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Back Riveting Problems on Rudder Stiffeners

Hi all.

I'm having a problem with rivets falling over while attempting to back rivet the stiffeners onto the rudder skin. After completing all the rivet work on the Horizontal and Vertical stab, I did not anticipate this giving me grief. I have double checked the rivet length and even confirmed that the 3-3.5s that the plans call for are correct with Vans. I noticed that my back rivet set couldn't be centered due to the inside radius of the stiffeners. I tried again on a test piece after shaving off some of the slip guard on the back rivet set, but I'm having the same problem. I do notice that its a bit difficult to keep the gun from slipping while applying solid pressure, but even on ones that I detected no slippage, they're still falling over. I'm so surprised that I've not yet encountered any other stories of similar troubles. My HS and VS went very well and I'm extremely happy with the quality of the work. I'm at a loss for what to do next. Am I doing something really stupid? Any advice? Has anyone else had difficulty with back riveting stiffeners?

I've attached an image so you can see what I mean.

Thanks.
 

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Just guessing but could you apply some thin tape like rivet tape on the manufactured head side to ensure the rivets stand up perpendicular to the surface? Even a slight lean could be causing this.
 
Measure to be really sure . . .

I am starting over after looking closer at your photo . . .
Rivets look like they are extended too far, measure and verify that you are using the correct rivets. 1.5 diameters should be showing.
 
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Inspect the face of your rivet set closely. I had to “reface” my rivet set when it started bending some rivets over. I bought a new backriveting set when I could no longer get consistent results.

You might try pre-squeezing your rivets with a rivet squeezer a little. The rivets look like they are loose in the dimples. This could contribute to the problem you are having.
 
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C frame dimpler does a nice job riveting these if you have one.

I filed part of the plastic down a bit on my back rivet set to help it fit a little better. Hopefully one of those two options help.
 
I do have tape already holding the heads of the rivets in position on the outside of the skin.

Those rivets will all come out. I have no intention of keeping them. I just posted the photo so you all could see the problem.

I will double check the measurements. I do believe it's 1.5x the diameter, but I agree they look too long to me, and that's what my gut is telling me the issue is. My only doubt was that Vans confirmed with me that they are the correct size, and I hadn't seen any other complaints.

I will also double check my back rivet set to make sure there's nothing strange going on there.

Thanks for the tips.
 
Yes, I have already shaved down the rivet set to fit inside the radius of the stiffeners. I do have a C-frame. It would be a bit difficult to work with the bend rudder skin, but I'll keep that in mind as an option if I really can't get the back riveting figured out.
 
When I back rivet, I grip the plastic tip with my thumb and index finger to keep it from moving while the gun pounds on the rivet. Not sure if you are already doing this.
 
Put the plate on a concrete floor, add plywood around it to support the skin. Put the set on the rivet to be driven and hold the collar down tight with your free hand (to keep it square). Hold the opposite skin up with your head if need be! Lean into that sucker and pop off a few hits. Should be all it takes.
 
Check dimensions -- Rivet Length...

Interesting -- the rudders between the -8 and the -7,-9,-10,-14 are slightly different in structure and skin thickness (-8 is 0.020", the -7/9/10/14 is 0.016". and the stiffeners are 0.025" and 0.020" respectively).

The -7/9/10/14 plans call out AN426AD3-3 rivets for the stiffener to skin attachment. The -8 rudder plans that I have show AN426AD3-3.5 rivets.

It's possible that these dimensions have changed -- or the material you have doesn't match the specifications of the parts in Section 4.

From your picture, the rivets look longer than necessary. You might try a -3 rivet, and see how it holds up.

Also consider using the smaller diameter backset, I believe it's about 1/2" in total diameter and has a gray/black plastic retaining bushing.
 
Thanks for the tips. I attempted the first go with the back plate on my workbench. I will try as suggested on the concrete floor.

