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.
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