Tuesday, November 21, 2006

AV - Riveting 101, Part 2

So... you've got some coupons and you got some rivets.

Down the middle of one of your 2x6" coupons, drill a series of five #31 holes. (No, not like that. You've gotta back-up the metal with something. Here... put it on this piece of 2x4. Hold the drill-motor vertical. No, don't push on the thing! The weight of the drill-motor and the sharpness of the bit is all it takes to make a clean hole. Okay... now you got it. Go ahead and do the other holes.) [If you're using a punch you're about to discover it's principle limitation. Normally, you use a punch to create holes in a part when doing flat-work on the bench. When it comes time to fasten that part to some other part, the matching holes are usually drilled. But for the purpose of this exercise, if you don't have a drill but do have a punch, stack the coupons and punch both at the same time.]

Gottem drilled? Now feel the holes.

Feel that turned edge? That's the infamous 'burr'. Shot Hamilton in the gizzard and completely upset the course of American history.... (Why are you all looking at me like that? Oh! Sorry... I thought it was Thursday... [Aviation Sheetmetal... Mon-Wed-Fri. American History, Pre-Civil War Era, Tue-Thur])

To debur the holes take a 5/16" drill bit in your bare hand, hold the coupon in your other hand and press the drill-bit lightly into the burred hole and give it a twist. You want to remove the burr without chamfering the hole. (A little harder... Not that hard... ) Use your fingers as your gauge -- feel to see that the burr has been removed.

Deburring with a drill bit assumes you're using a bit with the standard 118 degree included angle. You may also use a fine tooth single-cut file, with the understanding that you'll only do so when the hole is so positioned -- and the file wielded in such a manner -- that you won't scratch the surrounding metal. The popularity of 'patent' deburring tools is that with some of them, you can debur both sides of the panel at the same time (!). This allows you to debur holes which open into voids or other spaces where you can't get at the other side.

Now take a second coupon and drill a matching line of holes. Call when you're done, I'll be outside copping a smoke.

Didn't work, did it :-)

Unless you've got a CNC computer screwed to your ass there's no way in the world to produce two matching rows of holes using only a hand-drill. Unless... Did you try using the first coupon as a template? Drilling thru it? Ummm... slipped, eh? Okay, let's try clamping the two coupons together. You can use a C-clamp, a G-clamp, a spring-clamp or a pair of visegrips. Of course, it'll probably bugger up the metal. Buggering up the metal with clamping marks is considered bad form since it can make the plane crash. (So you glue little cork pads to your clamps, or buy the real thing, which comes equipped with little nylon pads.) So go ahead -- clamp the coupons together and try again.

By the time you've drilled the third hole it's becoming obvious that clamping the edge of your coupons only works for one or two holes. After that, they start to wander. (Edge clamping will work... if you clamp the entire edge. This works well for match-drilling small parts. Simply fit your bench vise with soft jaws and clamp both parts in the vise.)

What you need is some form of clamp that centers on the hole itself!

This need became evident in Roman times, when rivets were used to join panels of armor. By the late 1700's when boilers arrived on the scene, metalsmiths already had a variety of tools for positioning and aligning rivet holes. Most of the tools you'll be using for aircraft work are simply modified forms of earlier tools which accomplished the same task. Most of those aviation-related sheetmetal tools were developed in Germany in conjunction with airship fabrication, the frames of which were Duraluminum (yes, it's a proper noun although not in common usage). Both Duraluminum and all-metal airplanes originated in Germany.

Try using a sheet-metal screw. Go on; try it. Number six sheet metal screw, half an inch long... goes into that #31 hole a treat and takes a fair grip on the two coupons as well. Of course, the head of the sheetmetal screw is going to leave a mark... (Another trick is to use the sheetmetal screw to fasten the parts to the bench-top(!) or backing-block (ie, the thing you're drilling against). This is perhaps the most common method when drilling and works equally well using clecos.)

Feller named Parker came up with the solution for the scratches. Special sheetmetal screws having a little rubber or nylon washer made right onto the screw. Manufactured by the Kenton company (Kelon? Something like that). We called them P-K's. Handy.

