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Please Demystify Dimples for me

Roarks

Roarks

Active Member
Okay, I have a few aircraft technician handbooks, read about dimples... and am even more confused.
I have read about coin dimples, modified radius dimples, and hot dimples. As far as I can tell what RV building folks are doing is some type of modified radius dimple...

So I bought a die set, bought a squeezer, and tried it out... I don't think I'm doing it right. So tell me where I went wrong.

1/8" c-sink rivet to be used.
-Material is clecoed together.
-Drill hole for dimple pilot
-lightly deburr hole
-put dimple die set in my Tatco squeezer... it didn't fit...
-okay male die got shorter on the stem below the spring till it fit in the squeezer
-squeeze the dimple onto the clecoed, deburred hole.
-put rivet in
-drive rivet.

Is that right or... was I supposed to dimple each sheet individually? ?? My books say that the dimples on modified radius won't nest together properly.

So what am I missing?

Thanks,
-Super newbie that wants his first tool box practice kit to be right.
 
Thanks BobTurner!
Is it the same die for both top and bottom? I'm guessing it has to be because I haven't seen a "top" vs "bottom" die on any of the tool sites? I ask because these dimple tools don't seem to be of the coining type...so what are they? Everything I have read, or watched (http://youtu.be/dyXEjn7f330 @4:35) says to do it "right" ya need a 100º & 110º dimple dies.
So after you match drill everything, ya take it all apart, deburr, diple where needed, re-cleco, and rivet. Does the the process go for every other rivet operation also? Drill, remove clecos, deburr, re-cleco, then rivet... I had it in my head that once clecoed, ya match drill, then you're ready to rivet, no need to separate the sheets.
 
Always need to separate sheets to deburr after drilling.

Cleco together, drill, separate, deburr, then dimple, prime if desired, re-cleco, rivet. Perfectly acceptable to use the same dimple dies for both the top and bottom sheets.

If there's anyone in your area who has experience building metal airplanes, make friends with them. Invite them to help you learn. You'll learn more in an hour or two from them than you will in weeks of going it alone.

You're asking the right questions and learning. That's what this is all about. Have fun!
 
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Important step

This is an important step to getting nice looking
Flat skins. Most often a squeezer does not create enough
Force to get a proper dimple. You should get a perfectly
flat skin around the dimple with no sign of defection around
The dimple. If you see deflection, try making the dimple with a hammer
And c frame. If you still see some deflection, buy a good set
Of spring back dimple dies from Cleveland Tool.
Buy also a tank die. You will need it for your tanks.
Use the tank die to dimple the material the skin is nesting into.
It will nest better.

When I was a new at this I made the mistake of
Under dimpling and thought the problem was to much
force when riveting when not getting smooth skins.
It was a dimpling issue. If you want your plane to look nice,
Do not under dimple. Use some force. Put the c frame on a solid
surface. Don't use a rubber hammer. Use a heavy weighted nylon hammer.
The orange ones they sell at harbour freight. Or better yet get a Drd2 dimpler.
 
Cleco
match drill
take apart
debur
dimple
reassemble and cleco
rivet.
Only debur once after drilling. there is nothing to debur after a dimple.
 
Buy a DRDT2.

I thought it was a lot of money for a machine that puts dimples in a hunk of metal, but for the amount of dimples in the plane it is well worth having it. Can't imagine doing an entire slow build with the C-Frame....

-Dan
 
advise

Welcome

krw5927 said:
If there's anyone in your area who has experience building metal airplanes, make friends with them. Invite them to help you learn. You'll learn more in an hour or two from them than you will in weeks of going it alone.

You're asking the right questions and learning. That's what this is all about. Have fun!
Click to expand...

+1
Kurt is right. Find a local builder. Also, practice on scrap both setting and drilling out rivets of various types 426 & 470. You might want to build the other Vans practice kit as well. It teaches techniques the tool box does not.
I hope your tool box comes out perfect. Mine is pretty bad!
 
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Welcome! If you have a set of preview plans, read Section 5 of Van's instruction manual if you haven't already. Van goes into nice detail about the basics of dimpling and countersinking that you'll need to do on an RV. It was a huge help for me when I learned how to rivet. (It was Section 5 for the RV-3 and RV-10, not sure if it's still called Section 5 in the newer kit instructions or not...)
 
Now I am confused . . .

