What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

My First Row Of Skin Rivets. Is The Gap Normal

I am a first time builder. Started riveting the top skin on tonight after putting in the first 10 rivets I started checking my performance. I found that the skin is bent in at the rib there is no noticeable dents anywhere. Also I didn't feel that I was pushing hard at all.
First time trying to post an image so I hope this works.
25qan49.jpg

The above image is just to show how I laid an aluminum bar against the skin.

35m4iet.jpg

See the gap behind the bar where the rivets are. Is this normal should I be concerned? Am I pushing too hard on the rivet gun? Or just being too fussy.

eite9k.jpg

The above image is just to show that when I moved the bar over a rib that had not been riveted there is no gap at all.

Thanks in advance any and all help will be appreciated.
Mike
 
I had the same problems

Mike,

It is hard to tell from a photo, but it looks to me like your holes may be under-dimpled. I don't see the slight ring around the dimpled hole and the skin surface should be flat right up to the dimple. I can see distortion in the reflection around the holes on the left side of your first photo. If the skin isn't flat before your set the rivet, it won't be after.

I had this problem when I first started, after I figure out how to get a nice crisp dimple the skins are much flatter.

Hope this helps,
 
Mike
I see that you are located in Chatham. I live in St Thomas. Do not proceed any further, you need a bit of help. Give me a call 519-631-1369

Tom Martin
 
I've also seen cases where too high of air pressure on the rivet gun can cause the same effect. I have several spots on my plane where the rivets look like the example, because I had increased the air pressure for some 1/8" rivets and forgot to turn the pressure back down.
 
A few suggestions...

Rivet gun setting - As others mentioned, too high will cause a larger dip. Even with the proper setting you will see some dip though.

Dimpling - Its hard to tell, but the dimples do look a little rounded in your pictures. I found that using a C-frame and really giving a good whack (techinal term) is the best method to get a crisp dimple. Also make sure you have high quality dimple dies. I've tried dies from both Cleveland and Avery and found them to be good.

Back Rivet - This is by far my favorite method to get smooth consistent dimples. The top wing skins lend themselves to this method very well. The flat bucking bar will rest on the outside skin and the set is used on the shop end of the rivet. Since the flat bar is on the outside of the skin, it creates a very level surface with almost no dip at all. Check one of the major tool suppliers for a long backrivet set and bucking bar. I think I got mine from Cleveland.
 
Power, mass, momentum, stress, strain

A few things to be aware of when setting your pressure and how firmly you hold the rivet gun. A layman's understanding for the purpose of this post; stress is the amount of force that can move metal, but not so much that it permanently deforms. A strain will permanently deform. So the trick is, stress the metal, strain the rivet. Generally, setting pressure at the gun should not be enough by itself to deform the metal, however being heavy handed with the gun can be just enough extra force to start bending metal. This issue presents itself when the gun hits twice and the bar only makes contact once, which can be heard, or an incorrect balance of holding pressures between gun and bar. Now this can be counter intuitive, holding the bar to tight makes you hold the gun to tight, then the bar doesn't get a chance to "bounce" usings its monentum to help absorbe the energy into the rivet, and that energy ends up deforming metal.
A large area flat swivel set helps with this to.
I wouldn't be overly concerned about the quality of work so far. I see this kind of stuff on production airplanes.
 
Last edited:
A Short Tutorial

Stress is the amount of force that can move metal, but not so much that it permanently deforms. A strain will permanently deform.

Stress is simply force divided by area, with those dimensions, i.e., pounds force per square inch. Stress does not know whether the material being stressed is in its elastic of plastic range of behavior. It's merely a force on an area.

Strain is defined as a movement, such as stretching or compressing, and might either be elastic or plastic. It is caused by stress. Elastic strain will disappear when the stress is removed and plastic strain will, to some extent, remain. For most engineering applications, strain does not have a dimension because the actual strain is commonly presented as divided by the length of the material being strained.

Stress and strain in many materials are related by Young's Modulus, which is simply stress divided by strain, with the strain in its common non-dimensional form. For metals that are not stressed to the point that they yield or deform in a plastic manner, each type of metal has the same Young's Modulus. For most aluminum alloys, it's right around 10 million psi, depending on the alloy. That is, if you put ten pounds force uniformly on a one inch cube, stretching it, it would stretch one millionth of an inch.

Dave
 
Dimples

I agree, the dimples dont' look right. Are you using a DRDT-2? I've had better results with a C-frame dimpler that you smack with a hammer.
 
