Prior to riveting, drilling is done, after which de-burring and cleaning operation is done. Air operated drill guns are used. Straight and angular drill guns ( 30o & 90o ) are available. After drilling counter sunk operation is carried out to accommodate CSK rivets correct depth of CSK is required. Else the rivets will be either depressed or protruding.
Riveting can be explained by using the classic "swinging
balls" physics demonstration. One ball swings in and strikes the other
balls, the ball on the other end swings off, and the process repeats back and
forth.
Riveting is just like that. On one end, is the rivet gun. On the other end is a bucking bar ( Bucking bar is a chunk of steel of some convenient shape and weight, with a smooth striking surface ). In the middle is the rivet itself. The pneumatic rivet gun impacts the rivet, which acts as a medium and sends the impulse through to the bucking bar resting on the other end. The bucking bar swings away from the rivet. The small force of the bucker holding that bar in place "swings" the bucking bar back at the rivet. Now here's where riveting diverges from the ball demonstration, if the rivet gun had shot a single impulse, the bucking bar would strike the rivet and then the gun would get pushed away in turn, conservation of momentum and all that. But the difference is that the gun is held firmly in place and is sending a continuous stream of impulses through the rivet. As soon as the bucking bar hits the rivet, another impulse from the gun counteracts that momentum and cancels it out. In the process, the momentum canceling effect is absorbed by the rivet, which gets smushed a little bit. This happens several times per second, and the rivet gets driven down and flattened.
The most common types of rivets fall into the following categories:
The universal rivet is used where a smooth exterior surface is not needed. It is also used in areas which do not have to fit flush with another part. Such areas of an aircraft might be in the interior. The round head of the rivet could interfere in areas that have to meet close tolerances. The diameter of the rounded head by a general rule is about twice the diameter of the shank of the rivet.
Countersunk rivets are used in areas where the surface finish is important or where parts have to fit closely together. A countersunk rivet usually has a flat head and from the top of the head to the shank the rivet is chamfered. The standard chamfer is 100 degrees. The hole in the material must also be made to account for the chamfer. When joining materials with countersunk rivets it is important that the seam between the materials is either at or below the place on the rivet where the head and the shank meet. Otherwise the joint will fail do to stress fractures at the edge of the rivet head. Countersunk rivets are used on the exterior of aircraft to reduce the drag.
Blind rivets are used in places where it is not possible to place a bucking bar on one side of the sheets being joined. These places where only one side of the joint is accessible are places such as the wings and tail. There are several types of blind rivets. The most common type is the Cherry rivet. Cherry rivets can have universal or countersunk heads. These rivets are hollow with a stem which protrudes from the top of the head and a small bulb at the bottom. The stem is pulled through the center. Depending on the type of Cherry rivet the stem either breaks when the bulb has sufficiently spread the bottom end of the rivet or the bulb deforms and the shaft is pulled through the hollow center. Another type of blind rivet is the Du Pont explosive rivet. In these rivets the shank is filled with an explosive compound and the head of the rivet is heated to ignite the explosives. The explosion forces the bottom of the rivet which extends past the edge of the material to expand to be larger than the diameter of the drilled hole.
Solid rivets:
These are driven by a rivet hammer ( either on head of shank side ) and bucked on opposite side by using bucking bar. Bucking facilitates the formation of head. Solid rivets need access on both sides. Aluminum rivets may be installed by using a rivet squeezer. Squeezed rivets are preferable, because there is no damage to sheet metal and we have a well formed heads. Also it could be operated by a single person. The first few blows should be hard. The objective is that the shank portion should swell full up the holes and a well bucked head is formed.
Precautions:
Riveting Techniques:
There are several common methods or techniques for performing riveting operations. There is the standard hand riveting with a bucking bar or a blind rivet. Three basic alternative methods of riveting are cold riveting, hot riveting, and automated riveting. Each method is used to achieve different characteristics.
In the standard riveting process and in cold or hot riveting a bucking bar is used at the bottom end of the rivet to cause it to form a head when the rivet is driven through the hole. Bucking bars are of different weights depending on the size of the rivet being used. In the cold riveting process the rivets are kept in a refrigerator until they are ready to be used. The rivet is driven while it is still cold. While it is cold the rivet remains soft and is more malleable. The rivet will cure at room temperature and become hardened. This process is necessary for rivets produced from certain aluminum alloys. The hot riveting process is done for the same reasons. The difference is the rivet will be at room temperature before its use. When the rivet is needed it will be heated and then driven while it is still hot. When it cools again it will return to its hardened state. Automated riveting processes are cheaper, but do to lengthy setup time they are usually limited to one rivet type. Automated riveting can include the hot or cold riveting methods.
The most common types of riveting other than the standard methods are cold riveting, hot riveting, and automated riveting. Each of the riveting methods have advantages, whether it is strength, ease of production, or cost.
Countersinking of Rivets:
For external surfaces of aircrafts, protruding rounded rivets is not used, rather CSK rivets are used to achieve aerodynamic efficiency. Normally 100o CSK rivets are used for thick skins and 120o CSK is used for thin skins. Correct depth is maintained by using proper tools ( Depth control equipment ). Some times from the design point of view, both sides would have to be countersunk. Counter sinking rivets over a concave or convex surface causes a bird eye.
Advantages of Riveting:
Riveting has several advantages over other types of fasteners. Rivets are relatively inexpensive, they offer several surface finishes, they are easy to assemble, they may be used to join different materials with different thickness, and they are strong. In the case of the aircraft industry rivets offer other advantages. In the aircraft industry aluminum alloys are the most common material used to produce rivets. This allows for lighter fasteners and resistance to atmospheric and chemical corrosion.
Rivets are usually constructed with straight shanks. They are not threaded like bolts or screws and are therefore easier and less expensive to produce. Rivets can come with various head configurations which offer different surface finishes. They can be produced such that the head of the rivet is flush with the surface or some types can have the head ground to be flush with the surface. Standard rivet types have a round smooth head which can be polished or painted to match the surface of the fastened material. Rivets can be assemble in a variety of methods. There are hand tools and hand guns which are used for riveting and there are large machines that perform riveting operations. The advantage of the hand-held rivet guns is that they are very maneuverable. They can also be used on curved or varying surface contours.
Rivets can be used to join different types of materials. A rivet can join metal and plastic, whereas other fasteners may have to be specially made to join metal and plastic. If the materials are of different thickness’, rivets could still be used to join them. Depending on the type of rivet it may be preferable to have the rivet head on one side or the other, but rivets could fasten the materials together. The rivets which are used in the aircraft industry are generally strong enough to meet the needs of the product although they are usually weaker than a bolt. Aluminum alloy rivets offer several advantages to the aircraft industries. They are lighter than bolts which aides in the industries’ strive to make the lightest products possible. Because many metals used in the aircraft industry are aluminum alloys, the aluminum rivets protect against chemical corrosion which could be possible if a steel or other metal bolt was used. Aluminum rivets also are protected from ordinary atmospheric corrosion.
Last updated on Thursday, December 18, 2003 , 07:01 PM