Pre-Punched holes

[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]

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]

Originally Posted by dan

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]

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]

Originally Posted by dan

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]

Originally Posted by PJSeipel

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/~gilal...ec/rivet_a.htm

gil in Tucson

[dan]

Originally Posted by PJSeipel

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]

Originally Posted by az_gila

... 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.