Why does my new spiral cutterhead for my jointer leave grooves?

I recently bought an aftermarket spiral cutterhead for my jointer. Now when I use it, the face of each board shows slight grooves running along its length. Am I doing something wrong?
—Travis Adair, Cincinnati, Ohio

Probably not, Travis. We've found similar results in our tests of spiral cutterheads. The small individual blades that corkscrew around the cutterhead have slightly convex cutting edges that fight tear-out, but also leave the linear striations you're noticing. So if you're used to a nearly finished face out of your straight-knife cutterhead, you might be disappointed by the additional sanding burden added by the spiral cutterhead.

Still, a spiral cutterhead brings several advantages: Where tear-out-prone straight knives can wreak havoc on highly figured woods, spiral heads shear the wood without damage. Also, straight knives can be a hassle to replace and adjust. Not so with spiral cutters. When one edge of the small four-sided, self-indexing cutters becomes dull or nicked, simply rotate them 90° to bring a fresh edge to bear. That adds up to four times the cutting life. And that's not counting the fact that many of the replacement spiral cutterheads use long-lasting carbide rather than high-speed steel. At pennies per replacement cutter, you won't break the bank when you need to replace one, either.