Aircraft Riveting Review
There are five types of aircraft primary structures that have been employed through aviation history. These are Wire Braced, Truss Type, Monocoque, Semi Monocoque and Sandwich. Wire Braced and Truss Type structures were popular right up to the middle of the Second World War, then Monocoque and Semi Monocoque structures became the norm. The aviation industry is again going through a transition, this time to sandwich structures. As a mechanic you need to have a firm grounding in at least four of the five types of aircraft structures (forget about wire braced unless you want to build a Wright Flyer replica or similar).
For Truss Type Structure you will generally need to know how to weld steel tubing (a topic for another article), for Sandwich Structures you need to be familiar with exotic composite cloths, panels and epoxy resins (again a topic for another article). Now we are left with Monocoque and Semi Monocoque structures, which will require you to have a fairly detailed depth of knowledge of riveting tools, types and techniques (the subject of this article).
Unless you are employed in the manufacturing environment where you are working under the direction of concise engineering drawings that specify the location, spacing and type of fastener that you are to use in each location and you are drilling the first installation holes in a new piece of structure, you are most likely to be proficient in riveting because of the need to perform structural repair. This article specifically addresses riveting as it relates to structural repair.
Generally rivets are used where shear strength properties are required. If a joint is subject to a tensional force, a rivet is unacceptable and a bolt would be used instead.
There are three types of rivets: Solid Shank, Blind and Pin (Hi-Shear). Solid shank rivets are most common and consist of a manufactured head (mushroom head and countersunk), a solid shank and a driven head. Blind rivets have similar design properties as solid shank rivets, however the formed head is made by drawing a stem or mandrel through the hollow shank of the blind rivet. There are three types of blind rivet designs: hollow pull through rivets, self-plugging — friction lock rivets and mechanical locked stem self-plugging rivets.
The first two types of blind rivets described are generally not as strong as solid shank rivets and therefore their use must be carefully planned based upon the strength of joint required.
Mechanically locked stem self-plugging rivets are normally considered as strong as solid shank rivets in most applications and therefore may be substituted, unless otherwise noted by the aircraft manufacturer in it's Structural Repair Manual.
The third type of rivet design, Pin (Hi-Shear) rivets are designed to replace bolts in high-shear strength applications — allowing assembly time and weight to be reduced without compromising required strength. In sheet metal repair, after the basic repair scheme has been drawn up and the patch/splice repair metal type has been selected, it will be necessary for you to select the correct type of rivets that you will use, correct rivet size and determine how these rivets will be laid out in the repair. As a general rule, the rivet diameter (shank diameter) should not be less than three times the thickness of the thicker sheet of metal being joined. Ordinarily, rivets with a shank diameter less than 3/32" are never used in structural assemblies that carry any significant stresses. The rivet shank length (Grip Length) is determined by measuring the thickness of the combined materials being joined together and adding 1 1/2 times of the rivet shank diameter to this measurement. Whenever possible, select the rivet that has the same alloy number as the metal that you are joining. By doing this, you will be eliminating the majority of problems that can occur in the future with the joint that you are about to complete (i.e. joint is less susceptible to corrosion and to fatigue).
When laying out your rivet pattern, you must first calculate and mark out the edge distance of the first rows of rivets in relation to the edge of the repair. This distance should not be less than two and not more than four-rivet shank diameters away from the edge.
Rivet Pitch should next be calculated and laid out. What this means is that rivets cannot be placed closer than three rivet shank diameters and no more than ten rivet shank diameters (normally) away from each other. Diagonal rivet pitch is usually maintained at 75 percent of the pitch of the rivet rows, however, the minimum diagonal rivet pitch allowable is two and a half rivet shank diameters.
When the rivet pattern is non standard on the aircraft itself and there is no real guidance provided in the structural repair manual for the aircraft that you are working on, you should try and duplicate the pattern that was in place before the damage occurred, but you must lay out the repair in accordance with data that is acceptable and approved by the FAA Administrator, i.e. AC43.13. 1A/2A (recently updated to AC43.13b) and or a repair issued by a Designated Engineering Representative (DER) certified under FAA Form 8110-3. Once the layout for the repair has been completed, you are now ready to select the correct size drills for the rivets that you are to use. To make a clean hole that is free from burrs and is the correct size and shape, it is customary to first drill the hole undersize with a pilot drill and then either drill to the correct size of the rivet shank diameter being used, or use a reamer to achieve the correct hole size, or in the case of a highly stressed component, use a hydraulic powered mandrel to upsize to the correct diameter and to stress relive the hole.
Always use a center punch to mark the position of the required rivet holes. This will ensure that you do not wander and dance across the metal surface, causing damage to the corrosion resistance coating of the metal and creating stress risers in the surface that may turn into cracks later.
Once drilled, return with a drill that has a much larger diameter than what you have just drilled and slowly and lightly rotate this drill in your fingers over the mouth of the hole to remove any burrs that may be present. Be careful not to remove any of the base metal during this operation.