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Tank skin countersinking...

aarvig

Well Known Member
I just finished countersinking my tank skins. I was very diligent to be checking the depth of the countersink with a rivet as I went along and went just ever so slightly deeper than flush. However, on a couple of my countersinks I noticed went deeper than intended, one of them actually chipped a very small amount of material off the edge of the hole, it appears knife edged. In fact, a lot of the holes appear knife edged. Did I snarf these skins? What should I do about this hole with the small chip in the edge?

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Here is one showing the rivet depth?

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oops rivet?

Drill the hole the next size up and use an oops rivet, same size head but next size up shaft. That should remove your knife edge, once covered in tank sealant no one but you will know.
 
A countersunk hole for a 3/32 rivet in 0.032 material is essentially a knive edge by mil-specs, but OK by FAA standards.

Are the holes really too deep?
 
Also, purposely countersinking too deep reduces rivet strength even more, because the rivet set can not press the rivet tightly into the bottom of the countersink.
I know that some builders like to go deeper to account for some tank sealant under the rivet head but in this location there shouldn't be any (nor is any needed) if the baffle is installed correctly.
 
A countersunk hole for a 3/32 rivet in 0.032 material is essentially a knive edge by mil-specs, but OK by FAA standards.
Are the holes really too deep?

Knife edge for sure. The MS spec says the head is slight thicker than a 0.032 skin:



Note the 0.1790" diameter of the countersink if the head is perfectly flush. The 0.1760' dimension Gil referenced is the spec diameter; it puts the rivet head a little above the panel surface, which is structurally better. Diameter can be measured in the field. Make up a comparison plate; a variety of carefully measured holes, made with an adjustable reamer, a ball gauge, and a micrometer. Place a hole over the countersink, look down through it with a magnifier, and compare diameters.



 
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Dont sweat it

Several options, easiest is to make it a -4 (1/8") rivet and drill it to a #30, then the existing countersink will not be knife edge. The larger diameter rivet is then shot with the head a bit "high" and then shaved or sanded down. You can even shave it before shooting it.
 
Also, purposely countersinking too deep reduces rivet strength even more, because the rivet set can not press the rivet tightly into the bottom of the countersink.
I know that some builders like to go deeper to account for some tank sealant under the rivet head but in this location there shouldn't be any (nor is any needed) if the baffle is installed correctly.

Really? I didn't realize that and I'm not far from this step. Guess I had better go back and RTFM!

Thank you
 
Gil, I don't have a real accurate way to measure the countersinks. Is there a tool to do this?
Dan, are you saying the only way to get the countersink done properly on .032 is to end up with a slight knife edge because the rivet head is thicker than the piece of .032?
Should I replace this skin?
 
Dan, are you saying the only way to get the countersink done properly on .032 is to end up with a slight knife edge because the rivet head is thicker than the piece of .032?

Correct. Every tank in the RV fleet has knife edged countersinks here, unless they were built with oops rivets. Well, a few have been done with dimples.

Lots of them are flying with countersinks that were cut too deep, but it's not ideal.
 
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Correct. Every tank in the RV fleet has knife edged countersinks here, unless they were built with oops rivets. Well, a few have been done with dimples.

Lots of them are flying with countersinks that were cut too deep, but it's not ideal.

If it can be done with dimples why not go that route?
 
Correct. Every tank in the RV fleet has knife edged countersinks here, unless they were built with oops rivets. Well, a few have been done with dimples.

Lots of them are flying with countersinks that were cut too deep, but it's not ideal.

Thanks Dan. It's kind of what I figured but I wanted to clarify for sure.

Looks better then mine. Rivetitt! :)

Vlad, you always find a way to cheer me up:D! Thanks!
 
I agree with DanH. That is what the plans call for and lots of RVs flying with knife edge countersinks in the tank baffle. Along with the sealer plenty of strength in this area.
 
I agree with DanH. That is what the plans call for and lots of RVs flying with knife edge countersinks in the tank baffle. Along with the sealer plenty of strength in this area.

