Source: 2016 Q2 Beartracks, Mike Swain
Disclaimer: Making these lenses brought me to an emotional low in my build. I really struggled to get a lens I was happy with and I was tempted to give up. I know ya’ll like a challenge, so here is how I did it.
First, I needed the shape of the lens. To know how big I wanted, I made some cutouts of poster board to represent the size of my landing light and nav/strobe. I used these to determine how far back to go so I would end up with enough mounting area for the lights. Once I had this I drew the cut lines on the wingtip. Keep in mind your position lights (red & green) are supposed to be visible 110 degrees.

Once I had the wingtip marked up with a fine sharpie (photo above), I used a Dremel cutoff wheel to carefully cut the lines, but I left small uncut areas in the corners so the piece we want to remove stays securely in place for the next step, which is making the lens mounting flanges. I’ll be saving this cutout piece to build the form for the lens.
I marked parallel lines to the cut lines on the inside of the wingtip, 3/4″ inside and 1.25″ outside. The idea is to have 1/2″ flanges after trimming and the 1.25 overlap outside gives good bonding area to the inside of the wingtip. I cleaned the area inside the cut lines with air then wiped with solvent to clean any dust from cutting. Next I used clear packing tape to cover the entire area inside the cut lines, this will prevent the fiberglass from bonding to the piece inside the cut lines (photo below).

I now cut some fiberglass cloth so it would span the lines marked previously. It should protrude about 3/4 inch into the area where the lens will be and 1.25 outside the line where it will bond to the wingtip. I ended up using 4 pieces to go all the way around, I found it easier to work with multiple pieces. Once I had the glass cloth shapes cut, I made 6 more of each shape for a total of 7 layers. I scuffed up the area outside the cut line where there is no tape (careful not to move or mess up the tape) then cleaned with some alcohol. For the flanges, I wet out the cloth on a piece of parchment paper then put it in place, lining it up with the lines previously marked.

Once your flanges have cured, you can carefully cut the rest of the lens area out. Be careful not to cut beyond the thickness of the wingtip, you don’t want to get into your new flanges. Once you’ve removed the material you should be able to work the piece free since the cloth will not bond to the packing tape underneath. Be careful not to damage the cutout, as it will be the form for your lens. Now you can trim the flanges back to .5 inches.
You’ve now got a wingtip with a big hole.

You now need to form the 4 “walls” that provide mounting surface for the lights and finish out the space. I used heavy card stock, tape, and trial and error till I got the walls laid out. I stole some of my kids play-dough and used it as a form to transfer some of the complex curves onto the cardstock where the walls meet the wingtip. I then covered the cardstock in clear packaging tape so the wall layup will not bond. I taped the 4 cardstock pieces carefully in place from the outside, these will now be the forms for the fiberglass.

I used some thickened epoxy (left) to create filets all around the interior of the wingtip along the backside of the cardstock where it meets the wingtip. This is so the fiberglass does not make sharp 90 degree bend. I then put 5 layers of cloth with about a 1″ overlap to get a good bond. Once the layup is dry, you should be able to easily remove the card stock (below).



I used 1/16″ mirror Plexiglas to cover the walls, so I simply lightly sanded the walls after I removed the cardstock. If you plan to paint, you might need to spend some more time and filler to get a nice smooth surface.
Forming the Lens
I thermoformed my lens from .093 Lexan (Polycarbonate) using my shop vac as a source for vacuum, so first step was to build a vacuum box. I used some 3/4 MDF I had laying around and built a box 14″x22″x3″. Why that size? From what I could gather online, you want roughly 30% more material than your form to allow for stretching, etc. It was an educated guess. It turned out well to have the top frame fit in the oven well (more later). I cut a piece of pegboard to fit inside. I then glued and brad nailed some pieces of wood around the inside of the box so the pegboard would sit flush with the top. I also glued a couple piece of wood to support the pegboard in the center. Cut a whole in the middle of the box to fit your vacuum hose. Now line the top perimeter of the box with weather stripping to provide a good seal.
You need some sort of frame to hold the Lexan. I used some 1×2 pieces cut to the same size as my box. I used my Kreg jig to make a quick square frame. I then cut 4 identical pieces and match drilled .25 holes towards the outside of the frame, 2 on each side to be used and secured with screws and wingnuts. It was a pretty crude frame and you might do better, the idea is to be able to hold the piece of Lexan securely and form and maintain an air tight seal against the vacuum box. Mine was essentially a clamp between 2 pieces of wood. I lined the portion of the frame where the Lexan sits with some thin .0625 weather stripping.

