Tuesday, November 21, 2006

AV - Riveting 101, Part 3


Got your gun? Retainer? Rivet set? Regulator? Bucking bar?

Do you realize how lucky you are? You picked up the phone, made a few calls and all this stuff appeared on your doorstep in less than ten days. Not very expensive, either... assuming you've bought used stuff... which is fine for learning.

Now try that in Old Patagonia.

In effect, America's aerospace industry has subsidized the tools and materials you need to build a metal airplane. That's the lucky part. Ding your bird in the boondocks of the world and it's liable to stay there even if the required repair is relatively minor, because the tools and materials and skills simply are not available.


Okay, lemme ask you again: Do you have your gun? Rivet set? Retainer spring? Regulator? Bucking bar?

Have you got any idea in the blue-eyed world what I'm talking about?

Early on, I suggested you obtain copies of the catalogs from Aircraft Spruce & Specialty Co, and from U.S.Tool. One of the reasons for that suggestion is that the catalogs serve as excellent TEXT BOOKS when it comes to defining rivets and riveting equipment.

I also made mention of the enormous variety of riveting methods and equipment, and of the procedures specific to using each of those equipments effectively. The catalogs will give you a better understanding of what I meant.

Rivet guns are sized according to the diameter and hardness of the rivet they can set in a given amount of time. This is a pretty rubbery classification since by adjusting the air pressure you can drive a small rivet with a big gun, or a larger, softer rivet with a small gun. For general maintenance or building a homebuilt, you should do fine with a 2X or 3X gun.

The major difference between a pneumatic hammer designed for riveting and one designed for cutting off mufflers or whatever is in the trigger -- actually, in the valve the trigger actuates. For riveting, you need perfect 'throttle control'. This usually involves a valve having an orifice that varies in a uniform fashion as the valve is opened. On cheap 'chatter guns', such as the stuff sold by Harbor Freight and others, the valve is a simple hole, an on-off switch offering virtually no control at all.

There is an almost endless number of rivet sets, the variety dictated by the need to set rivets in corners, the bottoms of channels, behind stringers and so forth. For our purposes you'll do fine with a straight 470-4 set, 3-1/2" long. Used, expect to pay about three bucks, new about five. (But to add a whiff of reality... lightplanes typically use -3's, -4's and a few -5's... so you might as well order a whole kit of sets if you're serious about riveting.)

Rivet sets for guns have a standard shank size -- .401" The set slides into the barrel of the riveting gun and is held there -- loosely -- by a retainer in the form of a coiled spring. Retainers come in two flavors, Beehive and Quick-Change. I'm not as quick as I usta be and I've got several guns so I tend to use the beehive type.

Being springs, retainers tend to wear out or break fairly often so ALWAYS keep a spare in your kit. And if you use the quick-change type (often abbreviated QC), be sure to buy or make yourself a Quick-Change lever... or you'll find changing sets to be not quite as quick as you'd think.

The regulator screws into the air-inlet on the gun. It is basically a valve. Some guns have them built-in, some don't. The external regulators tend to be a bit more accurate than the ones which are built-in so it's customary to use the external type.

It is also common practice to fit all of your air-tools with in-line connectors. Harbor Freight carries good-quality brass connectors at a very attractive price. You put the female on your hose(s) and the males on your tools. Be sure to use a dab of Permatex or similar sealant when you thread the fittings into place.

Bucking bars are the hand-held anvils against which you upset the rivet. Their basic graduation is by weight, with a pound and a half being about the lightest that will prove usable in regular work. The heaviest commonly available buck is around nine pounds but you don't see them very often. The general rule is that the larger/harder the rivet, the heavier the buck. For general repair work and homebuilt construction, your heaviest buck will be on the order of three pounds with a one and a half pounder probably being used for about 90% of your work.

Other than weight, bucks are shaped to allow them to access the shank of the rivet to be upset... which means they can take virtually any shape at all. Indeed, for some repair work the first task is to manufacture a buck that will allow you to do, by hand and in the field, what the factory did on the bench with automatic tooling.

