Adam Cherubini, NJ
>I've had this exact problem but I wasn't smart enought to know recognize it. Not only does the slot have to be straight, but the slit for the blade below it has to be inline with the slot for the spine. I got these two slightly off and it was enough to create a beautifully tragic S curve in the blade. I just assumed I hadn't straightened the blade (so I tried to fix the blade- when I figured out what i did wrong, I had to refix it!)

I guess there are always some manufacturing tricks using machines and jigs. In this instance, I just tried to do a better job. Sometimes i have to just try harder.

The saw in question, (my sash saw) I had the blade slit right but the spine slot wrong (maybe like your tool). So i corrected the slot with a paring chisel. I ended up with a gap on one side, so I cut a very thin piece of wood and hide glued it in.

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To make the slot, I mark both sides with a marking gauge, then saw to the lines. Using a thin firmer chisel, I pare away the stuff between the lines. Since i saw the slit first, it isn't hard to remove the material. I learned that straight is more important than tight. I also learned that the slit for the blade must line up with the slot for the spine.

Adam