Sealer strength is actually quite low compared to any structural adhesive. The specification requires minimum tensile strength of only 200 psi (standard cure, no fuel), and allows as low as 50 psi after 14 days soaked in fuel at 140F. Hysol 9430 structural adhesive, for example, is 5300 psi.

Of far more interest in this discussion is the elongation specification, 200% minimum, which means it must be able to stretch to twice its thickness without adhesive or cohesive failure. The typical sealer film thickness between two riveted sheets is 0.001" to 0.003" in the rivet vicinity, thus the maximum relative sheet movement without failure can be as little as 0.002". Assume a good case, 350% elongation before failure and a 0.003" sealer thickness. Max shear movement before adhesive or cohesive failure would be 0.0105".

The test sample on the qualification report in front of me went to 460%, more than twice the minimum. That's good, but do remember this particular sealant sample is a lab creation, mixed and applied as perfectly as possible. Even this perfect sample would fail at 0.0046" in a 0.001" film.

With those numbers in mind, go back to post #6 and study the illustration of a countersink cut 0.005" too deep.

Out of lab and into the real world, paint it a dark color, fuel it up, and leave it parked in the sun. Like tensile strength, the minimum elongation specification goes down with the addition of fuel and heat. Given 72 hrs @ 140°F in reference fuel, plus 72 hrs @ 120°F in air, plus 7 days @ 250°F in air, the minimum elongation spec is only 75%. The test sample on the qualification report made it to 159% before failure.

That's why overly deep countersinks in 0.032" skins don't make good tanks.
 
I spoke to Vans technical support this morning regarding the knife edges on these holes and they were pretty resolute that the skin should be replaced. I was told that it would go together fine but the headache that would be created if the rivets started working would be pretty high. New tanks and paint on a flying plane. So, I'm biting the bullet and ordering new skins.:(
 
Really? I didn't realize that and I'm not far from this step. Guess I had better go back and RTFM!

And it will tell you to CS.

Knife edge for sure. The MS spec says the head is slight thicker than a 0.032 skin:

I spoke to Vans technical support this morning regarding the knife edges on these holes and they were pretty resolute that the skin should be replaced. I was told that it would go together fine but the headache that would be created if the rivets started working would be pretty high. New tanks and paint on a flying plane. So, I'm biting the bullet and ordering new skins.:(

Mmmmmm - not sure where these two quotes leave us when dimpling is frowned upon. Leave em high and shave I guess:confused:


Correct. Every tank in the RV fleet has knife edged countersinks here, unless they were built with oops rivets. Well, a few have been done with dimples.

Lots of them are flying with countersinks that were cut too deep, but it's not ideal.

And mine will be one. Shoulda, woulda, coulda, but dimpling is looking more appealing in retrospect.
 
After countersinking my first tank at the baffle interface, I dimpled the other three. Had no problem getting the baffle in position and riveted, and all of them have passed leak check. The rivets in the dimpled holes also seemed to turn out a bit better.

Then again, I'm not yet flying.
 
Mmmmmm - not sure where these two quotes leave us when dimpling is frowned upon. Leave em high and shave I guess:confused:

The manual says to countersink to fit the head of the rivet.

The OP originally posted that he purposely C.S. slightly deep and that on some holes it seemed that it cut even deeper than that.

If the holes are properly C.S. for a net fit of the rivet head it will be a knife edge on the skin. That is considered acceptable at this location. Rivet head faces that are below the skin surface will not result in a tightly fitting rivet head and should be avoided.

I have mentioned before....
The primary reason C.S. is specified is because the baffle seam is the location where leaks are most common.
Many builders do not dimple properly. This results in a skin and baffle flange that is not entirely flat. Since a bead of sealant has to placed and then the baffle slid in place blind, having the parts not perfectly flat makes the leak risk that much higher.
 
The manual says to countersink to fit the head of the rivet.

The OP originally posted that he purposely C.S. slightly deep and that on some holes it seemed that it cut even deeper than that.

