Though really amazing engineering, I'd say not all of them show "how they pulled it off". I'd like to know how the Byzantine geared mechanical calendar was "pulled off", especially those gears.
The gear teeth are cut with a file. For the angularity, draw a circle with a compass and subdivide it by measuring linearly with a measuring tool. This can be done larger than the part, and the teeth locations marked with a straightedge. By cutting the teeth where marked, you avoid a stack-up of error.
