[Brambo]

I'm just starting and RV-7 and was discussing different building techniques with some other builders when one mentioned that some builders of the new kits were not drilling out holes that were to be dimpled. It seems the dippling process enlarges the hole just enough for the rivit to fit. This would obviosily save time (no drilling or deburring), but I wonder if there are any drawbacks to this method. Any thoughts?

 

[dan]

Crackety crack

The only drawback would be stress cracks on every rivet hole. But sure, go right ahead!!!

Give it a try on ONE hole (just one, this is just an experiment) and take a magnifying glass to the hole after you dimple it. Like what you see? I thought not!!!

 

[TJoyner]

Page 5-4 of the builders manual covers this area. Apparently there are "shear marks" left by the punch process that can cause cracking around the hole due to the stretching the dimple dies will cause if not drilled first.


TJoyner

 

[az_gila]

Give it a try on ONE hole (just one, this is just an experiment) and take a magnifying glass to the hole after you dimple it. Like what you see? I thought not!!!

And then try a light deburr and repeat the experiment.....

Then try a #41 drill and a light deburr and repeat the experiment....

Let us know what you find...

gil in Tucson

Tech. Counselor

 

[dan]

Loose fit of rivet in hole

I hear builders talk about how "loose" a 3/32" rivet fits in a dimpled #40 hole. Yup. It fits loosely. But people talk about that like it's a bad thing.

Part of the process of squashing a rivet is that the shank swells to fit the hole. That's actually the first stage of deformation as you set the rivet. That happens before a shop head is formed.

If you use a #41 drill bit, you save two thousandths of an inch assuming you drilled the hole perfectly, or assuming you keep your #41 reamer exactly square to the work.

To those who believe using a #41 instead of a #40 is some panacea, like it's going to add longevity to your airframe or lend to a stronger joint in some fashion...get real!

I'm not denying that a 3/32" rivet will fit a little tighter in a #41 dimpled hole than in a #40 dimpled hole. But the difference is made up in the shop head. In other words, the rivet will swell more in a #40 hole, and the bucktail will end up being sliiiiiiiiiiiiiightly smaller than if you used a #41 hole. More rivet "volume" is consumed swelling up the sliiightly larger hole...less available to form the bucktail.

Big deal? I think not. You won't be able to tell the difference in shop head size, and as far as the rivet/hole is concerned, the rivet swelled to fill the hole. So why the fuss?

Hey, do whatever you're comfortable with. But I'm sticking with the #40 drill bit. KISS.

 

[Brambo]

Thanks for the info. I was pretty sure it wasn't a good idea, but thought I ask.

 

[PJSeipel]

I'm not denying that a 3/32" rivet will fit a little tighter in a #41 dimpled hole than in a #40 dimpled hole.

Assuming you have a #40 dimple die, with a #40 pilot on the end of it, don't you end up with the same size hole whether you drill it #40 or #41?

 

[az_gila]

Rivet lengths

Assuming you have a #40 dimple die, with a #40 pilot on the end of it, don't you end up with the same size hole whether you drill it #40 or #41?

It's probably a tolerance issue... my dimple dies (multiple sources) fit a #41 hole. If the die pilot doesn't slip into the hole, use a larger hole.... If the die slips in, a bigger drilled hole will form are bigger hole after dimpling. the die pilot should not be used as a punch...

As far as Dan's comments on the rivet expanding to fit the hole, he is correct, but if the hole is larger than that specified for the rivet, then a longer rivet is probably called for.

I found (old kit, no pre-punching, #41 holes usually) that a rivet length that was a 1/2 size longer than the plans usually worked better - this is for the smaller flush 3/32 rivets.

A longer rivet gives more tolerance on rivet "hitting time" since a larger diameter shop head can be formed without reaching the minumum shop head thickness. The only drawback to slightly longer rivets is the chance of "tipping", but operationally this seems much less likely with flat heat rivets in a dimpled hole.

Make it easy on your self, rivets are cheap, buy the half sizes and give yourself the extra tolerance, especially if you are a beginner. Rivets that are over driven, or under driven, so that the shop head tolerances are not met will be weaker.

As always, rivet specs. are available here...

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

gil in Tucson

 

[dan]

Assuming you have a #40 dimple die, with a #40 pilot on the end of it, don't you end up with the same size hole whether you drill it #40 or #41?

The pilot is 3/32" or thereabouts, as is the rivet shank. #40 and #41 are both a little larger than the pilot & rivet shank.

 

[dan]

Use primer? May need longer rivets

... if the hole is larger than that specified for the rivet, then a longer rivet is probably called for.

Likewise, if you have a smaller hole, then a shorter rivet is called for. But I don't believe #41 versus #40 makes any noticeable difference on the rivet length required.

