Riveting: Science or Art ?
by H. Dean Chamberlain
Reprinted with permission from the FAA Aviation News
In the era of high-tech composite aircraft and space age Kevlar' and carbon fiber structures, the lonely, unappreciated rivet still plays an important role in aviation. The question is, when was the last time you really looked at a rivet? Do you ever remember preflighting one? After all, there is no nut to come loose, or safety wire to break, or cotter pin to fall out. Rivets are just rivets. Right? Wrong!
With many general aviation aircraft approaching more than 25 plus years of service, proper maintenance and repair are important for continued airworthiness. Good sheet metal work, including riveting, becomes even more critical in aging aircraft as these aircraft become more susceptible to corrosion, cracks, and metal fatigue.
Although rivets are not rocket science, some basic mathematics and knowledge of science are required to install them. Riveting is also an art as I recently learned. You might say it was a 'riveting' experience. By understanding how rivets are installed, you can learn what is a good rivet job and what isn't. This will give you an idea about how well your aircraft is assembled, and an idea of the quality of any repairs.
Installing rivets involves two distinct actions. One is the proper selection of the rivet to be installed. The second is the proper selection of the rivet to be installed. The second is the proper methodology. This includes mechanical skills and a certain amount of ability. Although this article is a general review of riveting, any riveting done on certificated aircraft implies that the person doing the work has the appropriate certificates (if required), the training, and the supervision necessary for working on certificated aircraft.
When you are building or repairing a metal aircraft, the question of how to join two pieces of metal or material is one of the first decisions the designer, builder, or maintainer has to answer. Common methods of joining anything together include nuts and bolts, screws, and rivets. A key element in this decision is how often the parts must be taken apart. If you need to separate the two pieces of metal frequently, you probably would want to use a bolt and nut or possibly a screw. For a more permanent attachment or connection, you might want to use a rivet. The reason is highlighted in Webster's Ninth New Collegiate Dictionary which defines a rivet by saying 'a headed pin or bolt of metal used for uniting two or more pieces by passing the shank through a hole in each piece and the beating or pressing down the plain end so as to make a second head.' As you can see, once a beaten (driven) or pressed second head is formed on a rivet, a rivet is not something you can easily remove and replace.
In most cases, the question of how to join two or more pieces of metal has been determined by the manufacturer. You simply follow the aircraft's parts manual or construction or maintenance manual or engineering blueprints, if available. Then you use the same materials the manufacturer used in making the aircraft or as recommended by the manufacturer. In many cases, the manufacturer's repair manuals also tell how to repair the aircraft. Where life gets interesting is where the manufacturer has not published data for a repair.
In that case, a person can check for 'approved data' in other sources. One source is the FAA's various advisory circulars or manuals.
Rivets are used in many aircraft because they are cheap, work well, and are semi-permanent in many applications. Because of these factors, rivets are the fastener of choice in many metal aircraft repairs. There are many types of rivets and detailed repair instructions for repairing anything from a simple hole in sheet metal to repairing critical structures. If repaired improperly, they could lead to catastrophic airframe failure. This article is only going to use a simple repair to highlight basic rivet procedures. Complete details with illustrations are contained in FAA Advisory Circular 43-13-1B, Acceptable Methods, Techniques, and Practices-Aircraft Inspection and Repair. The AC is approved data for minor repairs, if the conditions listed on the first page of the AC under the 'Purpose' paragraph are met.
The basic science is the calculation of what kind of rivet to use and how strong should it be. Yes, rivet strength is one of the most important factors in repairing something. Many people may be surprised to learn that an aircraft repair can be too strong. For example, the original engineering of the aircraft may have been called for a specified amount of flexing in the part. If a repair to that part exceeds the strength or rigidity required to permit the flexing, then the part may fail because it is now stronger or more rigid than the original design requirements. Or other parts may fail when the 'overbuilt' part transmits too much energy to them. As FAA Advisory Circular 43-13-1B states, 'Aircraft principal structural elements (PSE) and joints are designed to carry loads by distributing them as stresses. The elements and joints as originally fabricated are strong enough to resist these stresses and must remain so after any repair.' The AC then goes on to say, 'All-metal aircraft are made of very thin sheet metal, and it is possible to restore the strength of the skin without restoring its rigidity. All repairs should be made using the same type and thickness of material that was used in the original structure. If the original skin had corrugations or flanges for rigidity, these must be preserved and strengthened. If a flange or corrugation is dented or cracked, the material loses much of its rigidity, and it must be repaired in such a way that will restore its rigidity, stiffness, and strength.'
So how does one learn how to design a correct repair? The first step is to check for any aircraft data to engineering data for the aircraft. If that fails, you can contact the manufacturer, a properly certified engineering representative, an FAA certificated airframe mechanic, or an FAA airworthiness safety instructor for advice. You can also check FAA published data such as advisory circulars, appropriate airworthiness and design regulators, and commercial manuals for repair data. If you are an FAA certificated airframe technician, you learn how to rivet as part of your initial training, so you should know where to find data for making riveting repairs. When data is not available, particularly for older aircraft where the manufacturer no longer is in business, the current version of FAA Advisory Circular 43-13 is the textbook on aircraft repair, including information and standards for riveting. Aircraft type clubs for specific makes and models of aircraft are great sources of information for older aircraft. The Internet is one of the best ways to locate such aircraft type clubs.