If the holes are properly C.S. for a net fit of the rivet head it will be a knife edge on the skin. That is considered acceptable at this location. Rivet head faces that are below the skin surface will not result in a tightly fitting rivet head and should be avoided.

I have mentioned before....
The primary reason C.S. is specified is because the baffle seam is the location where leaks are most common.
Many builders do not dimple properly. This results in a skin and baffle flange that is not entirely flat. Since a bead of sealant has to placed and then the baffle slid in place blind, having the parts not perfectly flat makes the leak risk that much higher.

Guess I read it as the guidance for skin replacement from the mothership was due to knife edge when it may have been due to knife edge AND CS deeper than flush. Thanks, Scott:D
 
I was told to replace the skins due to the knife edge only. I was told that I could build the tanks with the existing skins but that the likelihood of the rivets working loose in that area was pretty high since they were all CS'd to the same depth. So, since I don't want to have to do the tanks again 6 years down the road, I am biting the bullet and ordering new skins. They didn't mention anything about the depth of the countersinks. He said that looked OK but the knife edge condition is absolutely undesirable. So, it must be that these rivets have to be left a little high...:confused:...then shaved, although that is not specifically mentioned in the manual.
These earlier model (3,4,6,7,9) manuals need to be rewritten in my opinion. This is a perfect example where lack of clarity has cost me a few hundred bucks. The manual clearly states to leave the CS's flush with the surface. Even at flush you are knife edge. I went SLIGHTLY deeper to accommodate for pro seal so mine are even more knife edged. So the only reasonable answer is to leave them high, which is not mentioned anywhere in the manual.
I think I will dimple the new skins. I'm going to research this first...
 
Why not just use oops rivets here, and utilize your existing tank skins? You'll get rid of the knife edge by up drilling to #30.

Instead of spending a few hundred bucks you can get a pound of the darn things for 30 bucks from spruce.
 
I just finished countersinking my tank skins. I was very diligent to be checking the depth of the countersink with a rivet as I went along and went just ever so slightly deeper than flush. However, on a couple of my countersinks I noticed went deeper than intended, one of them actually chipped a very small amount of material off the edge of the hole, it appears knife edged.

These earlier model (3,4,6,7,9) manuals need to be rewritten in my opinion. This is a perfect example where lack of clarity has cost me a few hundred bucks. The manual clearly states to leave the CS's flush with the surface. Even at flush you are knife edge. I went SLIGHTLY deeper to accommodate for pro seal so mine are even more knife edged. So the only reasonable answer is to leave them high, which is not mentioned anywhere in the manual.

Aaron,
In your first post you wrote that you purposely went a little deep, and then it seemed that some of them went deeper still.
If you had done them for a net flush fit per the plans you would have been fine.
As DanH pointed out in great detail in his post (much better than I took the time to do) just .001 too deep is not a good thing. Are there other RV's flying that way (or worse) probably. In the end the final decision has to be yours.

I don't mean this as a flame, but you clearly didn't do it per the plans. The fact that you made the initial post shows that you must have thought something wasn't good. I will talk with tech support to clarify what is desired here because knife edge holes are expected but for a good rivet it must not be below the skin surface.
The bottom line is they are not going to lead you towards a decision that may result in a problem later, and then you may want to blame them.

The bottom (other) line is flush means flush. To make sure it doesn't go too far, .001 or .002 high would be better to shoot for and then sand them down during final prep for painting.
 
Aaron,
In your first post you wrote that you purposely went a little deep, and then it seemed that some of them went deeper still.
If you had done them for a net flush fit per the plans you would have been fine.
As DanH pointed out in great detail in his post (much better than I took the time to do) just .001 too deep is not a good thing. Are there other RV's flying that way (or worse) probably. In the end the final decision has to be yours.

I don't mean this as a flame, but you clearly didn't do it per the plans. The fact that you made the initial post shows that you must have thought something wasn't good. I will talk with tech support to clarify what is desired here because knife edge holes are expected but for a good rivet it must not be below the skin surface.
The bottom line is they are not going to lead you towards a decision that may result in a problem later, and then you may want to blame them.

