Project No-Flight Update
March 2010
Returning to work on the project after a much too long hiatus ... the long awaited "How to (attempt to) make headlight covers - Part 1".
I decided long ago that I didn't want the covers to simply overlap the body at the openings like they did on most of the road race cars (take at look at the restored Penske C2). I looked into installing "L" mounts on the side panels of the headlight openings but finally concluded that to get the covers to bend to the body contours smoothly I was going to need a ridge for them to sit on. Since I didn't want the ridge to be noticeable, and the car was already painted, I decided I'd make small clear end panels that would be recessed under the body line by the thickness of the headlight cover and fasten the covers to them as well. I've had a box of 1/4" Plexiglas (old display shelves) in the garage for years so pulled two of them out, covered them in masking tape to protect them, and fit and cut them for the end panels. The left and right sides of the body don't have quite the same profiles at each headlight opening (not a very high quality replacement front end and it didn't originally have any headlight openings) so I scribed each piece to fit the body contour, protecting the body with masking tape, and then cut out and trimmed the end pieces.
I had purchased a sheet of .060 Plexiglas for the covers long ago and after some rummaging in the garage found where I had stored it. Using the tape on the body at the openings to protect the paint I taped a piece of cardboard over the opening to make the rough pattern for the cover panels. I marked it from the underside and then cut it for a pattern. I cut the pattern oversize to my marked opening so I would have material to trim to fit the covers, which gave me enough margin that the single pattern would work for both sides.
I used the pattern to mark right and left covers on the Plexiglas sheet. And after messing up the outline for the right side (which had to be marked upside down so the bend lines would be on the working side of my sheet) rough cut them with a cutoff wheel then trimmed them closer with my band saw.
I had been told that you could bend the Plexiglas cold in a sheet metal brake, but since I don't have a sheet metal brake I decided to clamp the panels in my workmate. Wisely (ok, by pure luck) I decided to take one of the scrap pieces and try to bend it first. It bent about half way then snapped cleanly into two pieces. Time for take two. I had purchased a set of plastic bending heat strips previously (back when working on how to make the curve in the side windows) so I took another scrap piece and clamped it, along with the heat strip, into the workmate. After letting it heat till it was hot to the touch on the side opposite the heat strip, about 3 minutes, I slowly started to bend the panel. You can feel it start to soften and relieve once you get some pressure on it so I slowly bent it to 90 degrees, pulled it out of the workmate and held it bent while it cooled.
The test piece done I setup for the same process on the actual headlight panel.
I should note you will hear a cracking sound as soon as you start to bend the heated piece which is the paper covering stretching and breaking and not the Plexiglas itself.
To get even pressure across the length of the panel I held a piece of 2 x 4 against the panel on the side opposite the heater and bent it (heater on the inside of the bend) to once again form a 90 degree bend. When I pulled it from the workmate I held it at slightly over the 90 degrees while it cooled. The final bend looks to be close enough to the actual sharp edge on the body, and the panels are flexible enough to curve to the gradual curvature of the upper and lower body surfaces.
To get a little more curvature I decided to clamp the covers to the end plate forms and heat them gradually with an industrial heat gun. I put some temperature indicator strips on them to try to monitor the temperature they were getting to but never really got a clear reading from them. After heating them I left them clamped while they cooled. The process did help and when done they needed much lower pressure to wrap to the forms (they have about a 1/4" gap at the end when in their free state).
Compare the curvature in this picture to after the initial bend above to see how the additional forming helped (and how I over-curved the one edge and have to re-heat and straighten it).
I drilled and cleco'd the end plates to the body and sanded a couple high spots till I got them to conform to the outer body surface profile. Then I took an offset scribe (another McMaster-Carr find) and scribed them with a line offset the thickness of the Plexiglas covers, 1/16", and then sanded them down to the scribe line. Remounting them shows them inset to the body profile by the cover thickness.
Before Trimming
After Trimming
With the end plates recessed I was able to trim and fit the covers. For the first one I removed all the protective paper so I could place it over the opening and see where to make the initial trim lines (with masking tape). For the second one I only removed just enough of the cover to see where to trim it, which made it easier to work and not worry as much about scratching it. A note, the band saw works ok for rough trimming the pieces but leaves a very rough edge (and in some places small cracks) so the best way was to trim them about 1/4-3/8 oversize and then sand them to the final lines using a bench mounted disk sander. I trimmed them down to leave a small gap, about a thin piece of pattern cardboard width, so that slight flexing of the body or covers would not compress and crack them. I apologize for the crappy picture but it was hard to hold in place and take the picture at the same time and the camera didn't want to focus on the clear cover very well.
