Why you shouldn't put plywood on a jointer

Just recently, I was puzzling over a very regularly spaced series of nicks in my jointer knives that was leaving a series of ridges. I wondered what sort of metallic thing could have nicked the knives and leave so regular a pattern....

...until I held a piece of Baltic birch plywood up to the wood, and sure enough, the spacing of the ridges matched the glue lines exactly.

So as it turned out, I had recently discovered what an excellent job the jointer did at cleaning up the edges of the birch plywood. Works much better on that than on regular plywood.

I guess that wasn't such a good idea after all.

I measured the height of the ridges with a dial indicator, and they were only 0.001" high (or about 0.025 mm). A few strokes with a cabinet scraper is enough to take care of them without affecting the dimensions of the wood.

I seems my planer knives always have nicks in them. I'm just not careful enough, and I can't be bothered to take them out to sharpen for every single project. A carbide tipped helical cutter head would perhaps be the solution, but I'm too cheap for that.

Fortunately, this jointer is 12" (30 cm) wide, so it's easy enough to avoid these nicks until the knives really need sharpening. And for really wide stock, I can always clean them up with a scraper.

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