The bottom (other) line is flush means flush. To make sure it doesn't go too far, .001 or .002 high would be better to shoot for and then sand them down during final prep for painting.

Yep, I read you loud and clear. I did intentionally go a bit deeper and that was the deal breaker. If you look at my picture, it is barely deeper than flush but enough to knife edge those holes and even enlarge some of them.
As I said, I was told by tech support to replace the skins to eliminate risk of leaks down the road. I think this is absolutely sound advice and I intend to follow it.
However, I do know that the plans are not always clear on the process and this is one area where specifics could be added to the manual to eliminate confusion. If it really is mission critical to set those rivets slightly high it should be specified in the manual. Setting the rivets high is not mentioned anywhere in the manual. There literally are a couple of sentences in the manual regarding countersinking and they are in a different section.
Another area is the tank and leading edge skin/rib assembly process. The manual outlines a process that is entirely opposite from what tech support recommended. These skins have already been replaced once due to that issue. Without getting to public regarding this, Vans has dealt appropriately with me regarding this but I don't know if the problem has been dealt with in the manual. I certainly know it is an area of frustration for many builders.
Anyway, life goes on, I learned an (unfortunately expensive) lesson and I hope to build leak free tanks...:D
 
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If it really is mission critical to set those rivets slightly high it should be specified in the manual.

It's not. What is critical (if one cares to do it right) is for the rivet to not sit low in a countersink. If it does, when the rivet is installed, it can not be set with it sitting tight in the bottom of the counter sink. If this is still not clear, take another look at Dan's detailed post. Particularly the diagram at the bottom.

If you had countersunk to a depth that resulted in the rivet being exactly flush you would have done what the manual said, and it would have done what is detailed on page 05-08 in Construction manual Section 5 If you look at this document you will see that it shows the hole knife edge if machine countersinking on .032 material. It also says that for material that is .040 or thinner, dimple countersinking is preferred, but that in certain instances you can machine countersink as thin as .032 for an AD3 rivet. That is what is recommended in this location because of the reason stated previously.

I only mentioned 1-2 thousandths high would be a way someone could assure they did not have any that were too deep. In reality, just a few slightly deep would probably not be a big deal. You said you purposely made them all deep and some were even deeper. If that wasn't actually the case then you might have been fine.

Building RV's is a learning process. Very few of us have the skills and knowledge required when we first start (myself included). Chalk this up as a learning experience.

Here is a tip I developed many years ago to that can be used as a shade tree method to determine how deep a countersink is.
- Countersink a hole
- Insert a rivet
- apply a piece of vinyl electrical tape over the the rivet (do not stretch the tape) and rub it down well.

If you see a raised spot in the shiny surface of the tape, the C.S. is too shallow. If you see a low spot in the tape, it is too deep. If you can see nothing (can not see where the rivet is under the tape) it is probably perfect.

The unfortunate thing is that I would guess you countersunk deeper because some well meaning builder or builders, (possibly right here on VAF) suggested it as a way to avoid having rivets sitting proud because of sealant under them. This may shock some people... not all advice from other builders is good (whether it is meant well or not).

I agree it sounds like you got some conflicting info regarding the countersink depths. Maybe it was a mis-communication, maybe not. At this point I don't see how it matters in your case because you specifically said you had already C.S. deeper than what the manual specifies. Tech support does monitor activity here, and the person you talked with may have already seen your original post (though I do not know that is the case), and gave you a recommendation based on that.

BTW, I grew up right in your neighborhood. Learned to fly at KANE (North Star Aviation), and hung out at Benson airport on weekends when they were flying gliders. That was more than 30 years ago... the airport has changed a lot since then. Up late with a pounding sinus headache.... hopefully this got me tired enough that I can sleep now.....
 
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In 0.032" sheet, both a countersink depth resulting in a perfectly flush rivet (CS diameter 0.0179") and a countersink cut to the exact standards of MIL-R-47196A(MI) (CS diameter 0.0176") result in a knife edge. The knife edge is not a problem.

