RV10Rob

RV10Rob

Well Known Member
Hi, all... last weekend was cracked rivets; this weekend, it's cracked elevator rib dimples. I'm back riveting the ribs to the skins and getting several cracks in the dimples. Here's a particularly bad one (most of them are barely visible):
ribcrack.JPG


Notice I'm a little off-center on the riveting, but it doesn't seem that bad. Also, I think my deburring is good. Any ideas? It's not too cold this time :)

-Rob
 
Similar cracked dimple issue

Rob, your picture looks quite familiar.

I put a crack into a dimple in the top half of the front HS flange, and I still haven't decided what to do. I did get some good suggestions (see http://tinyurl.com/yrglnl ) and I'm thinking that I'll probably stop-drill and put in a universal rivet on each side, for additional strength (luckily, I have no plans of making a show a/c).

Please post any advice you get (e.g. from Van's), and good luck with the "fix".

Regards all - - Tom
 
I did my elevator ribs not so long ago in chilly UK and had no problems. Looks from the photo like you have an oversize hole and have "caught" the edge as the rivet expanded. Are you perhaps being a little over-zealous with the deburring? The skins are very thin and just a half twist with the c/s tool is all that is required.
 
Not sure what the cause of that crack is, but you could probably just drill out the hole to #30 and put an oops rivet in there.
 
doubtful,

i dont think a 30 will clean that up. your only talkin .031 which equates to .0155 per side.
are you drilling the holes or reaming...some like to ream but this technology is designed for drilling....with a 30 or 40 drill bit.
not a 31 or 41
is that already an oops rivet? if so next time when using an oops on a thin skin dont set it all the way. or it can stretch and crack. not always but occasionally. YMMV good luck
 
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Deburring

Rob... post a picture of one of your typical holes after deburring....

Also, how many "hits" did you have with your 3x (my guess...:)...) rivet gun to set the rivet?

Could you be hitting the rivet with too few "big" hits?

gil A
 
Rivet too short?

Although difficult to tell from the pic, it looks like your rivet was too short. A short rivet can swell in the hole instead of forming a nice shop head, especially if you hit it too lightly, too many times, and work hardened the rivet.
Your dimple also may not have been formed all of the way, and/or, your two pieces seated together well before you riveted allowing the rivet to swell between the pieces, making the rivet look too short.
Same result, too much swelling of the rivet in the hole, and the shop head does not develop properly.
 
Thank you all for the replies. It does seem like the holes are a little big. I don't think I was too aggressive with the match drilling and deburring, but I might be wrong. Here's something I don't understand: for the 3/32 rivets, if you match drill with a #40, the rivet fits. If you then deburr slightly and dimple, don't both of those operations enlarge the holes? Would it make more sense to match drill with a smaller bit?

To answer the other questions...
- The picture is not of an "oops" rivet; just a standard AN426AD3-3.
- The rivets weren't too short before driving (pretty much exactly 1.5x width from top to the top of the dimple).
- I'm using a 2x gun... hard to remember the setting... maybe 30# and 1-1.5 seconds each?
- I don't have a picture of a deburred, dimpled hole, but I can take one when I get home

For the one, I'll just replace the rib. Most of the other cracks are tiny, so I figure I can drill out and use an "oops" rivet for those (unless there are too many for one rib).

I don't doubt my ability to fix the problem; I'm more concerned about figuring out what I'm doing wrong in the first place.

Thanks again, all...

-Rob
 
some say yes

RV10Rob said:
Thank you all for the replies. It does seem like the holes are a little big. I don't think I was too aggressive with the match drilling and deburring, but I might be wrong. Here's something I don't understand: for the 3/32 rivets, if you match drill with a #40, the rivet fits. If you then deburr slightly and dimple, don't both of those operations enlarge the holes? Would it make more sense to match drill with a smaller bit?

