Friday, April 5, 2013

Wing Tip Ribs - Dimple Dilemma Solved – Thinking Outside the Box And Then Some

Time was spent in the shop Thursday not so much for doing RV-12 assembly, but taking care of lose ends and solving a troubling dimple dilemma that required resolution prior to wing tip closeout taking place.

One lose end was the fit of the landing light mounting ribs. I believe in a previous post, it was mentioned that the wing skin tended to pull in a little when securing the landing light ribs to the wing using Clecos … this was because there was a small gap between the landing light mounting ribs and the wing skin. It was not too bad, but bothered me for I would prefer not having the airfoil being distorted at all, if it can be helped.  The issue was resolved more to my liking by playing with both landing light ribs until a uniform gap was obtained. Then shims were made out of 2024-T3 aluminum .050” thick to close the gap between the ribs and the skins. Once the shims were drilled and dimpled, they were primed and are now ready to install. The rivet holes in the W-1203L outboard skin for mounting the landing light ribs were then dimpled. The landing light components are now ready for the landing light to  finally be permanently mounted.

The next task was dimpling the most challanging component of the build ... the wing tip ribs. The effort to dimple for flush rivets on the RV-12's outer skins has thus far been, for the most part, a piece of cake … time consuming to be sure, but not difficult. However, the wing tip ribs posed a huge challenge and required an unconventional solution. Yes, there have been a few tight spots requiring creativity such as the C-Clamp method. Last Fall when I was fighting to stay ahead of the cold temperatures and priming the components for both wings, I could tell just by looking at the W-1204E & W-1204F forward and aft wing tip ribs that they were going to be … challenging. Because time was tight, I elected to forge ahead and primed the parts figuring a solution to the problem could be worked out eventually. I’m happy to report to other RV-12 builders willing to take on the challenge of dimpling their RV-12 project, that even the wing’s tip ribs can be successfully dimpled without the need to deform the metal to gain access.

Over the last couple of months, the DOG Aviation think tank had been using the few working brain cells remaining to analyze different scenarios that could possibly be used to complete the dimpling of the two outer wing tip ribs. The tip ribs are the termination point for the wing's upper and lower tip skins. Each of the tip ribs are shaped differently and as such, required a different approach for successful dimpling.  The method of using a nail, with the special dimple dies that have holes drilled in them for a hand rivet puller would not work ... just not enough clearance for a long nail to be placed into the rivet hole.  Yes, I could have used the C-clamp method I came up with (and successfully used previously), but there were lots of holes to do and I really only want to use that method when there is no better options … because the dies tend to rotate as the clamp is tightened, which is not good on the primer or metal. So for me, that approach is a last resort method.

Thinking outside the box – dimpling of the W-1204E & F ribs. Because a lot of thought went into to the problem, the creative dimpling solutions utilized for each rib worked almost flawlessly.  One side of the W-1204E-L rib could be done with the pneumatic squeezer as usual, but the angle of the other side prevented access. For dimpling the one side where conventional squeezing would not work,  a “no hole” yoke (a type of yoke with no hole that acts as the rivet set and compresses the rivet directly) in the pneumatic squeezer. The “no hole” yoke has a thin nose which fortunately barely fit into the area being dimpled along with a female dimple die.

                             Tape was used “stick” the female die onto the no hole yoke on the pneumatic squeezer.

The yoke pulled in on the female dimple die from behind as the pneumatic squeezer ram outfitted with the male dimple die pushed into the metal being dimpled from the front. The point of the male die holds alignment because it slips into the female die when the dies are squeezed together as the dimple is formed. Plus, the tape once compressed from the first squeeze held the female die in place and it never fell off during the dimpling of the remaining holes.

The W-1204F-L tip rib was the huge challenge because it has a very narrow channel so there is little space to work with. To give the readers an idea of the clearance, it is tight for a finger and if a pop rivet were placed in the upper and lower holes they will touch one another. Here again, the C-clamp method could have been utilized, but the dies tend to twist as the C-clamp is tightened and will remove primer … plus, there are lots and lots of holes so a better way was required. Because the upper and lower holes are aligned, it presented a unique opportunity which played a large role in the ultimate solution.

Thinking way way way way outside the box resulted in the crown jewel of the project to date … as far as creative tool usage goes. (I’m still patting myself on the back for coming up with this idea.)  Taking the adage from one of the recent AT&T TV ads where “more is better” the DOG Aviation think tank applied that to dimpling the W-1204F-L wing tip rib by using TWO  female dies  back to back INSIDE the rib’s channel and placing a “carefully crafted” nail through the dies to hold centering. A standard yoke was used WITHOUT a rivet set so the alignment nail went up into the yoke's hole as the male dimple die penetrated the metal being dimpled. The head of the nail was first ground down to allow it to pass into the hole where rivet sets would normally be attached to the yoke … yet enough of the nail head was left so it would not pass through the holes in the rib and would also keep a washer in place. The length of the nail proved to be some what critical … it had to be long enough to keep the parts together yet allow the yoke to be positioned in place. Once the yoke's hole was positioned over the nail, a finger nail was used to push the nail alignement pin up into the hole in the yoke clearing the way for the male dimple die on the pneumatic squeezer to slide into one of the female dies inside the rib's channel.
This initial setup worked well for setting dimples in the W-1204F aft wing tip rib, but needed refinement … the tape made it difficult to remove the dies after the dimples on both sides were set.

Because the rib was actually squeezed from the outer sides, a washer was placed on the nail for the yoke to push against, thus preventing marring the outside surface of the rib as the male dimple die attached to the pneumatic squeezer's ram compressed the opposite side of the rib into the female die. The two female dies inside the channel of the W-1204F-L wing tip rib were almost a perfect fit … but still a little loose, so one washer was used to take up the additional space so the rib would not deform while the dimples were being created.

The beauty of this idea was, once the first hole was dimpled, the nail was removed and then slid in from the other side so the opposite side could then be dimpled without the need to reposition the dies. Brillant! This idea worked great because both the female dies were positioned facing towards the outside. It was a joyous moment at DOG Aviation when it actually worked as envisioned!

                             The components used to set the dimples in both sides of the W-1204F-L wing tip rib.

The first two side to side dimples were made with the two female dies stuck together with tape as depicted in picture two photos previous. At the time, this seemed like a good idea for easy install/removal of the dies. However, once both female dies contained dimples, pulling the dies out together proved to be problematic without flexing the metal. So a slight procedural modification was made which worked out perfectly. Basically, the washer taking up the gap was moved in-between the two female dies so after each side of the rib was dimpled, the washer was removed allowing the dies to be easily removed one at a time. This method worked great!!!!! … And did not mar the Akzo primer, so that method was proudly used with much glee for all the remaining dimples.
The final setup used to set all but the first two dimples in the W-1204F aft wing tip rib. Two female dies placed back to back inside the W-1204F-L wing tip rib with a washer placed between them for spacing and easy die removal!
Setting dimples into the W-1204F-L aft wing tip rib with the pneumatic squeezer using the previously described setup.

Think it is finally time to actually begin closing out the wing tip!