We've finished up the left spar for our Xenos motor glider, and that means starting what seems like a whole new airplane project to get to work on the right one. While they have almost all of their component parts in common, there is a lot of asymmetry in the design because they overlap inside the fuselage. That means that not only do parts get laid out in mirror images from tip to root, but the stack-up of web sections is reversed fore and aft. So while we learned a lot on the left side, we have to make sure that we don't blindly assume that we know how this one goes together.
One of the lessons learned from the first one was that with the root end of the web made up of six layers of prepunched aluminum, it is actually hard to get your drill to go straight through all that material and come out perfectly aligned. We figured that out pretty quick on the first one, since there is just no way to put this through the drill press - and the answer was to use a drill block to make sure that you get a good start.
This block was a gift from a neighbor with new milling machine, looking for projects - it is simply a block of steel with random holes drilled for each of the sizes commonly used in aluminum airplane construction. About an inch thick, you simply can't get the drill going crooked, unless you let debris build up between the work surface and the block. Keep that clean, and every hole will be straight and true.
I've already had to add a couple of holes to this block because we have some unusual fasteners sizes here and there - but it doesn't make any different where int eh block the holes are located - it works just fine. You can build one yourself with a drill press - and it is a really useful tool to have on hand.