To answer the other questions...
- The picture is not of an "oops" rivet; just a standard AN426AD3-3.
- The rivets weren't too short before driving (pretty much exactly 1.5x width from top to the top of the dimple).
- I'm using a 2x gun... hard to remember the setting... maybe 30# and 1-1.5 seconds each?
- I don't have a picture of a deburred, dimpled hole, but I can take one when I get home

For the one, I'll just replace the rib. Most of the other cracks are tiny, so I figure I can drill out and use an "oops" rivet for those (unless there are too many for one rib).

I don't doubt my ability to fix the problem; I'm more concerned about figuring out what I'm doing wrong in the first place.

Thanks again, all...

-Rob
Click to expand...

but this technology is designed to work just as all the books show. you should only debur 1 to 2 turns max with light to no pressure the idea is to simply remove the burs not counter sink the hole. if you enlarging the hole you are overdoing it.
 
Is this a problem you're more likely to see on driven than squeezed rivets? Is it something that would be rare on the thicker skins (.032 and thicker)? It would seem so to me, but I'm just getting started. Seeing this issue crop up makes me a little nervous now because I didn't closely inspect all of my driven rivets all the way around their circumference when putting my vertical stabilizer together.
 
No worries

tim.randles said:
Is this a problem you're more likely to see on driven than squeezed rivets? Is it something that would be rare on the thicker skins (.032 and thicker)? It would seem so to me, but I'm just getting started. Seeing this issue crop up makes me a little nervous now because I didn't closely inspect all of my driven rivets all the way around their circumference when putting my vertical stabilizer together.
Click to expand...

I think this degree of cracking is unusual, but to my knowledge, there has never been a structural failure in an RV due to rivets unzipping themselves as a result. Crack probogation has also not been linked to this issue that I am aware of either.
You are not the first to not fully inspect all of their dimples to this degree.
If I had ANY cracked dimples, I never noticed them and I doubt you and I are alone here.
 
Summary

Looking again at your photo and the other posts I would make the following observations:

Be careful with the deburring so that the hole does not get enlarged or too thin on the edge - just a half twist and return on each side with a 3 blade c/s.

Yes, dimpling will enlarge the hole slightly. However, despite the match drilling, nothing fits perfectly and the slightly oversize hole makes getting the rivet in easier. Make sure you get sharp dimples so the rivet sits nicely in the middle.

If 30# means 30psi, I would say you are using too little pressure. I use about 50 on a 2x gun and a shorter burst. The shop head looks poorly formed and work-hardened.

Not that I am particularly experienced but I have just finshed my -10 elevators, in the cold, with none of the issues you are having.

Hope this helps.
 
Cracked dimples

Rob,
1. Might be abit too aggressive with the deburring. Just clean the hole don't bevel. To eliminate this 'beveling' Avery and others sell #40 & #30 Reamers. Save yourself a BUNCH of time and just ream the holes - separately - you do not HAVE to 'match drill', except where no hole previously exists. Just about ALL of the builder's assist people use this technique. I have since starting my 9A and it is wonderful. Rarely, if ever, will you need to debur a reamed hole. Your crack may have happened while dimpling. Check your dimple dies to make sure that they are clean. I found that SB(spring back) dies give me a consistantly better dimple. Was your skin flat, supported, while dimpling?
2. My 9A elevator, I'm almost postive of this, used 3-3.5 rivets, you stated 3-3 which would be short. Can't imagine a 10 uses shorter rivets than a 9A. Also remember-the plans are only a GUIDE. Not Gospel. If you think that a longer rivet is needed-use it. I found many places that a longer rivet was more suitable and was happy with the final result. Just remember that too long of a rivet will want to lay over, and that IS bad.
3. One rivet does NOT an airplane make. These planes are so OVER engineered that some goofs are factored into the design. Strive for perfection, but being reasonable will keep you sane. If there is any builder out there, even Van's, that says every one of those 12,800+ rivets are perfect-I'll buy them a beer.
4. Don't get discouraged. It's a learning curve. You're asking good questions, and you've come to the right place. We've all been there, or are currently there. Just ask Scott at Van's about how many calls I made over the past two weeks about my slider construction.
Mike H 9A/8A
 
Thanks again for the advice. I ended up drilling out the few ribs I'd done. I then experimented by drilling two new #40 holes in one of the ribs, deburred only one of them, then dimpled them both. Both new holes were roughly the same size, and they were also the same size as the holes in the ribs I hadn't yet riveted. I then drilled, deburred, and dimpled a few more new holes and riveted the rib to a scrap piece of skin with no problems. Now I'm wondering if the cracks may have happened during the deburring process.

