4. Potentiometer drive – extreme rapid prototyping
The potentiometer was a small ceramic trim pot 6mm square designed for adjustment with a screwdriver. The connection with the trim-pot and drive shaft could not be achieved in plastic due to high torque and sharp edges of the pot digging into the plastic materials. It was also difficult to attach anything to the pot as the hole through the rivet was only 0.8mm diameter. I had made an effort to measure and describe the geometries involved in order to establish the space/function parameters which a solution had to live within.
The solution to this problem was a drive tag, shaped like a T and manufactured from etched beryllium copper. The etching process was over etched so as to produce three thin knife edges with sufficient resilience to be force fitted into the hollow rivet by press and remain permanently.
This idea came to me – the Eureka moment – on a business trip to South West Wales as I was driving and listening to music. I immediately stopped at the nearest telephone box (No mobiles then!) and telephoned my draughtsman in Winchester and described the idea to him. He immediately produced a drawing and artwork which was faxed to Photo Planar in Hersham, London for manufacturing.
The photomechanical milling was done on the nightshift and when I arrived at work the following day, a motorcycle courier was waiting for me outside the office with a little bag of etched drive tabs. The first one went into the rivet on my press tooling without a problem and this assembly never gave any trouble. Rapid Prototyping has been around for a very long time!