Yes, plans call for 3-3.5s. I will measure the pieces' thickness to verify the total thickness. I'm going to take some time setting up a practice piece to make sure I get it right before attempting the actual rudder again. I think I'll order some 3-3s to try those out on my practice piece as well.

Thankfully, I was able to successfully drill out the three bad ones without any issues. Holes still look great!

Thanks for all the suggestions. I'll get to work on it.
 
I had the same problem and abandoned the back rivet method and went to a gun and bucking bar.. if I had to do it again, I would have used a #41 drill before dimples. I feel the thin skins stretch more and the holes were loose on the rivets. Maybe the other suggestion of pre squeezing a little could help fatten the rivets?
 
Gun Pressure

What is the pressure of your gun? I set mine at #40 for -3 and #50 for -4"s.
Do you have a rivet gauge? Rivets do look a little long. Good Luck!
 
Back riveting

Those are close. Maybe a tad long but driveable. I prefer a tad long anyway. Don't try to drive them all at once with high pressure. Dial it down a little. Hit a couple taps and look. The rivet clinches in the direction the gun is tilted. It's opposite what you might think. If it looks like it's clinching, tilt the gun the opposite way. Sounds nuts andbI can't explain it. Try on scrap first.
 
When I back rivet, I grip the plastic tip with my thumb and index finger to keep it from moving while the gun pounds on the rivet. Not sure if you are already doing this.


+1 The rivet seems to smear, as opposed to clench, if the tool moves while pounding.

I had the same problem and abandoned the back rivet method and went to a gun and bucking bar

I was so pleased with back riveting I wish I could do the whole project that way.
 
I had a similar problem on my -14 rudder but figured it out and now find backriveting is the best way in situations like this.

Firstly, you have to stop the set moving by holding the collar onto the skin with your fingers. Put tape loosely around it so the roll pin doesn’t get you in the fingers.

But the big reason I had problems - the whole assembly was moving across the plate when i was riveting. Sort of vibrating around. I noticed it when I looked at a time lapse of a few rivets. Once I weighted the whole skin down with lead shot bags - all the rivets were perfect!

Maybe this is your issue?

I also only used 20psi for these tiny rivets.
 
Terminology

So a dash 3 rivet could mean two different things. It could me a rivet with a diameter of “3” as in AN426AD3-x. It could also be the length as in AN426AD3-3.

Often times people say, “ Use a dash 3 rivet”. Which means use a AN426AD3- whatever length. So be careful.

Lengths can be adjusted to be a little different than the plans call out but the diameter, AN426AD3, should be the same as plans unless one made a mistake and had to drill a slightly bigger hole.
 
What I do

Backrivet them with a 1/8 universal set..the cupped face makes a beautiful bucktail and also helps center the tail if your slightly off. Every greybeard sheetmetal guy in aviation will tell you this trick.
 
Backrivet them with a 1/8 universal set..the cupped face makes a beautiful bucktail and also helps center the tail if your slightly off. Every greybeard sheetmetal guy in aviation will tell you this trick.

Cool - I never thought of this!! I'll have to try it.
 
What is the pressure of your gun? I set mine at #40 for -3 and #50 for -4"s.
Do you have a rivet gauge? Rivets do look a little long. Good Luck!

Check your pressure and make sure the pieces are pushed against the backplate surface with your off hand. Press medium to hard with the gun/back rivet set. Oh, and yes, please get your self a rivet gauge. Like the black one from Cleveland. If you have a couple rivets that are a little long and don’t have the next size, a rivet cutter or a belt sander will do the trick. https://www.cleavelandtool.com/products/one-piece-rivet-gauge
 
+1 on the suggestion to get it off your bench and onto the floor. Had similar problem as yours and once I moved the plate to the floor, it made ALL the difference in the world. Guessing the softness of the wood vs. the hardness of the concrete makes a big difference.

Also +1 on starting with a low pressure and working your way up. I only buck -3 rivets at 35psi on my setup.