OK, I think Roarks is onto something. I have been building a while and continually seeking to improve my finished rivet look. While working the rivets around the firewall a spacer had to be used for mounting the cloc's for cowl fasteners. The spacer did not want to fit neatly. I actually tried some experiments of dimpling two pieces, separately, and then matched them together, and they don't nest well. I tried, then, to dimple them together (after separately) and that worked to settle them to nest better. Finally I took my deburring tool and (after experimenting) made about 4 rounds and they nested perfectly.

So, I was thinking about a die that would form the dimensional dimple needed for the second layer a little better so the nest would work. Is that what this tank die does?

I am an engineer and really don't want to get into all the details, but dang-it, I want the finished rivets to look nice and if my training glossed over something, a little enlightenment would be appreciated.

PS -I am using Avery dies, and pneumatic squeezer.
 
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I've 're-dimpled' a few spots during final (riveting stage) assembly when the rivet didn't sit quite flush or sat at an angle, but I would blame that on either my bad technique in the initial debur/dimple operations, or some slight misalignment issues during assembly (problems were usually in highly curved areas).

I did the 're-dimple' process by inserting a rivet in the stack, holding a female die in a drilled bucking bar on the back side, & driving the rivet with a short burst from the gun. This seems to 're-align' the nested dimples.

If you want to buy a multi-hundred dollar extra tool to dimple, that's fine, but don't let anyone convince you it's needed to make perfect dimples. A c-frame & a heavy mallet (even rubber works; I can testify) with enough driving force will make perfect dimples, and can be very fast, once you get your technique right & really get into the swing of things. (insert groan here)

Charlie
 
You guys are overthinking this. The basic procedure posted by Mark,
(ie. Cleco, match drill, take apart, debur, dimple, reassemble and cleco, rivet) done with standard dimple dies (and perhaps tank dies on the fuel tanks for added depth) built thousands of beautiful RV's.

Nothing wrong with substructure dies and whatnot, but they are not needed IMO. The dimples will nest once you rivet as the clamping force is significantly higher than with a cleco.

And to the OP, ditch the generic youtube videos and checkout the EAA how to videos at www.eaavideo.org if you haven't already.
 
David Paule said:
Also see this
http://www.vansairforce.com/community/showthread.php?t=101941

It'll answer a few questions, I bet.

Dave
Click to expand...

Thanks Dave,
that fits my experience perfectly. I usually just spin the dimpled holes on the substructure with my deburring tool to make them nest perfectly, but that gets tiresome and suspected there was a more precise way. It was a real problem on the firewall perimeter with skin, firewall, spacer, and cloc to rivet.

I am too close to the end on this bird, if I build again, I know what to do. I think the next plane would only take 1/8 the time.
 
KatieB said:
Welcome! If you have a set of preview plans, read Section 5 of Van's instruction manual if you haven't already. Van goes into nice detail about the basics of dimpling and countersinking that you'll need to do on an RV. It was a huge help for me when I learned how to rivet. (It was Section 5 for the RV-3 and RV-10, not sure if it's still called Section 5 in the newer kit instructions or not...)
Click to expand...

Thanks Katie.
It has actually been recently updated. The updated version is currently only posted in the RV-14 updates section of the web site but it is relevant to all of the kits.
It can be found HERE.
There are more additions being worked on that will expand the info on other subjects (Rivet removal, being one in particular).
 
Let me throw in a plug for the EAA SportAir workshops. http://www.sportair.com/index.html

Before I started my -7 my son and I attended one in Frederick MD and you'd be amazed at just how much you can learn and how many questions can be answered in a weekend. It was worth twice the price.
 
Just for clarifications sake because I saw no mention of doing so by anyone on this thread:
Make sure you DEBUR BOTH SIDES of the hole! This is not to say that you are not doing so but none of the previous posts have pointed out the importance of doing so when deburring.
 
BillL said:
Thanks Dave,
that fits my experience perfectly. I usually just spin the dimpled holes on the substructure with my deburring tool to make them nest perfectly, but that gets tiresome and suspected there was a more precise way. It was a real problem on the firewall perimeter with skin, firewall, spacer, and cloc to rivet.

I am too close to the end on this bird, if I build again, I know what to do. I think the next plane would only take 1/8 the time.
Click to expand...

Bill,

I was afraid I might be on to something like you said...

I have gathered up just about every "aircraft" technicians handbook I can get my hands on and none of them say it's okay to use the same dimple on both sheets.

I used to work at a place with a fairly big maintenance department... and none of them knew about dimpling which I found hard to believe, until one guy showed me the line "CAUTION: Form countersinking equipment (coin and modified radius) is normally operated only by certified operators..."

Sounds like you had unsatisfactory results, but figured that's the way it was... If you build again what would you do?

