Smoking Rivets - A Matronics Excerpt by Noel Simmons

The anatomy of the smokin’ rivet.

 What is smoke?  The black residue that streams down wind of a rivet is Aluminum oxide, the second hardest substance on earth with the first being diamonds.  Aluminum oxide is a by product of corrosion that naturally occurs.  Aluminums is considered to be self lubricating, meaning it will continue to sluff, and the aluminum oxide being harder than the base metal will exponentially carve more aluminum oxide out of the base metal,

 A rivet that has been properly set will eventually smoke given a structure subjected to vibration that has been under engineered, meaning not enough rivets per inch.  I have seen, and repaired these structures (again and again), mostly engine nacelles on jets, fortunately or unfortunately how every you want to look at the subject, Van’s aircraft does not have any under engineered structures that would qualify for the under engineered place to look for smoking rivets.  Yes all rivet joints move to one degree or another so a rivet can be set in such a way that it will smoke.  In conclusion smoking rivets on RV is a builder flaw.    
How to set rivets that will smoke.

First let us look at the parts that Van’s sends us.  The Punching process is extremely actuate, Fin Power CNC punch press have a tolerance of .004 in 8 linear feet. Van’s has CAD people that really know their system and can tell the punch press exactly where to put holes.  Given the .004 tolerance there are some places on the aircraft that have the same hole in 4 sheets of aluminum that each have this tolerance so you will see holes that are a little hard to put a reamer through, but is still very accurate.
The punching action causes aluminum to “flow”.  That is the cause of the volcano on the exit side of the sheet.  This flow is not like the burr created from using a drill bit.  You must remove the volcano completely with out countersinking the base metal. 
Below is how to cause voids and entrap, well let’s call it ‘Stuff’ for lack of better words, which promote the corrosive environment to create copious amounts of SMOKE.
  1. Use a drill bit instead of a reamer.
    1. Just for giggles take a ¼” drill bit and begin drilling a hole in a piece of .032 sheet metal.  You will see that the hole that begins to develop is triangular, and as the drill bit finally passes the hole is not truly round.  This is obviously a start of voids in the rivet joint.
    2. USE a Reamer they turn triangular holes into properly sized round ones.  Reamers should be used everywhere on the van’s pre-punched holes
  1. Don’t deburr/ deburr to deeply
    1. The punching process causes a volcano like structure on the punch exit.  Not only will this cause a void but will chip the rim of the volcano into the joint acting like grist in a roller mill.
    2. The head and or shop head will sit up on the volcano and will not properly clamp the rivet joint.
    3. Deburring to deeply is a very, very common mistake RV builder’s make due to the punching process.
    4. Look at some of the heavier aluminum that has been punched with 1/8” holes.  You will see one side that is pressed in and the other side will be coned out like a volcano.  If you take a 100 degree countersink or some of the other rotary deburring tools and cut this volcano off to the point that there is no aluminum that protrudes above the base metal you will have a shallow countersink.  This shallow countersink WILL NOT be completely filled by the expansion of a rivet.  This is the stating point of corrosion
    5. We use sand paper to deburr.  The sand paper will remove the volcano with out causing a shallow countersink.  Two notes WE PRIME, WE don’t build polished aircraft.
  1. Dimpling / countersinking the sub structure with the same dimple as the skin.  
    1. Easy test, take two small pieces of scrap aluminum and drill #30 holes, Deburr.
    2. Dimple each with your #30 dimple dies.
    3. Mate the two pieces and you will see that they don’t fit very well.  This cause lots of voids and is the primary thing that RV builders do to cause smoke.
    4. Take those same two pieces of aluminum and dimple them together using your #30 dimple dies.  
                                                              i.      Better fit isn’t it!  They don’t rock like a bobble head doll
    1. Point here is that most people don’t dimple the substructures (ribs) to “receive” the overlaying dimple.
    2. We take a small ½”X 1/2” scrap and attach it to the male dimple die to dimple all the sub structure.  This eliminates the rocking caused by having two improperly formed dimples pinched together by a rivet.
    3. I have seen people dimpling with the plastic sill in place, bad idea.
    4. I have seen people afraid of over dimpling so they hit the dimplier once instead of twice (real hard).  Dimpling is a forming process that must be complete; a half dimple will cause the skin to warp, bad idea.
Sorry if this is a little anal, I have spent many years trying to get the best looking rivets I can.  I have piles of scrap that I drill holes in and look at with a magnifying glass. Rivet and inspect, change the technique a little here and there then drill and rivet inspect until ,In my opinion we do some of the finest riveting on RV’s.  Every airplane we do get’s a little better and a little faster.   
Noel Simmons
Blue Sky Aviation, Inc.