[Boatanchors] Casting Knobs

J. Forster jfor at quikus.com
Tue Jun 26 20:50:35 EDT 2012

> -----Original Message-----
> From: boatanchors-bounces at puck.nether.net
> [mailto:boatanchors-bounces at puck.nether.net] On Behalf Of Bob Jackson
>> Is there a primer on how to cast radio knobs, e.g. what materials to
>> use,
> where to get them,
>> step 1,2,3... etc?

I'm not aware of such a site, but casating out of resin is routinely done.

A friend used to make custom knobs and other specialty parts for General
Radio, and I worked with him a few times.

Step 1: Design and make a master of what you want out of aluminum (or
possibly a hard, machinable plastic) using a lathe and Bridgeport. In the
design stage remember you have to get it OUT of the mold when the part is

Step 2: Spray the part with Teflon mold release, then embed the master in
silicone rubber (two part rubber that uses a catalyst).

The easiest way is to put a knob master on a shaft and suspend it in the

To avoid blowholes, put the mold an rubber into avacuum chamber for a
copuple of minute4sw and pump until the bubbles stop,.

Let the rubber set. Then remove fron the pipe, cut the rubber mold into
two pieces with a knife, and extract the master. A short length of PVC
pipe non a piece of Masonite will form the exterior of the mold.

       |  <....pour rubber in
    |  |   |
    |  O   |.... pipe
    |      |
===================  ...Masonite

Step 3: Spray the silicon mold inside with mold release, reassemble,
reinsert into the pipe. Mix the epoxy and pour into the mold, pump down,
then leave to set.

Step 4:  Remove casting and finish on lathe or mill.

The epoxy is usually mixed from clear thin resin, catalyst, and a filler,
like powdered stone or another mineral filler, and a color




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