Tuesday, November 21, 2006

AV - Riveting 101

Riveting 101 ...or Meet Mr. Smiley :-)

Wanna learn how to rivet?

Go down to the hardware store -- or whatever passes for a hardware store in your community -- and buy yourself a packet of copper rivets. They come in a variety of lengths and copper washers are usually included in the packet.

Now find yourself a small hammer (...no, smaller than that... about 8oz).

Have you got an old shoe? An old leather shoe? Okay, then howzabout an old handbag. Or even some heavy canvas. Okay, good... use that. (I never thought of riveting cardboard... I usually just weld it... )

What are you going to use for your anvil? (And yes, an anvil makes a good anvil. But real anvils are kinda thin on the ground this season.) No, wood won't work for an anvil, not even lignum vitae. (Yep, it makes good prop-shaft bearings for aircraft carriers. But it don't work worth spit as an anvil.) Well-made bench-vise... that oughta work. Face of a sledge-hammer will do too. You work it out.

Last thing you need is a hole to put the rivet in. Lotsa folks forget that the hole is part of the process. When you're riveting leather, canvas or cardboard the hole isn't a very important part of the process -- not like it is when you're riveting metal -- but you can't rivet without a hole so go make one. Make it as big around as your rivet. You can make it with a punch or an awl or a drill. Got it?

Okay, put the rivet through the hole, put a washer on the rivet and squeeze the rivet down tight against whatever it is you're riveting. How much of the rivet's shank is sticking up? That's too much. You only want two diameter's worth of the rivet's shank sticking up past the washer, so cut it off.

Now hit it with the hammer. (Okay, then hit it again!) Keep hitting it until the shank is hammered into a button about twice as wide as the shank.

Do a couple more.

Now try doing it using a heavier hammer. You'll notice that you don't have to hit it so many times. You should also notice that you have to hit it more accurately -- one good blow and the thing mooshes over, not always the way you want. Lighter hammer gives you getter control -- the thing don't moosh so fast -- but the more times you hit it, the harder it's going to get. Work-hardening. Evil thing. Hit it too many times, it'll get so hard it splits. So you must seek Harmony, Grasshopper. You must balance the mass of the hammer against the force of the blow... and moosh that puppy over with about three well-placed raps.

('Moosh' and 'raps' are technical terms. You'll pick it up as we go along.)

Now try it using a rivet with a longer shank. Screwed it up, right? So try the light hammer. In fact, try anything you can think of but if the shank is too long it's gonna bend and you'll end up with what's called a 'club-head' -- a bad rivet.

Here's why: The shank is a metal bar. To deform it -- to create the 'shop-head' -- you must deform that metal bar. But if the aspect ratio of the metal bar -- if the ratio of length-to-diameter -- is beyond a certain norm, the odds are the shank will bend to one side instead of being uniformly deformed.

So what's the magic ratio? It depends on the hardness of the rivet but it rarely exceeds one-and-a-half times the diameter of the shank, with harder rivets being a bit less, softer ones a bit more. (They rivet steel, you know. Bridges, skyscrapers, ships... Rivets are ubiquitous in any industrialized society. We even wear them :-)

Here's you're first Pop Quiz: How big does the shop-head have to be?

Come on... you can work it out.

Lookit the rivet. How big is the manufactured head? Does the shop head need to be bigger? Smaller? The same size? (And just what does that sucker mean by '..big'? Is he talking how thick or how big around?)

Answer: Not as big as you think.

Okay, it was trick question because copper rivets in a pair of Levis aren't really the same thing as aluminum rivets in airplanes. But the principle is the same: You deform the shank of the rivet to form the shop-head, which must end up with a diameter larger than the hole... which is just common sense. Unfortunately, common sense turns out to be surprisingly uncommon stuff.

That's Lesson One: The basic principle of riveting is to deform the shank of the rivet so as to form a head which is larger than the hole in which the rivet is inserted. (In the case of copper rivets and leather, the 'hole' is actually the hole in the washer.) Rivets also squeeze the parts together but I'll get to that in a minute.

