After removing existing fasteners, the layout process can begin. From the data gathering step earlier, you will be aware of the material required and general repair guidelines. There are many factors that could be discussed pertaining to repair layout from varying materials to complex bends, but we will focus on making a regular surface patch with 2024 T-3 aluminum using AD rivets. It is sometimes helpful to sketch the repair on the surface of the skin. This gives a good idea of how large the repair needs to be, where the existing fastener holes are, and what holes need to be drilled in order to maintain all required rivet pitch limitations and edge distance (ED).
Once sure of the repair patch needed, it can be cut out from stock. It is a good idea to make it a little bit bigger than necessary during this initial stage. Then, after all holes are drilled, you can go back and trim it to maintain ED. If cutting the piece from a large sheet of stock, it is easier to cut the original piece at least 1 inch larger than desired and then go back and trim the excess — still leaving a little of leeway for the final trim. Also, avoid snipping all the way through the metal with the shear tip as this will cause rough areas and may tear the metal.
All existing holes need to be picked up first in the repair piece to ensure that all new holes drilled do not interfere with the existing holes and that all dimensions are maintained. Existing holes can be picked up by the following methods.
A hole finder can be used to pick up existing holes. The benefit of this method is that it is fairly simple. The tool is slid under the repair patch and into the existing hole, and the drill bit is guided by a bushing on the top side of the surface. Several things need to be kept in mind when using these tools. First of all, use the appropriate sized hole finder. Using a hole finder smaller than the hole can cause the hole to be drilled off center with resulting re-sizing and possibly elongating the original hole. Also, appropriate cleco fasteners need to be installed as holes are drilled to prevent slippage.
In some instances, it is difficult to use a hole finder. Another method to locate holes is to use bisecting lines. Lines are drawn on the surface of the skin extending out from the center of the hole to beyond the area where the patch will cover — usually at about 45 degrees to each other. Then once the patch is placed over it, the lines can be used to mark where they bisect and a pilot hole can be drilled, which can then be drilled to final size. Some areas of caution need to be addressed when using this method. The lines need to intersect at the center of the existing hole. Also, ensure that they extend out far enough beyond the repair area in order to provide an adequate line up with the repair patch in place.
If space permits, it is sometimes possible to back drill through existing holes in the surface. A few words of caution should be mentioned on this method. The drill bit should be the same size as the existing hole. Drilling a smaller pilot hole in this situation that is off center will be difficult to pull back to center. Also, drill straight on and be extremely careful not to elongate the existing hole. If you have someone back you up by holding the repair piece in place, make sure that your communication is clear on where you are drilling so as to avoid drilling through their hand. Even if they are using something like a wooden block to support the piece, it is still possible for an accident to occur.
Once all the existing holes are picked up, you can begin the process of drilling any new holes necessary for the repair. It is a good idea to first mark on the repair piece where the holes will be to get an overall view of them and make sure all dimensional limits like rivet pitch and ED are maintained. Also double check the existing structure to ensure none of the proposed holes will interfere with existing structure behind the surface like stringers and bulkheads or lines. Once you are certain that the hole locations are adequate, you can drill them.