This main spar coutersinking technique was developed by Dan Checkoway (US) and also used by Richard Horan (UK). I have published them here to reinforce how good the technique is and also to add one extra bit in the design of the alignment tool.
Problem: If you simply use a #30 Countersink (CS) through the spar flange it will loose its centreing ability as the CS gets deeper and chatter producing an elongated circle.
Solution: Cleco a peice of 1/8" angle (alignment tool) to the spar - this gives the CS pilot a centering hole as the CS deepens. The technique is simple - clamp the alignment tool, uncleco, CS, check fit, then unclamp and repeat until bored then take a break.
Alignment Tool - The holes were marked by placing the angle under the spar flange then drilled; centre hole is a #30 and the outer holes are #40. The ones at 45 degrees are for the wing walk section. Top tip - place the holes in the centre of the tool, which is about 10cms long. The holes under the masking tape were a failed attempt so please ignore. On the other side of the angle I have holes for a K-1006 nutplate for #6 screws. Cont/
