Each strut needed two holes drilled. The holes needed to be accurate so that the pins were the right distance apart. An error in the hole placement would multiply through the whole dome. An 1/8" error would be over an inch off when it all the struts were connected. A inch isn’t much on a 30 foot dome, but trying to stretch the last steel strut one inch doesn’t work that well.
The hole on one end of the strut was punched, drilled through with a pilot bit and then drilled with a 1/2" bit.
In order to make the second hole as accurate as possible a special jig was made. The first hole is used to align the drill with the precise position for the second hole. On the drill press table five places for a small post are drilled. The post can be moved for the different length struts. A strut is place so that the hole drilled first slides over the post. With this method each strut has the holes drilled within a 1/16" of where they need to be.
