EAA Sport Aviation - January 2012

hole in the base for the bottom set is 3/4 inch to 1 inch from the edge and centered in the channel. Measure your throat depth back from this hole to locate the front vertical tube. The front face of the tube should be at the exact distance you noted earlier to set the throat depth. Use a small 1/2-inch piece of scrap tubing to mark the post and bolt-hole location.

Using a 1/2-inch tall piece allows your marker or punch to reach the channel easily. The second vertical post is simply a stiffener for the frame, and its position doesn’t matter much, so just make sure it’s perfectly in line with the front set hole and the front post. Once you have the holes located on the base channel, drill them out with a pilot bit and use them to transfer to the top tube after marking a centerline on the top tube.

The holes for the vertical support post bolts only go through the bottom of the top tube. The front hole for the spring-loaded set needs to go all the way through. Don’t drill this hole unless you have the set on hand to make the proper size hole. A hole that is too large will allow the set to wobble, and this will open up opportunities to damage the material being dimpled.

Once you have all of your holes drilled to full size, insert the front 7/16-inch bolt through the bottom of the base channel, slide the front vertical post over the bolt, then put the top tube in place. An extension magnet is useful here in getting the threads started because access down inside the tube is obviously too tight for fingers.

Once the nut is started on the threads, use a flat screwdriver stuck down the tube to jam the nut in place while you tighten the bolt end with a socket or wrench. This is the difficult one, and once it’s done, the second vertical tube can be put in place and the process repeated. The nut on the rear bolt is much easier to access to tighten it down, but it will still need to be tightened from the bolt side (underside of the C-channel) in the same fashion.

The sets can then be put in place, and there are several options for the base set. I have one that sits down on the base for dimpling skins, and another set that sat on top of an extension so I could dimple channels. The top tube set can either be spring loaded or free falling. To make it spring loaded, simply add an adjustable collar and then install a spring between it and the top tube. You can set it at whatever height desired.

Once materials are on hand, this C-frame can be constructed in less than one hour. You may want to take this even further and build a C-frame dimpling table, but that’s a project for another time. Don’t forget to paint the C-frame to protect it from the elements.