BTW, I'm about to back rivet all the stiffeners on the elevators here when we get back from OSH. Been two years since I did the rudder so I'm sure I will dork the first few ones up until I get a feel for it once again.
 
I found that I had to make sure the set was parallel and concentric to the rivet and I had to hold the set against the part with my fingers on both the set and part as others mentioned.

Doing it on the floor would have taken all the fun out of it. I have the standard 5/16” thick plate in a carpet cutout on a 2” thick bench.

I was able to set the -3.5 length rivets on my —8 with minimal/ no rework.
 

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I would also recommend measuring the length of the rivet to be squeezed. Even in areas where Vans calls out a rivet length, it can at times be a bit long. Using a dial caliper, I measured most of mine to ensure that the end to be set was indeed 1.5 X the river Diameter. There were times when you had to round down to the next shorter length rivet to get it to come out to the right diameter and height after bucking or squeezing.
 
Pressure

I recommend some testing. I see so many different pressures posted. It varies so muchnfrom builder to builder and setups. I set my gun regulator to 23psi for #3 rivets. Even less for a back rivet set up. Sioux 3X.
 
That looks a lot like some of my early attempts at riveting.

I will tell you what I learned from the guy who got me straightened out (a former crew chief for a well known Reno Racer). --However, LET ME SAY THAT this was his advice to me. And, I'm not going to guarantee this is what's going to help you.

1. The 2X rivet gun that came in my "RV Builder's Tool Kit" from a popular supplier didn't hit hard enough, or fast enough.

2. The backrivet set from my tool kit was not good quality, the face was slightly convex instead of being ground flat, and the spring collar was cheap and marred the work surface when it shouldn't.

3. My working pressure was too low. I need to get the rivet set with just a quick blip. The more blows the shop head gets, the faster it becomes work-hardened.

4. And last but not least, I need to develop a "feel" the gun, so that I consistently am perpendicular to the work piece and the set isn't dancing around. (practice, practice, practice!!!.....)
 
Rechecking all measurements

Section 4 describes the -8 rudder skin thickness as .020 and the stiffeners as .025. I have confirmed that the parts I have are these thicknesses. Thus, the total material thickness is .045. 1.5 times the rivet dimeter of .0937 plus .045 gives me a total rivet length of .1856.

This means that AN4263-3.5 rivets called for in -8 plans are .0344 over in length, and the next step down in length AN4263-3 are under length by .0046. In my mind, the choice of which rivet size to use seems obvious. The length called for in the plans is clearly over and largely causal to the problems I've been having with them falling over when attempting to set. The shorter length is just slightly under, but it is by a very small margin.

I'm just still confused why Vans support confirmed that the size called for in the plans is correct, and why I have yet to come across any builder who has run into the same issue and come to a similar conclusion. From what I can tell looking at other builder blogs, they have used the size called for in the plans without issue (this is confounding to me).

I'm waiting for an extra Rudder stiffener piece I ordered to arrive so I can set up a truly analogous test. In the mean time a fabricated one: same bad result on every 3-3.5 I attempted, some minor problems with sloping and inconsistency on the 3-3s I attempted, but much better, and actually seeming possible.

The only other thing I thought might be an issue is the hole size. I do feel that the rivets are a little loser in the holes than maybe some other sections. Going off the hole tolerance example for the 1/8'' rivet of -1mm +2mm, I figure the hole tolerance of a 3/32" should be 2.3-2.6mm. I sampled 5 random holes on my skin where the stiffeners attach and got consistent measurements of 2.50-2.56mm. That does mean the holes are slightly on the large side but should still be within tolerance.

I'm not ruling out an issue with my technique, but I am also quite self aware of my own competence level and past quality of work. I'm just trying to figure out why this thing that apparently other builders call "the easiest riveting yet" is giving me so much trouble. On a builder blog O'Reilly RV8, he writes, "Tonight I began to back-rivet the stiffeners onto the rudder skin, back-riveting is easier than i thought, I wasn't able to locate the rivet size on the drawing, so i went with AN426-AD3-3.5, which seems to be the right size." O'Reilly, if you're out there, please let me know why and how. Thanks.
 