Moving right along.... :-)

Have you got some real rivets? Some sheet metal? Drill motor & bits? Clecos & a pair of forceps? Prolly not. Which is why I started with copper rivets and an old shoe :-)

You've seen & handled copper rivets. You know airplanes use aluminum rivets. And I mentioned steel rivets. But rivets also come in plastic, bronze, brass and virtually any other malleable material you can name. Besides coming in an almost infinite variety of materials, within a given type of material you will find rivets of different alloys and tempers.

The idea I want to get across here is that there's a whole world of rivets out there and while the principles of riveting will remain the same, that world of rivets is supported by an equally large variety of techniques and procedures which are specific to a given type of rivet, method of installation and the object being fabricated. So stay loose and just follow me through.

The object of this message is to expose you to the basics of riveting. In our version of riveting-reality, the rivets will be used to fasten pieces of metal together. So let's start with that.

If you want to learn to rivet you're going to need some metal you can fasten together with rivets. (And there's a guy jumping up, waving his hand and shouting 'What about fiberglas?' ) Didn't I just tell you to say loose? (Cheez... some students... )

Go find some flashing material. Try the hardware store again. They sell rolls of dead soft aluminum about .015" thick disguised as roof flashing. (ie, 'Poor Boy leading-edge Material :-) Or find yourself some galvanized roofing tin. Or something. Aluminum siding will work. Doesn't have to be smooth. Ditto for those over-priced pieces of aluminum in the DIY rack, over where they got aluminum angle, tubing & screen.

Cut it into strips about 2" wide. Cut the strips into pieces about six inches long. Hereafter I'll refer to these as 'coupons'.

(To cut the stuff you can use whatever turns you on but for thin aluminum, why not try a paper cutter? Use your snips to whack off a piece about six inches wide and as long as your piece of aluminum will allow then sneak down to the office and swipe the paper cutter. That big steel blade makes a treat out of slicing aluminum into coupons... or chunking out lengths for forming nose-ribs and the like.)

Once you have your coupons you must dress their edges and round their corners. You ALWAYS do this. It's one of the things that sets an aviation metalsmith apart from the rest of the world's tin-benders. To dress the edge you can use one of those patent GeeWhiz deburring tools but a file will also work. A vixen file is a bit too coarse for most thin sheet stock so use a mill-cut file. Small part, use a small file... 8" is about right. And put a handle on the damn thing! (Yeah, some of mine are corn-cobs & masking tape but at least they're handles of a sort.) To dress the corners you use your snips to 'dyke' the corner -- to snip off a neat triangle. Then you snip the two new 'corners'. First snip is a 45, the others are half that... and only an eighth as long. (No, don't lay it out, just snip it off -- make it a habit. If you need a guide, use the 'moon' under your thumbnail.) Once you've roughly rounded the corners with your snips, smooth it with a pass of the file. Waste of time? Some folks think so. Chris Heintz sez so. (Page 13 of the CH-701 assembly manual under the heading 'Some More Tips': "...do not WASTE TIME to round off each corner." ) I've added the emphasis. If your only interest is in saving time then I suppose anything goes. But if you want to build an airplane -- a good airplane -- you focus on such details. That 'wasted time' will amount to several pounds of reduced weight with no loss of strength. So make it habit.

How are you going to put the holes in your coupons?

You can drill them or punch them. Initially, for the purpose of this training exercise, punching will be quicker. But if you want to build an airplane you'll need a drill-motor, an assortment of drill-bits and -- if you want to do it right -- reamers for each size rivet you'll be using.

The effectiveness of your drilling and the roundness of the holes you produce will be reflection of how closely the tool-speed matches the hardness of the material. For aluminum you must use a fairly high tool-speed and for a small diameter drill-bit, say, an eighth-inch, that means spinning the thing about 2,000 rpm. Smaller bits, such as #40... what you'd use for the pilot hole for rivets 3/32 in diameter, you need to spin the bit faster; bigger holes, a little slower.

