I'm still slogging my way through my slow-build wings. Today I wrapped up (or thought I did) getting the top wing skins ready for riveting to the wings. In section 16, page 16-02, figure 1, the plans show the locations where you must drill #19 holes for #8 screws, with the aft-most hole the only one that gets dimpled. There is a cautionary note about being vigilant with your deburring because these larger dimples have a propensity to crack unless thoroughly deburred. So far so good. I had just wrapped up step 7 where I dimpled all the #40 holes that had not previously been countersunk and just so happened to run my thumb over that aft-most dimple for the #8 screw and it felt rough. I thought, that's weird. Upon closer inspection the hole now looked oblong too. I got out my magnifying glass and took a closer look, and sure enough there were two new cracks in that dimple. @#$%%^$#@@!!!!!!! It was perfect before, and now it was cracked. Upon further examination I discovered what happened. When I dimpled the two adjacent #40 holes for the nutplate, the bottom (female) dimple die was too wide and "pinched" the skirt of the #8 screw dimple. That's why the hole was now oblong, and caused the flange of the dimple to crack.
Prevention is simple. Use a narrow diameter female dimple die so the edge of the female dimple die does not encroach on the screw dimple.
I've used the small diameter dimple die in tight locations, and where you are dimpling a curved surface, but it never occurred to me that dimpling the nutplate holes for the aft-most screw hole would be problematic. I called Van's to see what could be done short of buying a new wing skin and starting over (by the way, Eric was very helpful and offered a potential fix, if I can avoid screwing it up), and while I was on the phone with them I suggested revising the plans to include a warning about using the narrow die. So for those of you coming up on this step, BE CAREFULL to use the narrow die in this location or you will end up with cracks too.
I'll probably add something in the Plans Gottcha thread about this too.
Empennage Complete Except for Tips