High quality bucks are forgings having polished, induction hardened faces. But they remain just anvils -- a mass of metal. You can make your own bucks (and will make at least some) from mild steel, suitably polished. If you're familiar with Kasenite or other surface-hardening compounds -- or if you're a real machinist and know about case-hardening -- it's common practice to harden the working face of locally fabricated bucks. And of course, the working face is always polished. Whatever surface finish on the face of the buck, it will be transferred to the shop-head. Smooth is good. Anything else flirts with rejection since relatively minor surface scratches may be seen as stress-risers. Fortunately, there's a simple test for the surface quality of your shop-heads: They should look like mirrors. And continue to reveal a mirror-like smoothness when examined with a 3x glass.

Okay, last time... Got your gun? Rivet set? Retainer? Regulator? Bucking bar? Then let's go to work.

Got a block of wood? Got your ear muffs on? Safety glasses? (Shit happens. Don't mess around.) [I use those Lexan things that fit over my regular glasses.]

Give your gun a drop of 3-in-1. Make this a habit. Do it a couple times a day during the course of riveting. Most guns don't wear out, they rust out. Don't waste money on an in-line oiler or other fancy stuff intended for the factory floor, they'll only contaminate your hose, rendering it useless for painting. Instead, spend some time making good WATER TRAPS for your air supply. Commercial air-tool oil is primarily a rust preventative, lubrication is only a secondary function.

Insert a 470-4 set into your gun (ie, a set that matches your rivets) and secure the retainer. Now press the set against the block of wood (flat on the bench, please) and gently squeeze the trigger. You should hear a slight hiss... (awright! Stop fooling around and turn on the air compressor!)

A gentle squeeze of the trigger should produce a measured 'thunkathunkathunk' from the gun. If you got a rapid 'brappp!' it's getting too much air. So dial it down. When just lightly touching the throttle, thunkathunka is good, brappp is bad.

Thunkathunking okay? Then squeeze harder -- open up the throttle a bit more. The gun should provide a SMOOTH response from low speed to high. And yes, the high speed will sound like a 'brappp!'

No brapp? Then check your line pressure. The gun requires a given volume of air per blow or cycle, whereas the cyclic rate reflects the amount of pressure behind that volume of air, as in how fast it can re-cycle. In adjusting your gun you are taking into account your compressor's output, the size of the gun, the type of work you'll be doing and any restrictions that might apply to your air supply, such as a long run of small-diameter hose. (In the latter case, the usual fix is to place a tank -- a 'local reservoir' near the work, plumbed to the compressor via a check-valve. You may then leave the compressor at some distant location and still have adequate pressure & volume at the gun. You can make yourself a local receiver out of a portable compressed air tank, the sort of thing used for filling a flat tire. Handiest place to find a suitable non-return valve is from an auto-body paint supplier, whose rice-bowl revolves around compressed air equipment.)

- - - - - - - - - - - - - - -

Here's a Rule. Violate it at your peril.

NEVER trigger a rivet gun UNLESS the set is firmly placed against... something, be it a panel you're working on, a block of wood or whatever.

They aren't joking when they call it a gun. And retainers tend to snap when over-extended.

- - - - - - - - - - - - - - - - -

Once you've got your gun adjusted, put the block of wood in your vise so's it's sticking out to the side, like a driver signaling for turn. Got your bucking bar? Okay, hold the bucking bar against the back of the block of wood, put the rivet gun against the front and trigger the throttle. Do it a couple of times.

Now think about it.

The gun needs to be perfectly perpendicular to the block of wood. And the face of the buck must be perpendicular to the axis of the rivet-set. Adjust your stance as needed. No, you can't lean over like a willow in the wind, it'll throw off the geometry of your arms. Try to keep the mass of your upper body more or less centered over your hips. Stance is important. As with golf or marksmanship, a proper stance skews the odds of success in your favor.