The point about using a slightly longer rivet than called out is a good one -- because often Van's rivet callouts are too short. But! My experience is that this is often due to primer. If you don't prime, or if you go extremely light on primer, Van's called-out rivets work fine. If you prime heavily, you may need a longer rivet. I think this is what gets most builders, because most newbies tend to over-prime (thickness/coat wise).

My hangarmate, who is not a newbie but is extremely conservative, goes pretty heavy on primer. He almost always needs one length size up rivet when we rivet his airframe.

 

[jcoloccia]

I'll just make a point here: Other than a couple of failures early on (that have since been addressed), ALL of the structural failures that I know of have been due to yanking on the stick too hard or forgetting to install rivets/bolts entirely. I stand to be corrected, but I don't know of any RV's that are falling out of the sky due to underset rivets, squashed rivets, holes being out of tolerance by a couple 1/1000ths of an inch, etc etc etc.

Van's not stupid...I suspect he's taken into account that most of the people building his airplanes have never seen a rivet gun before. This is why there are 4500 RV's flying around the world.

I'm not saying do shoddy workmanship, but personally I don't stay up nights worrying about little stuff like this.

Take it with a grain of salt...my RV's a couple years away from flying (that would make it 4501 )

 

[jcoloccia]

The only drawback would be stress cracks on every rivet hole. But sure, go right ahead!!!

Give it a try on ONE hole (just one, this is just an experiment) and take a magnifying glass to the hole after you dimple it. Like what you see? I thought not!!!

fyi: I did this on some scrap when I first got my DRDT (was too lazy to drill out the hole just to do a stupid test). About 1 in 10 holes had visible cracks. The rest of seemed OK but I didn't look that closely.

Once again, Dan's right

 

[plaurence]

The only drawback would be stress cracks on every rivet hole. But sure, go right ahead!!!

Give it a try on ONE hole (just one, this is just an experiment) and take a magnifying glass to the hole after you dimple it. Like what you see? I thought not!!!

I'll second that. Also, be careful when dimpling the The fuel tank holes for the # 8 screws. If one dimples the # 19 hole with the # 8 dimple die, you may end up with some unacceptable cracks. One way to limit this is to drill the holes out to a # 17 drill bit. The drawback here is that the dimple will be slightly of fcenter in the enlarged hole. I minimized this problem by leaving the protective
plastic on, drilling the 1/8" holes with a couple of intermedate drill sizes to a # 19. Dimple the holes with the plastic in place. Remove the plastic and re dimple.I ended up with only two dimples that had minor cracks and filed them out with a jeweler's file.


Peter Laurence

 

[az_gila]

Quality matters...

I'll just make a point here: Other than a couple of failures early on (that have since been addressed), ALL of the structural failures that I know of have been due to yanking on the stick too hard or forgetting to install rivets/bolts entirely. I stand to be corrected, but I don't know of any RV's that are falling out of the sky due to underset rivets, squashed rivets, holes being out of tolerance by a couple 1/1000ths of an inch, etc etc etc.

I'm not suggesting lack of sleep for anyone....

What I am suggesting is that if there is a better way of doing something, then do it that way... We are building aeroplanes here.

I know of several RV that DO have loose, smoking rivets - another concurrent thread talks about a RV-4 with cracks in the wing skins radiating from rivet holes (dimpled, I bet). I've seen flying planes with bad rivets that get loose, and the repair effort is a pain when the RV is all assembled and painted. Poor electrical connections cause a lot of difficult troubleshooting and down time for flying planes - often caused by cheap automotive crimps...

Our goal should be to do the best job we can.

If I went out to see a plane under construction as a Technical Counselor, and the builder did not have that attitude, I personally would try and persuade him otherwise, and if that didn't work, I would not visit him again.

Luckily, it's never been a problem for me....

An "it's OK, this bit doesn't matter" attitude spreads out to a general loose attitude. We don't need to build show planes, but we do need to build structurally sound planes, and we don't know the design margins that Van used in every area.... Those of us who yank-and-bank need to be extra careful...

The old saying "It's always easier to build it right first time than fix it later" is still true for RV building.

gil in Tucson

 

[ptrotter]

There is a lot of room for personal choice and innovation when building an experimental aircraft, but in my opinion, construction techniques is not the place for this. The methods that Van's recommends are tried and true methods that are standard in the industry and have been developed over years of experience and testing by people who are much smarter and better equiped to make these determinations than most of us amateurs. If a #41 drill were really a better choice then that would have been determined by Boeing or Lockheed or some other entity that builds to very high standards. Everyone should have a copy of "AC 43.13-1B" and the "Standard Aircraft Handbook". These give a pretty good idea of the accepted practices. There is plenty of room for improvement in even Van's aircraft, but construction practices ae probably not the best place to spend time.


Just my opinion

 

[Captain_John]

WHOA!

No way! I am just skimming through this thread and cringing as I read some of the thoughts that are actually set down before me!

