Tuesday, November 21, 2006

AV - The Hole Story

Mention riveting and the first thing most folks think of is a rivet gun. Of course, it isn't a gun at all, it's a pneumatic hammer but that's what they think of and usually in the negative, too. "I don't have a riveting gun. I guess I'd better build a Varieze." Trying to explain that most rivets are squeezed rather than hammered just gets you a blank look known as Plastic Fixation.

The opposite of Plastic Fixation is the Popper Syndrome. That's the guy who uses pop rivets for everything. A certain Famous Designer, who sells his personally approved pop rivets for about eight cents each damns real rivets with faint praise... then uses about a thousand of them in his wing spar. He also makes much of the fact you don't need all that noisy expensive equipment, such an air compressor, if you use his Personally Selected, Deliberately Deformed Pop Rivets. But a little farther down the page he offers to sell you his Personally Selected pneumatic pop rivet puller for no more than twice the price of the identical thing from Harbor Freight. In doing so he coyly forgot to mention that his Personally Selected pneumatic pop rivet puller happens to need one of those noisy, expensive pieces of equipment known as an air compressor.

No wonder the newbies get confused. Lucky for them I came along when I did because the Truth about rivets is the Hole, not what you put in it.

Folks tend to forget that no Riveting Kit is complete without a collection of holes. And not just any holes; holes that are a snug fit on the rivet, hopefully in the right places and more or less round. Helps a heap too, if they don't have a lip. Make your holes with a .22 pistol, one side always has a hell of a lip. Use a Whitney-Roper punch, the lip ain't so bad but it's still there. Drill a hole, it'll always give you some lip.

Lips on holes is a bad, bad thing. If your hole has lips it won't fit flush up against whatever it's supposed to. Nor will the rivet set flat. They send inspectors to Cackle Choir Practice jus' so they can cackle with glee when they find a bad rivet. Out comes the Terrible Yellow Crayon and a big bilous X appears to mark your guilt.

Mr. Newbie always looks a bit stunned when he discovers he not only has to drill about a zillion holes but is requested and required to de-lip them. Or de-burr them, to use the modern vernacular. (We stopped calling them lips when they started hiring women to build airplanes. Hell of a note. I won't even bother to tell you what they call a prick punch nowadays.)

Newbies are always calculating things. I think it has something to do with trying to get a grip on the slightly fantastical nature of a flying machine made entirely of metal. One of their most asked questions is, "How many rivets does it have?" Lord only knows why they ask such things. "Enough!" is never the right answer to them. They want Hard Facts. So you whip a number outta your ass and they nod and you go on with explaining why we frown on wearing shower shoes to work. Just when you get to the good part with the color photos of the guy's toes laying there chopped off by a sheet of forty thou half-hard, Mr. Newbie pipes up, "So that means you need to drill a gazillion holes!"

Wrong. The correct answer would be seven and a half gazillion. Here's why. You start with a pilot hole, usually laid out and drilled into a pattern or jig. The pattern gets clamped to the work and is used to spot the locations for the holes that will accept the rivets. Once the holes are spotted, you drill them through using a pilot bit, which means a drill bit a tad smaller than the rivet size. The drilled holes are deburred then the mating part is matched to the drilled part and the drilled holes, still in the pilot diameter, are used as a guide to drill through the mating part, which you keep fastened together with PK's or clecos. And of course, the newly drilled holes must be deburred, especially if additional parts are to be secured with the same rivets.

Only when all of the holes at pilot diameter are drilled in all of the parts do you step up to the finished diameter and even then, you generally drill and deburr each hole in every component, opening up the holes to the rivet size with a reamer just before you insert the rivet. Why ream? Mostly because the spec sez to but for the rest of us, it's because rivets and drills and people come in all sizes and one batch of AD4's is liable to be just a tad fatter than another batch. (‘Tad' is a technical term, usually defined as half a gnat's ass or .0005" if the wind is from the south and it's a Monday. Don't worry about it for now; you'll pick it up as we go along.) Unlike rivets that are supposed to be the same size but aren't, reamers come in every size under the rainbow, so you pick the one that matches that batch of rivets. But most homebuilders don't ream. Neither do they sew.

The whole point here is that one rivet doesn't equate to drilling one hole and deburring two sides. At a minimum, each rivet is going to require about three drilling operations and the deburring of four sides. So stop calculating and get busy.

The basic tool for deburring a drilled hole is a 5/16" drill bit. If you do a lot of this sort of thing you probably got one in the right-hand pocket of your work apron with a lump of masking tape around the shank so some idgit don't chuck it in a drill, because you've taken the trouble to strop the edge and idgit's always dull the drills you love. And if you do much of this sort of thing you'll get sort of attached to things made out of aruminum, which is the same thing as aluminum but pronounced differently. That attachment will cause you to rub your hands and various parts of your body against the thing made out of aruminum and when you find a lip. Or a burr. You'll whip out your 5/16" drill bit, give it a twirl and the burr will be no more.

