Wings 33, Total 496 - Fabricating Tie Down Bracket Spacers

Last night I had the dubious pleasure of attempting to drill a really big 1 inch diamter hole smack in the middle of a 1.25 x 2 inch rectangular piece of 1/4 inch thick extruded aluminum bar stock - a total of 4 times. Gee, where to begin. I have a ton of pics that I will post tomorrow - too tired to deal with it tonight.

I purchased a 1 inch diameter hole saw and mandrel from HD - smallest one I already had in my possession was 1.25 inches, so had to buy one. If you do the math you end up with a lightening hole that leaves about 1/8 of an inch on 2 sides of the spacer, and about 1/2 inch on each of the other 2 sides. Those are the critical sides because you have to leave enough material to drill the nut plate holes for an AN 3 bolt on either side of the spacer.

Since I was going to be driling a rather large hole in a rather small piece of thick aluminum, this was a job to be done on the drill press. I spent an enormous amount of time trying to igure out how best to clamp this thing to my table top so I could hit the mark dead center and prevent any wondering of the blades on the hole saw. I ended up using those 3 foot long pieces of aluminum angle that I used to countersink all the nut plates as a fence to hold the spacer in place. What a pain in the ass it was to get everything clamped up right, but I finally got it there.

Then I began the drilling. The pilot bit (1/4 inch) went in with no problem. Once the hole saw blades hit the spacer - different story. The cutting blades were heating up so badlly that the aluminum shavings were melting an filling the gaps between the hole saw blade tips. I had to stop, clean, oil, and drill the hole several times before I finally broke through the 1/4 inch thick spacer. Now, can you guess what I did wrong? More on that in a moment.

I managed to get all 4 spacer holes drilled out to the correct dimension and location, and made a mess of my basement floor, work bench, and everything within a 4 foot radius of the press. Metal shavings went everywhere. After spending a good 1/2 hour tonight cleaning up the mess, and thinking about why I had such a hard time drilling them out with the hole saw, it suddenly dawned on me. A quick reference to my drill press manual revealed a chart that contained drill speed settings based on drill bit size and material type.

Sure enough, and the drill bit size got larger the bit speed got - you guessed it SLOWER. My drill press set up for "as fast as possible," and did not give a second thought to changing the speed by adjusting my belts, since my last use of the press to drill the lead counterweights for the elevators seemed to go OK at the higher speed. Problem was that the lead counterweights were drilled with a MUCH smaller drill bit. In other words, with a 1 inch wide hole saw, I needed to take the press down to the slowest speed I could possibly get, and I never did that. So, future builders - if you hit this step, make sure you slow down the drill press before you start burrowing large holes into thick aluminum. Seeing smoke rising from the metal within seconds of beginning the cut, even after heavily oiling it, should have been enough to give me a clue as to what was needed to resolve the problem.

Oh well, "live and learn" is just part of the process of building an airplane. After spending most of my evening tonight cleaning up my mess from last night, I also managed to clamp and drill the first of several holes in each tie down bracket. The process is basically to mark and drill only one hole in each bracket initially. This is the outermost hole on the top side of each bracket. The remaining holes are drilled by using the predrilled bolt holes in the wing spar. So as long as you get the first one correct, and you clamp the spacers in the correct location, drilling the remaining bolt holes should go fairly quickly.

What may trip up some folks is that they only show you the left side bracket. You have to be intelligent enough to figure out that you need to reverse the pictures when working on parts for the other wing, or you may end up with 2 left wings! So far I am keeping up with this, but it is difficult sometimes, because the wing part of this project has you constantly switching between about 6 different plan views for one thing or another. Sometimes you have to work hard to retain a sense of right vs. left before cutting or drilling something.

So far so good. Pics and more progress tomorrow for certain. Also expecting my Nobel Akzo primer to arrive from Aircraft Spruce tomorrow. Then I get to play with my new larger capacity spray gun!