I've been using #426 and when I run out even the flimsier #409 it works OK. It wears out after a while, but not after 12in. What are you trying to cut into? Just the regular ~1mm thick steel? If you're having trouble, try to cut in different ways - like cutting at a different angle and applying varying amounts of pressure - this worked for me. There are also other tools like a jigsaw that may be better to make large cuts into the case. The Dremel is better for shaping or smoothing or making small cuts.