Back Riveting problems on RV-7 rudder

I am trying to back rivet the stiffners to the rudder skins. The back rivet set (with plastic guard) diameter is 0.75" which is too wide to be able to center the set on the rivet head. The side of the plastic guard interferes with the side of the stiffner and therefore won't center on the rivet. The result is sort of a "glancing blow" on the rivet and one ugly rivet. I thought about removing the plastic guard on the rivet set but can't do that without tearing it up. Can you buy "narrow" (like <1/2") back rivet sets? Should I just use a straight rivet set with out the plastic guard?

Capt Riney

Capt_Riney said:

I am trying to back rivet the stiffners to the rudder skins. The back rivet set (with plastic guard) diameter is 0.75" which is too wide to be able to center the set on the rivet head. The side of the plastic guard interferes with the side of the stiffner and therefore won't center on the rivet. The result is sort of a "glancing blow" on the rivet and one ugly rivet. I thought about removing the plastic guard on the rivet set but can't do that without tearing it up. Can you buy "narrow" (like <1/2") back rivet sets? Should I just use a straight rivet set with out the plastic guard?

Capt Riney

Click to expand...

I don't remember this being an issue. What is your distance from the edge and curve of the stiffeners? I remember holding it in place with my fingers.

Is ham Tools

Mark,
Call the folks at Isham Tools (http://planetools.com/rv-tool-kit/

Their back-rivet spring-loaded bit that comes in their kit is a touch narrower, and features a metal guide barrel rather than the wider plastic one you're dealing with. I found it much easier to use on those narrow rudder stiffeners. Call them and order one over the phone. They're great folks and make superb tools.

Guard

How about grinding one side flat till it clears?
Mine has two opposite sides ground flat. Works in twice as many tight spots.

What Wirejock said. Although the one that came with my tool kit from Brown Tools already had one flat side.

I've used the back-riveting set without the plastic part a few times. A 1/2" box-end wrench fits around it and can be used to hold the parts flat and together. Don't know why an open-end wrench couldn't be used the same way if you can work two-handed and stand the crudeness of the method.

Dave

I also had issues backriveting the stiffeners. The back rivet set requires a bit more air pressure. In a couple of situations, I just went back to good old mushroom set and bucking bar. Those seemed to come out just fine. If one method isn't working, try another.

Same as David Paule

I found the same problem and decided to take the plastic and spring out! It's easy just need to take the axle out and it's done!
Then it will be easy to work centered on your rivet! Take care of the air pressure; too much and your skin will be destroyed!
Enjoy

Nicolas

Use your concave rivet set. I recall reading somewhere that this is a valid method to set the the rivet even though it leaves a rounded shop head - the internet can't be wrong, can it?

Back riveting, to achieve acceptable results, needs the spring and sleeve in order to keep the assembly tight until/as the rivet clinches.

When needing maximum structural strength, it is not an acceptable method.

But that need is rarely necessary and, if it were, the diligent engineer would likely add a note on the plans.

It's totally acceptable to "home-brew" a smaller diameter back rivet set. I have one in the .375 diameter range. It is what I'd use for close work like described in this thread.

FWIW

mjb

My back rivet set can't be more than 1/2" diameter. Pretty sure I got it from Cleaveland aircraft tools. Having said that, I have taken the plastic part off on occasion.

Bevan.

Marc Bourget said:

....When needing maximum structural strength, it is not an acceptable method....

Click to expand...

Incorrect.

If the parts are snugly together and the dimples formed correctly and the rivet head formed per that spec on Van's download page, it's as strong as if it were formed in any other manner.

It's true that the strength of riveted joints varies with the thickness of the metal being riveted, but that's true for the strength of joints with any kind of mechanical fastener in aluminum.

Dave

Same issue

Mine also came from Brown and is to large for the stiffeners on the practice kits. The rivets came out with a slight angle because as you hammer the rivet down the rivets set would slip a little bit off. I tried to hold it with my fingers.

Haven't done any sniffers on the empennage yet. I was kind of hoping this issues was just on the practice wing because it is very small. Let me know how you solved it.

I also filed a flat on one side of white p!astic ,did not effect other use Tom

David Paule said:

Incorrect.Dave

Click to expand...

David,

I have a lot of respect for this forum and it's members. I didn't throw that out as an "off-the-cuff" opinion. My foundation in sheet metal and riveting began with a 15 year mentoring relationship with John Thorp - augmented by regular discussions with other Structural Engineers and research related to Forums given at OSH and Sun'nFun. The forums I gave at Sun'nFun qualified for Continuing Education Credit to IA certificate holders.

The following is from a Boeing structural engineer, discussing just this question, posted in an Aerospace Engineering Forum:

"" I suspect that the mechanics want to reverse drive to ensure max gun-energy directed to deform the tail. If this is the reason, then stop the presses... as You indicate there will be hole-fill issues galore.""
They will be mushrooming the tail without swelling the shank.

reinforced in another paragraph:

"IF this is the case, then what has happened is that the shop is circumventing rivet conventional installation protocol in-favor-of "just getting the job done [any-way they can]". This is a shabby way to run an aerospace company."

point expanded by the following:

"Hole (or hole/csk) preparation???

As usual, hole preparation [IE: straightness, perpendicularity, roundness, roughness, deburring, inner-laminar chipping, +plus all countersink issues, etc] can make a BIG difference... grossly affecting installation soundness.