BTW: I'm a Mechanical Aerospace Engineer... who happens to work at a machine shop... I could make a set of top and bottom dimple dies. ;)
 
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Roarks said:
Bill,

BTW: I'm a Mechanical Aerospace Engineer... who happens to work at a machine shop... I could make a set of top and bottom dimple dies. ;)
Click to expand...

The best dies are "springback". They are made to slightly "over dimple", to allow for the fact that the aluminum will 'spring back' slightly after having been pressed.
 
Roarks said:
I used to work at a place with a fairly big maintenance department... and none of them knew about dimpling which I found hard to believe, until one guy showed me the line "CAUTION: Form countersinking equipment (coin and modified radius) is normally operated only by certified operators..."
Click to expand...

Sorry, Roark. I'm not sure what you're implying here. Can you explain what you mean?

It sounds a little like you may be questioning the structural integrity of dimpling. The only complaint I've ever heard discussed at length is about aesthetic concerns, not strength or safety. In fact, I believe I've seen multiple claims that dimpled flush rivet joints are significantly stronger than machine countersunk ones, even from the hands of "uncertified" builders.

And that's a good thing, since the vast majority of the flush rivets in some 9,000 flying RVs are dimpled.

Update: See Bill Marvel's riveted joint strength tests for one example of the differences between dimpled versus undimpled (although it appears he didn't compare dimpled versus machine countersunk). Anyone else have any writeups for strength tests?

Update 2: Here's the 1942 NACA dimpling test doc (declassified!) which specifically discusses nesting issues, found via Cleaveland's site and their discussion of the design of their springback dimple dies.

--
Stephen
 
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Fatherson... I have no idea what made you think that I was questioning strength.

The question is: Do you use the same dimple die for the top and bottom skin?

If yes... where did you learn that was okay? I'm curious.

From what I have researched, using the same dimple die (100º be it spring back or normal) for both sheets being riveted is wrong. I'm looking for evidence to the contrary.
 
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Roarks said:
Fatherson... I have no idea what made you think that I was questioning strength.
Click to expand...

Well, you're right. That's why I asked you to clarify what you meant. I did make a subjective leap that you might be concerned about strength of the joint, even though you literally were only questioning if the two skins would nest properly.

In my understanding of my readings on the nesting issue, the concern is that if the two skins don't tightly nest, then that reduces the sheer strength of the dimple, and thus increases the sheer load on the rivet, with an overall reduction in the strength of that joint.

If you are not concerned about the relative strength of nested, dimpled skins, then what is your concern?

The question is: Do you use the same dimple die for the top and bottom skin?

If yes... where did you learn that was okay? I'm curious.

From what I have researched, using the same dimple die (100º be it spring back or normal) for both sheets being riveted is wrong. I'm looking for evidence to the contrary.
Click to expand...

It's not the springback portion of the dies that causes the skins to nest properly, but rather the relative sizes of the diameters of the male and female dies (as measured at the widest part of the impression, basically the "top" of the manufactured head of the rivet). See the NACA doc if my summary is unclear.

As for me, I use separately milled substructure dies for the bottom dimple, and standard milled dies for the top skin, but that's more because I like tools, like and trust Cleaveland's dies, and enjoy being an anal-retentive engineer, and not because I believe there will be any significant difference in the strengths of my joints.

Plenty of perfectly sound RVs are flying that were dimpled with one set of dies (well, you know what I mean: one set for each rivet shank diameter).

Your mileage may vary (but I seriously doubt it'll be by any noticeable amount). The fun of experimental aviation is you can build to whatever standard you're most comfortable. The docs I've seen, the trust I have in Van and his company's engineers, and almost 9,000 flying examples are sufficient proof for me.

--
Stephen
 
John R. Graham said:
This old WWII-era instructional video from Disney / Lockheed implies that it used to be accepted commercial practice that the top and bottom sheet dimples are formed with different tools. After discovering that, I decided to get a set of substructure dies from Cleaveland Tools.
Click to expand...

Awesome. Thanks for finding that, John. I recall seeing the Disney video early-on in my "aluminum apprenticeship." Perhaps that's what led me to buy the Cleaveland substructure dies, like you did. (I don't recall why I did, exactly.)

It's interesting to note though that the Disney / Lockhead method of improving nesting requires a different angle for the substructure dies. As far as I can tell, all the Cleaveland dies are 100* (except the 120* pop-rivet ones, which aren't related to this discussion).