Well... sorta handy. You gotta screw the thing in... then you gotta screw it out again before setting the rivet. And sometimes the screwdriver slips and you scratch the panel... which ruins the whole thing. So they made hex-head P-K's. Run them in with a nifty little socket-tool, takem out the same way. Of course, that's an extra tool to keep on your belt. And those threads do bugger up the hole a bit. But they really do keep the parts aligned and don't scratch the work. (Hint: A piece of masking tape makes an adequate temporary washer.)

Nobody uses P-K's any more... unless they need to. But the use of threaded temporary fasteners is a valid procedure and was the industry standard for more than thirty years, before being displaced by the more useful (but far more expensive) spring-type sheetmetal fasteners introduced by Cleveland Equipment Company. The use of P-K's... or sheetmetal screws and masking... remains a valid method for the homebuilder... and for doing emergency repairs when you don't have a lot of clecos handy.

(Pop Quiz #2: Why do we call 'cleco's' clecos? Hint, hint... read the last paragraph :-)

P-K's will allow you to match-drill the second coupon. Then you remove the P-K's, take the coupons apart and deburr the second set of holes. At this time you'll notice that the P-K's have left a burr on the original holes(!) Some days it don't do to get outta bed, eh?

Okay, put the two coupons back together using your sheetmetal screws which we are calling P-K's. Chuck a #30 bit into your drill-motor and lets rivet that thing up.

Of course, you gotta figure out which rivet you're going to set first. I mean, you got five holes, right? Where you gonna start?

Ever stitched up a wound? (Don't look like that. It's a handy skill to have in your warbag. Ditto for knowing how to give shots and starting an I.V.) To close a wound with stitches you generally start in the middle. The idea is to keep things nice & even. You do the same thing when you're setting a row of rivets. So remove that middle sheetmetal screw and drill the hole to rivet-size with your #30 drill. Any burr? If so, remove it. Now insert the rivet, position the assembly on your anvil and use a hammer to form the shop-head.

Hey! Nice job.

So keep doing it. Remove the screw to one side of your rivet, drill, deburr and set another rivet. (Hey! This is going pretty good!)

Now go to the other side of center and do it again. Then back, then back again.

Five nice neat rivets in row.

Okay, so they aren't all perfectly symmetrical. That will come with practice. And that's what I'd like you to do -- practice. Do at least five pairs of coupons. You don't need someone looking over your shoulder, you can tell a good set from a bad one; just do the best you can. And the more often you do it, the better they'll become. (Don't sweat the mistakes. We all make them at first. Now's your chance to make all the mistakes you want, the coupons ain't gonna fly.)

Working with coupons and P-K's, you will have seen how the sheetmetal screws touch the anvil when you try to head the rivet. The usual solution is to use a narrower anvil. But in the real world there are riveting tools called 'hand-sets'. These are round or retangular steel bars having a polished face that is machined to match the head of the rivet you're setting. In use, you can clamp the hand-set in a vise and use it to support the head of the rivet while you form the shop-head with a hammer. Or, you might turn the work over, pressing the shank of the rivet against the anvil and hammer on the end of the hand-set. Hand-sets are fundamental to riveting. I'll have a bit more to say about them later.

Working with two flat coupons, the riveting is very easy. But most aircraft components aren't flat. Take a coupon and bend it length-wise -- turn it into a six-inch long piece of angle stock. (Just clamp it between something and fold the edge over with your hand.) Now you may call it a stringer, if you wish :-) Drill five holes down the center of one of the flanges then go ahead and rivet it to another coupon.

The flange limits your access and will make the riveting more difficult. You may find you need a hammer with a smaller head. Indeed, the head of the classic riveting hammer is made from hex or square steel and is typically about 3" long from the handle-hole to the face. As with rivet-sets and bucking bars, riveting hammers came in an almost infinite variety, each shaped to allow the riveting of a particular shape of panel or stringer. The same is true for their matching hand-sets.

