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.
The repair piece should be secured by clecos prior to drilling the new holes. Then it is a good idea to drill pilot holes first. These can then be stepped up to the final size once you are certain all original holes are picked up, all repair fastener holes are laid out, and all holes are verified as being in the proper location to be used in the repair.
Finishing touches before installation
Now that all holes are in place on the repair, a final trim can be done to achieve the required ED, generally 2 to 2 1/2 times the diameter of the fastener (from the center of the fastener to the edge of the repair piece). Too little ED can result in cracks forming from the edge rivets to the edge of the repair. Too much ED can cause the edge to lift up from the surface.
Any chips, burrs, and foreign material should be removed from the mating surfaces. Surface treatment should be accomplished including chemical conversion coating, primer, and sealant as required.
Now the repair piece is ready for installation. Although there are many factors affecting repair installation, one of the most important factors is proper bucking bar techniques.
Bucking bar weight
When working with solid rivets, using the proper bucking bar weight is essential. Table 2 shows recommended bucking bar weights associated with various rivet sizes. Using too heavy a bucking bar can result in flattening out the bucktail beyond limits, and too light a bucking bar will result in work hardening the rivet before the desired bucktail flattening is achieved.
In addition, the proper bucking bar shape is crucial. If the bar does not have the correct shape, it can deform the bucktail. The face of the bucking bar should always be held at right angles to the rivet shank.
The person holding the bucking bar should hold it in place until the rivet is completely driven. Failing to do so may cause the rivet set to be driven through the surface.
Do not bear down too hard on the rivet shank with the bucking bar. Allow the weight of the bucking bar to do the work. The hands should be merely a guide to the bar, supplying the necessary tension and rebound action.
These have been a few basic tips on sheet metal repairs. With a little patience, some practice, and guidance from an experienced mechanic, you can be well on your way to becoming a sheet metal artisan.
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