Mike,

An hour with Tom will make all the difference in the world. We can talk about pressures, angle to the dangle, how to hold your mouth right, 2X or 3X, back riveting sets, tungsten bucking bars, riveters tape, tank dimple dies, primer application and metal temperatures, but nothing helps quicker than getting with someone who has built one. Enjoy the build.
 
Mike,
Your skin looks normal for rivets set with a rivet gun. As others have said, if you can back rivet, you'll see a difference. I just finished my wing top skins on my RV14, one done the normal way and one back riveted and the difference is noticeable - but I think once it is painted, you won't notice the difference as a casual observer. I had help with back riveting - from experienced builders. Structurally, there's no difference. Also, as others have said, too much air pressure to set a 326-3.5 rivet will produce more skin deformation than is necessary, ask me how I know. I use about 55 psi with 75 feet of small diameter hose coming off the compressor (some pressure loss) and don't press hard with the gun, whether using a flush set or a back rivet set.
 
All of the riveting tips given so far are helpful, but I think riveting technique is causing only a small part of what you are seeing and are not happy with.

The lack of flatness evident while using a span wise straight edge on the skin could be caused by a number of things.
The most common is not having the bend angles of the flanges of the wing ribs adjusted to 90 degrees.
This will try and force the skin out of plane/alignment.

BTW, the photo with the straight edge is not really indicating an unusual amount of waviness in the skin. I have a feeling you are checking because you don't like how the riveted areas look. I think that is probably being caused by something else....

In the third photo in your original post, it is obvious that the dimples on the skin are not fully formed.
With experience this can easily be detected using the skin reflection.
In that photo, there is a reflection of a gray hose or extension cord on the floor. Note where it comes out from under the foot and intersects through a dimpled rivet hole. The reflection is all distorted as it passes over/through the hole. It is indicating that there is an approx 1 " diam. dished area in the skin around the hole. This is a classic indicator of a not fully formed dimple. It appears most of the dimples on the skin look the same. If a dimple is formed correctly on a flat skin (this skin was flat until it was clecoed to the wing frame), the skin will still be perfectly flat right up to the edge of the dimple (before riveting)

If you already have dished areas in the skin at each hole before riveting, once you rivet (regardless of how skilled you are), it will only get worse.

I would suggest that you remove the rivets (please get some experienced help to teach you if it is your first time), and re-dimple the skin.

This time look carefully at the reflection of overhead ceiling lights in the skin while it is on your dimpling table. There should be no distortion of the reflection in the area around a properly formed dimple. Re-dimple a hole out in the middle of the skin (If you are using a C-frame tool, don't be afraid to hit it hard... if using something else, it is not adjusted/working properly) and compare the light reflection to the others near it. I think you will quickly see the difference.
Post back about how it goes, or if you have any questions.
 
Last edited:
What kind of hammer do y'all recommend for the C-frame when dimpling? I just used what I had handy -- a 16 oz (or maybe 32, I don't remember off hand) plastic dead blow. One good whack seems to do the job.
 
What kind of hammer do y'all recommend for the C-frame when dimpling? I just used what I had handy -- a 16 oz (or maybe 32, I don't remember off hand) plastic dead blow. One good whack seems to do the job.

What hammer you use is far less important than knowing how hard to swing the one you are using.
Probably just about any hammer can work, but some will take more effort and/or more hits to get a properly formed dimple.

The work surface the the C-frame tool is resting on is also a major factor.
If you have it sitting on a concrete floor (very solid, unyielding surface), it takes a lot less energy to make a crisp, well formed dimple, than if you have it resting on a lumber framed particle board work table.

The key is to learn how to tell when the dimple is formed properly, and then adjust your technique to repeat that consistently with what every set-up you are working with.
 
What kind of hammer do y'all recommend for the C-frame when dimpling? I just used what I had handy -- a 16 oz (or maybe 32, I don't remember off hand) plastic dead blow. One good whack seems to do the job.

Dale,

On my C-frame set-up I use the plastic (looks like polyethylene) head hammer that it came with, but I agree with the previous poster, knowing how to use it is more important. Using light multiple strikes with the hammer lets me listen to each blow - the sound of the second blow is much different (more solid sounding) than the sound of the first since the first blow is doing most of the dimpling. If the third blow sounds identical to the second, then I know there is no more dimpling left to do; if not, a 4th strike will usually do it.
 