General Rule

When the rivet length comes up short of length required by the formula the next size up is selected. If you have a way of cutting a 3.1, 3.2 length rivet then that’s your solution (sounds tedious and unnecessary). If you think you can create the manufactured head to the dimensions in the mil spec with the under length 3 instead of 3.5 rivet then that’s your solution. When everything is in spec and the rivet doesn’t go right, it’s either tools, technique, or both.
Your hole diameter is fine.

I built my tail at Synergy South, had zero issues with the 3.5 length rivet. As many as they do there I’d think they would be doing something different if it were a problem.
 
Rivet set

Is it possible the face of the back rivet set isn't square? I've seen bad tools. It wouldn't take much to clinch a rivet. Polishing the face perfectly square may help.
Make some scrap and test.
 
I have used it in the past without issue. It squares up when the sleeve is on a flat surface and when the head is pushed down onto a flat surface as well. It does not appear that the sleeve can be removed to measure the square of the head more precisely without permanently dismantling the sleeve.

I just ordered the Avery back set from Spruce to try that out. The one I have is the 1/2" with black sleeve from Cleaveland.
 
What gun are you using? What pressure are you running? Is it possible to take and post a quick video of you back riveting a few of those rivets so we can see if there are any anomalies the brain trust here can figure out? Do get a rivet gauge so you don’t have to do all those computations every time.
 
First of all I'm so amazed at all of the amazingly helpful suggestions. Thank you all!

I'm so embarrassed to mention this, but I haven't even considered the air pressure. I have the 3x gun from cleaveland. At the beginning, I had difficulty finding any answers to what pressures the various tools should be operated at. Some how I landed on 90psi. I operate all my pneumatic tools directly from the regulator on my compressor: drill, squeezer, and rivet gun all at 90psi. Thus far this has worked without issue, so I never even thought about it again.

I will absolutely start experimenting with lower pressures. Do you all think it's important to have a separate regulator on the tool, or is the regulator on the compressor adequate?

I have an extra rudder skin that arrived damaged, so I've prepped some stiffeners and that old skin to use as my test. I'm just waiting for my new back rivet set to arrive, then I will run my test. I will try to get some good footage of the riveting test.
 
90 psi definitely seems high to drive AN3 rivets. You're going to get 50 different answers but I have a 3x gun and use 35 psi for backriveting AN3 rivets.

This is a bit off topic but something that isn't said often is that different rivet sets may require different pressure settings. I've found the larger the rivet set the higher the pressure required due to the larger mass. Just something to keep in mind going forward... It's more apparent when you are using a longer set or a double offset set.

Also, I only use the pressure regulator at the compressor and adjust it as necessary. I've seen many builders like to have a small regulator at the gun but that doesn't work for me b/c I can't see what pressure I'm setting.

Hope that helps.
 
Pressure

I have a Sioux 3X with a pigtail and RTI mini regulator.
30 psi for #4
23 psi for #3
20 psi for back riveting
Squeezer runs 90 off another regularor
 
I can’t find my Cleveland aircraft tools sourced back rivet set but I’m pretty sure the face was slightly countersunk. I assumed it was on purpose to center the tool on the rivet. Maybe someone else can look at theirs.
 
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What works for you

You need to experiment a bit, your compressor, gauges and the like will all be different.

For the standard long back rivet set adjust the compressor to about 30psi for a 3-3.5 rivet. Then go from there. 2-3 ish second to set the rivet. If its too quick turn it down, too slow, turn it up.

Adjust by 5ish PSI each time. Will depend on the set and the rivet.
 
