BY CHAD JENSEN, EAA 755575, EAA HOMEBUILDERS COMMUNIT Y MANAGER
IF YOU ARE GOING TO BUILD a metal airplane that uses flush rivets,
you will need a variety of tools to put dimples in the skin so the rivets sit flush. One tool that comes in handy is a C-frame dimpler.
These are available for purchase at many different tool outlets, and
they typically range in price from $150 to more than $300. They are
great tools; I built one of my own in the early stages of my RV- 7 project, and it worked flawlessly. After my RV was finished, it became a
loan-out tool to my friends and local builders.
The beautiful thing about the particular C-frame that I built is
that it didn’t require any welding. The C-frames that are available
for purchase have a welded top and vertical tube, attached to the
base with bolts. When I built this one I hadn’t yet learned how to
weld, and when I saw a version of this on a forum somewhere, it
sparked the idea to build one for myself. Steel tubing has gone up
in price since I built mine, but at the time (2005), I spent all of $22
on steel and hardware. The dimple die holding sets ran another
$25 or so.
First figure out how wide your widest metal skin is. This will give
you the throat depth for your C-frame. That dimension should equal
1 inch longer than half of the width of the skin. So, if your skin is 36
inches wide, make the throat dimension 19
inches to cover each half of the skin with a
bit of room to spare.
Once you have that measurement, buy
your steel. You’ll need a base for the frame
that is made from thick C channel. I used 1/4
inch. The rest is made up of square tubing,
and you’ll need the same length of top tube
to match the base. The top tube I used was
1 inch square, and the vertical support tubes
were 3/4 inch square, with an inside
diameter large enough for the 7/16-inch
bolts to fit through them (bring a bolt with
you because different wall thickness will
make a difference). The height of the vertical
posts is somewhat arbitrary, but 6 inches is
what I used.
The base of the frame should have the
front end cut at an angle to allow for bent
skins to fit up against the base to get at holes
near the bend. I put some felt along the front
edges of the base to protect the skins. The