(page 20-02) Fuselage center section...                      

A scary-looking piece considering how much it'll cost if I manage to screw it up.  It first had to be clecoed to the bottom skin to ensure proper lateral alignment of the fore and aft center section bulkheads.  The backbone of this piece is a massive (compared to every other piece I've worked on to date) aluminum channel with 3/8th-inch-thick flanges.  The aft bulkhead is clamped to the flange of the channel, and then 72 #30 holes are match-drilled through the flange, providing 72 opportunities to kill this piece and 72 stress concentrations as the channel fulfills it duty of supporting the bending and shear loads produced by the wings and landing gear.  My track record squeezing long rivets is not enviable, and if some of the holes are not precisely perpendicular to the flange surface, it's hard to get a good squeeze without bending the rivet.  Bent rivets, of course, have to be drilled out and redone, often leading to an enlarged hole, which.....well, you get the picture.  I started the drilling process with fear and trepidation.

Turns out I was worrying way too much about the drilling.  For each hole, I held a thick carpenter's square against the flat surface close to the drill bit, providing an excellent visual reference to the perpendicular.  I'm confident that all the holes are close enough to perpendicular, but I may change my tune when the squeezing commences.

I drilled and clecoed the holes shown in the first picture, then un-clecoed the bottom skin and layed the piece flat on the table as shown so I was drilling vertically downward.  This made the task much easier than I first imagined it would be.  The lesson I took from this is to buy the best quality drill bits available.  Some I got from an unknown supplier proved not up to the task.  The bits that came with the tool kit from Isham have been excellent.