Most sundials cannot be relied on to read more accurately than plus or minus 16 minutes. The reason is due to the planets elliptical orbit and the tilt of the Earths axis. This results in solar time not being the same as civil time. The two are related by an equation known as the equation of time. This sundial corrects for this by selecting the date on the outer ring and rotating the ring to align with the date on the inner ring. The arm is then swung round to cast a shadow and image of the sun on the screen centre line, the time then being read on the outer scale which is accurate to plus or minus one minute. All parts are brass and are machined and engraved on my Stepcraft 420.
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I use a 0.44mm end mill with a total depth of cut of 0.3mm. The brass is placed on a hotplate and hard wax from a stick (used by clockmakers for dials) is applied. The excess wax is immediately wiped off using a paper towel leaving the wax in the grooves.
Thanks for the explanation. I tried to make a plate for a compass with 1 degree markings. The diameter of the disk was 100mm. I used a graver, think that was the fault, because slight different depth due produce a different width of the markings. Next time I'll use a normal end mill.