[David G4EBT]

I've had it in mind for some time to have a dabble at casting replacements for Bakelite knobs, and thought it might be of interest to share what I've been doing.

Two questions arise at the outset - what material to use to create the mould, and what material to use for casting. In this regard, I decided to try 'alginate' which is a harmless, odourless, flexible material which is in powder form and is mixed with water. Bizarrely perhaps, it's much favoured for making moulds of new-born babies hands and feet to create plaster casts for keepsakes.

When used for that purpose, the baby's hands or feet can be wriggled free, and a perfect mould remains. Thus, the material will replicate every detail of say a radio knob with ridges around the perimeter, recessed lettering, and even if the knob sides at sloping 'the wrong way' (Roberts R200 etc), the specimen knob can be easily extracted from the alginate. (I believe an alginate product is more conventionally used for taking dental impressions). The product I've been using is known as 'Alginart' and comes in 450g, 900g or larger packs as desired. a 450G pack for knob casting moulds goes a long way.

Turing to the casting medium, there are of course all sorts of resin products used extensively by model makers and the like, but there are two products I had to hand which I thought I'd give a try - namely, Ronseal 'High Performance Wood-filler', which - among other things contains styrene, and uses a catalyst hardener. It sets very hard in minutes and can be sanded, drilled, or turned on a lathe. The other product is 'Super Steel' - a Plastic Padding product, which despite its name, is not electrically conductive, which could otherwise pose a hazard if used to create a knob for a live chassis radio.

To date, I've only used the wood-filler - the Super Steel might give better results as it's much more runny so will most likely be easier to place in the mould.

An essential requirement is of course that when finished, the knob must be perfectly concentric to the control shaft to which it's fitted, so a means of lowering the specimen knob into the alginate is called for, and a means of inserting a brass ferrule of the desired internal diameter (eg, 5mm, 1/4" etc). I'm sure that there is any number of ways of holding the knob upright, but the method I chose was to create a little stand using materials that I had to hand - namely, M8 screwed steel rod and nuts, plywood, acetate sheet, and for a casting cup, the top off a 300ml aerosol can.

I've attached a few sketches and pics to show the completed stand, and will add some further posts to the thread on some knobs I've made so far.

Hope that's of interest.