tank rivets -- solution for sitting proud

[aparchment]

Hey gang:

I just finished some more tank stiffeners, and the rivets are sitting way too proud for my liking. I am thinking of drilling them out and doing them over.

How flush are yours with the skin surface, and what did you do to get them that way.

I am thinking of using a punch to push the next set in tight before I tape over them and do the back riveting.

Suggestions?

Antony

[cnpeters]

Funny, I just did the tank stiffners tonight. The rivets were slightly proud, not from sealant me thinks, but seemingly from a little slop in pressing down the stiffner/skin to the back rivet plate. I'm letting it go, since they are on the tank bottom and out of site, and I follow Van's standard answer to all such things - "build on".
My fuel flange rivets were slightly proud - I forgot to cleco first . These I'll probably remove and re-do the flange so it is a little tighter. In truth, not necessary, but I am seduced (no comments) by the cap/flange and want it perfect. Lastly, if you feel the rivets are slightly proud due to sealant, then you may want the fuel tank dimple dies sold by Cleveland and Avery, as they dimple a slightly deeper (.007 if I recall) hole to compensate. I have heard pros and cons (money), but at least two accounts where the builder tried both yielded no significant difference.

[cjensen]

Mine were slightly proud as well. Didn't do anything about them. They are not sticking up high enough to make any difference at all, and they are on the bottom.

I DID use a pin punch to push the rivets in as far as they would go on all the fuel tank rivets, but it seemed to only make a difference on the rib rivets. The stiffener rivets, being backriveted, didn't seem to make a difference as to where they ended up being set using the punch to press them in before taping.

[ptrotter]

There are a couple of things that can be done. First you should use the special tank dimple dies from Cleveland Tools. The are very slightly deeper to account for some proseal to get under the head. Second, it helps to run your hand deburring counter sink tool a couple of turns in the dimple as it make the rivet fit a little better. For the ones that are already done that you don't want to remove, you can use a rivet shaver to take a little of the head off to make it flush with the skin.

[Spike]

Are you putting the proseal in the holes before you insert the rivets? I would figure that you should put the rivets in, tape them, and then add the proseal to the backside before dropping on the stiffeners. Ill hopefully be there in a few weeks.
John

[LooseNut]

I'll repeat Paul's advice.

Use the tank dimple dies to get a slightly deeper dimple, both on the skin and the stiffener.

Also, use a counter sink cutter and "clean up" the dimples in the skin. You can turn it by hand as Paul suggested, or put it into the countersink cage and run in in your drill. Either way, you just want to take a tiny smidge out, just a little dust really. Just to crisp up the dimple a little.

If you don't have the tank dies, you might just try the counter sink cutter trick in the "regular" dimples. I used the tank dies, but, I have a feeling you could get good results even without the special dies.

[aparchment]

Hmmm, so far I have taken the recommended approaches:

- Used tank dimple dies
- slight countersinking prior to inserting rivet

I am going to try using the punch to seat the rivet before back riveting.

We'll see where that gets me.

Antony

[bglentz]

Hey Guys, It sure has been a while. After a move, job change I am now thinking about picking up the rivet gun. In my search for information about building I can across some posts from various places by those who drive rivits for money. I kept a file with all these helpful snippets and I thought of a couple while reading this thread. Sorry for the long post but I thought I would run this by you guys. I don't know who wrote them but here they are for your review.

Number 1:

A lot of discussion centers around deburring of rivet holes. After drilling and installing hundreds of thousands, if not a million rivets I can say with some authority you do not need to deburr rivet holes in .020 or .032 aluminum. What is left in the hole is not a burr, but is actually a chip left over from the drilling operation. You can easily remove chips by disassembling the parts, putting on a pair of gloves and lightly running your hand across the drilled sheets. What must be avoided is removing the aluminum material. The holes should be crisp and clean, not deburred by removing material. If you do have an occasional burr (a piece that you cannot remove by rubbing) you can remove it by taking a countersink bit and lightly turning it by hand to remove the burr.

