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Need help with numerous cracks on dimpled tank skin!

AX-O

AX-O

Well Known Member
My second mistake so far, but my first major one.

I dimpled my tank skins with #8 dimple dies per Vans instructions. The #8 screws were not sitting completely flush on the skin so after all the holes were dimpled, I squeezed them again with the dies a little closer to each other. The first few holes looked good so I continued and re-squeezed the rest. After completing both tank skins I went back to check my work. To my disgust, I saw cracks on many of the dimples. The cracks start at the drilled hole and stop before reaching the flat part of the skins. My stomach has been turning nonstop. I can?t stop thinking about it. My questions are the following. Can I drill the holes wide enough to get rid off the cracks and then use #8 dimpled washers? Would that be strong enough? Would that give me enough edge distance? Should I just bite the bullet and buy 2 more skins (approx $100 each, plus S&H, pull fees and redo all the completed work)? Would that add a lot of drag and just look stupid?

I will call Vans later today.


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I had this happen in a different (non-visible) area at the wing roots.

The only explanation that I could come up with was the low temperature (40F), but that seems weak.

Since they were non-visible, I stop-drilled the cracks and declared victory.

Vern Little
 
I think you should order new tank skins. I would expect the cracks to lengthen with time in service. Using dimpled washers would be quite hideous.

What size did you drill the holes before dimpling? Were the holes deburred?
 
Kevin Horton said:
What size did you drill the holes before dimpling? Were the holes deburred?
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The holes were deburred and I followd Vans directions. I can't remember the drill bit size (don't have the directions infront of me).
 
I can't imagine shop temp being the problem. My first thought upon seeing these pics was that the drilled hole was too small or perhaps not deburred. Too small a hole will definitely cause cracking. As for the fix, I'd say you gotta do it over. The screw is going to put that crack in tension and just rip it right open.
 
Dimpling

Just thought I would ask, did you use rivit dimple dies or screw dimple dies? The skin was just streched too far, I agree with one of your replys that the maybe the hole was too small, also coming back later and dimpling them again may have work hardened the skin. But to get to it, the only fix is new skins, with the pre punched it won't take very long and a lesson learned. I too have made things over, nothing new to anyone.

Happy riviting.

RT
 
SportAvServ said:
Just thought I would ask, did you use rivit dimple dies or screw dimple dies?
RT
Click to expand...
I used Avery #8 dimple dies and a pneumatic squeezer.
 
Are you sure that your dimple dies are 100 degree? I have found that you cannot dimple 2024 to 82 degrees, which is a standard non-aviation countersink.
 
Just called Vans. The gentleman I spoke with had actually looked at the picture on this forum. He told me to leave it alone and keep working. No need to replace the skins or stop drill it. He said that as far as safety goes, it is fine. The tank is supported in many other places. I told him my concerns about the crack making it all the way to the edge of the skin. He said the worst that can happen is the crack runs to the edge of the skin and it would not be appealing to the eye, but no safety issues. He said he has never heard of a builder using dimpled washers to fix the problem but he knows of cracks on tank skin dimples. He said that the #8 dimple stretches the aluminum a lot.

He also told me that he personally knows of an RV-6 that has been flying like that for the past 12 years and the holes have not cracked yet :eek:

All that said. I may still replace the skins. $200 may equal peace of mind.
 
If you do reorder ... make sure you order the right skin part #.
I thought about replacing one tank skin and ordered a replacement for Partain to deliver with the fuse kit; I ordered it based on the part # on the plan. Unfortunately the page I looked at had the older non-prepunched part number on it and so that's what I got! I don't have the plans / part #'s in front of me now but just a head's up...

Thomas
 
One question?

When you did the initial dimpling how did you do it, and did the pilot on the male die slip easily into the drilled hole or did you have to force it in or drive it in (assuming they were initially done with a C-frame tool).

If the pilot does not freely fit the hole it has a strong tendency towards cracking.
 
I ordered a T-701 L and a T-701 R today. Call it peace on mind. Gave me an excuse to order a calendar.


I have been talking with my girlfriend about making a bar for the house. I wanted to build something that looks like a wing for the top. I guess I now have a very expensive bar top.
 
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Be sure to use a #19 drill bit to enlarge the holes before dimpling with the #8 dimple dies.
 
Dimple Cracks

Axel...

I do not use a pneumatic squeezer but I have seen cracks very similar to this when the squeezer frame runs into some structure, a rib, sparr or whatever, while the squeezer jaws are closing causing the dimple die to be forced slightly sideways in the hole as the dimple is being formed. This can produce a very small crack on the "outboard side" of the hole. This is not very noticeable but if you use a magnifying glass and look at the back of the dimple you can see the crack. I know the leverage from a hand squeezer will make this slight side pull very difficult to feel and, with a pneumatic unit, it would probably be impossible to feel. Now I have some dies and a squeezer yoke ground down on one side for additional side clearance just to prevent this in tight situations. I noticed slight side scuffing marks on some ribs which led me to the cracks I had made and they have not grown at all but I discovered this condition very early in construction. You may(?) have this condition. Stuff happens.

Dick DeCramer
RV6 N500DD
170 Flight hours
RV8 wing kit
Northfield, MN
 
dont think they would go any further,,,,,,,but

i dont think these would go any futher as they would be going to stronger thicker material, the screw will have them so clamped down i doubt they will ever do anything and probally never be seen. anywho, i am a freak and can never let things like this rest and would wake up a couple of months from now in a cold sweat and then order new skins and ribs and cap flanges and so on. go ahead and fix them now and its just two skins. my .02 :rolleyes: which is the rationalizing that cause holes in my wallet.

oh yeah that cold temp has caused some weird things to happen. but then ive done alot in the cold that didnt. however on this particular item you are at the threshold of cracking anyway now as the temp drops things are brittle and shrunk. i really doubt these would crack on a 90 deg. day. but i wouldnt wait. had rivets that would shear when squeezing one night at 40 deg. came back the next day at 65 and no problems hmmmmm. (they were on the aft deck and i was using the squeezer.)
also next time use the c frame so the dies are evenly loaded. (dont have a drdt)but that would even be better as it SEEMS to be less offensive to the material.
 
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I have a DRDT-2 and may use it on the old skins as a trial piece. If that does not work, then I will barrow a C-frame.
 
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