The Art of Sheet Metal Repair
A primer on basic sheet metal techniques
By Joe Escobar
Sheet metal work can be considered an art. In this art, some mechanics produce work comparable to Michelangelo, while others are on a etch-a-sketch level. So what does it take to progress to a skilled sheet metal artisan level? Well, the two main qualities that all of the highly skilled sheet metal workers I have worked with possess are knowledge and patience. Knowledge comes from learning all of the factors that affect good repairs: defect evaluation, layout, and installation techniques. Patience is essential in making sure the job is done right the first time around. A back-to-basics review can be helpful in developing your sheet metal abilities.
First things first
Hole Size, Standard Rivets Rivet Diameter (inches) Recommended Drill Sizes Hole Diameter Minimum Limits Maximum 1/16 #51 (0.067)0.0620.0723/32#40 (0.098)0.0930.1031/8#30 (0.128) 0.1250.1355/32#21 (0.159)
#20 (0.161) 0.156 0.171 3/16 #11 (0.191)
#10 (0.194) 0.187 0.202 7/32 #1 (0.228) 0.218 0.233 1/4 F (0.257) 0.250 0.265 9/32 L (0.290) 0.281 0.296 5/16 O (0.316) 0.312 0.327 11/32 S (0.348) 0.343 0.358 3/8 V (0.377) 0.375 0.390 13/32 Z(0.413) 0.406 0.421
A key factor in sheet metal proficiency is damage evaluation. This involves a visual inspection. Key to successful damage evaluation is not only being able to note the obvious defect such as that gaping hole in the wing or the dent on the side of the fuselage, but being able to evaluate any hidden damage. Is any structure underneath affected? What about adjacent structure? Are there any hidden dangers like fuel cells, wiring, or other components? Having the ability to assess all these factors is a crucial factor in developing a repair.
Determining the repair criteria involves three steps — identification, determining allowable damage, and repair. The resources and approved data vary by aircraft and operation, but generally include the manufacturer’s structural repair manual, maintenance manual, and AC 43.13-1B.
In the identification step, the type of metal and thickness of the damaged part is identified. Not only is this information necessary for repair fabrication purposes, but knowing the material composition and thickness is needed if non-destructive testing is necessary for determination of material loss.
The next step would be to research allowable damage and determine whether the damage such as cracks, gouges, dents, etc. are within allowable limitations.
Finally, if damage is beyond acceptable limits, a repair needs to be initiated. In some instances, prior approval for the specific repair can be used as reference. In other cases, an approved repair may have to be developed.
Beginning the repair
Caution: Before any disassembly, it is important determine if the structure to be disassembled or removed is essential to the support or rigidity of the complete structure. If so, support the remaining structure prior to disassembly to prevent distortion and permanent damage to the structure.
The first step in the repair process is the removal of existing fasteners, some of which may have already been removed during the damage evaluation phase. Sometimes, the paint layer may have to be removed in order to see the rivets clearly. If removing paint by abrasive methods, be sure to use a mild abrasive that will remove the paint coating without removing any metal material.
When removing solid rivets, weaken the rivet head by drilling it. A drill bit of the same size as the rivet should be used (see Table 1 for drill bit sizes). Drilling must be through the exact center and to the base of the rivet head only. After drilling through the head, a pin punch can be used to pry off the head. The rivet shank then can be carefully driven out using a punch.
Once again, taking your time is essential to preventing damage to the structure. Some practices to avoid when removing solid rivets:
• Drilling all the way through the metal
Although it may be tempting to blaze through rivet removal by drilling all the way through the rivets, it should be avoided. It is too easy to get off center and cause hole elongation. The result is either having to use oversize rivets or having holes not completely filled by the rivet, which causes a weakened repair. Another potential problem with this method is that it may result in drilling through components or structure on the back side of the skin.
• Using a chisel and hammer
Another tempting way to speed up the rivet removal process is using a chisel and hammer to remove the heads of button head rivets and then punching the stems out. This should be avoided for two reasons. First of all, damage to the skin can result. Second, distortion or cracking of the rivet holes is likely.
Take your time
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 one 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.
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.