Friday, March 13, 2009

Cutting Aluminum

Aluminum is marvelous stuff for building airplanes. It is as strong as mild steel -- and even stronger in some cases == yet weighs barely a third as much. The advantage of aluminum over other materials is best illustrated by a design such as the Teenie Two, which has an empty weight just over three hundred pounds. What's remarkable is that when you subtract the weight of the converted VW engine and the landing gear, you're left with barely a hundred pounds.

That hundred pounds represents the aluminum used to fabricate the airframe. And while that sum includes some extrusions, most of the hundred pounds is made up of sheet aluminum.... and of just two thicknesses, .020" and .040", making the Teenie Two one of the most practical and least expensive of any homebuillt. (So why isn't the sky filled with Teenie Two's? Because the plans... all seven pages of them... are as screwed up as Hogan's goat.) Great little airplane; horrible set of plans.

When you set out to make an aluminum airplane one of your first chores is to cut the aluminum. This holds true for every metal airplane. Indeed, a large percentage of the time needed to make such an airplane is taken up by cutting the aluminum.

There are lots of ways of cutting aluminum and as a novice tin-bender we're expected to know quite a few of them, from using a large pair of tin-snips to milling out stacks of ten ribs at a time using nothing more sophisticated than a router.

Oddly enough, one of the most common methods of cutting aluminim -- and a method used virtually every day by everyone from school kids to housewives -- is not to cut the aluminum but to fracture it.

Think I'm kidding? Pick up a cool one and pop the top. You've just cut the aluminum top of the can by fracturing it!

Working with sheet metal we often are tasked with making a cut that is perfectly straight, or at least within the spec allowed by most homebuilts (which happens to be one-eighth of an inch in thirty-two feet... or a sixteenth over sixteen feet.) Which is pretty damn straight. To achieve that degree of precision we simply clamp a straight-edge to the metal to be cut and transfer the 'truth' of the straight-edge to the work-piece by scoring the work-piece with a suitable scriber.

The most commonly used scriber is just that -- a machinist's scriber (think of a dart without vanes). That works fine for small jobs, such as scribing the air-foil of a propeller, but as the thickness of the material increases, we are forced to score (or scribe) the material more deeply. This causes us to reach for scoring tools designed specifically for that task. A nice example of this is the bottom tool in the upper photo. The tool has a silicon carbide cutting edge and is perfectly flat on the left-hand face. This makes the tool suitable for scoring metal up to .032".

The box-cutter (ie, the yellow tool) is probably the tool most commonly used for scoring aluminum sheet. Unfortunately, the blade is not flat-sided; you must learn to hold it at the correct angle. An X-acto Knife or #11 surgeon's scalpel is a better choice when scoring beer-can stock for templates and the like.


About here someone usually sez: "I don't get it." I've been talking about scoring and scribing and telling you which tool is best for this & that... but I haven't mentioned how the metal is actually cut. Because it's not, of course. Cut, that is. Once the metal is scored we simply flex it back & forth. Not very much... not enough to bend anything. But to just flex the metal. That's all it takes for the metal to fatique... and then to fracture, right along the score-mark.

Long piece of floppy metal? Then clamp a stiffener to it; a piece of wood will do. In fact, flexing even a small piece of metal is sometimes easier if you clamp a piece of wood to it, to act something like a handle as you flex it up & down.