I have designed this sextant in the UK, where units are in a bit of a muddle. Most full size engineering is conducted in millimetres. The drawings are in millimetres (unless stated otherwise). However most of the general population use inches for rough dimensions, and model engineering suppliers are well and truly wedded to inches as their preferred dimensions. Unfortunately, the only people prepared to sell materials in the small quantities for this sextant are model engineering suppliers. That's why you will see some utterly ridiculous thicknesses specified (eg the frame is 3.2512mm, because that's the metric equivalent of 10swg plate).
If you wish to work in inches, remember to divide the millimetre dimension by 25.4. In most cases, you can substitute the nearest sensible inch dimension for the metric one. However, DO NOT, UNDER ANY CIRCUMSTANCES CHANGE THE TOOTH PITCH OR RADIUS OF THE GEAR TEETH. Sorry I shouted, but I must make it plain that the tooth pitch and radius is VITAL to the operation of this sextant. If you wish to use a different thread pitch you'll need to redesign the sextant yourself. The thread and radius of the sextant body are inextricably linked - it must be sized so that:-
Radius = 360 x Thread Pitch / 3.141592654
Note:- If working in inches the pitch is the distance between crests NOT the number of crests per inch - so for 20 TPI the pitch is 0.05".
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