Intro: Gearshift Knob
This big car project yielded a lot of Instructable material. It is a reproduction of a legendary 1963 show car scratch built by Ed Big Daddy Roth. Picture 1 is my version of the car. Revell even made a very popular plastic model kit of the car immortalizing it. Unfortunately it was so poorly built (Revell was pushing Roth to make one car per year for model kit prototypes) that Ed had it destroyed about 4 years later. I fell in love with it as a young teen at the time and always had a passion to reproduce it. I was driven to make it as accurate as humanly possible which was doubly dough since there is very little information on the car out there.
The subject of this Instructable is the gearshift knob. I remember seeing this chrome and plastic barrel knob in the day but they are now EXTREMELY rare. The word 'rare' is code for 'not popular'! After much searching I finally found a very nice example on Ebay. Picture 2 is a closeup of the actual knob I needed.
Step 1: Making the Plastic Ends
The problem I had was the one on Ebay has red end caps but I needed clear caps (photo 1). In view of the fact that this is the only one I have found in over 10 years of looking, I knew I had to make my own clear caps.
The first step was to make a mold so I could make some clear castings. Using the original red cap as a pattern, I used modeling clay a to make dams to cast a mold half for the convex half of the cap. I chose urethane rubber (a 2-part resin like epoxy and cardboard or silicone) for the molding material. Urethane resin comes in a variety of hardness from a very rigid plastic to a very stretchy rubber. The mold is a medium softness so it will de-mold easily. Urethane is a bit cheaper than silicone and a LOT tougher and doesn't degrade over time like silicone does.
Once the resin on the first half of the mold set, I removed the clay and cardboard, turned the half-molded part over, sprayed it with release compound (so the new resin wouldn't stick to the set resin), and cast the second half. Photo 2 shows the pattern and finished rubber mold.
Now I needed to make the final parts. I first cast a couple parts using polyester casting resin (the stuff your Aunt Betsy makes those horrid grape cluster lamps shades by casting the grape berries in Christmas Tree ornament bulbs) but the color was quite yellow and I am sure the plastic would continue to discolor with time. I finally went back to the trusty urethane resin for a rigid, water clear compound and that turned out to give perfect results. Photo 2 shows the two type castings, the polyester ones on the ends, urethane ones in the middle.
Result was a perfect rendition of the original. All I had left to do was build the entire rest of the car and save up the national debt to pay for all that chrome!