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Batch of Cracking Rivets...

Jarrett

Member
Hey folks! While riveting the gussets to my 14 firewall everything seemed to be going smoothly. Plans called for AD4-7 which seemed to be a tad on the long side, but I didn't want to veer off the plans too much in a critical area and I am using a pneumatic squeezer. After setting the rivets on my first gusset I noticed what appeared to be multiple cracks on the rivets 45 degrees from the shanks and the shop heads seemed rough. I've attached a bunch of pictures (I used flash the highlight the roughness:

https://photos.app.goo.gl/BfeeyiGycqroHaVV8
https://photos.app.goo.gl/Vf3SitYeWByBxgNz6
https://photos.app.goo.gl/mcfWMWBVaXWfQc2q6
https://photos.app.goo.gl/6SpDrEGieQcongHE7
https://photos.app.goo.gl/1Tfxek3XJuuZqTVu9
https://photos.app.goo.gl/QrWBcSy6UBda1Guz7
https://photos.app.goo.gl/3bHmNGFJC1fu4mL6A

On the thousands of rivets that I have set on the plane so far, I have never experienced this issue. I did some more digging and found that it could be a bad batch of rivets. I found that builders in the past have experienced similar issues with rivets marked NR on the manufactured head. I pulled out some rivets marked P to see if this would make a difference. After setting these rivets there was no visible cracking and smooth finish:

https://photos.app.goo.gl/BPzpspJ9oXcPPAjo8

I gave a call to vans with the pictures I took and they said given the thickness of the material I would likely cause more damage removing them, so it would probably be best to leave the majority untouched, however for the next gusset I should order a new batch of rivets. I'm still concerned with the rivets in the first gusset. The mil-spec says:

Cracks in driven heads (1) which fall within a circle which is concentric with and 1.10 times the nominal shank diameter, or (2) intersecting cracks, or (3) five or more cracks

As far as I can tell (1) the cracks do not fall within the 0.1375 radii (2) the cracks are all 45 degrees of the shank and only 2-3 intersect (same with potentially five or more cracks) (3) out of the 24 rivets only 3-5 might have 5 or more, but it's hard to tell. Now I really don't know what to do... As far as I can tell the majority fall within the mil-spec, but I really don't like cracked rivets. If I remove them I'll probably do more damage than good. I could completely reorder that side of the firewall, but that'll take time. Curious what everyone thinks. Thanks!
 
Photos

Hi Jarrett, the appearance of the rivets in the photo seems OK to me. If there was cracking we'd expect to see radial splits in the flat face of the shop head, but this is not evident in the photos. Maybe it's more obvious up close? The edges of the shop head may look a bit "bumpy" but not obviously broken. How do the diameters of the shop heads compare to spec (1.5D) and can you achieve this diameter by squeezing less?
I also think it's not worth the risk of drilling out and replacing any of these rivets, based on the photos.
Cheers Paul.
 
If it was mine, I would drill them out. Whatever manufacturing process went awry to put the material into that condition, could have put the rivet outside its spec for strength. That’s a pretty important part of the airplane. I messed up a few rivets in that location and drilled them without causing damage so it can be done. Go slow, drill on center, on square, and not too far into the hole.
 
The MFG head markings (or lack of)

The "AD" rivets will have a single dimple in the center. If there is no dimple at all, its quite possible the vendor stocked "D" or "DD" rivets typically used in airliner skins. Those rivets are most often softened by controlled annealing by the user prior to shooting. Its very common to see them crack if not annealed, and I suspect that may be your situation. AD rivets are never annealed and should seldom cracks unless significantly too long and oversize bucktail.
 
Too bad there is not more specific data on the strength of these "rivets" They look grainy and the lines look like sheared grain boundary's.

If Vans Engineering says they are ok, then fine, but was it really engineering?

I (just me) would give a call to Tom at General Aviation Hardware - he is an RV builder and would probably help you become satisfied with some data. As a quality supplier of rivets of all kinds, he surely has a manufacturer contact for this specific issue. No slight to Vans, but a good second opinion.

Too bad we don't have some aerospace fastener quality engineer on here to weigh in and see us straight.
 
Honestly, I don't see the cracks in any of these photos, maybe it is time to check my eyes again.
 
In the last photo, what all the crud under the shop head? Not deburred, if so those will have to be redone.
 
Honestly, I don't see the cracks in any of these photos, maybe it is time to check my eyes again.

A “V” shaped crack is visible in the second photo. This is the typical crack on this bad batch. Sort of slides a chunk of the shank off in the shop head.

The stuff under the head is just primer.

I’d drill them out. Use a smaller drill (#40) to drill the shank so it can compress in the hole. They will drive out pretty easily.
 
I see the cracks you're talking about.

Could be several things
-bad batch of rivets
-rivet initially too long
-work hardened and then hit again to make it flatter
-gremlins

Having said all that, I get the desire to make stuff perfect, but that mil-spec gives guidance for a reason.

Also, the Mil-Spec is to be used in the absence of superseding manufacturers guidance, not the other way around.

Given that Vans is telling you that they're okay, and I believe you said you've evaluated them and they fall within the guidance of what's allowable in the mil-spec, seems to me like theres not really a justification for changing them out.
 
Removing - a tip

If you do attempt removing, it seem's Van's concern is that due to the thickness of the material it would be likely that you'll need to fully drill out the rivet, and thus most likely go slightly off and oversize the hole.

My tip: proceed as usual - make a hole just deep enough to get a pin punch in and snap off the head.

Try to (with moderate force) knock out or pull out the rivet. Most likely won't budge. In that case I use an undersize bit. So if these are 4-7's you'd normally use a #30 bit for the hole. Use something like a number #35 and go down the center of the rivet. This will often weaken the rivet enough to allow you to pull or push out the tail without damage to the holes.
 
If you do attempt removing, it seem's Van's concern is that due to the thickness of the material it would be likely that you'll need to fully drill out the rivet, and thus most likely go slightly off and oversize the hole.

My tip: proceed as usual - make a hole just deep enough to get a pin punch in and snap off the head.

Try to (with moderate force) knock out or pull out the rivet. Most likely won't budge. In that case I use an undersize bit. So if these are 4-7's you'd normally use a #30 bit for the hole. Use something like a number #35 and go down the center of the rivet. This will often weaken the rivet enough to allow you to pull or push out the tail without damage to the holes.

Just like the rivet removal process documented step by step in Section 5 of the construction manual....;)
 
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