Sunday, July 1, 2012

Necessity – The Mother Of Invention

Dimpling the rudder ribs (and other control surfaces) will be a challenge but with a little creativity, it can be done without relieving the bends dimpling then re-bending the ribs which I feel is the wrong path to go down.

Earlier today I took the time to work through a small issue that I knew was going to arise at this point in the construction … because of my decision to dimple the parts for flush rivets. I remembered from working on Pete’s RV-9A that the last couple of rivets on the trailing edges of the control surfaces can be tough to get at for dimpling. I think the RV-12 is even tighter yet. The last two holes on both sides of the narrow end of the rudder ribs will require special treatment … because there is no room for the usual methods of dimpling. Even while work was been done on other items there was a lot of thinking going into what sort of jig or tool would work to dimple the small end of the ribs that make up the control surfaces. Using scrap metal, my first easy idea of squeezing the dies together with a small vice grip (no room for a regular vice grip) did not work well … was not able to get achieve enough pressure to get a deep enough dimple for a flush rivet to sit flat … scratch that idea.

My second idea (which was actually my first but required grinding metal off a tool so it became my second choice) turned out to be the best overall. A small 1/8” nail was cut in length to go through the 120 degree dies I had custom made with holes in them. The length of the nail was made just shy of the total depth of the dies when placed together. The purpose of the nail is to keep the two dies centered on the hole being dimpled when they are squeezed by a C-clamp.

The C-clamp needed a portion of the back of the clamp ground away on a grinder to allow for clearance at the very narrow space available due to the taper of the rudder ribs. Fortunately, Van’s offset the two rivet holes at the narrowest point on the rudder ribs … because of this nicety, the dimples at the narrow end of the ribs won’t create a clearance issue and interfere with one another after dimpling. That said, should you decide to build your RV-12 with flush rivets, I would strongly suggest dimpling the hole with the least amount of clearance first.
                                 My crude but effective way of generating enough brute force to dimple the ribs.
               The short nail stub keeps the dies aligned with the hole while the C-clamp squeezes the dies together.

Clicking on the photo above to make it larger you can see the first dimple I made by placing a nail into the dies to hold alignment, then squeezed the dies together with a C-clamp. The C-clamp had material ground out of the back side to gain enough clearance to fit into the small opening at the narrow end of the rib.