Source: 2014 Q4 Beartracks, Russ Erb
While in Oshkosh I spoke to fellow builder Caleb Irhig about the problems I had riveting the main spar splice plates. See picture below. This picture was taken after I had drilled out all of the -4 rivets in the splice plate that I found unacceptable. As you can see, my success rate was miserable.
Fortunately, this was a very early attempt at riveting, before I had much experience, and I was able to learn from it. The redo on this spar and the first attempt at the second spar had a very high success rate. Here is what happened (as well as I can remember something that happened in about 1997). This is an operation very different than riveting together two 0.025 sheets. Instead of the material being 0.050 thick, this is 0.282 thick. That is significantly more mass by comparison.
The problem was that I kept smashing the manufactured head (smileys and other problems) instead of forming the shop head. What I think happened (remember, this was 17 years ago (yikes!)) was that I had just finished driving all of the -5 rivets in the spar caps. These had to be driven very hard to upset the bigger rivet. I think I attacked the -4 rivets similarly with much too high a pressure on the rivet gun.
There are two issues at work here. The long rivet presents a lot more material that has to yield in compression. This is like pushing on a longer spring. Remember the upsetting not only forms the shop head, but also increases the diameter of the rivet to fill the hole that it is in. This would seem to require hitting the rivet harder.
However, the manufactured head, where the impact is occurring, is trapped between the rivet set and the spar spice plates, which by comparison are rather massive. The spar plates themselves start to act like a bucking bar, causing the rivet head to get smashed between the set and the plate, rather than letting the compressive force go through to the intended bucking bar. This can be compounded if the spar plates are held in space very rigidly. This would seem to require hitting the rivet softer. As best I can remember, make sure the spar is not rigidly attached to something far more massive (like a work table). The long rivets will take more pressure (harder hits) than similar diameter but shorter rivets.
If the shop head does not form in a reasonable period of time (maybe 1-2 seconds) you probably need more pressure (hit it harder). If you start smashing the manufactured head without getting a good shop head, you probably need less pressure. The gap between these two possibilities gets smaller the longer the rivet is. I think that’s why you don’t see many very long small diameter rivets. By the way, I’m not a fan of “back-riveting” because it tends to push the end with the manufactured head out of the hole. I never could get it to work consistently.