FINAL POST
I wanted to close the loop on this series of posts to give a final readout of the project, with hopes that anyone wishing to duplicate this can skip right to the finish line without suffering the experimentation.
Plexi (Acrylic) Overlay Panel:
As indicated previously, in my plane the back side of the original acrylic panel turned out to be painted.
The final copy of the acrylic panel has been completed and installed during 62C's annual inspection. The panel was cut/engraved on a Glowforge laser cutter/engraver. I tested three different suppliers for the acrylic, but the final acrylic was sourced from Glowforge itself and I determined it to be the best. It is 1/8" thick. As near as I can tell it is UV-resistant. The original acrylic that was in the plane measures 3/16" thick (see "future considerations" section below).
The "back" side of the acrylic was first sprayed with four coats of custom-mixed automobile base coat enamel which I had color-matched from a can of the original clay/maroon paint by a local auto paint supplier. The acrylic overlay was then cut/engraved 24 hours after the final coat of paint was applied. The engrave file was cut/engraved in "reverse image" so that it engraved through the paint just into the acrylic.
Laser settings used (these are pretty important, as I did an unusually large number of tests to determine the best engrave depth, etc):
Outline and Holes: use the standard Glowforge setting for "cut".
Lettering: use "Engrave" option with the following settings - Speed: 600; Power: 16%; Lines per Inch (LPI): 450 LPI.
Grid lines: use "Score" option with the following settings - Speed: 300; Power: 20%.
The lettering was then painted into the engraved portions using an automobile detailing brush. Two coats of paint were applied to ensure opacity. I originally tested four different types of paint and found that pretty much any type of acrylic paint worked great. I used "Golden Fluid Acrylic" brand acrylic procured from a local Michael's. White is "Titanium White" and the yellow is "Benzimidazolone Yellow Medium". 12 hours after the final coat had dried I applied a protectant coat of automobile 1k clear coat as a protectant (although pretty much anything will work).
Alignment of the holes had been worked out previously with draft copies of the overlay, though we discovered some fine-tuning had to be done during the actual installation; for that purpose we used a Dremel with a sanding drum. A new mixture cable was installed due to the old one being unserviceable. Additionally new carb heat and starter cables were installed because we needed to replace the "silver" knobs that were in the panel with originals (see section on "Knobs" below). There were minor variations in fitting diameters that needed to be accommodated. In the end, we also discovered that the throttle cable needs to be replaced as the old one is also unserviceable, and we are currently waiting on delivery of that cable.
Because the acrylic was 1/8" thick we did not need to bevel the top edge of the acrylic panel.
The new acrylic panel was written up in the log as an "owner produced part" as part of the annual inspection.
Acrylic Panel - Future Considerations:
I do have some lingering concern over the 1/8" thickness of the acrylic and its ability to stand up over time. The engine controls exert a lot of the tension at the attach points and the motion of the cables during flight, over time, may lead to localized cracking. My first impression is that with the strong aluminum panel behind it, it should hold up and not be subject to stress cracks. If I was to do this over again I might spend more time sourcing an exact 3/16" sheet, and may also have spent additional time testing standard acrylic vs. polycarbonate for rigidity. Although an option, I believe 1/4" acrylic would be too thick.
While I do not have immediate concerns over paint fading, I will be curious to see how the yellow and white acrylic paint holds up. The automobile paint is designed not to fade, so I do not have concerns over the base color. HOWEVER - it should be noted just how much the original panel faded from maroon to its dull brown equivalent over seventy years. Time will tell how the new acrylic panel holds up.
Knobs:
In a recent post (
viewtopic.php?f=24&t=15840) I explored casting new knobs for the panel to replace missing originals. As original knobs are difficult, if not impossible, to find, I used the knob from the cigarette lighter to create silicone molds, then poured resin castings. For the draft versions of the knobs I utilized press-fit brass threaded inserts to accommodate the cable ends.
In the final product, I used EpoxaCast 650 high strength resin and the ExpoxaCast HT (high temp) hardener. 650 is designed for industrial high-impact tooling applications and the addition of a high-temp hardener raises the critical temp of the part well over 200-degrees F. Rather than press-fitting the threaded inserts I designed a mechanism to allow the inserts to be held in-place during the curing process, so they are an integral part of the knob.
The resin cured in-mold for 24 hours and then is further cured for two hours at 175 degrees F, followed by 3 hours at 300 degrees F. The feel and hardness of the knobs is indistinguishable from the original. As a test I performed some unscientific pull-testing using a vice, a threaded rod, and pliers, and was unable to get the part to fail. The only way I was able to fail the part was by viciously beating the hell out of it with a hammer that put the opening scene of 2001: A Space Odyssey to shame. I am thoroughly convinced the aircraft will disintegrate before the knobs will.
During the mixing process the resin was color-matched using resin coloring pigment by my patient wife. It took us (her) three times to get it right, as the color changed from the original mixing towards the brown end of the spectrum during the heat curing process. In the end we were able to get knobs that are a direct match to the knobs currently in the panel. For your reference, the carb heat and starter knobs are new.
The new knobs were also signed off as an "owner produced part" in the logs as part of the annual inspection.
Future Considerations - Knobs:
None - it was a lengthy process but I'm proud of the knobs we were able to create.
Photos:
Photo 1 - the original acrylic panel and silver knobs:
IMG_7550.JPG
Photo 2 - compare of the front side of the original and new acrylic panels:
IMG_8068.JPG
Photo 3 - compare of the back side of the original and new acrylic panels (note: the new acrylic panel is actually a back-up copy of the one installed in the aircraft, and did not have the final protective coat of paint applied to the backside, as I wanted to show how the lettering and lines were filled in):
IMG_8069.JPG
Photo 4 - the "nearly" completed install. Note the new acrylic panel is in place. The new knobs for carb heat and starter are in place. The new throttle cable has not arrived, thus there is an open spot on the panel. The decorative instrument panel overlay has not been installed):
IMG_E8048.JPG
A huge thanks to member Barry for providing the original template, which gave me a great starting point to begin the creation of the design file, as well as the large number of members who provided guidance and input during this process. It is very much appreciated.
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