BTW, an oops rivet will eliminate the knife edge (which is not a problem in this case), but it will not fill an over-deep countersink.

 
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The unfortunate thing is that I would guess you countersunk deeper because some well meaning builder or builders, (possibly right here on VAF) suggested it as a way to avoid having rivets sitting proud because of sealant under them. This may shock some people... not all advice from other builders is good (whether it is meant well or not).

BTW, I grew up right in your neighborhood. Learned to fly at KANE (North Star Aviation), and hung out at Benson airport on weekends when they were flying gliders. That was more than 30 years ago... the airport has changed a lot since then. Up late with a pounding sinus headache.... hopefully this got me tired enough that I can sleep now.....

Yep on the advice from well meaning builders:(. You win some you lose some I guess.
Benson airport is literally a mile from my house. They still are a very tight knit community but they are not flying gliders there anymore. Now they are at Osceola, WI. I think they had an off airport landing and it is getting to residential around Benson to risk that anymore. KANE is my favorite. N, S, E, W runways and tower control with a great view of the Twin Cities once airborne. Its hard to beat!

In 0.032" sheet, both a countersink depth resulting in a perfectly flush rivet (CS diameter 0.0179") and a countersink cut to the exact standards of MIL-R-47196A(MI) (CS diameter 0.0176") result in a knife edge. The knife edge is not a problem.

BTW, an oops rivet will eliminate the knife edge (which is not a problem in this case), but it will not fill an over-deep countersink.


Dan, Thanks for the pictures. It makes total sense. The rivet sits down in the hole and does not contact the sides of the countersunk .032 tight enough to allow a solid hold, which is why I am going for a do over.
Thanks for all your help gents!
 
As DanH pointed out in great detail in his post (much better than I took the time to do) just .001 too deep is not a good thing.

To make sure it doesn't go too far, .001 or .002 high would be better to shoot for and then sand them down during final prep for painting.

Are we really saying the skin rivets at the tank baffle have a flushness tolerance of +.002/-.001? A three-thousandths tolerance band? On a 2117-T4 soft rivet that likely deforms this much or more when setting? The aero-smoothness requirements of near-mach transport caterogy aircraft in acoustically-sensitive and flow-critical areas like engine inlets and fairings are not this strict.

How are homebuilders supposed to even measure this repeatably and precisely (on a contoured skin, no less) without many thousands of dollars in calibrated measurement equipment like the big manufacturers (and even they have trouble doing it)?

If this is truly the requirement, then not only is the manual in desperate need of a re-write, but the tanks just plain need to be re-designed. It's not reasonable to assume that any homebuilder, no matter how particular, will be able to meet such a requirement.
 
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Are we really saying the skin rivets at the tank baffle have a flushness tolerance of +.002/-.001? A three-thousandths tolerance band?

No.

I am saying that the countersink depth has a tolerance of + 0.0000, and - what ever you are ascetically willing to except (within reason).

The point is (for the fifth time in this thread?) that on a countersunk hole, if the hole is deeper than the thickness of the rivet head, the head will never install tight in the bottom/recess of the countersink.
When dimple countersinking this is regulated for us by the consistent depth of the dimples made by the dimple dies. When machine countersinking it is regulated by the person doing the countersinking.
My suggestion of adding a couple thou was to assure that it wasn't too deep (tolerance + zero) but not overly objectionable in how much the rivet heads protrude.
 
If it helps, mine are a combination of slightly proud and perfect (accidental no doubt). You can not tell they are proud unless you get right on the deck and examine at just the right angle.
"Precision" machine countersinking is an advanced skill, and this is one of the few, if not only places it is required, or recommended.
It is amazing to me that people like the legendary Art Chard countersank the entire wing, perfectly.
I'm not worthy!
 