Riveting also means drilling the properly sized hole and, if removing a rivet, following the proper procedures. AC 43-13 explains in detail why you must drill the proper size hole when installing new rivets and why it is important to avoid damaging or making a rivet hole larger when removing a rivet. If you make or find a damaged or oversized hole, the AC also tells you what you must do. Did you know rivets expand to fill the hole when installed? The AC also explains why rives should fail before they can cause the underlying metal to fail. The basic rule is the properly installed rivet should fail before the underlying metal rips. Rivets are easier and cheaper to replace than the underlying metal. This requirement sets the maximum strength for a rivet and a repair. Too strong a rivet and layout and the underlying metal fails, too weak a rivet and the joint fails.
So how do you determine the correct strength of rivets?
First, you have to select the right type of metal rivet. According to AC 43-13-1B, in aircraft manufacturing and repair, the most common rivets are standard solid-shank aluminum rivets with a universal head. These can be used for both interior and exterior applications. The AC says MS20470 is the standard protruding head rivet in the United States. The standard for countersunk head rivets is the MS20426 100-degree countersunk rivet. Countersunk rivets are used to provide a smooth aerodynamic surface and for where a smooth finish is required.
For those not familiar with rivet nomenclature, the type of rivet as well as the special markings on AN-type aircraft solid head rivets identifies the rivet by type and size. The part number of a rivet is MS20470AD3-5. The MS means military standard number. The 20407 are a universal head rivet. The AD shows it is made out of 2117-T aluminum alloy. The 3 shows it is 3/32nd inches in diameter, and the 5 shows it is 5/16ths of an inch in length. The coded head marking for this rivet has a dimpled dot on it. The heads of rivets are marked or coded to indicate the type of metal in the rivet. The AC shows the head marks used for the most common rivets.
One important benefit of aluminum rivets installed in aluminum aircraft is that similar types of metals in contact with each other reduces the risk of galvanic conductivity between the two metals when wet. This is a fancy way of explaining the risk of corrosion between two aluminum metals. Similar metals reduce the risk of corrosion between the rivet and adjacent metal. If the two were dissimilar metals, you run the risk of localized corrosion where the rivet penetrates the joint. Such corrosion would eventually cause the rivet to fail thereby weakening the joint or destroying the underlying metal. This is also why it is a good idea if you know how to tell if a rivet has any corrosion developing under its protective paint.
Based upon approved data or the recommendations made in the AC, you select the type of rivet material. How do you know which style of rivet to use once you select its metal type? This is where things get interesting. Let's assume you have made the choice between the need for a flush mounted rivet and a protruding head rivet. Remember, the AC says, 'Replace rivets with those of the same size and strength whenever possible.' That choice also determines how you drill and finish the holes required for the rivets. For example, flush mounted rivets require the correct type of countersunk hole. Remember when installing countersunk rivets, the main portion of its body is below the surface level of the metal it is installed in. As a result, countersunk holes have their own standards for strength and installation requirements. The trade off for using countersunk rivets is the reduced drag involved because there are no rivet heads sticking up in the airflow. The downside is the extra effort and work involved in installing them.
That is why in most low-speed aircraft, the common universal-protruding head rivet is generally used. This type of rivet is generally used. This type of rivet requires less preparation work in drilling and finishing the holes required for installing the rivets. But protruding head rivets do add extra drag to the aircraft. But normally, this should not be a factor in slower speed aircraft.
At this point, you have selected the rivet material, aluminum, and the design, the universal-protruding head. Although some special riveting applications may require special processing or handling techniques, AC 43. 13-1B states the most common aluminum aircraft rivet can be used as is.
Since the basic purpose of a rivet is to hold at least two pieces of material together, you want to make sure the rivet is strong enough and installed properly to meet its design purpose. One way to ensure this is to follow the recommendations listed in aircraft repair data, the aircraft construction data, AC 43. 13-1B, or data provided by the manufacturer.
So much for the bureaucratic disclaimers, do you know why the mechanic who worked on your aircraft used a ring of rivets on your last repair? Why not an X design or some other design? Why a design at all? It all goes back to basic math and science. You have a hole or damage in some sheet metal. You want to repair it. You know the type of sheet metal involved and its thickness. Tables listed in the AC provide data needed to determine the number of rivets required based upon the permissible strength of the proposed repair. For example, a certain type of repair may be rated at 70 percent of the base metal strength. Using this information, the number of rivets required can be determined for that specific repair. In rivets, you trade strength for quality. You can use fewer large and thereby stronger rivets or more smaller but weaker rivets.
