These knives were too dull to cut. I tried them on a very thin piece of plastic that the tool crushed rather than cut. The extreme tips of the edges are in contact, but there is a 1-2mm gap between the remaining working length no matter how hard the handles are squeezed. So. Do I have to grind the edges to reproduce and sharpen them, or do I (no pun intended) cut my losses and return this inefficient tool?