Screw threads, slots and watches

Have you ever noticed when you screw in a group of wood screws into your flat pack wardrobe that when they are fully tightened the slots are always at different angles to others in the group? The same effect is seen in any engineering structure. The engineers do not worry about the bolt hexagons on car engines or gas turbines, say, being at different angles to each other.

This means that the start or finish of all screw threads, be they wood screws or more high technology screwed fasteners always vary in their position relative to any other feature of the fastener such as slots or the hexagon of a bolt. This is because the production engineers exert no control over where on the screw blank or in its matching hole the rotating thread die starts to cut to form the thread. Sometimes, in alternative processes, the blank rotates and the die or even a still single point tool is still. Relatively speaking, all these are the same. See if you can visualise all this.

Lots of engineers have good 3D visualisation. It may well be a helpful capability for any engineer.

Whole Graphic
Shows a model and real watches

The Lad was reminded of all this when he saw recently an advert for a famous make of expensive, high quality watch. There had been some sort of retro styling decision letting the engineering be partially visible. The watch has screw heads visible around the watch face.

The plaster or talc model shows every one of the screw slots tangential to the crystal bezel outer circle. So that’s what the marketing people seem to prefer or want.

Model
Slots aligned in model

But note that a close look at the ‘real’ watches shown below have the screw slots at random. It seems as though the engineers have not accommodated the marketing vision. I wonder why.

Real
Slots not aligned on real watches

Normal slots are at random angles. The Lad is not a production engineer but I am sure that, with some effort and careful manufacture of the screws and the matching female thread and control of the bezel flange thickness the watch engineers could arrange for the slots in the actual watches to align tangentially with the bezel circumference. A better way would be to arrange a collection of under-head washers of different thicknesses and choose a suitable one to orientate each screw perfectly. Such washers are commonly used for other purposes by design engineers. They call them shims or adjusting washers.

You only have to contemplate the magical miniaturisation of any watch movement to realise that the first approach would be well within their engineer’s capability. Don’t even get me started on the vanishingly minute screw thread diameters that they use routinely. It makes The Lad’s engineering look like that of a giant.

By the way, in what way is a wood screw fastening different to that of a machine screw fastening?

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