One has to take various critical measurements - rim diameter, hub flange diameter, distance from each hub flange to the centre (the rear flanges are offset by the cassette or freewheel). These measurements are then fed into the calculator along with the number of spokes, and the length of spoke is calculated according to which lace pattern is to be used.
A vernier caliper is all that's needed for hub measuring, but rim diameter is trickier, and is best effected by cutting 2 spokes 200mm from the threaded ends. A nipple is then threaded onto the spokes, and the spokes inserted into opposing eyelets. The distance between the spoke ends is measured, and 400mm added to the figure. As a rim may not be exactly round, an average is taken from a number of measurements. It's all quite critical, with small tolerances. The spokes I have been given are only 2mm longer than the suggested length, but I would run out of thread before they were tight enough, and the couple of extra millimetres would protrude beyond the end of the nipple. This, in itself is not critical on the rims I am using, as the nipples are set deep in the rim, but a professional would never do this, whatever the circumstances.
As learning curves go, it's been a good one. The groundwork I covered by reading both wheelbuilding books has been solid and useful. Dismantling and relacing the wheel on which I am working has also been a very useful if time consuming exercise.
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