Evaluation of rivet head formation with flat and dished head

So far I have been using slightly dished nose piece heads. The Gesipa rivet have a flattish head, and if fact a slight circular forming mark is made on set rivets. This is not unsightly, however for external rivets I will switch over to a flat rivet gun nose-piece. The Gesipa rivet guns have a flat nose piece.

My concern here is the engineering quality of rivets when set with this gun. I have used a hand rivetter with flat heads to emulate the result that would be obtained with flat nose piece.

I have confirmed that the dished nose-piece does make contact with the entire topside of the rivet head, so some forming does occur thoughout the head.

For both 4mm and 4.8mm rivets, the head is formed distinctly flatter. This is also evident in the underside of the rivet head. It is also clear that while the flat head reduces the gap between the surface and the underside of the rivet head near the shaft, it by no means closes this gap to anything like a gas tight or actual contact. For both dished and flat nose-pieces, the contact is primarily around the periphery of the rivet head, and in the latter case this contact extends further to the rivet shaft. Also the flat head does in some cases form an actual contact between the rivet head and the surface over a region extending close to the rivet shaft, however this seems unpredictable and a random matter.

The initial conclusion is that the choice of flat or dished nose-piece does not have a large affect on the nature of the contact between the rivet head and the surface. In fact the dished nose piece creates a more consistent contact pattern. The dished nose piece does leave a greater gap between the rivet head and the surface compared to the flat nose piece, close to the rivet shaft.