Try it again. Visualize shooting something from the gun straight through the block of wood into the bucking bar. Get the feel for the proper alignment in your hands and arms and feet. You should have a sense of being poised but stable. As with drilling, you are not doing the work, the tool is doing the work... you're only there for guidance. You aren't pressing the gun against the wood with any great amount of force, nor are you pressing the buck toward the gun very strongly. 'Firm' is the best definition I can offer but you will have to translate that into your own terms... and the rivets will tell you how well you've done.

Now go make up some coupons. Flat work only, for the time being. (And yes, you can make them any size you wish... and with as many holes as you wish. But five or six rivets at a time is more than enough in a training environment.)

Make up your coupons complete, right through to insertion of the first rivet. Position a coupon in the vise in the same orientation as with the board. Position the set... then the buck... now feel how the set indexes on the head of the rivet.

Initiate the hammering with a gentle squeeze, allow the hammering rate to increase -- but not up to a full-blown 'brapp' -- and STOP. Should take less than a second.

Don't move. That's the 'follow-through'. The set should still be perfectly indexed on the rivet, the buck still perfectly centered and YOU HAVEN'T MOVED... except for your trigger finger. (Why the 'follow through'? Because of inertia. You aren't aware of it but your MUSCLES have become tense as they did the work. They have balanced the thrust of the gun against the inertia of the buck. If you allow yourself to move in an uncontrolled fashion, the set will come off the manufactured head and may go galloping off across the panel... or the reflexive reaction of your bucking-arm may cause you to bend the panel.... or some damn thing. The key point to be learned here is that a form of 'follow-through' is required. What you call it... and how you deal with it... is up to you.)

Okay, inspect it.

The whole story -- everything you need to know about your technique and the setting of the gun -- is right there in that one rivet. And you are the Mechanic-in-Charge, the man on the scene.

Dead soft rivets -- 'A's -- are about as hard as butter. Odds are, you hit it a little too hard... the shop-head is 'pancaked' -- too wide and too thin. So here's your options: You can try a lighter buck or you can lighten up on your touch... not hold the trigger down so long... Or you can reduce the air pressure... (I prefer to adjust the pressure; I use about the same 'touch' for all rivets. But it's a personal thing... do what feels best for you.)

Whatever you decide to do, do it. Then go back through this cycle. And keep doing it until you start seeing nice, symmetrical buck-heads.

If one comes out too high, don't go back and hit that it again. Do a new rivet. Why? Because you are learning. At this stage of the learning process the object is not to produce a perfectly set rivet, the object is to learn how to accomplish that feat. Going back over your work simply adds noise to the signal at this point. The goal is to learn how to head the rivet with one smooth brappp.., rather than how to do it with a series of brappp's.

The rivets themselves provide the history -- the feedback -- needed to interpret what you are doing wrong so that you may eventually do it right, every time. In this process, the rivet is secondary to the process of riveting. You are trying to master the process. The rivets are just a means of keeping score, as it were.

The significance of this learning process escapes many people because it runs counter to the modern-day philosophy of 'Everyone's a winner!' as well as the modern-day definition of 'education', which has virtually nothing to do with true learning. Riveting is a senso-kinetic skill. It isn't something to be memorized and regurgitated up during Finals, you must train your muscles to accomplish the task without direct, real-time feedback from your brain -- your nervous system simply can't react fast enough... just as when learning to ride a bicycle.

Another reason this point escapes a lot of people is because they don't understand that in the process of acquiring information -- which, upon integration becomes what we call 'knowledge' -- all data, negative as well as positive, is valid data. Indeed, you must have both or you can not define either. ("Okay, that's what we call 'falling off the bike.' That's bad. Now get back on and try it again, except this time try not to fall off.")

So make your mistakes. Make a lot of them. And in making them, learn from them. There is no 'failure' as such when you have done the best you can and learn from your mistakes. But if you tackle a task with the assumption you must do well from the outset -- that to do less makes you a loser -- odds are you lack the required attitude to do well at anything.