I mention this because most cheap electric drills don't spin fast enough for aluminum work. Working on real airplanes, you'd use a pneumatically powered drill-motor, adjusting its speed by adjusting the regulator that feeds air to the drill.

So how fast is fast enough? Time for some homework :-) The standard tool-speed for aluminum is a hundred feet per minute; for 4130 it's only 40 so you can see the material has a lot to do with it. To find out how fast you need to spin your drill-bit, multiply the diameter of the bit in inches times .2618 and divide that into the tool-speed for that particular material. ('tool-speed' is also called 'cutting-speed', 'peripheral-tool-speed' and 'surface-foot speed' . The tool-speed for different materials is listed in the various handbooks but 100 for aluminum, 40 for 4130 and 60 for mild steel will cover 99% of the drilling you'll encounter building a metal airplane.)

Didja notice the part about the diameter of the drill-bit?

Different size rivets use different size holes :-) But the really funny part is that in aircraft work we seldom drill the size hole we really need. That's because no drill makes a perfectly round hole unless the thickness of the material is about seven times the diameter of the drill-bit. (If you think about, that would make most airplanes kinda heavy :-) So we start with a pilot hole, a tad smaller than what we really want. We deburr the pilot hole(s) and check the fit and keep everything aligned using fasteners in the holes themselves. Then just before we do the actual riveting, we remove the fastener from that particular hole, ream it to the proper size and set the rivet. IF we used the right sized pilot hole and the right reamer, and if we did a proper job of deburring, the reamer will leave a perfectly round hole without raising a burr. Neat, eh?

(Why ream? Because the more closely the rivet fits the hole, the stronger the fastening. Boeing and Grumman and NASA ream every hole for every fastener, bolts as well as rivets. And so do good aviation metalsmiths repairing a Piper or Cessna. But with the typical lightplane needing 6,000 to 8,000 rivets... Just how good is 'good enough'? Would a drilled hole suffice? In most cases, yes. Indeed, we simply use the drill-bit as a reamer.)

So... what drills will I need?

Obviously, you'll need drill bits to match the rivets you're using. And for most lightplane riveting that means you'll be setting rivets having a diameter of 3/32", 1/8" and a few 5/32", rarely larger. (This is important because it means you can get by with using a relatively light riveting gun. I'll get into that in a minute.)

Since the AN code for rivet diameter is in units of 1/32th of an inch, that means 3/32 is a dash-three ( -3), 1/8 is -4, 5/32 is -5 and so forth.

And to make suitable holes for those rivets you use a set of three drills -- pilot, nominal and clearance.

For -3's (ie, 3/32") the pilot drill would be a #43 or #42, the nominal (ie 'normal') bit would be a 3/32" and the clearance bit -- the one you'd use to open up the pilot hole before setting the rivet -- would be a #40 or #41. (Why the variation? Because of the manufacturing tolerance in the shank diameter of rivets. Tolerance is fixed rather than proportional; a variation of .005 in a -3 reflects a significant percentage of the diameter, whereas the same variation in a -6 would probably go unnoticed. Also, by convention, certain rivets commonly referred to in fractional-inch sizes [ie, 5/32 or .1562"] are actually larger [ie, .1565", for example].)

For -4's you'd pilot with a #31 and clearance with a #30.

For -5's you'd pilot with a #23 and clearance with a #20.

So go buy some. Just those sizes I've listed, plus a few for the 'nominals', but at least a dozen of each size. Call up an industrial supplier, such as MSC or Enco. You'll be happier with American made stuff. Given the number of holes you need to accurately drill, the price difference between a high quality American-made drill bit and cheaper -- but usually shoddier -- goods is of little importance. Cheap tools are rarely a bargain.

( In each of these cases, if you were reaming the holes, you would select a reamer which most closely matched the batch of rivets you are using.)