Update on Backriveting

I ran my back riveting test, and I'll just start off by saying that I've had success. I think the biggest difference was made by lowering the pressure of the gun. I found that about 35psi seemed to be right for setting the 3-3.5 rivets, and 25-30psi on the 3-3s. Operating at the lower pressure made the gun much easier to control and thus I had less issues with leaning or slipping. I also did half with the Avery back rivet set I ordered. To my surprise, I actually got more consistent results and found is easier and more comfortable to operate than then the smaller set I was originally using. The white sleeve, although a larger diameter, is nicely radiused on the bottom, and made of a harder plastic. For me it felt more stable and harder to accidently lean.

The 3-3 rivets smashed nicely and although not very tall, are within mil spec tolerance. That said, once I was able to consistently smash nice heads on the 3-3.5 rivets, I decided to with that. It's what the plans call for, what Vans recommends, and what most of you all have done without issue.

Thanks so much for all the help, and I hope the actual piece goes as well as my test.
 
I finished riveting the rudder stiffeners with good, but not perfect results. I ended up using the back rivet set with the larger white collar. I had better control and stability with this set which led to more consistent results. I also operated the gun at about 32psi. About 50% set very nicely, the other half were not perfect, but within tolerance. I think there were 3 total that I was unhappy with and drilled out and replaced. 2 of those were on the trailing edge that didn't pull perfectly flush to the skin. Those drill outs and resets went beautifully, and I'm very happy with the end result.

Still not looking forward to the elevators, but when that time comes, I'll do lots of warm up time to dial in my technique again.

Thanks for all the help!
 
Cold rolled steel

Find a metal distributor nearby.
Cold rolled steel is cheap.
A big plate is useful for more than just back riveting. Buy the thickest you can handle.
1" square cold rolled steel makes awesome bucking bar stock. Have it cut into a variety of lengths.
https://drive.google.com/file/d/0B7...=sharing&resourcekey=0-Bonw_QIwhIbyoVSUcmK_dg
Buy some aluminum angle for making braces, straight edges, etc. Not for airplane use unless you know it's the correct stuff.
 
That's some good advice Larry!

I'll chime in here since I just riveted the stiffeners to my left elevator the last few days. I last used my back rivet set about two years ago doing the rudder. I don't remember having too many issues but it's been a while and even my builders log doesn't say much about that step.

So having read this thread with interest this week, I started off with the air pressure really low. For me this was a mistake as it took so long to set each rivet, it allowed many of them to tilt over or get smeared. I drilled out a lot of rivets this week unfortunately. I even just bucked a lot of the rivets on the one side of the skin because I was tired of screwing so many of them up.

Finally I decided to start upping the air pressure. I ended up at about 60 psi on my setup which worked well for me, YMMV. At that pressure all it took was a quick half second burst from the gun to set each rivet fully and acceptably. Once I figured that out, I was able to run down each row quickly and set everything. The very end ones, next to the trailing edge were still a pain and I dorked a few of those up but after this week I'm an expert on drilling out 426 3-3.5 rivets so no big deal.
 
Backrivet them with a 1/8 universal set..the cupped face makes a beautiful bucktail and also helps center the tail if your slightly off. Every greybeard sheetmetal guy in aviation will tell you this trick.
Works like a charm - thanks so much! Back rivet set is not nearly as neat.
 
If anyone is still reading this thread - check you C frame if back riveting. Min'es from Cleaveland tools and unfortunately there is 2/10 inch play at the top if measured from the top of the back rivet stem. I have to push the stem to one siide and squeeze the trigger to have the squeezer set sit flush. If the sets are flush and square, there will be no walking of the sheet - though I hold the bottom of the stem as a guage to feel how far the rivet sets. M
 
I really appreciate all the input on this forum. They definitely helped me improve my technique. But my main problem has to do with what N804RV and wirejock alluded to... my brand new Cleveland tools back rivet set had a concave face which I believe was pushing the shop heads over to one side. I removed the set from the collar and sanded the set flush.

Now my rivets look perfect, as long as I use the techniques listed above.

Thank you!
 
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