I have found this thread interesting as it seems to be a spot where there is some confusion about what an acceptable tolerance might be.
I took two pieces of .032 and drilled a series of holes and clecoed the pieces together.
I countersunk some holes of different depths and before riveting I measured the hole size of the countersunk piece. All holes were knife edged. The simple act of countersinking enlarged all holes. In theory a flush rivet should have enough material left to maintain hole size. Using drill bits to measure this increase in size the flush, +1 and +2, samples had a hole size of a #36 drill. This demonstrates why a knife edge is not a good thing.
As the depth of the countersink increased the size of the hole increased so that at a 10 thou increase in depth the initial #40 hole had increased to the size of a #30 drill bit!
If one assumes that one click on a countersunk is .0001, I did one flush, one at one, two, three, six and ten thou deeper then flush.
I riveted the pieces together with a pneumatic squeezer set the same for all rivets. In all cases the shop heads of the rivets were within tolerance.
I then cut the test piece in half, right through the mid point of all the rivets so that I could check for rivet head depth and also to see if the "countersunk" holes filled with a compressed rivet.
After examining all the different cross sections the rivets expanded to completely fill all the cavities with the exception of the deepest, 10 thou, hole. Even in this case the two sheets were firmly attached the head was noticeably lower then the surface. Also on this exaggerated sample the underlying material was actually countersunk a bit. This resulted in the lower surface of metal being drawn up into the upper sheet.
Rivet samples of flush, +1, +2, and +3 in depth all ended up with a flush rivet head and the hole fully filled with no deformation of the underlying sheet.
Even +6 thou deep ended up with a rivet head that was very close to being flush and the holes in both sheets were fully full of compressed rivet.
Obviously as the countersink depth increases there is less material left in the top sheet and thus the bond would not be as strong.
Based on what I could see there is probably a margin of error of up to three clicks too deep before there were any visual differences.
A difference of three clicks on a countersunk is indeed a very small amount, and likely within the range of what is possible with the hand tools that we use.
Countersinking less and leaving a rivet slightly proud would help decrease the knife edge and enlarged hole issue. Proud rivets are quite noticeable and would have to be either sanded or shaved using a rivet shaver. As you shave the top of the head then you also lose the diameter of the top of the head and likely the holding power. Thus there will be a point where being too high also decreases the strength of the bond
 
After examining all the different cross sections the rivets expanded to completely fill all the cavities with the exception of the deepest, 10 thou, hole.

That's a great experiment Tom, and I absolutely agree; rivet material should indeed flow to fill the oversize countersink. However, the mechanics might change a bit when the empty dry space is already filled with a viscous, relatively incompressible fluid.

 
Slightly Shallow Countersink.

So, we are supposed to countersink a skin that is 0.032" thick, but seat a rivet that has a head depth of 0.036", and at the same time not create a knife edge or countersink into the baffle material below the skin. Asking me to fly to the moon would be more realistic.
To get around this, I made the decision to countersink the skin 2 to 3 thou shy of the dreaded knife edge. This means that I'm going to have about 6-7 thou of rivet head sitting above the skin surface, which I will then shave down with my rivet shaver bit in a heavy duty CS cage attached to my 25,000RPM die grinder. If I botch my skin trying to shave the rivets back, I'll probably strangle someone, so stand clear. I'll then resort to dimpling the replacement skin and taking my chances with potential tank leaks.
Let me know if you think I am completely bonkers before I pull the trigger on this one (I've already countersunk to 3 thou shy of knife edge, but can go further).
It seems in an ideal world there would be a rivet that better suited this application (i.e. something in between an AD426AD3 and an NAS1097) along with closer rivet spacing to account for the decrease out of plane strength due to the smaller head.
Cheers,
Tom.
 
Before you make your final decision, just consider all the thousands of RVs flying around with the tanks built per instructions.
 
.....
Countersinking less and leaving a rivet slightly proud would help decrease the knife edge and enlarged hole issue. Proud rivets are quite noticeable and would have to be either sanded or shaved using a rivet shaver. As you shave the top of the head then you also lose the diameter of the top of the head and likely the holding power. Thus there will be a point where being too high also decreases the strength of the bond

The Mil Spec allows shaving up to 0.006 off the head of a 3/32 rivet with no structural effect.

