The engineer’s new coin

It’s 1968 and the Decimal Currency Board is planning decimalisation. This group of the great and good wanted to get rid of the Ten Shilling note. Change it, they decided, to a new coin that had not previously existed and worth what is now 50p: a value greater than the then biggest coin. Now a bigger value usually means a bigger coin. Not a good idea this time though: for, to fit into the purse or pocket, it needed to be not too large or too damagingly heavy.

What’s to be done?

Through the ages, for every State and its Mint, resistance to the counterfeiter has been vital. But modern coins need another feature too. They need to operate vending machines and be suitable for bank counting machines.

Therefore, their weight and thickness have to have the right relation to other value coins. But the major criterion, by far, is that when you pop it into the machine, however a coin is oriented, the machine recognises its value because the coin width is constant.

Ergo, the coin has to be circular. Right?

One of the great and the good was Sir Hugh Conway. He was a top notch engineer, having spent most of his career designing aero-engines and was a recent past President of the Institution of Mechanical Engineers.

Sir Hugh Conway.

We mustn’t hold it against him that the company portrait gave him a strong resemblance to a Stage leading man of the 1950’s. Sorry, Sir Hugh.

Anyway, Conway was familiar with the processes used by his production engineers. They are the Company team who carefully engineer the methods for making amazingly-accurate engineering devices.

There are hundreds of different machining processes. Grinding is one way of making some highly accurate pieces. There are sub-types of grinding: some for flat items, others for cylinders.

Trying to avoid the cry, ‘Too much detail!’, The Lad resists the nerdy approach. One sub-type, see image below, squeezes the ‘work piece’ between two rotating wheels. The ‘moving’ wheel moves towards the other removing the metal; the other ‘stationary’ wheel rotates the work piece. As the machine removes material, the work piece will remain circular, won’t it?

This is how centre-less grinding works.
This is how centre-less grinding works.

No, not always. The work piece certainly does stay a constant width. So that’s circular, then? Not necessarily. If the engineers don’t set up the machine properly, the work piece will become – not circular but a lobed, non-circular shape. Certainly it is a strange shape but one which has a constant width. To the production engineers demanding a perfectly accurate, circular cylinder, this was nothing less than a pain in the butt.

To Conway though on the Decimal Currency Board, it was an opportunity. Make the 50p piece that shape and it will still work a vending machine while yet looking and feeling different to people.

Try this video to see a spritely take on the shapes.

To see this strange, lobed shape in real life, just look at a 50 p piece from your purse. Another thing is to put the coin on edge on a table. Then roll it, very carefully to stop it falling over, between a flat rule and the table. You will feel NO BUMPS at all. I guarantee you will think that your eyes are deceiving you.

In October 1969 the 50p coin was introduced, with the 10s note withdrawn on 20 November 1970.Now, in 2013 in the UK, the 50p is not the only coin this shape; the 20p coin is another. It was an announcement a couple of weeks ago of a new coin design that reminded The Lad of this story.

Not all think it was a good thing. Here is a ‘mint’ example of a truly ridiculous, bonkers, journo screed lauding romance apparently – by someone who does not calculate very much – his partner probably does it for him.

Engineering influences our daily lives in some ways that you may not expect. So, there it is: a new coinage by the engineer – both literally and metaphorically.



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