In any case, my plan now is to examine all the remaining ribs carefully, use a half size longer rivet, "pre-squeeze" it slightly, rivet the ribs to the skin, and inspect again for cracks. A complete new set of ribs is on order in case this doesn't work out (and to replace the one with the big crack).
 
Well, after working at it some more, the only way I can get acceptable results is:
* use the -3.5 rivets "pre squeezed" in the squeezer. The plans call for -3s, but when the shop head expands it "pushes out" the sides of the dimple on one side, causing the cracks and/or a head that doesn't actually sit on top of the material. The larger rivets form a good shop head.
* use the 12" back rivet set, so it's super-obvious if I'm slightly not square to the surface. Probably not necessary, but it works well for this purpose.

Unfortunately, the ribs halves I drilled out have too many cracks to be usable, so I'll be making a new set (the other halves have all been riveted to the skins successfully). I really wish I knew why I've had so much trouble with a part that most people seem to not have issues with. It seems like it's really difficult to back rivet against these 0.020" ribs, stretching the dimpled hole, without cracking. Even the last couple I did have some black lines visible (may just be primer cracking due to stretched aluminum), but with so much stress on the holes, I can imagine there are at least cracks that can't be seen.
 
...and for the sake of comparison with a "good" rivet, here's more of what I've been dealing with:
badbackriveting.JPG
 
Strange....

Rob... on the top rivet in the picture, it looks like the inner dimple is collapsing into the external sheet dimple.... Really strange.

Have you tried using someone elses dimple dies?

Or is your deburring really a countersink and you are actually thinnning out the metal at the hole edges...

Can you post of pic of your typical deburred hole, before and after dimpling?

gil A
 
Standard Aircraft Handbook

Why does there seem to be a complete disregard for the Standard Aircraft Handbook's statement that "Deburring shall not be performed on predrilled holes that are to be subsequently form countersunk."?
Seems pretty cut and dry to me, you're adding additional steps, while weakening the structure. Maybe I'm missing something here.:confused:
 
Rivet Set Not Centered

In your pic I can tell your back rivet set is not centered on the rivet. This is what's causing the shop heads to push towards the rib web (and causing the dimple collapse as previously mentioned - we had the same problem). The ring left on the rib from the nylon 'cage' indicates you're not able to get the set close enough to the web of the rib. This is probably due to the cage's thickness. We sanded a flat spot on one side of the cage that allowed us to get the set over the center of the rivet without it interfering with the rib web. Also, as you mentioned, the longer set really helps with ensuring you're perpendicular. Good Luck!
 
You're right about not having the tool centered. I realized that may be a problem, so I've been paying special attention to keeping the end held in place while riveting. Since the set has a flat bottom, as long as its kept perpendicular and doesn't move, it shouldn't matter too much whether it's centered (as evidenced by the good shop heads I've been getting lately).

As for why everyone deburrs holes before dimpling, I see this has been discussed on this board before, with no real conclusion: http://www.vansairforce.com/community/showthread.php?t=12661. I tried skipping deburring for a few holes on a couple pieces of scrap, and the results weren't good--very rough edge and an occasional tiny crack already forming.

Here are a couple pics of a dimple before priming or riveting. Sorry, I don't have a picture of a deburred hole before dimpling, but I promise I'm not doing much deburring.
dimple1.JPG

dimple2.JPG
 
It's very hard to tell from a photo, but something looks odd about that dimple. My dimples are shaped like a frustum of a cone (a cone with the top chopped off) while the one in the picture looks more funnel shaped with sides that seem to curve inward. Where'd the dimple die come from?.
 