Then the guidelines specify how the rivets are to be laid out. There are minimum distances from the edge of a sheet of metal to the center of the rivet hole. Then there are minimum and maximum recommended spacing distances between rivet centers. Offsetting adjoining rivets makes the repair stronger than if the rivets are placed side by side. Offsetting rows of rivets minimizes the lost of strength of the base material in closely spaced rivets compared to adjoining rivets.
In some rivet layouts, such as circles; the installation data requires that specified angles be maintained between rivets. So not only do you have to understand basic math and enough science to understand the metal and design strength requirements used in the job, but need a minimum understanding of geometry and layout. All of which must be understood before you drill your first installation hole or insert your first rivet.
Once you have made all of the selection decisions, laid out your work, and started drilling the correctly sized holes, you need a means of attaching or holding the metal together so you can complete the job. One of the tools widely used in industry is the trademarked sheet metal holder called a Cleco' A, a small, spring-loaded clamp, or similar tool is inserted into a rivet hole between two metal sheets by a special tool that looks somewhat like a pair of pliers. The spring loaded Cleco' has an expandable tip that locks itself into the hole when pressure is released by the insertion pliers. The special pair of pliers compresses the Cleco' spring, which allows the tip to be extended and inserted and later removed from the hole. In a major job, you can have dozens or hundreds of Clecos' holding your sheet metal together. During the riveting process, you remove a Clecos' and install the rivet, and you go to the next Clecos'. There are different sizes of Clecos' for different size rivet holes. Each type of standard Cleco' fits a specific sized hole. Normally each size of Cleco' is color coded for ease of use. There are several types of devises used to hold material together. Clamps and special long-range Clecos' are used when needed. The important thing is to ensure that all of the holes in both sheets of metal are in alignment and that the parts are tightly held together.
As we said in starting this article, riveting is not rocket science, but like many things in aviation, it is easier said than done. There is a certain art to making a good rivet head. A standard solid-shaft rivet consists of a factory formed head and a shaft of a given diameter and length that you either drive with an air-powered rivet gun to form the second rivet head or 'shop head' using the correct rivet set, or you can use a squeeze-type riveting tool to compress the rivet and form a second rivet head. A correctly sized rivet set's 'face' should slightly exceed the 'face' of the rivet to avoid damaging the factory-formed head of the rivet. So far, so good.
The AC says a good rivet is one that meets the standards shown in the AC. Sounds simple enough. You just have to look at the illustrations in the AC that match the work. But like trying to make consistently good landings, it takes practice to drive consistently good rivets. Generally speaking, a rivet is properly formed when the thickness of the formed head or 'shop head' is equal to a minimum of one half the diameter of the rivet and the width of that formed head is a minimum of one and a half diameters. The formed head must be vertical and centered on the center of the rivet. The rivet or surrounding metal cannot be damaged or the rivet too loose or too tight in the hole. For example, a 'smiley' is great on a T-shirt, but one is bad on a rivet or surrounding surface. A smiley is where the riveting set cuts either the rivet or underlying metal and forms what looks like a smile or semi-circular cut in the rivet or metal.
Common names given to riveting problems include rivet driven at a slant, dolly head at a slant, one side of the rivet is flat, body of rivet too short, rivet not pulled tight or metal plates not closed, rivet too tight or metal plates bulged because of poor fit, riveting tool damages metal, or rivet head cracked because the rivet was too hard when driven.
In reviewing this article, one FAA airworthiness inspector wanted me to clarify why I said in one part of the article a rivet should be tight and then in the above paragraph, I said one of the possible riveting problems was it could be too tight. The answer is, if you are joining two pieces of metal together, you want the metal to be riveted firmly together, but you don't want the rivet to be so driven that it causes the metal surrounding the rivet to buckle or bulge.
Although you can buy small gauges to check the rivet for the proper thickness and width experienced riveters can judge a good rivet by sight. If it is a bad rivet, they drill it out and replace it using the procedure outlined in the AC. There is even an informal system of codes or taps used by some riveters when they cannot see each other such as when working on large sheets of metal or inside bulkheads, etc. One signal or tap tells the riveter or person with the rivet gun the other person is ready to buck or hold the bucking bar or dolly against the rivet. As the rivet is buckled or driven against the bucking bar or dolly by the rivet gun, a formed head or shop head is formed on the end of the rivet protruding through the hole. Then there is another signal to stop riveting when the proper sized head if formed. There is also a signal that the rivet is okay.
Although we have been talking about basic riveting in aircraft construction and repair, the rivet was an important historical symbol in American culture.
During World War II, one of the famous American war symbols from that era was 'Rosie the Riveter.' Rosie the Riveter represented the thousands of women who went into the defense factories to build aircraft, ships, and equipment for the war. The millions of rivets they drove to make the equipment needed to win WWII were vital to winning that conflict and marked a significant change in America society.
From its initial development to today's aerospace use, the simple rivet continues to play an important role in joining not only sheet metal together, but also an American cultural revolution that dates back to WWII. The mothers and grandmothers of many of today's pilots and mechanics helped win a war with simple, unappreciated rivet.