For the purpose of this exercise, we're going to drill the holes. Or punch them :-)

And use eighth-inch rivets.... dash-4's.

...which means it's time to mention the rivets themselves. Such as what kind of manufactured head you want and how strong they need to be. For the purpose of this exercise I'm going to stick with Universal Heads -- what you probably think of as 'round heads' (they're not actually 'round' in that the head isn't a hemisphere). The common AN designator for Universal-Head rivets is AN470. Flat-headed rivets fall into the AN42X series, with the terminal character reflecting the angle of the head. (For example, the copper rivets you got from the hardware store probably had a '420' head, meaning the included angle was 90 degrees. For most flush-head aircraft work we use AN426's, which have an included angle of 100 degrees. But forget all that for now -- flush-head riveting is a special case that we may -- or may not -- get into later.)

So you walk up to the counter and say "I'd like a pound of AN470's please." And the clerk starts to giggle, looks around for the hidden camera then runs off to tell everyone another homebuilder just wandered in.

AN470 describes only the configuration of the manufactured head. We must also tell the clerk what kind of rivets we want -- steel, copper, aluminum, etc -- and within a given type, the particular alloy. Then we need to specify the diameter and after that, the length.

So here's some common aluminum alloys for rivets: 'A' stands for pure, dead-soft 1100 aluminum. AD is half-hard 2117. D is hard 2117. DD is hard 2024. B is half-hard 5056.

Stop writing all this down -- you don't need to remember it. The rivets themselves are marked according to their alloy. A's... pure aluminum = no mark. B's have a tiny 'X' on the head. AD's have a dimple in the middle of the head. D's got a tit in the middle. DD's have a raised double-dash, like the hands of a clock pointing to 9 & 3.

Unless you're building bombers, you'll never see any D's or DD's. These are usually driven in their annealed state. Try to drive one that isn't annealed, you'll usually fracture the shank before you form the shop-head. (These are the so-called 'ice box' rivets. Ask if you don't know why.)

Most lightplanes are assembled with A's, B's and AD's. For the initial phase of this exercise I'd like you to use A's.

So now we can tell the clerk 'AN470-A-4...' which sez we want some pure aluminum, eighth-inch rivets with Universal heads. All we need to add is the length.

Having learned that rivet diameter is coded in thirty-secondths of an inch, you'll be happy to know that rivet length is coded in sixteenths.

So how long do you want them? Here's how to figure it out.

The standard dimension for the shop head is 1.5d x .5d, where 'd' is the diameter of the shank. Those dimensions are approximate but close enough for all cases so that you should tattoo it on your eye-balls.

Knowing the desired dimensions of the shop-head, you then need to work out the length of shank needed to create a shop-head half again as wide as the original diameter and just half as thick. So go ahead and work it out. Find the volume of a shop-head 1.5d wide by .5d thick. Come on... assume d is equal to 1 and run the numbers. (First you do the area of a circle having a diameter of 1.5... Remember, cornbread are square, pie are round... r = .75 so you got .75x.75x3.1416, which should come out to about 1.77. Then you multiply it by the height, or .5d and the volume of the shop-head comes out to about .88 Now go back and figure the volume of the shank, using the same formula... only this time r = .5 That gives you .5 x .5 x 3.1416, which works out as .78. Then just divide .78 (the volume for one unit of length of your mythical shank) into .88 (the volume of a 1.5d x .5d shop-head) and you'll come up with 1.12... or something close to it.

So you apply 1.12 to your nominal shank diameter of an eighth of an inch [for example] and it sez the protrusion of the shank should be .125 x 1.12... or about .140", which ain't much.

Now go measure the thickness of your coupons. Let's say they are .015 and you are fastening two of them together, so the total metal thickness is ..030... to which you add the protrusion of 0.140 and come up with a RIVET LENGTH of 0.170... which doesn't happen to exist. So you divide the unit-of-length (0.0625, which is close to a sixteenth of an inch) into 0.170 and come up with 2.72... which also doesn't exit. But a length of -3 does exist... so that's what you order.

