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How do I prevent ripples when jointing?

Q:

I’m suddenly seeing ripples along edges of boards after I joint them. Because this never used to happen, and I don’t think my technique has changed, I’m at a loss. What’s wrong?
—Bill Harris, Laramie, Wyo.

A:

Two things cause ripples (also known as scallops, shown above) when machining boards on a jointer with straight knives, Bill: excessive feed rate or a misaligned knife. Although you say you haven’t changed your technique, try the simplest tactic first. Grab a scrap board and joint the edge as you normally would, and then check the edges for ripples. Now do it again, but at a much slower feed rate, and compare the results. If the ripples go away, you’ve found the solution.

If that doesn’t do the trick, check the knives: Each one must be set at precisely the same height. One that protrudes a hair more than the others will make a slightly deeper scallop with every cutterhead revolution. Carefully reset each knife, securely tightening all the gib bolts, and you should have smooth sailing from there.

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Check your jointer knives with a straightedge resting on the outfeed table. Adjust each knife so at its peak it just touches the straightedge.

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