References....

captainron said:
Why does there seem to be a complete disregard for the Standard Aircraft Handbook's statement that "Deburring shall not be performed on predrilled holes that are to be subsequently form countersunk."?
Seems pretty cut and dry to me, you're adding additional steps, while weakening the structure. Maybe I'm missing something here.:confused:
Click to expand...

Maybe... but the approved FAA references mention that all holes must be free of burrs before riveting...

Do you really think that you will get a good dimpled hole if you start off with a big honking edge burr before you squeeze/hit it with your dimple dies?

Common sense says not....:)

The term "form countersinking" does not even appear in the FAA AC 65-15 document for Airframe Mechanics knowledge...

The Mil-Spec for riveting does give a requirement for deburring....

Burrs may be removed from rivet holes by chamfering to a depth not to exceed 10 percent of the stock thickness, or 0.032 inch, whichever is less.

... we are not talking about it even looking like a countersink with these numbers...

http://home.earthlink.net/~gilalex/rivet_spec/rivet_a.htm

Who approved the "Standard Aircraft Handbook"?

gil A
 
az_gila said:
Maybe... but the approved FAA references mention that all holes must be free of burrs before riveting...

Do you really think that you will get a good dimpled hole if you start off with a big honking edge burr before you squeeze/hit it with your dimple dies?

Common sense says not....:)

The term "form countersinking" does not even appear in the FAA AC 65-15 document for Airframe Mechanics knowledge...

The Mil-Spec for riveting does give a requirement for deburring....

Burrs may be removed from rivet holes by chamfering to a depth not to exceed 10 percent of the stock thickness, or 0.032 inch, whichever is less.

... we are not talking about it even looking like a countersink with these numbers...

http://home.earthlink.net/~gilalex/rivet_spec/rivet_a.htm

Who approved the "Standard Aircraft Handbook"?

gil A
Click to expand...

OK folks, de-burr away on .028" ribs, but not to exceed ten percent, or .0028", less than three thousandths of an inch! Common printer paper measures about four and a half thousandths of an inch thick, so exactly what tool is there to check this with? Anyone have a part number handy from Avery or Cleveland so I can order one and appease the FAA?
In lieu of finding a proper tool, my lovely bride has devised a formula for determining the maximum depth in .028 by measuring the outside diameter of the bevel. To not exceed ten percent, or .0028" of depth, you can measure the hole diameter, add .00871" to this number and the sum would represent the maximum outside of the bevel. Example: a .040" diameter hole could be de-burred with a 100 degree countersink or de-burring tool, and as long as the outside of the bevel is .04871" or less, you haven't exceeded the maximum depth.
Of course, this may prove to be difficult and tedious, but I guess the FAA will be pleased!
 
Sensible actions...

captainron said:
OK folks, de-burr away on .028" ribs, but not to exceed ten percent, or .0028", less than three thousandths of an inch! Common printer paper measures about four and a half thousandths of an inch thick, so exactly what tool is there to check this with? Anyone have a part number handy from Avery or Cleveland so I can order one and appease the FAA?
In lieu of finding a proper tool, my lovely bride has devised a formula for determining the maximum depth in .028 by measuring the outside diameter of the bevel. To not exceed ten percent, or .0028" of depth, you can measure the hole diameter, add .00871" to this number and the sum would represent the maximum outside of the bevel. Example: a .040" diameter hole could be de-burred with a 100 degree countersink or de-burring tool, and as long as the outside of the bevel is .04871" or less, you haven't exceeded the maximum depth.
Of course, this may prove to be difficult and tedious, but I guess the FAA will be pleased!
Click to expand...

Ron... the other sensible thing to do would be to just remove burrs....:)

The point of the spec. quote was to not have builders over use a countersink to remove these burrs...

The "finger test" would be good... if you can feel a burr, remove it....

Quite different from your proposal to just leave the burrs were they are....:rolleyes:

gil A
 
I'm just saying that it's almost impossible to put a cutting bit to this metal and not overdo it. I have been just reaming the holes and then running a shop rag along them- that seems to be all they need. I think the real key here is using the reamer. there is a lot of precision cutting area available here, not so with the iffy point of a drill bit.
 
