Figure 13-65.—Transferring rivet holes.
may be determined in either of two ways. The
dimensions can be measured during the inspection, or
the old skin can be used as a template for the layout of
the sheet and the location of the holes. The second
method is preferable and more accurate. Regardless
of the method used, the new sheet must be large
enough to replace the damaged area, and it may be cut
with an allowance of 1 to 2 inches of material outside
the rivet holes.
If the old sheet is not too badly damaged, it should
be flattened and used as a template. The new sheet,
having been cut approximately 1 inch larger than the
old, should then be drilled near the center of the sheet
by using the holes in the old sheet as a guide. The two
sheets are then fastened together with sheet metal
fasteners. The use of sheet metal screws is not
recommended since they injure the edge of the rivet
holes. The drilling should proceed from the center to
the outside of the sheet. You should insert sheet metal
fasteners at frequent intervals.
If it is impossible to use the old sheet as a
template, the holes in the new sheet should be drilled
from the inside of the structure. Use the holes in the
reinforcing members as guides, and insert fasteners at
frequent intervals.
This process is called back-
drilling. Before you place the new sheet on the
framework to drill the holes, make certain that the
reinforcing members are aligned and flush at the
points at which they intersect; otherwise, the holes in
the new sheets will not be accurately aligned. For the
same reason, the new sheet should have the same
contour as the old before drilling the rivet holes.
To duplicate holes from reinforcing members to
the skin, you must exercise extreme care or both
frame and skin will be ruined. Since most bulkheads,
ribs, and stringers depend on the skin for some of their
rigidity, they can easily be forced out of alignment in
the drilling process. The skin must be held firmly
against the framework, or the pressure from the
drilling will force it away from the frame and cause
the holes to be out of alignment. This may be
overcome by placing a block of wood against the skin
and holding it firmly while the drilling progresses.
Also, make sure that the drill is held at a 90-degree
angle to the skin at all times, or the holes will be
elongated and out of alignment. When you drill
through anchor nuts, a smaller pilot drill should be
used first. You must use care so as not to damage the
anchor nut threads. The pilot holes are then enlarged
to the proper size.
It maybe necessary to use an angle attachment or
flexible shaft drill in places where it is impossible to
insert a straight drill.
In case neither type can be
inserted, the new section should be marked carefully
with a soft pencil through the holes in the old section.
Another method of marking the location of the new
holes is to use a transfer or prick punch, as shown in
figure 13-65. Center the punch in the old hole, and
then tap the punch lightly with a hammer. The result
should be a mark that will serve to locate the hole in
the new sheet.
Still another way to locate the rivet holes without
a template is to use a hole finder similar to the one
shown in figure 13-66.
Figure 13-66.—Using a hole finder.
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