Project No-Flight Update
September 2010
Headlight Covers - Plan B
For the next attempt at headlight covers I decided to try cold-forming Lexan. I was told by one of the shop technicians that you can cold-bend Lexan so I bought a sheet of Lexan and proceeded to try to bend it by hand and using a V-block and press tube ... to no avail. Going back to the tech to see what I was doing wrong he showed me that you can cold bend Lexan easily, provided you have a sheet metal brake and make a tight enough bend. I didn't have a sheet metal brake and using the one at work would require marking and the making each bend on subsequent days, so I decided to see if one of the small hobby brakes would work. I bought an 18" brake supposedly good for up to 16 Ga steel from the standby McMaster-Carr.
The next thing was to figure out the best way to rough and finish cut the Lexan. I purchased a "plastic cutting" blade for my cordless circular saw, and it did cut the Lexan but left a very nasty edge. For rough cutting the large sheet in to blanks I ended up using a cut-off disk in my die grinder, quick and fairly neat edge but very dusty. In the end I did most of the cutting with a stagger-tooth blade in the band saw and trimmed the edges with the stationary disk sander. The sander will trim the edges neatly but leaves a melted and reformed plastic burr that needs to be removed. The best tool for that turned out to be a razor blade.
So armed with the old broken covers for patterns and the new Lexan blanks I setup the metal brake and was able to make the main bend to wrap the cover at the body edge fairly easily. The Lexan has a lot of spring-back so it you can't get much over 70 degrees bend from the brake. However once the initial bend was made I could "tune" it by pressing the bend open or closed laying one side on the workbench by hand fairly easily.
A few more difference between the Lexan covers and the prior attempt with Plexiglas covers. The Lexan will only hold a fairly sharp bend so the covers only have two distinct bends, the main bend at the body edge and a secondary bend to form a stiffener at the bottom. Also I could only make a fairly tight bend radius, so wasn't able to match the radius of the body very precisely. You can heat bend Lexan but need a professional bender as it takes about 350-400F to form it.
One of the nice features of the Lexan (at least the material I got) is the transparent scratch covering so it was easier to do almost all of the forming and fitting with the protective covering in place. Having the old covers for patterns speeded the process quite a bit so the two covers took about 4 hours to make and fit this time.
The resulting covers a nice stand-off units. When you get close you can see the mis-match between the body radius and the cover bend radius.
I don't have plans for another cover attempt at present. The Lexan is easier to work with than the Plexiglas as it has doesn't want to crack as easily, and although it takes a lot of force to make a bend that will say, gentle curves in the Lexan don't take much force so the ridge-plates I made for the initial cover attempt work fine to form the body of the lens cover to the body profile. I do see why the racers used a simple overlapping cover as it is much simpler as long was you don't mind the sheet metal screws through the body holding them on. If I had more material, time, patience and a professional brake I might be able to get a better match in a bend that I can get to take in the Lexan and the body line, but for now I think these will have to do.