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Dimples dishing after riveting

Working on my -14 tail cone bottom skin and noticed that I have some dishing showing up after I rivet. The dimples were super clean as you can see from the not yet riveted holes on the bulkheads and the edges of the sheet. Then after I hammer a rivet it seems to pull the skin in slightly. It may just be the lighting as you can only see it at certain angles. I haven't seen this show up so noticeably on any of the tail feathers. Is this normal? What might I have done wrong here that didn't show up elsewhere?

Ribs and stiffeners were dimpled with squeezer and skin was done with c frame and hammer (as were all other parts I've completed to date).
 

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It appears that the underlying structure was dimpled/countersunk too deeply.

Or possibly too much pressure on the gun (Hand pressure, not air pressure).

Have you tried back-riveting here?
 
Hey Mel, Thanks for the reply. I'm using a pneumatic squeezer for dimpling the ribs and it was set up in the same way as I have been doing to date without issue... So I'm not sure how it could have been dimpled too deeply. Also the stiffeners were tightly against the inside of the skin.

I haven't tried back riveting here only because the surface is slightly curved, the bulkheads would be difficult to manage if inverted, and this is how the manual calls out building the assembly.
 
maybe too much pressure

It could be too much air pressure on the gun or the needle valve on the gun needs to be turned in to limit the air flow.

Too much air pressure means the gun is striking with too much force and bending the underlying structure.
 
After dimpling, do you see a faint witness mark (circle around the dimple)? If no, you need to hit it harder (or adjust the squeezer to squeeze harder). It’s pretty easy to under-dimple. That’s my guess.
 
I think some slight dishing is normal for rivets that are hammered in thinner aluminum. Are you comparing these rivets to other rivets that you back riveted? Back riveting will give you non-dished rivets.
 
Another potential cause might be you are applying more force (pressure) to the rivet gun, than is being applied to the bucking bar.

Its a dance, apply a bit more force with the gun initially to hold the rivet in place, than follow up by equalizing the force (pressure) you apply to the bucking bar, finishing with gun & bucking bar pressure neutral if you follow what I'm trying to describe.

Good luck
 
Too much pressure would be my guess. Everybody’s compressor is different, but on mine, if I use more than about 35 psi on -3 countersunk rivets for skin to rib flange, I’ll see impressions like you have in your pics. You could try reducing pressure, and then turn it up slightly if it’s taking to many hits to form the shop head, until you get the right setting.
 
“Pulling in the skin” is classic example of when to add a shim. The rule is, if you can fit a shim in between the skin and the rib, it needs a shim.

Look at the curved top skin on RVs. If there are flats where the rivets are then there should have been a shim added.

Make the shims out of 0.020”, 0.025” and 0.032” scrap aluminum. Perhaps 3/4” square. Drill a #40 hole in the middle and dimple. You don’t need all that many for a project.

Carl
 
I have a different take, based on the photos - but I really wish I could inspect in person.

Work this out on scrap, first.


Try getting a fresh rubber mallet and, backing the shop head with a bucking bar (bigger the better) lightly and careful to make contact flush with the outer skin, "draw" the dish back to level.
 
Dimples

Post a photo without the clekos.
I would bet that's under dimpling.
 
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If the backside is accessible, take a wooden dowl and a hammer and lightly tap each dished rivet just enough to bring the surface back flush. This will not solve the root cause of the problem but it will make the appearance more pleasing to the eye.
 
I have the exact same issue with mine. Glad to see I am not the only one. Sorry to see that I am not the only one. I will be redoing mine before too long.

"I built it right 'cause I built it twice"
 
Plenty of opinions here. One of them could be right……. maybe. But all of them could be wrong. I say build on and don’t worry about it. Use your best judgement. Cosmetically, it matters. Technically it doesn’t. Try some of these suggestions if you want. They might work, or not, but finish it, paint it, fly it, and it won’t make any difference going forward, including when you want to sell it (I wouldn’t recommend that). A buyer has more important details to consider than if the skin rivets having a slight impression (which they won’t notice). They look at the paint job (90%). For you personally, you’ll stop noticing this after you have a couple hundred hours on it and other wear things start showing up. I say, fly the **** out of your airplane and don’t worry about it.
 