The cutout from the wingtip will be the form for the lens. This need to be reinforced since you will be pulling a good vacuum down over top of it, you want it to maintain it’s shape and it needs to be elevated slightly when it sits on your vacuum box. This is due to the Lexan being in a frame and not necessarily flush to the vacuum box. I used my card stock to mock up templates for some wooden supports I cut from scrap lumber which I epoxied into the cutout (clay version, shown right). It sits on the box open side down with what is the edge of the wingtip pointing straight up.
Once you have reinforced the form, you want to make sure the surface is perfect. Any imperfection will show in the lens. Use filler and sand if needed.
When you are ready to form, the first step is to “dry” the Lexan. Load a piece of Lexan in your frame and put it in the oven at 180 for 5+ hours. You can get steam bubbles if you skip this drying step. My frame was a perfect width where it would rest on the same supports used for holding the oven racks. Otherwise you will need to support the frame somehow in the middle of the oven. You don’t want anything below as the Lexan will sag when you raise it to forming temperature.
Make sure everything is in place before starting the forming step, because it happens fast. Vacuum box, vacuum on, helper, gloves to handle the hot frame, etc. I coated my form in petroleum jelly, I read somewhere it helps. When I was ready to form, I removed the frame and Lexan and bumped the oven temp up to 350. Once it reached 350, I put the frame back in. You have to keep a close eye on things because you are watching the sag of the Lexan as the indicator of temp. I found that once the middle of the Lexan was about 2.5″ below the frame it was ready. I had my vacuum box on the floor next to the oven. Open the door, pull out the frame and immediately push evenly down over your form. You have to eyeball things a bit, but your frame needs to come down and land on the weatherstripping around the perimeter of the forming box to make a good seal. Once the seal is made the Lexan will draw down. Rather than pulling a hard vacuum, I found that I got a better part by pulling the vacuum hose out of the hole as soon as the Lexan forms around the part and holding it close to the hole, acting like a partial vacuum, otherwise the Lexan would wrap around the under side of the part which makes it hard to get out. You have to be very quick as it happens very fast. It might be better to have a helper. Another approach would be to build a solid support for the part so the Lexan cant pull under. Pop the form out as soon as it cools enough to handle but hasn’t fully hardened. It’s easier when it’s still hot but solid.
Here is a video of the actual forming.
If you get the Lexan too hot, it tends to “print” easier, and it looks wavy and shows imperfections in the form easier. I found there is a very small sweet spot at 2.5″ inches of sag. Too cold or too slow from oven to form and it will be too stiff to properly form. Also, I had better results with the polycarbonate I ordered from Amazon, which turned out to be Bayer Makralon. The brand from Home Depot and Lowes I did not have much luck with. I also tried Plexiglas but found that it was very brittle and would crack easy when I tried to trim it. Keep in mind it smells up the house pretty good too, not to mention using the food oven for heating plastic may not be the best idea. My kitchen closes off from the rest of the house and I opened a window with a fan blowing out.
I pulled the formed part out of the frame and rough cut the excess material. Then I carefully marked the rough trim line by placing the lens in place. I then carefully final trimmed with my pneumatic 1/2″x24″ mini belt sander and a lot of trial fitting. I radiused the inside corners and sanded the edge with 220 grit when I was happy with the fit.
My lens shrunk slightly, so I ended up using some quick fair compound in the flange to get a perfect looking fit. Also, where I radiused the corners of my lens the cut in the wingtips are square, again I used some putty to make everything perfect.
Once you’re happy with the fit, you can drill the lens to the flange. I drilled and clecoed #40 first, then went back and opened them up for #6 screws and riveted on plate nuts using soft flush rivets. I originally planned to use countersunk screws, but I didn’t have good edge distance on a couple of holes and I read Lexan can crack if countersunk screws are used. I settled on some low head (.063) socket head screws (Mcmaster Carr P/N 90666A002). You can now mount your lights and add any trim. I trimmed the interior area where the lights mount in .0625 mirrored Plexiglas. I used adhesive caulk to hold it on. It was a long process but it was fun and I learned a lot.