This gives a 0.161 head diameter which can be checked with reference holes as shown in a way earlier posting by Dan H.
 
Thanks Miles, but with all due respect, I can't see how you can possibly countersink an 0.032" skin and install a 0.036" rivet head without countersinking to knife edge or into the material below, which is apparently a no-no according to Van's. The math just doesn't work, nor does it in reality. Sure, there may be thousands of aircraft flying like this, but it doesn't make it correct according to standard aircraft building principles, which is what we are supposed to be striving for.
Thanks Gil for pointing out the acceptable shaving depth. That should allow the 2 thou shy of knife edge. I'll go back and check my countersink diameters today to make sure they comply with the 0.161" target.
Regards,
Tom.
 
numbers

Thanks Miles, but with all due respect, I can't see how you can possibly countersink an 0.032" skin and install a 0.036" rivet head without countersinking to knife edge or into the material below, which is apparently a no-no according to Van's. The math just doesn't work, nor does it in reality. Sure, there may be thousands of aircraft flying like this, but it doesn't make it correct according to standard aircraft building principles, which is what we are supposed to be striving for.
Thanks Gil for pointing out the acceptable shaving depth. That should allow the 2 thou shy of knife edge. I'll go back and check my countersink diameters today to make sure they comply with the 0.161" target.
Regards,
Tom.

Yes, but the 0.161 should be a should be a minimum, not a target, dimension.
 
Thanks Miles, but with all due respect, I can't see how you can possibly countersink an 0.032" skin and install a 0.036" rivet head without countersinking to knife edge or into the material below, which is apparently a no-no according to Van's.

I am curious where the .036 dimension is from.... is that the high end of the manufacturing tolerance range for a -3 rivet?
The reason I ask is that when I have ever measured them, I have always gotten a dim. very close to .032. I measured some today and was getting about .033-.0335.
It is for this reason that the machine countersinking portion of Section 5 in the construction manual says that machine countersinking .032 material for a flush -3 rivet will result in a knife edged hole. It is for that reason that the recommendation is made, that when you have a choice you should dimple countersink all material .040 or thinner, except for special circumstances. The tank skins is one of those (few) circumstances.

As has already been mentioned a number of different times in this thread, the critical factor isn't the material becoming knife edge, it is that the counter sink not be extra deep.
Any extra depth beyond what would produce a perfectly flush rivet will result in a rivet that is not set tight in the bottom of the counter sink.
It can't be; because any rivet set used against the head of the rivet, that is bigger than the diam. of the rivet head (as they all are), can only push the top of rivet head down to be be flush with the surface of the skin.
 
What do you mean when you say that many builders don't dimple properly? What are the common errors? How can we ensure that our dimples are airworthy?
 
Dimensions

I am curious where the .036 dimension is from.... is that the high end of the manufacturing tolerance range for a -3 rivet?
The reason I ask is that when I have ever measured them, I have always gotten a dim. very close to .032. I measured some today and was getting about .033-.0335.
It is for this reason that the machine countersinking portion of Section 5 in the construction manual says that machine countersinking .032 material for a flush -3 rivet will result in a knife edged hole. It is for that reason that the recommendation is made, that when you have a choice you should dimple countersink all material .040 or thinner, except for special circumstances. The tank skins is one of those (few) circumstances.

As has already been mentioned a number of different times in this thread, the critical factor isn't the material becoming knife edge, it is that the counter sink not be extra deep.
Any extra depth beyond what would produce a perfectly flush rivet will result in a rivet that is not set tight in the bottom of the counter sink.
It can't be; because any rivet set used against the head of the rivet, that is bigger than the diam. of the rivet head (as they all are), can only push the top of rivet head down to be be flush with the surface of the skin.

Scott,

The 0.036 dimension is the depth of the 100 degree head - dimension "B" on the first page here -

http://everyspec.com/MS-Specs/MS2/MS20000-MS20999/download.php?spec=MS20426L.022196.pdf

I think he is saying that if he has a 0.004 proud rivet head then the countersink will not penetrate unto the lower material layer when the upper skin is 0.032.
 