So Cleaveland seems to be using the narrower-female-die-diameter method (like the NACA doc recommends) and then a wider-substructure-die-diameter to improve nesting (same angle, deeper dimple), instead of the shallower-substructure-angle method (like the Lockhead doc recommends). Perhaps I'm wrong about that. Mike Lauritsen frequents this forum, so maybe he'll weigh in. Does any vendor make 110* dies?

So to Roark's question, is there a definitive standard somewhere that says one of these methods is actually "wrong", or is this just a matter of different-strokes-for-different-folks?

(Not that I believe either answer is likely to make much of a difference. Even the NACA doc admits the angle of the dimple changes once the rivet is driven, deforming the dimples somewhat as it clamps them together.)

--
Stephen
 
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One thing I have noticed when using the "substructure" dimple die on the second hole - the hole dia is right at the top end of the hole tolerance. The hole is drilled with a #41 - any burrs are removed carefully.. before dimpling.... Just an observation..
 
There should be no angle difference in properly formed skin & structure dimples; simple geometry. If the dimple isn't fully formed on one of the pieces, that's a different issue, & different dies won't help that.

The only issue is whether the inside surface of the skin actually touches the flange of the structure. IIRC, there are even a couple of places in Van's instructions where the top layer is fairly thick, where he says to remove extra material in the bottom countersunk layer to allow full nesting.

Perhaps it's time for the reminder that this isn't rocket surgery. :)

Charlie
 
Adding a bunch of complexity to the dimpling thought process does nothing for demystifying dimpling, but I guess there is also nothing wrong with it if someone wants to invest the extra effort and $. In my opinion in most instances it does nothing for strength or longevity, but if it makes you feel better... go for it.

RV's have now been built for decades using equal size dimple dies for all material layers (what to do when there is more than two???).
A little know fact... when Van built the first RV-3, he did all of the dimpling during the riveting process. One hole at a time, he first drove the rivet against a drilled and countersunk bucking bar to reform the material, and then shifted the bar to drive the rivet. Pretty primitive even he admits, and it didn't look nearly as good as what has now been done for many years, but it worked.

The bottom line is that technique in both forming the dimples, and doing the riveting is 99% of the equation regarding strength and how good it looks.
Vs whether you used different sized dimple dies on different layers. In fact I would bet that it is undetectable by eye or strength measurement, if the same level of workmanship was used on either process.

Learn (I mean really learn) how to properly form dimple countersinks (my guess is, to some degree, probably 50% of builders don't). Then use your best riveting techniques to get it assembled. You will be flying that much sooner and the finish can be as good as anyone else has done.

BTW Section 5 of the construction manual has been updated recently. One of the changes was adding more info regarding dimpling and evaluating the finish quality. It is currently published on the web site HERE
There has been a lot of other stuff added also (electrical noise troubleshooting guide, etc)... Take a look.
 
Hi John R

Yes ..second hole, the hole below the top visible hole .... This was using the Cleveland 'sub structure' dies for the second hole ... another interesting bit of useless info ;-) .. I also have dies from Avery .. and I found a good 'nesting' of the two dimpled holes .. using the normal Cleveland dies for the first hole .. and dies from Avery for the second hole !! I guess that is why I am still building ... after lots and lots of years ...

rvbuilder2002 are spot on .... it all works .... and after all .. I am doing this because I enjoy it ... :) ...
 
rvbuilder2002 said:
BTW Section 5 of the construction manual has been updated recently. One of the changes was adding more info regarding dimpling and evaluating the finish quality. It is currently published on the web site HERE
There has been a lot of other stuff added also (electrical noise troubleshooting guide, etc)... Take a look.
Click to expand...

Scott,

You guys get my kudos on a much improved document. I wish I had this version seven years ago when I started.

bob
 
Technically fatherson is correct. The issue is that, due to the finite thickness of the material, and finite bend radii, the underside of a dimple is not an exact match to the topside. So ideally, for a second, lower piece of metal, you want to use a dimple whose top side matches the profile of the underside of the top sheet. On most RVs the material thickness is thin enough that the effect is small, and the final riveting can bend the metal enough to make them nest properly. As the material gets thicker, we usually go to countersinking. But in an industrial setting, where you might be dimpling thick pieces on hydraulic presses, this can be a concern.
 
Thanks folks! It was just something bugging me. I think fatherson said it best... it probably doesn't make a bit of difference... but I too have a engineer anal retentive tool loving personality (just picked up my compressor today!).
I think the best analogy I can come up with is that identical china soup bowls don't nest, but paper bowls nest perfect... when ya push them together. I guess I was just thinking that the aluminum sheet would be more like the china bowls.
Personally I think I'm going to get the substructure dies and try it out... and the DRDT is on my short list.
 
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