Since this is the last of the molds, in the photos below I’m showing each of the layups, for a total of three. Target thickness for the molds is 4.8mm, three times the expected thickness of the parts to be molded.
Probably the main thing to explain here is how I create the overlaps in the molds. The edge of a mold is marked on the body buck with duct tape, which will leave an impression in the mold for later trimming of the finished part. Then after removing the mold from the buck, I lay in a strip of 1″ duct tape touching the existing tape, then another strip touching that one. I then remove the first two strips of tape, leaving the edge for the next mold with a 1″ overlap.
Time to make molds!
I ended up applying ten coats of SikaFloor epoxy to try to build a hard base for further finishing. Even this gave me problems, though, as it appears that the two-part urethane foam continues to expand indefinitely. Every time I would finish a section, next time I looked at it, it needed more work. For a long time I just thought my eyes were getting more demanding, but I finally realized the body buck was slowly changing shape, bulging out between the ribs. Once I figured this out, I just tried to finish the molds as fast as possible. I also installed air conditioning in this part of the workshop, and kept it running at night to avoid temperature-cycling the pattern.
If you’re thinking of doing this yourself, a better way to do it would be to just fiberglass straight over the plywood forms, using tape or something to support the first layer of fiberglass while curing. About a 3mm fiberglass shell should do it. Then use body putty right over that, using standard auto-body finishing techniques. That way there’s no foam between the ribs to push outward and mess up the shape. The only time you need foam is when you’re really sculpting something, like the sidepod air intakes. Oh, by the way, plan on about 1,000 hours of work.
After the floor epoxy there were several rounds of primering, puttying and sanding, followed by two coats of black two-part epoxy paint. This was sanded with 400, 800, 1200, and 2000 grit wet-or-dry sandpaper, then machine-polished with rubbing compound. The top layer was 9 coats of “Hi Temp Mold Release”, applied by hand strictly according to the instructions.
When I started to think about how to split up the body panels, I realized that the “horse collar” head surround would be impossible to remove when the car was finished, as it would interfere with the main roll hoop. This necessitated going all the way back to the SCCA rule book, where I took another look at the minimum cockpit opening specifications. I found that I could meet the minimum cockpit opening size with a fixed head surround, but I had to cut the “arms” off it. So, you get to see that surgery in the photos below.
Here’s what we’re building, sort of. Rather than build a CAD model of an actual assembly of stringer and rib parts, I extruded cuts into the solid model so that the slots will appear in the correct places when I make cross-section drawings at the appropriate locations. This is actually harder to visualize than you might think. I wasn’t sure it would go together flawlessly, especially since the cuts in both the ribs and stringers went to two different depths depending on the typical height of each region. I didn’t want long floppy sticking up from the cuts:
The suspension uprights have gone through a long evolution, but I’m zeroing in on the goal.
First Design: Machined Billet Aluminum
This design uses radial-style brake caliper mounting. Needed to be redesigned when someone ordered the lug-mount calipers and had them delivered all the way from the USA. Probably a Freudian slip as they’re less expensive.
Second Design: Fabricated Steel
Third Design: Cast Aluminum
Here’s the final result of literally hundreds of design revisions, ensuring that the upright is strong enough and as light as possible. This design is made possible by the new technology of 3d printing, which will be used to make the master “plug”, from which molds will be made to cast the actual part. Note that the steering arm is not an integral part of the upright, but is modeled together with the upright because the FEA runs much faster this way.
Finally, the Master Copy
The upright had to be split into four pieces for 3D printing; split vertically so I can make two mold halves and remove the masters from the molds, and split horizontally to fit the 3D printer. The 3D printer extrudes hot ABS plastic in X-Y layers onto a heated Z-axis stage with 0.3mm resolution. The print is slightly rough and the parting plane is slightly warped, which will have to be corrected with auto body putty, primer, sanding, and paint. The cast aluminum blank will still require several machining steps to cut off the gate and sprue, drill mounting holes, bore the bearing hole and retaining ring slot, and mill the brake caliper mounts. Still far better than trying to machine individual parts (or even a mold pattern) this complex, which would be approximately impossible and semi-infinitely expensive.