But eventually you simply run out of room -- you can't get at the rivet with a regular hammer. For example, take a coupon and bend it into a 'U' about an inch across the bottom. Now figure out how to set a rivet with the shop-head in the bottom of the 'U'

If you're like most folks, you will have reached for a drift or flat-faced hand-set that is long enough to reach the rivet in the bottom of the 'U'. But unless you have a third arm and prehensile tongue, you also discovered that the parts were more difficult to hold and position than in previous examples.

You've already seen the need for positioning and locating the components prior to riveting. Working with angles and deep 'U' channels should bring home the need for some means of positioning and holding the work. 'Relative to what?' ...someone always asks, which is a good question, given the three-dimensional nature of aircraft structures. Do you position the work relative to the anvil or relative to the hammer?

The most correct answer is that you should try to position the work relative to YOU --- to the person doing the riveting. You will do your best work with both feet on the deck and the panel at work-bench height. And yes, this is a bit of a trick question because up to now the anvil has been fixed in position and we've been positioning the work atop it. But the anvil -- the mass against which the rivet is 'bucked' or deformed -- does not have to be fixed in position. So long as it has sufficient mass, we may even hold it in our hand and do a good job of forming a shop-head, thanks to the laws of inertia. But before freeing you from the bench, let me tell you a bit more about hand-sets.

Hand-sets used to be like mother's milk to an aviation metalsmith. But with squeezers and pneumatic guns having become ubiquitous in the trade, the traditional hand-set has largely vanished, replaced by a simple block or bar of steel, drilled to accept the die from a squeeze-set or rivet gun.

The sets for squeezers look rather like a rivet, being a button of steel with a shank extending from the bottom. The top of the button is shaped to match the head of a rivet and is normally hardened and polished. The shank, which comes in various standardized sizes to match the wide variety of squeezers and tools, is three-sixteenths (actually, .187 ) in diameter for small tools (and smaller rivets) and goes up to three-eights (.375) for the larger. Among that range, as a homebuilder, you will probably find the three-sixteenths sets (ie, .187" shank diameter) to be the most useful.

To make yourself a hand-set you simply chuck a suitable bar of steel into the lathe and drill a #13 hole about five-eighths deep in one end. If you were an apprentice, you would be expected to knurl the thing and polish the hammering face but in the real world you will find yourself creating hand-sets as needed, from whatever stock is on hand and in whatever shape is needed to insure the rivet is properly headed.

Which brings me to perhaps the most important bit of information you're liable to get from this exercise: Most rivets are not set in what Conventional Wisdom insists is the 'traditional' fashion using a rivet 'gun' and bucking bar. The overwhelming majority of rivets are set using various forms of squeezers, or in the case of homebuilders, are headed by hand. The pneumatically powered riveting hammer and bucking bar is largely reserved for what is called 'panel work', where it's impossible to use a squeezer... (although aircraft manufacturers have built squeezers so large the entire panel can be pushed through them).

The popularity of squeezers and presses over guns & bucking bars is well justified. A squeezer takes no special skills yet produces a shop-head of perfect proportions and uniform quality. Since the object of riveting is to create the best possible fastener, anything that promotes that goal should be used.

If you haven't already realized the obvious, let me say now that I can't teach you to rivet via email. Indeed, I can't teach you to rivet even if we were in a classroom together. Riveting is a senso-kinetic skill, akin to riding a bicycle. Riveting, like riding a bike, is something you can only learn for yourself. What I'm hoping to do with these articles is to show you the natural progression which lead to riveting as we know it today. That progression begins with forming the shop-head by hammering directly on the shank of the rivet. The next step is to form the shop-head using a pneumatic rivet hammer and bucking bar. According to Conventional Wisdom that would appear to be the logical goal of any treatise on riveting but the reality of riveting is that most of it does not involve the use of rivet guns and bucking bars. Explaining why this is so and introducing you to some of these extremely valuable tools and techniques, is the ultimate goal of these articles. The fact that you might just happen to learn to rivet along the way is merely frosting on the cake :-)

---- to be continued ---

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