I was going to mount the covers with screws in the 1/4" thick endplates and even made a special drill jig to be able to drill and tap the endplates exactly on center but once I looked at how close the edge distance in the thin covers would be I decided to go back to part of an earlier plan and mount right-angle aircraft nut plates. They will be riveted to the Plexiglas endplates (instead of the body itself as I was thinking back when I ordered the nut plates) so the fasteners will be inset further from the cover edges and I can get at the back of the Plexiglas end plates before they are installed to buck the rivets holding the nut plates. I looked at bonding the covers to the end plates but decided not to since if I broke one then I would need to make new end plates as well, and I wasn't comfortable that VHB tape would hold them over a long term. I also wondered if the ends of the light recess bay were "exactly" parallel and if I could get a bonded on-piece cover assembly in and out or not.
You can see in the photos that the covers do not seal across the bottom. I had never intended them to as the open covers should not have fogging problems and to close the bottom they would need to included a dished profile since the headlight frames protrude slightly below what would be a straight edge across the bottom of the nose panel. I plan to add a slight doubler of a thicker strip of Plexiglas across the open edge to stiffen it.
I received the rivets to attach the nut plates to the Plexiglas end pieces and promptly broke the first piece (there will be more to come). Aircraft nut plates are designed to install with 3/32 solid aluminum rivets. When I went to set the first nut plate rivets they promptly cracked the Plexiglas. The rivet body expands along it's whole length when compressed to set so they fit tight into the normally metal hole and the pressure was too much for the Plexiglas (obvious afterwards) and cracked it. I was able to use the broken piece as a pattern an make a copy so not too much of a setback. I drilled the nut plates to 1/8" and used pop-rivets to attach them to the Plexiglas endplates after first doing a test using the broken end piece. With the larger head size of the pop rivets I had to use only one of the two rivets normally used with the nut plates.
The next problem was with the pop rivets I had purchased to attach the Plexiglas end plates to the body. I had gotten aluminum rivets with aluminum mandrels so they wouldn't put too much pressure on the Plexiglas, but they didn't hold the ends tight to the body. I had to switch to aluminum rivets with steel mandrels to get enough clamp pressure. I used my broken end piece as a test with the new rivets before attaching the endplates to the body just in case. I finally was able to install the end plates pretty much as originally intended.
With the endplates attached I did some additional trimming and went on to install the cover pieces. The first discovery was some micro-cracking along the edge of the left cover that had not been visible with the masking tape protection in place. The band saw cuts leave some very small cracks along the cut edge and on that edge I had not left enough material to sand off when trimming so didn't get all of the crack area removed. I decided to try to install it anyway as a learning exercise, which is what I got. The piece of course cracked further once I had it installed. I also discovered how hard it is to figure out where the fastener holes need to be in the Plexiglas, The covers are too reflective to look straight down on the nut plates to find and mark the mounting holes, and the parallax when looking at a slight angle resulted in the holes not being quite where intended. Even with the planned allowance of the floating nut plates I didn't get the last mounting hole on the left cover where I wanted. The fit on the outer edge of the covers aligns well with the body, but there is more gap than I want at the inner edge.
You can see the resulting bow in the bottom edge, and another problem area. It isn't too much of an effect on the mounts at the top edge of the cover, but is for the mounts on the bottom edge. The nut plates are a right angle to the endplates, but the body curves a lot going between the inner and outer ends of the headlight cutouts across the bottom. I'm going to have to remove the lower nut plates and bend them to align with the body curvature before making the next cover attempt.
I had added a third mount along the center of the top edge to stiffen the panels in-lieu of adding a doubler, and just to complete the need to start over the right cover cracked out when I went to drill the hole for that fastener.
So, a lot of lessons learned from this attempt but it's back to order some more materials and start over. I can use the existing covers to make a little better patterns for the pieces for initial fitting than before, although they aren't going to be much use to help with locating the mounting holes. I drilled the mounting holes oversize so the screws would have some movement room, and I think I'll go up another 1/64" next time. I think I am going to make the bottom edge longer and see if I can put a second small back-bend along the bottom to act as the stiffener. And I may make then next attempt with Lexan instead of Plexiglas and make an attempt at cold forming (which the aircraft tech at work assures me can be done). Finally, I'm going to have to wait for the final fitting until I can get help as it really takes at least 4 hands to hold them in place to get them trimmed to where I can (I hope) get the fasteners aligned properly.
Stay tuned for the infamous Plan B.