Custom lenses - DONE!
My Lexan lenses are done, and they turned out pretty good. Here's the basic process I followed:
Using 3/16 Lexan (polycarbonate) sheet, I machined a .032" thick step-down around the perimeter of the lens. This was done in my CNC mill, and it was really no problem getting a clean chip-free cut in the plastic. The main problem in cutting this stuff was it had a tendency to "goo up" around the mill instead of cutting cleanly. The fix to that was slower surface speeds (100 ipm) and higher chip loads (around .004"). The resulting cut was perfect.
Here's how my blanks looked:
Then a tool was made to fit the leading edge shape. Using RZBill's advise, I just used particle board and .020 aluminum sheet. The sheet is wrapped tightly over the particle board form and held in place with screws on the bottom side. This pic shows how the tool holds the blank in place. I'm relying on gravity to do the forming work.
Polycarbonate does need to be dried out thoroughly before it can be heated to forming temperature. Failure to do this is said to contribute to bubbles forming while heat-bending the plastic. This requires baking the blanks at 250F for 24 hours. Forming temperature is from 350-400F (GE Lexan thermoforming manual).
I got an old oven for $40 (spent the money myself Bill !) and stuck an oven thermometer inside it to verify the temp. After the 24 hour bake, I raised the temp to 350 and prepared the tool. I decided to carefully clean only the mating surfaces of the tool and the blank. No felt.
350 wasn't hot enough, so I went to 400F. That was the trick. The lexan formed fully to the tool in 6 minutes. No bubbles, no problems. I took the rig out of the oven, and used a straight piece of wood to help hold the free edge down against the tool while cooling. Here's the results:
They look a little better in the pics here than they do in person. Optical quality is very good (which was my main concern) but the post-formed lexan did retain some uneven shrinkage along its bottom edge after cooling. Its not really significant; only about 1/16" of "bow" along that lower edge and this is an 11" wide lens. Good thing is it fits the airfoil on the wing like a glove!
Probably I could do better with more practice....
Edit: The 3/16 material is pretty heavy for a lens such as this. After forming, the lens is very stiff and if not made *exactly* to contour, would be very hard to fit inside the leading edge. Also, the 3/16 material took a fairly long time to reach forming temp internally (5-6 min at 400F). Knowing what I know now, I'd recommend using 1/8 material for the lens. After cutting in the .032 recess, that would still leave a .093" flange which would be plenty. It would form faster (and maybe with less chance for problems), and fit better. Just a few stray thoughts.... have fun!