In order to clean up a rough...
In order to clean up a rough cut, we always scribe a line on the aluminum. You can use commercial layout compound, or in a pinch, a felt marker.
If you examine the chart on page 127, you'll discover drill bits are available in fractional sizes, measured in increments of 1/64 inch along with a pretty good number of "in-between" sizes. Those in-between sizes are based upon a system that sizes the bit from 80 through 1 (80 being the smallest, 1 being the largest). There's more here, too: An alpha system exists, which begins with "A" and ends with "Z" ("A" being the smallest, "Z" being the largest). The alpha size system takes over once the numeric system ends (fractional system notwithstanding).
There's another thing to consider before you grab your drill index: Bolts are usually undersize. For example, a high-quality 3/16-inch bolt (a good example is the AN-3 fastener we use to fasten doubler plates) is supposed to measure 3/16 inch or 0.1875 inch, but it doesn't. Typically, it measures 0.185-inch. This means a No. 13 drill bit is the exact size, but as you might have guessed, you'll have trouble getting the bolt into the drilled hole if everything is identically sized.
Another problem-and it can get pretty big when you're dealing with aluminum-is the burr that remains around each hole. It's a good idea to eliminate these burrs, simply because they're a place for stress risers to begin. Some folks deburr the drilled hole by chattering a drill bit around the circumference. Sure that process wipes out the ridge but it also destroys the surface around the drilled hole, and more often than not enlarges the drilled hole. The aircraft folks recommend this: A deburring bit or countersink can be used on each side of the hole, using your hands as power rather than an electric or air tool, to remove the burr. This process bevels the edge and removes the ridge left by the drill bit. If, however, you flat sand the piece you're working on, much of the ridge left by the drill bit is removed (We almost always flat sand our various brackets).
Edges and Details
The rough-cut edges can now be filed and sanded. How do we do it? The old-fashioned way, we don't have a belt sander in our tool collection, so the finish work is done by hand. A good set of files work wonders. We don't have an aluminum cut file, but apparently they work well. You'll find that aluminum has a tendency to stick to the teeth of a regular file. One solution, when using conventional files on aluminum is to clean the teeth regularly. Believe it or not, simple blackboard chalk is often used on files to help keep them from loading up. On aluminum and other soft metals, you can drag the file on the pull stroke and that helps clean the file. That is sometimes called "draw filing." When draw filing you are supposed to hold the file in both hands and then draw it toward you. That's not easy when you have one hand holding the part, so we just draw file with one hand. This process tends to work best when the work piece is smaller in thickness than the file is wide. Obviously, if you have to remove a lot of material, the coarsest file in your collection should be used. As you get closer to the final size, move toward a finer tooth file.
Need to round an edge? The way we do it is to first "paint" the corner with a felt marker. Next, we place a flat washer on the surface of the bracket, and trace around the circumference with a scribe. The bigger the washer, the larger the corner radius. You can chop the corner with a saw (jig saw or hacksaw), and then file it to size. Or you can trim it with a die grinder (using cutting oil). But be forewarned, a die grinder will remove copious quantities of aluminum in short order.
When we're finishing a part, the last thing we do is to block sand it. First, the edges are finished. Usually we begin with a relatively coarse grit paper (120). The sanding blocks can be anything, but body shop-style blocks work great. Typically, we'll increase the sanding paper grit (getting finer and finer) as the work progresses. The flat surfaces are blocked using various grits of papers. Obviously, the grit number determines the finish you prefer. The parts you see in the accompanying photos were finish blocked with 320-grit paper.
For a closer look at the process, check out the accompanying photos.
The idea is to file while...
The idea is to file while constantly checking your progress. Files load up with aluminum, so you'll have to constantly clean them as well.
Rounding corners isn't difficult....
Rounding corners isn't difficult. There are several ways to accomplish it. This one is simple: Trace around an appropriately sized flat washer. File to fit.
The edges (file cut) are first...
The edges (file cut) are first finished with 120-grit (or so) paper, then we go over the entire piece with 320 grit. It leaves a nice finish on 5052 aluminum.
Here's a look at the fuel...
Here's a look at the fuel filter doubler plate along with the battery charger jack mount. This is easy stuff to build at home.
This is the doubler we built...
This is the doubler we built to mount the fuel pump. It also covers a couple of previously drilled holes in the trunk.
This is a combination firewall...
This is a combination firewall hole block-off/mount for the Roll Control we're using (not yet plumbed).