside dimensions versus the inside dimensions. I initially made a template using the outside of the windshield. I thought it over for a few days (not because I was gun shy or anything!) and realized that the inside of the windshield rests against the windshield frame (this isn't rocket science!). I found out that there is a significant difference that the 1/4" thickness of the windshield adds to the curved surface! So make your template based on the inside dimensions of the windshield!
All about Windshield materials
A windshield is made up of two thin sheets of glass sandwiching a layer of plastic. The correct name is laminated safety glass.
You need a windshield with no shading preferably. This is the dark colored band at the top of the windshield. I ordered a new one (yes, you can order new windshields!) but had to return it. The shading was 3" in the center but 4 1/2" at the sides. If I cut it 3 inches, I would have what looked like paint overspray in the top corners. I did eventually find a windshield with straight 3" shading across the top that was usable. I met the owner at a junk yard of all places - cost $100.
If you do a search on Ebay, you'll find that there are a few vendors that sell 1953-54 Chevy/Pontiac windshields. The price range is from US$285-$350. The hardtops/convertible windshields are different from sedans/wagons (posts) models. they come in green tint with shade (I believe the shade is blue), green or clear.
Front Windshield
I have an old broken 54 windshield resting against the windshield frame. This shows the difference between the original windshield and the new opening. It's about 3 inches. The windshield frame should be 1/4" larger than the glass for the rubber seal. This leaves 1/8" top and bottom. Steele Rubber Products makes the windshield rubber (1-800-544-8665). I'm not too familar with the rubber so rather than buy generic rubber from JCWhitney which is much cheaper (about 1/3 the cost), I purchased the Steele Rubber part. Here's some info on different methods of cutting a windshield:
Scoring and breaking - Not Recommended
I don't recommend scoring and breaking windshields. The rule of thumb is 3 broken/cracked windshields for every good one! Not the type of odds that I like.
The procedure is to score the windshield (front and back) where you want it to break. Pour some lighter fluid on it and light it on fire. Supposedly the windshield will crack where you want it. It just sounds too risky for me.
Router and diamond bit - Not Recommended
A friend just happened to be cutting ceramic tile with his router and a diamond bit. We figured that if it could cut through ceramic tile, then it should be able to cut through glass. I borrowed his bit and put it in my router and gave it a try.
RotoZip ceramic cutting router bit
As you can see from the picture, it cuts a very wide channel through the glass. It requires a lot of effort (pushing) to cut and it was very hard to control and guide, and created lots of spider cracks. I don't recommend it at all.
Dremel and diamond saw - Not Recommended
I was talking to my cousin who recommended using a Dremel tool and a diamond cutting wheel. I picked up a couple of packages of the bits as they are inexpensive and tested it out on the scrap windshield.
Dremel tool and diamond cutting wheels
Glass cut in one pass - lots of chips and rough edges!
Closeup of one pass cutting - not good!
The first cut, I tried to cut the glass in one pass. There are lots of chips and rough edges. The glass would glow red and it had very fine spider cracks appear. You can see some in the picture. Not acceptable.
Light multiple passes with the dremel tool
I taped up the windshield with masking tape on both sides, marked up the cuts and went to work. In about an hour, I had the windshield cut. It was quite easy but unfortunately not successful.
Windshield cracked in left corner
Crack in right corner
I was very disappointed because I had spent so much time researching and testing. I also was very careful to apply light pressure and do many many cuts. I used 3 blades to make the cut and changed them whenever they felt that they were not cutting. Again, I'm sure that it was the heat that cracked the windshield. It was difficult to make light passes in the corners. The only thing that I can think that may of caused the cracks is the direction that I cut.
If you move in the direction of the blades rotation, it takes a very light cut. If you go against the blades rotation, it cuts more aggressively. I was cutting in both directions, maybe I should of cut with the less aggressive direction by going with the rotation? Who knows, I've run out of windshields...
Water Jet cutting
Another option is to use something called a waterjet (abrasivejet) to cut your windshield. This is the ultimate method but can be expensive. I did have US quotes that were in the US $150 range. Not every city has a waterjet. They cut using a high pressure stream of water. Here's a source of information for waterjets
Here's some people who are into cutting windshields:
I charge $ 100 to $ 150 per windshield, depending on the complexity of the cut and the attention needed to the Z axis. Comparible costs to cut down windshields are around $ 400 to $ 500, this involves cutting by hand, sandblasting material away and wet grinding, a very involved process. I hope this helps. We have extensive experience using WaterJets for purposes other than originally intended, experimenting is the only way! Larry Berk Creative Cutting Service LARRY BERK lberk@redshift.com US location
Another source: Classic Glass
And one more:
We currently water jet cut frames for windshields in fork lift equipment. Our machine is five axis, so the curve of the windshield is not a problem. We account for angles and contours in the programming process. I would be happy to answer any questions you may have on this process, you can reach me at 920-739-1120, or email fcsinc@vbe.com. Jon Martin Fox Custom Supply, Inc.
Sandblasting - HIGHLY RECOMMENDED!
Sandlasting is the most reliable method to cut a windshield. My first windshield took 6 hours and 200 lb of sand to cut. I changed to a smaller nozzle and it took 1/2 the time and only 80 lbs of sand. It sounds like it is slow but you are cutting about 10 feet of 1/8" thick glass. Part of the time was taking breaks, loading up the 40 lb sand pot, replacing the sand mask face shield.
