Cutting And Trimming Plexiglas Canopies By Glen "Gee Bee " Breitsprecher (EAA 35503) 18415- 2nd Avenue, S.
Seattle, Washington SEEMS possible that IfatherTthreeHARDLY years have gone by since my and I became involved with making airplane canopies. Since that first canopy on Bob Hammer's T-18, we have produced around 150 canopies, windshields, and bubbles of various shapes and sizes. During this time, we have cut and trimmed quite a few — even added a couple of cracked ones to our old "boneyard" of ruined Plexiglas. We experimented with different tools and techniques and have come up with a few do's and don'ts I would like to pass on to aid those who have been looking at that untrimmed canopy with fear. Most builders are hesitant to tackle the job of cutting Plexiglas because they have heard stories of how wild it is to work with this stuff. I won't kid anybody. Plexiglas, when highly stressed, can be pretty wild to work; but with the proper tools and techniques, canopies and windshields can be cut and fitted without any problems. First of all, a suitable working surface must be provided. A 4x8-ft. sheet of 5/8 or 3/4 in. plywood or particle board surfaced with Formica allows smooth operation on the table top. A flat surface is a must to insure equal distribution of pressure on the partially trimmed part. A crack will start at a focal point of pressure. The cutting tool is probably the most important single item. I'll touch briefly on the ones I feel are the best. The best possible tool for cutting a formed part is a band saw with a non-ferrous metal cutting blade. The band saw has its limitations, however. The throat of the saw limits the width of material that can be fed. With a little ingenuity, most builders could cut and trim a complete canopy and windshield on a standard 12 in. band saw. The next best tool, and probably the most practical, is a router. This is the tool we use for most of the cutting because it is fast. Fastening the router underneath the plywood table and coming up through a hole with a 1/8 or 1/4 in. double flute bit allows 46
JUNE 1971
complete flexibility in moving the part. The secret in cutting Plexiglas is to burn your way through. Hence a router, turning around 30,000 rpm, is a perfect tool. A Skil saw, or hand-held circular saw, works on straight cuts but tends to nick the part. Remember, cracks start from even small nicks. "Can I use a sabre saw?" you ask. Well, it's been done by using a slow speed, but I can almost guarantee you'll crack your bubble at least once or twice. Don't use a sabre saw! You need continuous motion in one direction. The sabre saw is up and down, and one grab or bind and man!, you've had it! Now that you've got your tools lined up, take a tranquilizer, or a good stiff one, get a couple of helpers, and you are ready to make the chips fly. Before you start cutting, cover the entire canopy with masking tape or sticky paper to protect it from scratches. The accompanying pictures don't show us using any protection on the canopy, but you should. Mark your cut lines with a felt pen. Start by trimming off the forming flanges. The initial cut should be kept an inch or so away from the curve at the base of the canopy. This is extremely important. Never allow the bit to cut into the curved area so as to give a sharp, jagged, pointed edge. This is the area where cracks are most likely to start. Always keep the cutting angle at 90 degrees to the surface of the part! After you have taken the flange off to within a quarter inch of the curve, stop cutting! Before going any farther, immediately take a belt sander and dress down this bottom edge. Remove all traces of cut marks, nicks, and gouges. This removes potential crack areas. With the flanges removed the canopy is very flexible. This is the most critical time until you have dressed the edge and the part must be handled very carefully. Repeat this dressing procedure after every major cut. The bottom curve can now be removed with the router, a band saw, or the belt sander. You'll be surprised at how fast the sander cuts through Plexiglas. Keep in mind that during all cutting and dressing operations, the Plexiglas should be held as steady as possible to keep it from chattering — cracks, you know! The next step is to remove the blunt end, as on the T-18 canopy, or the windshield portion, as on a singleplace bubble canopy. Invert the canopy; start at the bottom on one side, and feed the part into the router. After a little practice on scraps from the flanges, it is quite easy to freehand this cut. This is where the help of your friends comes in. It's about a three man operation. Take it slow and easy, and you've got it made!
A f t e r the canopy has been trimmed, and your heart is back in the right place and the adrenalin has quit pumping, it is time to trim the windshield. If you got through the canopy
operation without any cracks, consider yourself an expert. The windshield is
"duck soup." Assuming you've already made a template of the windshield and it fits the airplane, proceed with the windshield. You can follow a straight line closer with a band saw and can control the cut better. The blunt end on the T-18 could also be cut off easier with a band saw by making several cuts as the blunt end is scrap anyway. All cuts should be made outside your final line. Take the excess off with the belt sander. To finish off the job properly, you should flame polish the edges. Get out your welding rig. Use your smallest tip with a pencilpoint flame and pass it along the edge. Once you start on a pass don't stop! This seals the edge to prevent any cracks from starting. Practice on some scrap pieces first to develop your technique. It's easy. I couldn't possibly cover how to mount all canopies because of the many different types and designs. The best move is to follow the designer's plans and be sure to drill all holes well oversize to prevent binding from thermal expansion. Scratch removal used to be a big problem, but not anymore. There is a product out now called Polysand. A kit should be available through your local Plexiglas dealer. Polysand does a fantastic job and is well worth the price. I've received many letters asking if my bubble will fit their plane. In almost all cases the answer is a qualified "yes." The word "adapt" should be familiar to most homebuilders. Pazmany and T-18 canopies are easily adapted to most two-place aircraft, and the large or small single-place bubbles are very flexible. For example, a builder using a small bubble on a Pitts, will use the front of the bubble as a windshield, slide the middle part back, and will discard the pointed rear portion. A Cassutt or Midget Mustang builder will take the pointed end forward. By the time a builder reaches the canopy stage, he's got a lot of cockpit time already and should have the canopy details pretty well worked out. If some of the canopy details elude you, check the T-18 canopy frame and installation details. Now that your canopy and windshield have been cut, trimmed, fitted and mounted, let's get that bird in the air.