Scott,

The 0.036 dimension is the depth of the 100 degree head - dimension "B" on the first page here -

http://everyspec.com/MS-Specs/MS2/MS20000-MS20999/download.php?spec=MS20426L.022196.pdf

Thanks, that is what I was guessing.

As I already wrote, I have measured this dimension (B in the chart) on -3 rivets many times over the years and have never seen it more than .002 over .032 so it is generally possible to C.S. for a flush -3 in .032 and only have it .001-.002 above the skin surface.
In this application (tank baffle flange rivets) I have no problem with the rivets being perfectly flush (this would mean the C.S was cut .001-.002 deep which would very slightly enlarge the knife edged hole).
As already mentioned, the critical factor is that the C.S. not be even .001 deeper than the rivet height as this results in a rivet that is never set tightly in the C.S.
Since it is difficult to be perfectly consistent with depth, that is why I recommend shooting for the rivet being a couple thou high. This also reduces the likelihood of enlarging the hole.
 
Thanks Scott. That's what I've gone for. I went back and re measured my hole diameters, and they were at 0.150", so I took them an additional two clicks deeper, taking the CS diameter to just over 0.160" and leaving exactly 2 thou shy of knife edge remaining. I spoke with Stirling at Van's yesterday and he said that most of the rivets on the prototypes sit a little proud in this location, then they are sanded back to near flush, or at rounded so that there are no sharp edges protruding.
Thanks Gil for stepping in with the dimensions source.
Tom.
 
I have read this thread intently, since this is right where I'm at in the build. So I plan to leave the rivets slightly proud, as discussed. Could someone please elaborate on exactly how to sand the heads down and/or shave them down?
 
I guess that one is answered

Ok Wow... Good reading. I now know all there is to know about rivet dimensions lol.

Makes it tough that the 3/32 has a head depth of 0.036" and using it with 0.032" thick material you must avoid the knife edge.

Guess the general consensus is leave the rivets proud. I'm on board with this too. Thanks guys
 
rivets

Not sure if this has been mentioned (didn't read all the posts) but GAHCo sells AD type rivets that have shanks that are 1/64" (.016") oversized with the same head size. Sure beats using an oops rivets IMHO.
Rick
#40956
RV10 completed, waiting for spring to test fly
 
Here is what you end up with

So...If you do chose to proceed with countersinking the skin instead of dimpling. This is what a 0.162" diameter countersink looks like.
IMG_7721.JPG

And this is how proud the rivet sits
IMG_7715.JPG


For me...I think ill make up the gap of the proud rivet with a fillet of proseal.

Thanks for the advice guys!
 
Makes it tough that the 3/32 has a head depth of 0.036" and using it with 0.032" thick material you must avoid the knife edge
I realize the chart says .036 but when I measure rivets it is less.
The tanks I build end up with the rivets just a couple thou proud.
 
Think about this (not a rivet war)

Virtually all rivets used in the production of heavy aircraft are of the "reduced head" classification, and the head is approximately the same diameter is the next size down shank size...in other words, the head dimension for a -4 rivet is nearly the same size as the head of a -3 AN rivet. The base number is NAS1097, or BACR15CE. They are not common to the lightplane/homebuilding group, but at my day job, its all I use. My RV-4 has .032" full span skins, all countersunk as well as the all the stabilizers. I used the standard AN-3 rivets for them, but could have gone with the 1097's if I was nervous about knife edging..I wasn't. For the record, I worked with Boeing on many skin related failures due to knife edging, so I am aware of what and when its a deal breaker. I used reduced head titanium #8 CSK screws for all my tank skin screws and panels throughout, so I don't have any deep dimples..they are a bit harder to get, but I have access to them/ The base number for them is BACB30VF2K(length). The chances that knife edging on an RV will cause anything other than a few smoking rivets years down the road is slim. For the tank skins, the Proseal will virtually eliminate any of that. Build on per plans, don't leave the rivets too high. If one sticks up, hit it with a Scotchbrite wheel on a die grinder and move on.
 
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