Perhaps there is a market here for a tool with a de-burring bit enclosed inside a sleeve so the cutting area only extends to the max cutting depth; in this case it would be .0028" the outer sleeve could be hardened and polished so as not to wear and to protect the surrounding surface. Kind of like a countersink cage, only much more compact and fixed depth. Probably only need two or so dedicated sizes for the most commonly used thicknesses of material. Maybe someone at Avery is watching this. I'd buy 'em.
 
The aircraft manufactures that are here that make Beavers and Twin Otters and others on their thin skins don't ream the holes they drill and deburr! I, like them use a small drill bit to deburr and it works just fine.
 
Dimpling

Hi Rob, from the last picture of your dimple it looks like it could have been over dimpled which could work harden the inner edge and when you squeeze the rivit it streches the inner edge and cracks it. How are you dimpling? using a C squeeze or one of the "C" frame hammering type? The only other thing that comes to mind if somehow the rivit is over squeezed or squeezed again at a later time after it has work hardened. I used to have to make Stainless Steel fuel/hydrolic lines, and if you didn't deburr and polish the end of the tube it would crack everytime when you flaired it, but you couldn't over do it, because the edge would be too thin and also crack. I have found most people over do a rivit, too hard on the dimple dies and hammer the rivit too long, using the thought that if a little is good a lot must be better. Always go back to the basics, re read the instuctions. Hope this helps.

To all on this web site, have a very Merry and Safe Christmas.

Randy
 
Clearification

Just though I would tell you why I think that the dimple is over done, I see a very solid ring left by the dimple die, usually it isn't that prevliant.

RT
 
Hi, Randy... the consensus seems to be that it's impossible to "over dimple" if the dies are used correctly. Otherwise, it seems like it would be impossible to get consistent results. For the record, I'm using the excellent DRDT-2.
 
Borrow another set of dies?

RV10Rob said:
Hi, Randy... the consensus seems to be that it's impossible to "over dimple" if the dies are used correctly. Otherwise, it seems like it would be impossible to get consistent results. For the record, I'm using the excellent DRDT-2.
Click to expand...

Rob... can you borrow another set of dies and repeat the test? gil A
 
Question for you. Are you drilling the holes with a number 40 drill bit or just dimpling the pre-punched holes? I have seen this happen when the hole is actually too small for the dimple dies.
 
I tested with another set of the same dies (cleveland, marked "CAT 426-3", which are the standard 3/32 dies), and got the same result. I also tried with the main squeeze--same result. They really don't have the same "volcano" look as in that picture... I'll try to take another picture when I get some more time among the Christmas chaos.

I also got out my sample aileron project from the construction class I took earlier in the year. For that, I used someone else's equipment, and the dimples look the same.

To answer the question, I'm not just dimpling the punched holes... I'm drilling to #40 first, and without exception the holes have dropped right onto the dies without any force needed.

Thank you all again for your help... I think I just needed some practice backriveting. Too bad my elevator ribs were the guinea pigs.

Merry Christmas!

-Rob
 
dimple problem

RV10Rob said:
You're right about not having the tool centered. I realized that may be a problem, so I've been paying special attention to keeping the end held in place while riveting. Since the set has a flat bottom, as long as its kept perpendicular and doesn't move, it shouldn't matter too much whether it's centered (as evidenced by the good shop heads I've been getting lately).

As for why everyone deburrs holes before dimpling, I see this has been discussed on this board before, with no real conclusion: http://www.vansairforce.com/community/showthread.php?t=12661. I tried skipping deburring for a few holes on a couple pieces of scrap, and the results weren't good--very rough edge and an occasional tiny crack already forming.

Here are a couple pics of a dimple before priming or riveting. Sorry, I don't have a picture of a deburred hole before dimpling, but I promise I'm not doing much deburring.
dimple1.JPG

dimple2.JPG
Click to expand...

