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Cracked dimples in J-stringer

G

gfb

Well Known Member
Hi Folks,

Was riveting the stringer behind the top skins on my left wing when I noticed that some of the rivets had small cracks in the dimples. I suspect I have a bit of tension in the stringer because of slightly high rivet holes.

Replace the stringer? Would involve drilling out 33 rivets on the top skin...
I count 7 dimples with cracks total. I'm thinking oops rivets?
IMG_3532.JPG
 
Small Shop Head

I think we need some more information. Did you drill the holes before dimpling? What pressure are you using?
Also, your shop heads diameter looks smaller than the dimpled hole, but that could be the camera angle.
 
Drilled to #40 before dimpling. Pressure for what? 90 psi at the compressor exit.

The two visible shop heads are ones we only tapped a few times as we noticed the cracks, plus the camera angle. They are not examples of the average shop head :)
 
I agree, the photo makes it look like something is seriously wrong here. Almost like the dimple die used was way too large. Dimples just shouldn't be cracking out like that. Add to that the j-channel doesn't look fully seated against the skin, and the rivets look under- (or inadequately) set, yet not long enough to set properly.
 
They are cracking when I buck the rivet. I looked at the next few holes down and it seems the holes/dimples on the stringer are just slightly offset above those on the skin, causing the tension on it which I believe is the cause of the cracks.

I'm thinking remove the cracked ones, drill to 1/8 and replace with oops? Thoughts?
 
Are you deburring after final drill and dimpling? Those rivet shop heads look too small, are you positive you're using the correct length rivets? Are the dimples too big? Almost looks like the shop head is filling the dimple and pushing them out.
 
az_gila said:
Is it possible that the stringer was dimpled with a #30 die by mistake?
Click to expand...

One of the possibilities I was alluding too. Run a rivet gauge over them and check the shop head. If dimpled too big, now that they are cracked I would drill out and replace, then go over ever other rivet and make sure you didn't make the same mistake.
 
Checked, all dimpled to #40.
Everything deburred after drilling, before dimpling.

Sig has an interesting idea. I wonder if because the hole is a little offset the rivet is filling under the dimple and cracking from pushing from behind.
 
gfb said:
Sig has an interesting idea. I wonder if because the hole is a little offset the rivet is filling under the dimple and cracking from pushing from behind.
Click to expand...

I would agree with this... If you are in fact using the right length rivet, the shop head on your "lightly" set rivets doesn't have enough material in order to complete the set. It looks like the taps that you have given them have driven the material back between the skin and channel.
 
I'm going to drill these out and see if there is a way to adjust my existing holes/dimples in the j-stringer without removing all ~30 rivets. I'd be happy to remove them but considering they are 95% good I worry about damage to the skin from the extra drilling. This is why I was thinking oops-rivet. It would allow me to open up the hole a bit, helping with the alignment... but now that I think about it, it would not re-align the dimple in the stringer.

Any other ideas?
 
rv6rick said:
To answer your question, replace the J channel.
Click to expand...

Yeah, this is the right thing to do so I should just do it. I'll carefully start drilling out rivets and see if I can do it without making any ovals.

Any pointers on how to position a final drill/dimple on the j-stringer considering the skin it attaches to is already dimpled?
 
Check your air pressure when you set these. For the small diameter rivets, you should only be using about 35 psi with a 3X gun with a mushroom set. If you are back riveting these with a longer rivet set, then the psi will need to go way up, since there is more metal in the rivet set to move with the air.
I agree it looks like the rivet length is too short for the holes. You should be using AN426AD3-3.5's and 3-4's under the wing walk doubler and where the J-stringers overlap.
 
Drilling rivets

Drilling #3 flush rivets is no problem.
Center punch the little dimple exactly in the center of the head.
Use a small drill bit like #46 to drill first. Set the bit on the center punched hole and turn it a few turns. Take a look. If it's not centered you can coax it where it needs to go easily. If needed tilt the drill a tiny bit to drill in the direction needed for a few turns then go back to perpendicular.
Run the drill a few turns slowly and look again.
Once satisfied it's going centered, drill the depth of the head.
Use a #40 with a drill stop set to only allow it to drill the depth of the head. Drill the rivet head.
Take an old #40 bit and pop the head off.
Use a small punch to drive the shank with light taps.
If you have access, a pair of side cutters can be used to grab the shop head and pull it.

Wish I knew this trick before doing the Service Bulletin on my HS. I learned it drilling the 500+ rivets to disassemble it to fix one egg shaped mistake.
 
