Dimpling the new tank skins - 3 hours

I waited thinking that my #19 reamer was going to arrive, but it didn't. So...I guess ill have to go to plan B on the tank skin holes. I decided to use 400 grit sandpaper on the dremel and open the #20 holes to #19. Its quite a tedious process...took 2 hours to do all the holes themselves. I know that I am deviating slightly from my test holes by increasing to 400 grit, but I feel confident it will be ok. The abrasive wears down quickly anyway to its only 400 grid for the first hole or two, but then it smooths out.

I then used my dimple die and made sure that each hole was large enough for the pilot, and re polished holes as needed. The benefit to all this was that there were virtually no burrs using this method.

I sprayed the piece down with WD40 and lubed the dies. I started to dimple with my stomach in my throat. In the end. Every dimple was perfect! The dimples did have sharp edges though as the holes are so crisp. I'll have to do something about that

I used a maroon scotchbrite pad on the dremel and buffed each dimple.  

Testing dimpling 0.032" thick material for #8 screws - Day #2 - 3 hours

Well tonight, Good news and good news. The replacement wing tank skins arrived from Vans. And I gave it another shot on the old wing skins and all 42 test holes where successful.

Using the dremel. I wrap a small square of adhesive back 600 grid sandpaper on a mandrel that i made (Cut the brush end off of a dremel accessory). I then insert it in the hole that I've drilled and while turning on the dremel I move it in a circular motion to help ensure that I do not make an oblong hole.

 
Using the felt tip and polishing compound to debur edges. I found after a few holes that this method doesn't really do a great job to remove sharp edges of debur. I don't think that I will repeat this process on the new skins.


 One old wing skin dimpled like mad!!!! Lots of test dimples in this one. but I think I have found the secret. Now to apply it to the new skins.
 
How I'm checking for cracks


I was glad to see that the new skins arrived, but I do have some knots in my stomach as to my ability to dimple these holes. Not wanting them to sit in the box too long, I opened them up and set them on the bench.


If anything, I thought that I should at least start advancing them. I may feel shaky about dimpling, but I am confident that I can at least drill out the holes. So I de-vinyled the edges and dug out the drill.  

Drilled all holes to #20 slowly working my sizes up starting with a #30 bit. No I await the #19 reamer to arrive.

Testing dimpling 0.032" thick material for #8 screws - 3 hours

So I have been practicing for the last two evening trying to solve how to eliminate the cracks. I've been on the phone with Vans for their advice, talked with friends, etc. Not to mention laid awake a bit a night.
For me, it doesn't matter if the hole is #19, #18 and #17, it can crack. That's not to say that going up a size or two on your pilot hole doesn't relieve some pressure, but I found that it wasn't the complete answer.

Just for kicks I drilled some #18 and #17 holes to see how it would go. I deburred the holes and dimpled them. They cracked.




So, for me that kinda solved the hole size issue. I did notice that there was a difference in hole sizes after they were squeezed. Its a bigger pilot hole, and after squeezing, surprise surprise, the hole in the dimple was larger too. Seeing that it did not eliminate cracks...I resolved to focus on the #19 holes. What seemed to be the common denominator in all my cracked holes was how well/clean the holes were drilled and the condition of the deburing. I holes were good, the cracked dimple was 1 in 28. Holes that were not as clean I got 1 in 10 cracked. It's easy to be sloppy with the drill in opening up the holes and its very easy to debur too much. My 3-flute deburring tool does not do a nice job when looking under a magnifying glass. The one hole debur tool is better, but its not perfect. 

I carefully drilled all the tank skin holes (39 test holes) up to #19. I started #25, #22, #20, then #19 (This process gave me brain damage the whole way...its not a great way to enlarge holes this way) Once I was done I then used a small piece is 600 grit sandpaper on my Dremel, (Just a small piece wrapped on the mandrel. I carefully polished the holes (It took a long time to prep the holes) but they looked great.

Here is what a marginal hole looks like. This is likely to crack. 


This is a #19 hole that was final polished with the 600 grit sandpaper.



 At the suggestion of a friend to lube the dies, I sprayed WD40 on the work and the dies and started squeezing. All 39 holes made beautiful dimples and no cracks!


For me this was a bit of a win. I feel confident that I am on the right track, but I still want to perfect the process. I'm still waiting for a #19 reamer to arrive and I may abandon the deburing tool completely. As you can see from the pic its not a perfectly clean edge on the dimple, they are some very tiny jags on the edge. I may use the Dremel felt and polishing compound to debur the edges instead.
 I am one step closer to solving this.