Rob,
What is causing the indented ring around the dimple? It actually looks in the photos that the skin is cut, like by a fly-cutter. I have never seen this before. I believe you, that you not deburring too much. The photo leads me to believe it is the dies that you are using. $28 to Avery/Cleveland/Brown for some Spring Back dies could solve your problem. Are there any RV builders in your area that you know? Gotta be some in the Seattle area. Why not try, before doing anything else, hook up with a local builder and let him try using your tools on hand. Barbara at Van's or your local EAA chapter should be able to lead you to some local builders if you don't know any. Maybe you should go this route and get someone on site that can get a 'hands on' view.
Mike H 9A/8A
 
mike109g6 said:
Rob,
What is causing the indented ring around the dimple? It actually looks in the photos that the skin is cut, like by a fly-cutter. I have never seen this before. I believe you, that you not deburring too much. The photo leads me to believe it is the dies that you are using. $28 to Avery/Cleveland/Brown for some Spring Back dies could solve your problem. Are there any RV builders in your area that you know? Gotta be some in the Seattle area. Why not try, before doing anything else, hook up with a local builder and let him try using your tools on hand. Barbara at Van's or your local EAA chapter should be able to lead you to some local builders if you don't know any. Maybe you should go this route and get someone on site that can get a 'hands on' view.
Mike H 9A/8A
Click to expand...

Thanks, Mike... Agreed, the photo does look like there's an indented ring around the dimple; however, that's not the case (it's just shiny). Next time I think I'll take several pictures from several angles to avoid confusion. :) I am using Cleveland dies, and my dimples don't look any different than other dimples I've seen.

In any case, I definitely agree issues like these are best addressed in person with a fellow RV builder.

-Rob
 
I am having the exact same problem. I think part of the problem is the dimple dies. I have a pair in a vice grip that I got from aircraft spruce that the dies don't close perfectly and the dimple is sort of odd shaped with one side higher than the other. I think this is causing some of the rivets to lay over. I'm going to check it over and maybe try a different hand dimpler. It's really only been 2 rivets that I noticed have cracked the dimple but quite a few that the rivet lays over. Still a good rivet but looks bad. Want to get this problem fixed soon.
 
I think something is wrong with your dies. The dimple looks too much like a angled cone than straight edges, sorta like the male die on the was ground undersize and the flat outer edges are contacting before the straight edges of the cone is formed in the inside of the female die. The colapsing dimples would also indicate something is not right, perhaps since the two sides of the dimple are not straight, there is a gap that is being squished down as you rivet. Perhaps it the picture, but is the backside of the dimple fully contacting the inside of the female dimple die as it should? This is the best picture I could find of my stiffeners, but it showes how the backside of the dimple is still sharpley formed after riveting and the expanded rivet head is on top of the dimple where the pic of yours appears to be flattening the dimple out. These dimples were made with Avery Spring back dies on a DRDT Cut and past this link, I left it sized really large for clarity. http://www.dslextreme.com/users/mwss/rv/100_0555.jpg
 
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Hmmm....

The close-ups of the dimpled holes actually look to be UNDER dimpled. That's why the curved look. The small part of the male dimple cone is bending (rolling) the sheet into the female die - just not all the way - instead of forming it in full contact between the two dies. You may have bad or mis-matched dies. I'd try another "proven" set and do several test holes in some same thickness scrap. Light rap, moderate rap, hard rap, and "your gonna break the table" rap - just so you can see the differences in what comes out.

As for the cracks?!? Dunno. POSSIBLY (not probable) the sheet is the wrong alloy or temper. Or a dirty die or one with a burr on it (defect). As above... try a set of "proven" dies. One last thing - some dies are not marked and can get mixed - a #4 screw female and a -3 male, etc. Close, but no banana.
 
I'm not 100% sure but now that I think about it, The one crack that I had, I think, was in a hole that I had to drill out the rivet because it was a bad one. I'll bet that when I did, the dimple was work hardened and and not in the right shape anymore and the replacement rivet cracked it when I bucked it. Like I said not 100% sure but it does sound like that would be a plausible reason.
 
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