What to expect when setting rivets with long-reach yokes
If the rivet is ANY over the 1.5 times length it wants to start bending. Generally Van’s specs are slightly under with extra rivets used to increase strength. The bending does not cause what you are seeing, but plays a part, see below.
I often scuff the rivet set that goes on the shop head on a concrete block, sidewalk, brick; something pretty smooth and flat, but not polished. Do it one way, rotate 90 degrees and do it again. This is an old carpenters trick to keep a hammer face from sliding off of a nail when struck. Since you are on the yoke side, you could rough it up a bit with some 80 grit sandpaper, but I would likely try putting a piece of masking tape on it first to see if that gives just the adhesion that you need to keep the rivet from sliding. Again, see below.
Finally the reason for both above… as you place a force roughly the weight of a car on that rivet, things want to do anything but stay neutral. The longer the yoke the worse things get, and this is why: Obviously the yoke wants to flex when force is applied, the longer the yoke, the more the moment arm. Like any mechanical thing, with the force applied the yoke wants to bend toward it’s weakest axis. In this case the yoke will want to spring sideways, top one direction, bottom the other. This is made easier if the rivet slips on the yoke, the roughness helps to limit this side slip.
This is more pronounced on the longer reach yokes, and the reason you can’t rivet with the DRDT. Back to the longer rivet, there will always be some tipping, the longer the rivet is, the more it would rather tip, than collapse. Once it starts to tip, the prior mentioned problems just are exacerbated, thus it is far better to use a slightly short rivet, than a slightly long rivet.
There is a final point to consider and that is the fit of the ram in the yoke. The reason that we designed around these yokes was not only their popularity, but the fact that the ram goes through the yoke with a pretty tight tolerance. The ram is guided by the yoke, thus flex in the yoke will also move the ram with it. Some systems have this section of the squeezer independent of the yoke, and the problem is just made worse by one bending one way, and the other bending the opposite. Look at the fit of the ram in the yoke, if the fit is loose because the ram is out of tolerance too small, or the yoke is out of tolerance too large, the ram will have ‘room’ to get an angle on it. Also and more importantly on the thin nose yokes, make sure that if you are using an adjustable ram, that you use a long set for a short rivet. This will keep the ram further engaged into the yoke, and give it a longer ‘arm’ to align with.