These practices have been time tested, documented, analyzed, scrutinized, NDT'ed, DT'ed, magnafluxed, X-Rayed and accepted as a military standard!

Far be it from me to say that we can safely skip a step. Dare I say, far be it from most anyone else!

This is to say NOTHING about the real reason for drilling the hole is to MATCH UP the parts!

Am I off base here? Is it okay to have an opinion?

CJ

 

[az_gila]

Tolerances

If a #41 drill were really a better choice then that would have been determined by Boeing or Lockheed or some other entity that builds to very high standards.
Just my opinion

Actually, I published the specs. you are referring to in an earlier post.
This has been worked out before, and the specified hole for a 3/32 rivet is 0.093 to .103 inch diameter
A #41 is 0.096 and a usual tolerance is -0.001 and +0.004 for general machining. So it is well within the approved specifications that Boeing, etc. work to.

This machinist's tolerance of 0.004 is using "jigs and fixtures", so our hand-held drill use may produce larger holes - how accurately can you hold the drill perpendicular to the work?

http://www.engineersedge.com/drill_sizes.htm

Since the hole does open up during the dimpling process, and the Mil-spec. (and Boeing's spec. probably) is actually for a drilled hole with straight sides, not the sort of stepped hole we make when cold dimpling two sheets, starting with a slightly smaller hole gives you a little more tolerance, moving you into the middle of the range.

Check the image on my web page intro. to make sure you are getting the correct driven head height... it's easy to make an error on double dimpled sheets with the Avery gauge.

http://www.home.earthlink.net/~gilalex/

Real life engineering (well, except for software...Dan... ) is full of tolerances, so we can make things just a little easier by erring on the low side of a hole dimension when we know that hole will get larger with a later step, moving you into the center of the tolerance range.

Does 0.002 make much difference? - maybe not, but is it any harder to do - not really.... so why not let the tolerances work in your favor?

I have tried #42 drills (0.0935) but that size did not let the pilot of my die slip into the holes, and at the -0.001 lower end would fall out of tolerance.

NOTE: I'm not suggesting any extra work here....

gil in Tucson

 

[ptrotter]

gill,

I think part of my point was "why bother" trying to re-engineer tried and true practices. Using a different drill size, or whatever, may not be bad, but it probably isn't better, or the standard would have changed.

There are plenty of other areas where one can make changes or improvements that would be better worth one's time.

I agree with your argument that a #41 drill meets the specification for a 3/32" rivet, but since it probably doesn't improve things, why confuse things by making people wonder if they are doing things properly.

In general, I suggest following the standards. If you want to deviate, make sure you fully understand the impact of the change.

 

[Tom Maxwell]

I'm Confused

Well I guess I am dumb and confused because I don't understand the statement that we need to use longer rivets. Duirng our class we checked our rivets with the rivet gauges each time we made a squeezer change or changed rivet lenghts.

My experience was that the 3/32 rivets were just right. Where there were two pieces involved (skin to rib or spar) the 3.5s worked great, if there were three pieces (skin and two flanges) a 4 worked great and if their were four pieces a 4.5 worked fine. I found the opposite to be true on the 1/8 inch rivets typically choosing a rivet 1/2 size smaller than what Van called out in the plans. Again we measured them with the rivet guages and got snug fits and adequate heights.

I guess if I should have been using longer rivets I can look forward to pieces flying off of my plane when I get it up. Oh well, nice knowing you guys.

 

[jcoloccia]

Actually, gil, my point wasn't to have a sloppy attitude about the plane at all...it was that I don't stay up at night worrying about the specifications and trying to tighten those up. So for example, if I have a couple of rivets that look a little over/underset, as long as it passes the gauge test, why would I spend the time and energy to second guess Van and all the other engineers that came before him? That's the whole point of the specs...it gives me a precise definition of "good enough" and trying to do "better" is not a good use of my limited workshop time.

Then again, my plane will never win any awards at Oshkosh but that's not important to me at all. Personally, I'd rather be passed over at Oshkosh 2008 than win grand champion at Oshkosh 2012.

 

[ptrotter]

Van's sometimes specifies shorter rivets on purpose if it is close. They do that because inexperienced builders have a tendency to bend longer rivets. They feel that a short rivet driven properly is better than a long rivet that is bent and has to be drilled out. In drilling out a rivet there is a likelihood of damaging the hole which is worse than a slightly short rivet. At least this is what Van's told me when I questioned some of their callouts.

 

[dan]

The gauge is not always foolproof

use what the rivet gague shows to be the right length and there wont be a concern if you did this and acheived the right shop head you cant possibly be/go wrong.

True in general, but if the hole is enlarged this goes right out the window. The "gauge" is just indicating the required rivet length for a perfect hole. An enlarged hole will consume more shank than a perfect hole.

I'm picking nits at this point...but just want to clarify that the gauge is not foolproof.