My 2 cents worth.Do you have a local builder that can spend an afternoon with you to help you with proper dimpling, pressures both hand and compressor etc.?
That would be the best way to figure out what’s going on and correct it.
I don’t agree with just build on and to heck with it. Take pride in what you’re doing. I don’t agree with putting shims between skins and substructure. Totally not necessary once you know how to fit the parts together as Vans designed.
Learn what a proper dimple looks like, check all flanges to webs are 90 degrees, proper bucking technique and back rivet whenever possible. I back riveted the entire tail cone on my work bench.
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The holes in the skin were likely under-dimpled. When using a C-Frame dimpler be sure you are hitting at least twice for each hole, the second with a firmer wack than the first. You can hear the difference. If you are using a DRDT=2, adjust the dies closer together. Dimpling a hole requires the aluminum to stretch at the hole. A riveted hole is going to stretch from somewhere so it's pulling in the metal outside of the under-dimpled riveted hole.
 
thanks all for the knowledge and advice.

A couple things. I'm 95% certain there is no under dimpling in the skin or the substructure. As I've done my whole tail the same with no issues. And yes I see the witness ring from the dies.

Also the skin lays perfectly flat on the substructure. No gap before or after riveting. And it doesn't seem that the gun is bending the flanges but it hard to tell.

The suggestions that make the most sense to me is that I may be putting too much pressure (hand pressure) on the gun or maybe also the compressor is set too high. This could have drifted in adjustments over the months. I'll try lightening both up and also resort to back riveting on the more visible side skins.

It's really not that severe but I wanted to make sure it wasn't a larger issue than cosmetics. I tested some paint over some of the dishes and they all but disappear. Bare metal brings everything out in the right light.

Thankfully its on the bottom! My plan is to build on.
 
Isn't some amount of dishing kind of normal? The only time I've been able to avoid it is while back riveting. I used to stress about it until I saw all the polished mustangs at Oshkosh.
 
c-frame underlying surface

I saw this same thing on my first vertical stabilizer (early 7a kit). I found out it was caused by using my c-frame on a work bench. When I move it to the concrete hanger floor it did not do it anymore. The work bench had enough bonce in it to cause the dimple not to be crisp.
 
Bobby, that is an interesting thought. I did do this on a concrete floor, but in a different spot than I did on other surfaces. This is important because the concrete must have been softer or has a defect because after this dimpling session I had made grooves in the concrete with the rails of the c frame. maybe I wasn't getting supper crisp hits....
 
My 0.02 as a repeat builder using the C-Frame for all skins.
Looking at the OP's second picture where the skin dimples can be seen, I see some very lite witness rings on some dimples but not all. I might also surmise that the dimples could be more crisp and would be curious to learn more about the hammer used, number of strikes/dimple, and estimated strike force. As another poster mentioned, the striker must listen for the tone to change. The tone change is very easy to identify and is generally a good indication that the dimple is complete. There should be a significantly pronounced witness ring. It is difficult to over-dimple. Quite possibly 0.03
Edited... I do acknowledge that there look to be significant witness rings visible in the first picture.
 
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C-frame

Bobby, that is an interesting thought. I did do this on a concrete floor, but in a different spot than I did on other surfaces. This is important because the concrete must have been softer or has a defect because after this dimpling session I had made grooves in the concrete with the rails of the c frame. maybe I wasn't getting supper crisp hits....

I always set up my c-frame so the business end is right over the legs of the bench. Then I hit it really hard twice with a 2lb dead blow. I destroyed four Harbor Fright dead blow hammers over the build. Lifetime warranty! :D
Some use a 2lb steel hammer and hit twice. You really have to get jiggy with it.
The other issue is the part needs to stay perfectly in plane with the dies or it will cause a dish. I ruined a top skin that way. My dimpling shelf wasn't properly shimmed. It's a 4'x4'x4" box. I forgot to shim it so the skin lays pefectly in line with the dies. That may also be what you're experiencing.
 
<snip>...
The other issue is the part needs to stay perfectly in plane with the dies or it will cause a dish. I ruined a top skin that way. My dimpling shelf wasn't properly shimmed. It's a 4'x4'x4" box. I forgot to shim it so the skin lays pefectly in line with the dies. That may also be what you're experiencing.

Did this myself - forgot to add the washer shim under the bottom post in my c-frame once, and it caused the female die to be just ever so slightly below the plane of the sheet. :mad:
 
When I did the aft fuse I back riveted the F-01486 stiffeners to the F-01473 side skins before clecoing them to the side frames. They can be inserted into the side frame notches after riveting. I can’t remember if I did the F-01486B but definitely did the upper stiffener. Just cleco them to the side skin and dry fit them to check for your self.
 
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