Dale,

On my C-frame set-up I use the plastic (looks like polyethylene) head hammer that it came with, but I agree with the previous poster, knowing how to use it is more important. Using light multiple strikes with the hammer lets me listen to each blow - the sound of the second blow is much different (more solid sounding) than the sound of the first since the first blow is doing most of the dimpling. If the third blow sounds identical to the second, then I know there is no more dimpling left to do; if not, a 4th strike will usually do it.

Hmmm.

I give it one good solid whack and the dimples appear to be fully formed, well defined, and with a distinct ring around them just like if I'd done them with a squeezer. I can't tell the difference between a dimple smacked once, and one I've hit twice. So I guess they're getting done OK.

I know you can under-dimple, but I've wondered if it's possible to over-dimple when using dimple dies. Other than beating on them hard enough to stretch the metal, I can't see how you could.
 
Dimple a row of holes. Now hold the metal/sheet in front of you and slowly rotate it in front of your eyes so that the sheet is flat directly in your view. If the dimples are done properly they will appear to disappear and the sheet will look perfectly flat with no distortion.
Test this by purposely under dimpling a row,now take a look. Then hit them one more time and check again. Continue this too many times and you will go too far and the distortion will come back as you stretch the skin around the hole.
Once you get the right number of hit/hits you will always get it right. I use a small ball peen hammer, tap,tap, whack, done.
 
Hmmm.

I give it one good solid whack and the dimples appear to be fully formed, well defined, and with a distinct ring around them just like if I'd done them with a squeezer.

Clarification...
The tip to look for the concentric ring around the dimple as proof of it fully formed, is not complete evidence that the dimple is fully formed.

When the dimple is almost but not quite fully formed, the skin will be in contact with the outer perimeter edge of the male die and it will leave a scuffed ring, but the entire flat face of the die will still not have contacted the skin. This contact is what finishes the dimple properly and returns the skin to being completely flat.
So, what you actually need to look for is the concentric ring, with uniform skin surface scuffing on the entire area within the ring.

If you are a builder that is using a single layer of masking tape on the male die so that you do not leave the scuff mark on the skin (because you are crazy enough that you plan to leave the airplane bare polished later), then you need to just learn to read the reflection of the skin (which I think is the most reliable technique anyway).
 
one more suggestion

Haven't seen it mentioned yet, but one way to fix slightly dimpled skin on the visible side is to get a hardwood dowel or steel rod approx. 1/2" diameter and if you can get access to the back side, place the dowel or rod onto the bucked rivet. Just a light tap, will take the dimple out. Very effective and can improve the appearance of the skin.
 
Looks like a classic flange not 90 degrees to the rib web. Keep your pressures below 40 psi at the rivet gun as well. Plus the prior posts have good info as well but many overlook the flange alignment.
 
Back riveting gives better cosmetic results

Mike,
Check for under dimpling as mentioned by previous replies. That said, I suspect that this is simply caused by the thinner skin bending to meet the shape of the thicker rib below it. The rib flange tends to be wavy, due to fluting. The action of the rivet gun pushes the skin into the rib flange. With conventional riveting techniques, the thinner material tends to form to the thicker part's shape.
If you are looking to obtain dead flat skins, forget conventional riveting. Back rivet. That way, the gun and the shop head tend to bend the rib flange flat to conform to the the skin/bucking bar combination.
Charlie
 
Mike,
Check for under dimpling as mentioned by previous replies. That said, I suspect that this is simply caused by the thinner skin bending to meet the shape of the thicker rib below it. The rib flange tends to be wavy, due to fluting. The action of the rivet gun pushes the skin into the rib flange. With conventional riveting techniques, the thinner material tends to form to the thicker part's shape.
If you are looking to obtain dead flat skins, forget conventional riveting. Back rivet. That way, the gun and the shop head tend to bend the rib flange flat to conform to the the skin/bucking bar combination.
Charlie

Charlie,

In this instance, the skin is thicker than the rib so I don't think this is an issue, though I agree that backriveting (when done correctly) gives a flatter finish.

I can clearly tell from the photos that the first problem (there may be riveting technique issues also) that needs to be resolved is the skin dimpling... you can't get a flat looking finish if you are using skins that aren't flat to start with.
 
Scott, what type of machine are the QB's dimpled with? I know it can't be a bunch of fillipinos playing whack-a-mole.
 
Scott, what type of machine are the QB's dimpled with? I know it can't be a bunch of fillipinos playing whack-a-mole.

I don't know how they do it now, but when I was there many years ago, they had a bunch of tables with C-frame tools.
 
Back
Top