Revved Up

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Pictured is a little gadget I picked up yesterday at the indoor antiques market in Church Stretton, along with the patience cards I mentioned in Deck of the Week, for the princely sum of eight quid. Since the failed experiment with the flaky digital tacho I got off the 'net which lasted precisely one hour before dying, I've been looking for a reliable way of determining the rotational speed of the bench tools in the workshop. I was aware of the type of tacho shown from articles in the Model Engineer mags that I've been triaging - a simple mechanical solution to a basic mechanical problem. This particular model is by Moore & Wright of Sheffield, which judging by its style, construction and the 'spectacle' case, is around sixty or seventy years old.

The method of use is simple: you just select one of the two removable 'ends', shown left and place it over the end of the rather beautifully ground shaft of the device. The pointy one is to locate in the centre drilling of a shaft or the workpiece, and its partner takes the point of a dead centre: in the mandrel of the lathe, say. The outer scale moves one mark per revolution of the shaft, showing one hundred revolutions of the shaft per each complete turn of the dial; which then moves the inner dial by one notch as it passes zero: the entire revolution of this second dial representing ten thousand revolutions of the shaft. Timed against either a stopwatch or the second hand of a wristwatch, the number of revolutions per minute at the shaft or chuck can be gauged. Simple, indeed, and no electronics or lasers to snuff it unexpectedly.

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