Figured it out after drilling out the I-channel. The dimples were slightly offset from the skin which left an odd gap. The rivets had swished themselves into this gap, pushing up on the simple, causing the crack.

New stringer drilled and dimpled, waiting for primer to cure. Thanks all for the help!
 
Just checking....

Are you clecoing every other hole? If you are using too FEW clecos, the stringer won't rest tight against skin. If there is a gap, the rivet will try to fill that gap, thus pushing the stringer further away. Likely stating what you already know, just sayin....

GB.
 
J Channel

I agree with Rick, for what its worth, replace the channel, this does not even look like a close call to me, there is definatly a problem here with the process, that caused this, I would replace it.
 
Closing the loop...

So last night I drilled out the bad J-channel. Was easier than i expected to drill out so many rivets by adding a drill stop right at the rivet head depth. Below is the removed stringer. Note the folded over rivets inside! the dimple. This is clearly what was causing the crack.

New j-channel is about done with primer and I should be riveting it into place tonight. Test fit last night was perfect so this should go very smoothly.

Thanks again for all the help!

IMG_3533.JPG
 
bruceh said:
Check your air pressure when you set these. For the small diameter rivets, you should only be using about 35 psi with a 3X gun with a mushroom set.
Click to expand...
Yes, aside from whatever other issues there may be, 90psi is way too high to be shooting AD3 rivets. For that matter,it's way too high to be shooting AD4 rivets.
 
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Drilling or reaming?

I had the same going on, but only on reamed holes. Also the dimple dies where out of manufacturer tolerances.
The dimple dies where making bigger dimels than with a T-Dimple Dies (used for Tank-Skin).

I believe the main thing was due to reaming the material got hot and changed the hardness. Then during dimpling, the dimple cracked.

Interestingly, if I dimpled with a C-Frame dimpler, they where fine, but if I where using the squeezer, then about 10% cracked.

Replace the parts, check your dimple dies, deburr after drilling, then it should be fine.
 
UH OH.......I wish I had known those this. My compressor is set at 95lbs and never moves :eek:.

Don't believe everything you read on the Internet :rolleyes:

alpinelakespilot2000 said:
Yes, aside from whatever other issues there may be, 90psi is way too high to be shooting AD3 rivets. For that matter,it's way too high to be shooting AD4 rivets.
Click to expand...
 
gfb said:
Closing the loop...

So last night I drilled out the bad J-channel. Was easier than i expected to drill out so many rivets by adding a drill stop right at the rivet head depth. Below is the removed stringer. Note the folded over rivets inside! the dimple. This is clearly what was causing the crack.

New j-channel is about done with primer and I should be riveting it into place tonight. Test fit last night was perfect so this should go very smoothly.

Thanks again for all the help!

IMG_3533.JPG
Click to expand...

GB, Don't know if you did but draw a line down the length of the J-channel so you can tell when you're in the center. In that old one the holes are a little too close to the bend
 
rv6rick said:
UH OH.......I wish I had known those this. My compressor is set at 95lbs and never moves :eek:.

Don't believe everything you read on the Internet :rolleyes:
Click to expand...
I honestly have no doubt that you could set a good rivet at any pressure, Rick. A less accomplished riveter, however, can do a lot of damage at 90psi. Of course, its the pressure at the gun, not the compressor, that really matters.
 
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Av8torTom said:
GB, Don't know if you did but draw a line down the length of the J-channel so you can tell when you're in the center. In that old one the holes are a little too close to the bend
Click to expand...
Tom may be on to something here. I agree that it looks like the rivet line runs a bit close to the radius of the stringer. It's right at the point where the stringer starts pulling alway from the skin. This might be why the rivets were setting between the skin and stringer.
Good luck on the next go around.
 
I would suspect that the female dimple die had a hard time fitting where it was supposed to go without contacting the radius before being able to seat properly.

This would force the female die away from the radius. The male die would follow, and an angled or oblong dimple would be the result.

Pure conjecture, of course. If one had a sufficiently small female dimple die it might be able to fit.

It's hard to tell from the shadows in the first photo if the female die might have contacted and scratched or dented the inside radius.
 
The issue was the line mentioned in the the plans at 3/8" was actually too close to the radius and did not leave enough space for the dimple to properly seat against the dimple in the skin. This made the rivet expand in between the skins and push up agains the dimple in the j-channel when being bucked, causing the crack.

I made a new j-channel and riveted it in place a few hours ago and it worked perfectly.
 
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