Over in the pattern shop, where you'll waste several years of your life trying to explain to the newbies why a one inch pitch on an eighteen inch panel means it should only have seventeen holes you'll probably use a dog-leg deburring tool instead of a 5/16" drill bit. Why? Probably because you made your dog-leg tool when you were an apprentice at El Segundo back in ‘37 and Don Sr. came through the shop one day, looked at your mangled hunka quarter-inch round and said, "Good job, son." But you'd never mention that sterling moment to a newbie. Some things are simply too precious to share. (Even without mentioning it, in time they come to realize there is something Special about your particular dog-leg deburring tool, especially since you'll rip a new asshole in anyone who so much as touches the thing. But I digress.)

Files are good deburring tools. There's probably an eight-inch, round edge single-cut mill file in the left hand pocket of your tool apron. Unlike a newbies file, yours has a handle, even if it happens to be a piece of corn cob wrapped with friction tape.

Dog-leg deburring tools have been a standard in the industry since it was invented by Mr. Doughleag in 1926. He worked for Mr. Loughhead up in Santa Barbara before the polo players ran him off. (Not Malcolm. He was the brake guy. The other Loughhead, Alan or Allan. Moved the plant down to Hollywood just in time to go bankrupt in the Great Depression.)

A dog-leg deburring tool is threaded quarter twenty-eight female so it will accept countersinks and deburring heads. Countersinks & deburring heads come in several flavors, all threaded quarter twenty-eight male. Just screw one into your dog-leg tool and flick your wrist, the dog-leg in the shaft causes the tool to spin around and the burr vanishes. Of course, so does your wrist, eventually.

What you need is More Power.

You can buy a power-operated deburring tool but be prepared for sticker shock, especially if you wander in with ‘Homebuilder' stamped on your forehead. Lotsa folks who cater to the Homebuilder Crowd have a pricing policy that makes Enron look like chump change.

So there you are, with a wrist as limp as Odie's tongue and seven and a half gazillion holes to debur. Whatcha gonna do?

Whatcha do is order a cordless screwdriver from Harbor Freight, item # 46394. Gonna cost you between $5 and $10, depending on which catalog you order from. It'll come with a wall wart to keep the 2.4v battery charged, a double-ended screwdriver bit, #3 Phillips on one end, quarter-inch straight on the other, plus a standard quarter-hex adapter.

To turn the El Cheepo Screwdriver into a hard charging steely eyed Powered Deburring Tool, wash the quarter-hex adapter with MEK, blow it dry, mix up a dob of J-B Weld and dribble it into the business end of the adapter, which should be stuck into a lump of modeling clay, fixed in a vise or grabbed with a sheet clamp so's it'll stand up straight. Now go find a deburring head, wash it with MEK, blow it dry and moosh it into the J-B Weld. Leave the thing sit overnight and there's your Powered Deburring Tool. (‘Dob' and ‘Moosh' are more technical lingo. You'll pick it up.)

The real thigh-slapper of rivet-hole deburring is watching those newbies do everything they can to create the largest possible burr. To go with their out-of-round holes. Which ain't in the right place by the time they get done. All because they're using the wrong drill motor.

There's a handy little formula that sez how fast a drill bit of a given diameter has to be rotating to cut a clean hole in materials of various types. Drilling aruminum, using a #41 bit, your drill motor has to be capable of spinning about 3000 rpm. At that speed a sheet of forty thou is virtually transparent to the bit. Just touch the trigger and there's the hole, nice and neat and round and with the smallest possible lip.

See those Jim Dandy drill motor kits at the Wal-mart store? The ones that swear they can chuck a 3/8" bit? That's the last thing in the world you want to use on an aruminum aeroplane. To be able to hog a three-eights hole in a half-inch steel plate the drill motor has to be geared down to about 1200 rpm and that makes it unsuitable for drilling rivet holes in aruminum. What you want is something that spins at least three thousand rpm.

The industry standard for drilling holes in aruminum is a high-speed pneumatic drill motor. But of course it takes one of those Evil Machines called an air compressor. Harbor Freight offers a couple of electric drill motors that spin fast enough for drilling aruminum. Their quality isn't very good but they should last long enough to give you at least one airplane's-worth of holes.

Drilling holes, you don't push on the thing. If you do, it's going to wander, or bump the panel. Or some damn thing. Drilling holes with a drill motor turning the proper speed, with a sharp bit in the chuck, the weight of the drill motor is more than enough feed to keep the bit cutting at its maximum rate. The bit and the drill motor will do all the work. They only hired you to hold the thing and move it from place to place. If they wanted something to push on the work they'd be using a drill press. (If you're worried about bumping the panel, put a sleeve of vacuum line hose on the bit, which is another industry standard that's been around since Jonah was a Seaman Deuce. Or you can buy one of those Patent Jobbies at Enron prices.)

So there's the Hole Story and the bell. Finish up Chapter Six and make two cowling flaps for a DC-3. Next Tuesday we'll get into how to make wing ribs out of old orange crates.


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