Number 2:

SETTING THE RIVET BEFORE DRIVING IT
-----------------------------------

The facts are that no matter what dimple die you use for dimpling 02Thin skin you will not get a professional dimpled rivet unless you finish the dimpling process when you set the rivet.

Everyone selling tools has the best dimple dies on the market! Some suppliers claim to have developed and hold the secret to the perfect springback dimple die after years of fine tuning. I don't know if they are dazzling you with their genius or baffling you with BS. What I do know is that the "perfect" 100-degree springback dimple die was perfected in 1942. Everyone that builds springback dimple dies today uses the original 1942 design with very little difference.

I constantly hear and read that dimples are too deep or too shallow and the dimple die is generally blamed for this. The real problem isn't the dimple die, it's the process for installing countersunk rivets.

In order to do a professional job you will need to start with the dimple die to make the initial dimple. This is done by impact or pressure on the dimple die set using a DRDT-2 dimpler, C-Frame dimpling tool, hand squeezer or pneumatic squeezer. After the dimple is formed in the skins, stringers, ribs, bulkheads and all parts for assembly you will need to go to the next step.

Lightly debur the holes. Very lightly so you don't actually cut any metal away, only knock off the rough burrs. After deburring cleco the parts together and your ready for the next step.

Make a tool. Yes, that's right. Make a dimple set block. Make this by drilling a hole and countersinking it to match your rivet head depth in a scrap piece of steel or bucking bar. Next, use the dimple set block.

Install a rivet in the skin, place the dimple set block on the back side and lightly drive the rivet into the dimple set block. This will "set" the aluminum and the rivet tight and flush. When you do this it only take two or three taps with the rivet gun at low throttle.

Next drive the rivet using a standard bucking bar and you will have the perfect countersunk rivet.

This is a lot more work than would be required if you had the "perfect" springback dimple die. Unfortunately, many claim to have improved upon the springback dimple die developed in 1942 by NACA. Few, if any, have come up with a new unique design for a tool that installs a rivet into aluminum sheet that hasen't changed since 1942. Drive your flush rivets using the dimple set block and you won't need to spend hours trying to figure out who makes the best 1942 springback dimple die or why your flush rivets just don't set right in the dimple.

Happy building!

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[svanarts]

Very good stuff Bob, thanks for sharing that. Feel free to drop those long posts on us any time!

[aparchment]

Stupid, stupid, stupid . . . DOH!

I headed down to Mass. today to work with CJ and Dave on my tanks, and after unloading my truck, pointed out my less than perfect results with my tank stiffener rivets. Both guys got a close look and John said noted that the dimples didn't seem deep enough. Dave pointed out that the bottom tank dimples didn't have the rim that were present on the top skin dimples.

Here is where the learning comes in. If you use the DRDT2, make sure you check the settings/fit of the dies as you move through your work. Apparently I let mine loosen as I dimpled the various parts, and I got less than perfect dimples on the bottom of my tank. You could actually see where the settings loosened up as the dimples got shallower and the ring around the outside of the dimple disappeared. Man that was dumb. I get so used to the "set it and forget it" ease of the tool that I tend to zone out when using it. Add to that the fact that it's usually late at night, and it's twice the potential for error.

There was no real harm done, just some time lost and a less then gorgeous set of stiffener rivets on one tank. It is a good lesson learned though. CALIBRATE YOUR TOOLS BEFORE YOU USE THEM!!!! DON'T MAKE ASSUMPTIONS!! Not only is it good practice for building, it's excellent practice for flying. Ask me about an the ILS I flew into White Plains one day when I couldn't figure out why my ADF wasn't working.

Have a good weekend all.

Antony

[JohnR]

Antony, that is an easy mistake ot make. I learned early on to make sure and snug the locknut down a little after adjusting the plunger. I just use my fingers and that seems to be tight enough to hold it in place. I never thought of that earlier whne you posted it. That small amount of coining around the dimple is always a good thing to look for.

Glad you figured it out.