So when do you go back and give a high rivet an extra brappp? When you know the first hit wasn't enough. You've hit it. Your muscles and your brain recorded the hit. Now the little man in your head sez 'Hit it again.' And you do what the little man sez.

The variation on this theme is when you are part of a riveting team. The Captain of the team is the bucker. He or she is often out of sight on the other side of the panel. You communicate by taps of the buck against the work. The usual code is one tap to say 'Hit it again; not fully headed', two taps sez it's good, move on to the next. Three taps is telling you to drill the sucker out and drive another. Which brings us to a crux in the art of riveting...

Wanna know the real secret of riveting?

It's drilling them out.

Drilling out dead-soft rivets is pretty much a waste of time. Oh, you can do it.. and you're gonna have to learn how. But A's aren't the best stuff to use when introducing you to the process. For that, I'd like you to obtain some AD's.

To learn how to drill out rivets you make a test coupon about eight inches wide by a foot long out of half-hard aluminum. On the bench, or using a drill press, layout and drill six rows of ten holes. Now fill each hole with a properly set rivet of suitable diameter and length.

Then drill them out... without damaging the coupon.

In the overall scheme of things the ability to drill out a rivet will always have a greater value than the ability to properly set a rivet. The reason is pretty simple: Even if your riveting skills are rather poor, so long as you can drill out rivets without damaging the part, you will eventually produce an acceptable finished product. (Bad rivetors are often referred to as 'blind hogs', apparently from the old saw: 'Even a blind hog finds an acorn now and then.' Of course, in the peace-time reality of today's world, a bad rivetor is more likely to be referred to as 'unemployed' :-) More important, in the real world, airframe maintenance involves repairs... and you can't do the repairs until you remove the rivets fastening the bad part.

To drill out a rivet you use a drill-bit that has a smaller diameter than the shank; typically you would use the pilot drill-size (ie, a #31... or even #32 for a 1/8" rivet, for example) to drill through the head of the rivet... and a few thousandths beyond. You then insert the shank of the drill bit or a punch of suitable diameter, into the drilling and gently wobble the head until it breaks off. The shank of the rivet may then be punched out. There are a host of precautions that must be observed when drilling-out, such as supporting the panel when punching out the shank and being sure your hole is properly centered.

It's vital that you drill in the exact center of the rivet. On AD's, the signature dimple often remains visible and may be used as the center. On others, it's usual to file a small flat on the head and use a center-punch. For flatheads, there's a little optical device made out of Lucite that allows you to center the rivet in the bullseye of the device then flick the spring-loaded punch, marking it with a high degree of accuracy. For rivets having raised heads there are a number of tools devoted to drilling-out, most of which will automatically center your drill-bit, limit the depth of the hole and prevent the rivet from spinning, all in one operation.


To add to the utility of this article, which I imagine is a rather boring read for the more experienced hands, I'm included drawings of a fairly typical Apprentice's Toolbox. Thanks to Mr. Ryan Young, you'll find them posted at...


As a point of interest I wrote ‘Riveting 101' several years ago complete with drawings, photos and assembly instructions for the toolbox. The article was submitted to several magazines, who either rejected the material or offered only a few cents per word. (Good magazines pay a dollar a word and up for illustrated technical articles.) One aviation rag wanted to print it without any payment at all :-) Since that time the assembly instructions and photos have gone adrift. If I find them I'll send them to Mr. Young to tack onto the drawing file.

The only tricky bit in the construction of the toolbox is offsetting the corners by the thickness of the panels. That is, the panels do not overlap each other at the corners. This allows the builder to use whatever thickness of sheet stock they happen to have on hand.

To demonstrate their use, the tray was assembled with flush-head rivets, some dimpled, some counter-sunk.

Wooden dowels, 3/4" in diameter, may be used for the handles in lieu of aluminum bar stock.

To make the box waterproof buy a small tube of ‘Boat Life' polysulfide caulk and treat the edges of the panels prior to assembly. You may pretend it's ‘Pro-Seal,' if you wish :-)

-R.S.Hoover, 9 September 2002

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