A little design story ... As designs evolve and become more refined, one often wants a component that doesn't quite exist. At this point, the options are
a) ACCEPT an existing component ( defeat, lameness )
b) MODIFY an existing component ( compromise, regret )
c) MAKE a component ( nice, but a lot more work )
An unusual need ... I wanted to copy a nice feature from an old Swiss tripod (left): its rubber feet are mounted on threaded rods which are lathed to points at the outer ends. The feet can be rotated up causing metal points to become exposed at the bottom. In effect, the tripod can rest on rubber pads or metal points, depending on the situation. E.G. for best stability in wind, you want the metal points, and on nice wood floors, the rubber is preferred.
Although there are lots of rubber feet in catalogs such as McMaster-Carr, I was unable to find a good solution, so I decided to MAKE.
Machining Hard Rubber - A Dead End ... My first inclination was to simply obtain some "machinable rubber" and with a few lathing and tapping operations be done. I read on the internet (risky) that rubber harder than 90A could be machined and got a piece of this to try. It was messy and frustrating. While barely cuttable and drillable, lathing left a terrible finish and tapping was impossible. I gave up.
The Molding and Casting Solution ... (see photo sequence at right) This simple open-mold project illustrates many aspects of technical molding and casting in general. The objective was simply to make a rounded cylinder with a cast in thread.
First, a "master" was made out of black Delrin (an easy to work plastic) on the lathe. The master defines the final casting shape. The threaded rod below the master ill create a seat in the mold for the core.
A negative of the master is created in silicone rubber (Smooth-Sil 930 from Smooth-on) - this is the MOLD. Silicone rubber has the outstanding quality of not sticking to stuff, as well as being stretchable. A housing is required to contain the silicone pour. Here it is an acrylic pipe on a Delrin base.
A "core" is something that sits temporarily inside a casting, here it is a length of aluminum 1/2-13 threaded rod. The core acts as a "space holder", resulting in a defined empty space within a casting (the thread).
The core is sprayed with mold release and inserted into the mold, then the casting pour is done. Here, an 80A hardness urethane (PMC-780 dry from Smooth-on) is used. The casting cures in a few hours if warmed and then can be extracted easily from the silicone mold. The core is a bit more difficult to remove, that's why the flats were made on it.
The castings are just what was needed. Below they are shown in their final context.