Practicing safety wire

My buddy Mike came over to put some work in on his Stewart S-51 Mustang. we mad some good progress and hit a small roadblock. The usual story for a build.

After we wrapped up, Mike showed me the ropes on how to use safety wire.
Step #1

 Step #2

 
 Step #3

 Step #4

Here is my go at it. My very first safety wire job. I still need practice.

And here is Mikes attempt with just using his hands to twist the wires...No Pliers!

Researching dimpling 0.032" thick material for #8 screws

I was floored by yesterdays outcome. Really took the wind out of my sails. I really hate the taste of defeat. It's easy to dwell on the negative I suppose. But...The bright side is that I did not rivet anything yet. So...Not a complete loss.

I called Vans this morning to talk cracked dimples. The conversation was...interesting. The answer is not black and white. Start over, or build on. Vans thinking is this....Its your choice what to do. Replace, or keep going. At the end of the day, what are you comfortable in doing.

One way to look at it is that cracks are not a good thing. But....this area in particular is prone to develop cracks, due to how much pressure the skin is under to form the dimples. So as far as Vans was concerned....Its up to me how to proceed. He did indicate that in his personal observations he hasn't seen a crack propagate out of the screw hole. Not to say it doesn't happen, its just that he hasn't seen it yet.

So...Now. Personally what do I want to do. Well, as much as it sucks... New skins are on order.

The question is how do you avoid cracks. Again...I asked Vans how they felt about the various options that some of the forums discuss. I asked about drilling to #18 or #17, then dimpling. I was surprised that he did not feel this was a wise course of action. His opinion was that opens the door for the dimple and hole to be out of alignment. This could cause the screw to sit improperly in the hole, which in turn can cause issues of their own. He didn't say no, but I could tell if it was him, that's not his choice. He did feel that the #19 hole was the correct course of action. I had also asked him about quality of dies and he did not feel that this was important. As long as the dies are cleveland, avery, plane tools, etc. I should be fine.

He did say that hole preparation is the key. He recommended polishing the hole....Really do my best to alleviate potential stress points before I dimple.

In talking with friends, some mention drilling the hole larger, some don't. One suggestion that seems very logical is to lube the dies to help the skin from being "grabbed" by the die. Mineral oil is suggested. Maybe WD40 will work.

Well...I know this. I have a TON of material to practice on with two botched tank skins. I think mentally I have to be prepared that I may end up with a crack. My goal....is to minimize this as much as possible.

New skins are set to arrive on Wednesday and a #19 reamer will come shortly as well. So I will do every sort of dimpling scenario in efforts to prove to myself how to dimple the thick material and not crack it. When the new skins come...I will dimple first, and then worry about all the match drilling and countersinking that I have to redo.

Drilled nutplates on W-423 joint plate. Dimpled #8 screws on tank skin - 3 hours

Using a nutplate jig I drilled the W-423 joint plate for the K1100-08 nutplates. I drew a line down the center of the #19 holes to provide a reference. I then used the jig to drill the holes.


Then, I dimpled them with the #8 screw dies. I was sure to use scotchbrite on the holes to be dimpled. The goal was to smooth out the hole as best as i could to mitigate the risk of cracks. After the hole was dimpled, I went over the hole with scotchbrite again to smooth the hole.

Everything turned out well. So I decided to give the tank skins a shot. I started off with the T-701/W-423 attachment holes. I repeated the process, but low and behold...I have 1 cracked dimple. Crap...
So I carried on. I completed the dimpling the spar attachment holes...About 1 hole in 10 is cracked. Crap! Not sure what to do now.

Test piece for #8 dimples - 1 hour

I've been reading about potential cracking of #8 dimples in the tank skin. I've decided to make a test piece out of material if the same thickness. I've drilled the holes #19 and some #18 to test various methods. I have chose to use my alligator squeezer to dimple the holes. I decided that it is in my best interests for a fast dimple rather than go slow. I want to avoid work hardening the dimple by going slow.

Both the #19 and #18 holes dimpled perfectly. No Sign of cracks, although the #19 holes were a smaller diameter after being dimpled.


I decided that with no signs of cracking on the #19 holes (and a smaller final hole diameter) i'll proceed in that direction.