I know what you are going through and sympathize with your frustration. When it happened to me, several years ago, I was trying spirals for the first time. There almost wasn't a 2nd time.
A couple things come to mind. First, it could be the blades. I believe that it's possible to get a bad batch that is more brittle than they are supposed to be. With very small blades, like you are using, the margin for error is quite small. However, the fact that you have tried 2 different brands would seem to indicate that blades aren't the problem.
Second, it may be that the set screw, in the blade clamp is retracted too far into the clamp block. If this is the case, then tightening the thumbscrew forces the blade into the set screw hole and stresses it. This problem usually reveals itself in the telltale "hockey stick" on the blade end. Again, with blades that small, it wouldn't take much to weaken them and cause them to break. Also, sometimes there are burrs on the set screw and or thumbscrew that can damage the ends of the blades. They can be sanded or filed smooth. However, since you have replaced the blade clamp, set screw and thumb screw, it seems unlikely that this is the problem.
Hope you find the solution. Good luck!
