5.2 EDGE FINISHING, HOLE DEBURRING & SCRATCH REMOVAL
Aluminum sheet of the 2024-T3 variety is relatively hard and brittle. Maintaining the high strength of this material in use requires
that care be taken in its cutting, bending, and finishing. Because it is a hard material, it is scratch and notch sensitive. This means
that sharp or rough edges, corners, and scratches can cause stress concentrations which will greatly increase the possibility of local
failure, usually in the form of a small crack. The problem with small cracks is that they soon become large cracks, one piece of
aluminum becomes two pieces, etc. Obviously, we do not want this happening in our airframe, particularly when separated from
solid earth by a lot of very thin air.
All aluminum edges and corners must be smoothed and radiused to prevent this stress concentration from occurring. Any sheared
edge, whether sheared by hand or by machine, has sharp corners and has a burr on one edge as shown in Figure 1. This burr must
be removed and the sharp edges rounded off. This can be done a number of ways; with a file, a Scotchbrite polishing wheel,
sandpaper, or an edge (de-burring) tool. In most instances, the tool, followed by a pass or two over a Scotchbrite wheel mounted in
a bench grinder, is the best and quickest method. A good test for the edge finish of aluminum sheet is to run your finger over it. If
you can't feel any roughness and there is no chance of drawing blood, the finish is OK. You should not be able to see the original
cutting marks on the material. In other words, if the sheet had been sawed (bandsaw or hacksaw), the saw marks should be
removed in the process of smoothing.
Corners, particularly inside ones, must be cut with a radius to prevent cracking.
See Figure 2. This radius can vary from 1/16'' for .016 thick aluminum to 1/8'' for
.040 aluminum. The radius edge then must be smoothed just as the straight edges
discussed above. A small round file works well for this. This is especially important
where a bend line is intersecting the inside corner in question. Cracks are likely to
occur at the sheet edge even if the bend radius is great enough.
All drilled holes, or prepunched holes that have been final-drilled to a larger size,
should also be deburred. Holes that were factory punched to final size can be
inspected and only deburred if needed (with the exception of large holes to be dimpled
for screws - see below). This is an easy but time-consuming chore, and can be done
with an oversize drill bit, either held between your fingers and twisted, or in a variable
speed drill running very slowly. Special swivel deburring tools are also available from
tool supply houses. These work better and are much quicker. Burrs around holes
are a problem mainly in riveting and dimple countersinking. The burr can prevent
a rivet head from seating properly and can make dimple countersunk holes more
prone to developing cracks radiating from the hole. Many novice builders deburr
excessively deep. Deburring should not produce a significant chamfer/counter-
sink on the edge of the hole. Be particularly careful deburring holes in .020 or
thinner sheet. By the time both sides have been deburred the hole could be
enlarged.
SHEARED ALUMINUM SHEET
TEAR MARKS
FROM SHEAR
SHARP EDGE
BURR
0.040 OR THICKER
FINISHED EDGES
FIGURE 1: EDGES
0.016-0.032
BEND RELIEF
AFTER BENDING
BEND LINES
FIGURE 2: BENDING
FLAT PATTERN
The finishing procedures just described will constitute a sizable portion
of the total building time. However, they are important for structural reasons as well as cosmetic.
Most of these holes, edges, etc. will be inside the airframe and out of sight when the airplane is finished.
This is no reason to consider them unimportant. The need for good edge finishing is most difficult to
impress on new builders unaccustomed to aircraft standards.
Scratches in the surface of aluminum can have the same weakening effects as rough edges, corners and holes. The alclad sheet
used is very easily scratched because of the thin surface layer of soft aluminum. Scratches within this layer will have little effect on
strength, but deeper scratches will. The greatest difficulty is deciding how deep a scratch can be before it is a potential problem.
The best approach is taking extra care to prevent scratches in the fist place. When a scratch does occur sand or buff it out no
matter how small. Very light scratches can be removed with #600 wet sandpaper. Deeper ones will require #400 (or perhaps more
coarse) sandpaper, followed by #600 for finishing. One thing to remember when removing scratches is that in doing so the
corrosion resistant alclad surface of the aluminum is also removed. Therefore any area that has been sanded for scratch removal
must be primed.
RIB FLANGE FACETING
As a skin wraps around the leading edge of the horizontal stabilizer, vertical stabilizer, or wing, among others, the skin must pass
over the forward edge of the rib flange. See Figure 3. Shape the front edge of the flange to prevent this edge from forming a dent in
the skin as the rib is riveted in place. A similar effect will occur at the notch between rib flanges. During manufacturing, as flanges
form over the curved edge of a formblock, the ends or surface of the flanges may remain straight or flat rather than conforming to
the curvature of the formblock. This results in faceted, instead of uniformly curved mating surfaces. A skin riveted on top of the rib
would appear faceted and a bump would occur in the skin in the area of the notch between the rib flanges. Shape-deburr the edges
of the flanges especially in the area of the flange radius as required. See Figure 3.
PAGE
REVISION:
DATE:
VAN'S AIRCRAFT, INC.
05-03
RV-ALL
1
04/15/13
FIGURE 3: RIB FLANGE PREP
SHAPE EDGE OF EACH
FLANGE TO REMOVE
"FACETING"
FLANGE RADIUS
FRONT EDGE OF
FORWARD FLANGE
