Spent a lot of time figuring out how to machine countersink the center hole for the platenuts. Decided the best thing was to make a guide for the center hole out of .100" aluminum sheet. Clamping this to the underside of the spar flang and then running the machine countersind through it would provide sufficient control of the countersink bit to go deep enough. Consulted a number of web sites to see what others had done.