AD470 - A - 4 - 3

Four-dash-three's are eighth-inch rivets, three sixteenths of an inch long. But don't stagger up to the counter and ask for a pound of them. Not unless you want stubby little rivets running out your ears. According to the catalogs, a pound of 4-3's is 1,734 rivets, a few more than you'll need for this exercise.

Another reason you'll seldom buy them by the pound is that rivets are relatively expensive -- a full pound would cost you about $20. But suppliers understand the realities of rivets and will usually sell them to you in two- and four ounce quantities. Two ounces of 4-3's should cost no more than three bucks... and will contain about 200 rivets. Indeed, many suppliers (such as Aircraft Spruce) sell assortments of rivets, often called 'Student Paks' or 'Maintenance Paks', containing an eighth-pound of ten or twelve of the more common sizes, for about $30 per pack.

So now you've got your metal and you've made your holes and you've even gotten some rivets... But unless you trim every rivet you use, the odds of coming up with exactly enough protrusion to form a 1.5d x .5d shop-head are somewhere between slim and none. Which is why, about a thousand words ago, I mentioned that those proportions are approximate. So which one do you shoot for? The 'perfect' diameter of 1.5d or the 'perfect' thickness of .5d?

In forming your shop-head you always shoot for the optimum diameter. If you don't have enough protrusion to make a 1.5d shop-head, use a longer rivet.

Did you notice that I rounded up when I did the calculation for shank protrusion? The strength of the shop-head is a function of both diameter and thickness but the tricky part of the 1.5d x .5d formula is that while a shop-head diameter of 1.5d is the maximum, a thickness of .5d is the minimum.

Starting to make sense?

The extra metal in the shank length appears in the shop-head as a slightly thicker head... which is about the only way that extra metal can serve any useful purpose. Because if you mash the shop-head flatter than a pancake, making it 2d or whatever in diameter, you will have created a weaker head, one more prone to fracture.


Now that we've gone through the calculations showing how the volume of the protruding shank determines the dimensions of a properly formed shop-head, you can see why a standard minimum protrusion of 1.5d will work in most cases. In the same vein, if you apply the typical rivet inspection gauge to the work you will see that while the gauge for the shop-head diameter of 1.5d is a go-gauge, the gauge for the 0.5d thickness of the shop-head is a no-go-gauge. That is, the head may be thicker but should never be wider than the minimums.


This is already too long so I'll break the lesson here. Next time I'll threaten you with some real riveting, using panel fasteners and so forth. In the meantime, try to obtain a wish-book from Aircraft Spruce and another from U.S.Tool. ASS Co will sell you just about anything you need... but you probably can't afford it :-) Varga, over in Aridzona has a good selection of rivets. Do a gopher search to track them down. Airparts, back in KC, will sell you 'sweepings' -- 1-lb bags of assorted rivets -- for about four bucks a pound. (In a production environment, anything that hits the floor has 'fallen out of spec' -- you don't build airplanes with stuff you pick up off the floor [although homebuilders do :-) ] The stuff is swept up and sold as scrap. Airparts has bagged it for you, leaving you the joy of sorting through a couple thousand rivets.]

In our next episode you're going to need some clamps, a rivet gun, some rivet sets and so forth. Call Jerry at The Yard in Wichita (1-800-888-8991) and see what he's got in the way of used stuff. A new gun from U.S.Tool will run you over $300 but a used 2X is generally available for thirty to sixty bucks.

If you know a better source for stuff, use it. Other than as a customer, I've nothing to do with the outfits I've mentioned.

If you belong to a local EAA chapter, ask around. There's tons of riveting equipment out there, most of it going bad from disuse. Don't worry about the air-compressor. Truth is, for 1/8th inch rivets, almost anything will do. It's air-drills and the like that need a big compressor. Setting rivets places only an intermittent demand on the compressor, giving it plenty of time to pump back up.


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