Why Planer and Jointer Knives can Lose Their Straightness

Repeated sharpenings can create stress-relief curves in cutter knives. February 19, 2006

I have been having problems with our planer and jointer for a while. Everybody has had a theory - bad gauge, bad knife replacer, bad worn out machines and bad table. Bad knives turned out to be the problem. They were not straight and some had bows and some had humps.

A bad knife sharpener is what we could all agree on. Our old sharpener took several sets back and re-sharpened the knives with no charge and even bought us a new set of each. Recently I decided to check out a new set (Freud 20" planer knives) and they were pretty crooked.

Do my question is this - is this standard? Our knife-setting jigs can't really accommodate for this feature. How do you go down the knives trying to bend the s-curves out?

Forum Responses
(Solid Wood Machining Forum)
From contributor R:
We had a similar issue. The knives were coming back with a bow. I can compensate with my gauge and experience but the guys in the shop just complained and didn't take action. I talked to my sharpening guy and he said they could straighten them out if we asked for it. Metal, like wood can release tension and bow when it gets ground down to certain points. Having metal-smith experience I should have realized it. I just ask them now to make sure the knives are straight and the problem has been solved.

From contributor R:
WKW or Schmidt knives can bow over time with the grinding releasing tension in the metal. I have several sets of each and other high grade blades. It's a natural occurring thing in steels. When I was doing metal-smith work this is something that was very common when cutting and working with any metal.

From contributor P:

It is more convenient for them to be straight, certainly. I have 4 Leitz blades for my Griggio, and one has a bow. It just means more of an effort to replace it in the block after sharpening. I think s-curves are pushing it a bit I wouldn't accept that from the supplier.