[end quotes]

I will add that last quote went on to estimate up to 10% less strength and much lower fatigue life (most important to RV builders). This shows the problem lies more with the substrate than the means used to form the rivet. The quality of the rivet installation depends on the execution of the maker of the parts to be assembled and the greatest variability lies with dimpling/countersinking for installation of flush rivets. I do not feel the instruction on riveting currently offered in the E/AB community adequate to sufficiently inform a builder.

I'm not acting as the harbinger of doom in making these statements.

My guess is that there is more than enough "margin" in the RV rivet spacing that the 10% loss of strength is of minimal concern. What I do feel is more significant are fatigue life issues - The recent SB on the rear wing spar is not directly related but it shows the consequence of the point I stress.

RV builders expend a tremendous, admirable, level of effort and detail in the construction of their aircraft. Their airframes, more than likely, will be around long enough to manifest these fatigue related problems if the considerations I raise aren't addressed.

FWIW

mjb

Warbo said:

What Wirejock said. Although the one that came with my tool kit from Brown Tools already had one flat side.

Click to expand...

+3. I used a disc sander to remove material for clearance. Even so, I found I had to angle the gun a little to get the shop heads to set nice and square.

Don't give up on back riveting!

I've got the Isham/plane tools spring loaded back rivet set also. It works great. It is maybe 3/8" dia with no plastic. I haven't had any access issues with it yet. I add a bit of duct tape to the edge to avoid marring the surface. It is one of the most consistent and quick ways for me to set rivets. The wife picked it up very quickly as well.

wirejock said:

How about grinding one side flat till it clears?
Mine has two opposite sides ground flat. Works in twice as many tight spots.

Click to expand...

Some back-rivet sets have collars that have a smaller OD, just small enough to fit in these places, but others have thicker material...if you have the thicker material, just grind it down (but smooth the edge that goes against the material being riveted so it's not a sharp 90 degrees, or you'll run the risk of putting a little mark in the aluminum).

Lots of tools will get "modified" during the build. I'd rather grind away on a tool than mess up a part, any day.

I too had a similar problem until I started using a spring-clamp to hold the skin down firmly so it did not move while I held the gun in the right hand and the tip of the set on the rivet. When things didn't move around the rivets set well. I didn't have to worry if the rivet was in the center of the set or not.

For me back-riveting seems to make the skin move on the table every time if I don't hold it somehow.

Marc Bourget said:

The following is from a Boeing structural engineer, discussing just this question, posted in an Aerospace Engineering Forum:

"" I suspect that the mechanics want to reverse drive to ensure max gun-energy directed to deform the tail. If this is the reason, then stop the presses... as You indicate there will be hole-fill issues galore.""
They will be mushrooming the tail without swelling the shank.

Click to expand...

I'd love to know how the physics of that works. Hitting the head while backing up the tail with a bucking bar, or hitting the tail while backing the head up with a bucking bar (or large flat plate) should result in the same forces, as far as the rivet is concerned... The rivet is otherwise free to slide in the hole, until you start hitting it and the shank starts to swell. It's not clear to me that it would start swelling differently depending on which way you drive it.

Not saying the Boeing engineer is wrong, but there were a lot of wild statements in those included posts with very little technical explanation to back them up.

Mushrooming the head vs. swelling the shank.

From experience, I think you see mushrooming with increasing age/hardness of the rivets. Back in the 70's Fullerton Air Parts would get rivets shipments twice a week. A day, or less, from the mfg, you could drive them at 55# with a short burst. Perfect "donut" shop head, no swelling of the adjacent skin. After 4 days, it was 80#, more hand pressure and a longer burst. Sometimes you'd get localized deformation of the nearby skin.

Sometimes, when things don't quite "fit", you can pull the parts together by lightly driving the rivet, which swells the shank to the point you can use a bucking bar with a hole drilled to "draw" the parts together and there's enough drag to keep them tight so you can proceed to form the shop head.

Bought some surplus rivets. They had been around for a year and the shop head, actually the end of the rivet shank, would form like the bell mouth of a trumpet - not the "donut" that formed when fresh. No way to "draw" with the harder "trumpet" end rivets.

Conclusion: The "trumpet" shanks don't fill the hole as well as the "donut"

shop heads.


FWIW

A follow-up.

Lots of solutions and diversions. Back to the OP - followup.

I have the Avery back set. 3/4" in dia at the plastic, 1/2" for the driver.

I took a small section of rib stiffener and measured, the rivet centerline is only .070" off center (.180 from the edge). The set is flat, so no wedging effect. There is another problem occurring here. The OP needs to adjust technique, equipment, pressures or other variables to fix the issue. Learning is part of the process. Knowing something can be done sets the goal - this can be done!

Now is a good time to practice by varying pressures and trigger time. Count the hits, hold the gun differently, quantify the results. I hold mine upside down with my palm over the center axis and pull the trigger with my middle finger. Develop your own technique, one that works for you consistently.

Alternatives are good, but won't overcome developed skill. A skill which must be improved to complete the airplane.

Bill's advice is invaluable.

Years ago, I made a lot of extra work for myself, and achieved less than optimum results (but good enough prompt me not to re-do the task) because riveting conditions change almost day to day. Rivets get harder, even one production run of alu sheet from another, certainly different thicknesses.

I now make it a practice to "practice with test tabs" Excellent use of shear drops or trimmings that otherwise can't be used in the structure.

As a closing remark, these differing results are reduced the fresher (softer) the rivets.

mjb