Planer Wear and Tear-Out Problems

Wear on the knives and the chip breaker could lead to tear-out problems when the knives are replaced. July 15, 2012

I just replaced the knives on a Jet JWP-15csw planer that I recently bought. Now it seems to have excessive tearout no matter how much I plane off. Even on alder. Any thoughts?

Forum Responses
(Solid Wood Machining Forum)
From contributor G:
I'm not familiar with that planer, but is there any chance the knives were mounted backwards? That would definitely change the shear angle of the cut and cause tear out. I'm not certain how foolproof the knife change can be on this particular planer. I know on other brands knife direction isn't always obvious.

From the original questioner:
The knives are in properly. On further investigation, I think the chip breaker is bent. I am going to try to find someone who works on these things to come and look at it.

From contributor J:
Well, if it didn't do it before, but it does it now, it definitely sounds tooling related. Please check the blades you put in the machine. Unless you damaged the chip breaker changing blades (I don't see how), it must be the blades. They are the only thing that changed, correct? Chip breaker damage on these machines is usually kickback related, running dull blades, resulting in a massive kickback, striking the chip breaker.

From Professor Gene Wengert, forum technical advisor:
The chip breaker does indeed get worn and that creates a larger gap. The new knives can also be on a cutting radius that is too big for the chip breaker. That is, if the knives and chip breaker were both worn and then you put in new knives and these new knives project out a bit more (0.005 or so), the chip breaker is actually expecting shorter knives so it is too high (with a top cutting head). Note that most of the wear will be in the middle, so we would expect less chip out near the edges of the planer if I am correct.

Note that chip out also occurs because the knives have too much rake angle, so they act like a wedge rather than a plow, by lower MCs, by high feed speeds and high stock removal, to name a few other reasons.

From Professor Gene Wengert, forum technical advisor:
Although it does not seem to apply to your case, we also see chip out, chatter, and other defects when the bed plate wears. (Wear is mainly in the middle and not the edges, as most lumber goes through the middle.) If planing rough lumber that has been air dried (which means a lot of fine grit in the surface), you will get bed plate wear, especially in the middle. The defects will be more obvious in narrow pieces; a wide piece will be held by the edges where we would not expect as much wear.