I've test cut an old windshield from an auto glass installer. I used a sandblaster with the smallest grain sand (Sil-1 - about 100 grain), I've also used Sil-2 and it worked well. I tried a siphon feed gun but it doesn't have a small enough nozzle and its blasting area was about 1 inch in diameter which is much too large. I was able to cut through the glass quite slowly with no problem. I test cutted through a crack in the glass successfully and even sandblasted a hole through the glass. When I reached the plastic laminate between the glass layers, I cut it with a razor blade and continued on through. Expect that it will take a couple of hours or more to cut if not a day or so.
Two problems that I had were the sandblaster nozzle size and the masking material. I was able to reduce the nozzle size significantly by buying a 40 lb pressure feed sandblaster for US $80 from Harbor Freight Tools . You can't reduce the nozzle size on a siphon feed sandblaster without defeating the siphoning action. The machine from Harbor freight is a surprisingly well made machine for the price.
A big improvement was to convert the pressure feed sandblaster to a PA (pressure assist) sandblaster. It has the best characteristics of siphon feed and pressure feed. The pot is only pressurized with about 5 to 15 psi to feed the sand using a siphon feed gun and blasting pressure needs only 65 psi. Works like a hot damn!
Rob Cummings came up with the idea after getting frustrated with siphon and pressure feed sandblasters. He's put a lot of work into the design and sells the plans on his website PA Blaster for a very reasonable price that includes more support than you can ever imagine and a community of over 1000 fellow DIY (do it yourselfers) that will assist you with pretty much anything that you want to do!
Masking Materials
Masking the windshield is really divided into two parts: the cutting edge and protecting the rest of the windshield. I tried a few different methods of masking the cutting edge and eventually came upon this method which worked surprisingly well:
- First a layer of masking tape that follows the cut line previously marked from the template. The masking tape has to be super sticky. It's job is NOT to peal from the blasting pressure.
- Cover the layer of masking tape with a layer of aluminum tape like used on heating ducts or muffler tape. Sorry, I didn't mark down the brand of tape
I used. Aluminum is soft and the tape holds up surprisingly well to sandblasting at 65 psi from 1 inch away! It's job is to keep the edge. I used
the handle of a razor knife to press smooth the aluminum - compare the two following pictures for smoothness.
A layer of aluminum tape over the hidden masking tape
- Cover the aluminum tape with masking tape and cover up the rest of the windshield with double thickness masking paper. Tape up every crease -
you don't want the air pressure to rip the paper and sandblast the good part of the windshield. Be ANAL about it!
Masking up the rest!
- Cover the edge with a rubber tape. For windshield number 2, I used the rubber tape that is used between a pickup truck topper and the box
but switched to a foam rubber tape for windshield number 3. Foam rubber is NOT that good. Real rubber tape is much better if you can find it.
Layer of grey rubberized foam (not the best choice)
- Mask up any edges on the foam and everything else in preparation for blasting!
Ready for blasting!
NOTE! You must mask both sides of the windshield because you will be cutting both sides! Alternative Masking Materials
For masking material I would suggest a product by a company called Hartco. It is called Sand Mask, and is used in the sign industry for making sandblasted signs. it's good up to about 90 psi and has a very aggressive adhesive on it to keep it from moving, and is made from pvc. check out Sand Mask .
Another source for masking material which I will try is the mask used for scribing cemetary headstones. They carve the granite/marble headstones using sandblasters and make very intricate designs. They should have a very good masking material.
Makng the cut
When you start sandblasting, you should be blasting away from the masked edge and along it. Typically, you'll be positioned reaching over the main body of the windshield which is very awkward. I would work a 6 to 8 inch line slowly back and forth. The idea was that if the windshield cracks, you've weakened it and it should follow the cut line. I never had a crack occur except when you just start breaking through to the center plastic laminate and it would be only about 1/4" long.
This picture dramatically shows that you can cut a 2 1/2" slice off of a windshield accurately!
You'll find that the cut is about 1/8" wide and about 1/8" from your cut line. Don't worry! It's better to be bigger, you can fine tune the edge with a sanding block and 40 grit paper. The sandpaper doesn't last long so change it as soon as it stops sanding.
Windshield with template on it, 40 grit sanding block and direction to sand along the cut line.
I cut my windshields in the back alley. Expect lots of sand every place so make sure that you are well protected, wearing coveralls and that the garage doors are closed! You are doing precision cutting and will end up with your face very close to the cut line, you will need a complete head covering sanding mask. I have a very cheap one which works quite well.
Faceshield Tip
A problem that I ran into was that the plastic face shield ended up getting sand blasted from the reflected sand and I couldn't see out of it! The solution was to cover the new face shield plastic in green painter's 1" wide masking tape (green pulls off easily). I would leave a 1" wide window to look through, when the shield became too difficult to see through, I would pull off the next layer down of masking tape and look through the new window. One face shield would last 3 hours doing it that way!
Pitfalls
A buddy of mine tried the sandblasting method on his pickup windshield. It was a very curved windshield and he decided to make some stress reliever cuts. Cut along the line where the windshield would be chopped then up. The theory was that the weight of the windshield would cause cracks so if he cut chunks out, the weight would not be present and crack the windshield. It cracked at the stress relievers! Sometimes you can't win. He bought another windshield and cut it without the stress relievers and it worked perfectly!