Planer is tearing-out -- on white oak, but not red

      What could be the culprit when a planer is tearing out on one type of oak, but not another? June 14, 2000

Q.
My friend has an older, 26-inch Baxter D. Whitney planer he is setting up.

We are getting an unsettling amount of tear-out when planing kiln-dried, rough-sawn white oak. It planes red oak just fine.

Is this normal? We have been told that we need a "blade angle" of 12 degrees. Someone else mentioned "back bevel." A third mentioned a 49-degree angle to grind the blade.

The third seems understandable, but how do we verify that this angle is correct?
What are the first two talking about?
And does any of these have to to with the tear-out we are experiencing?

FYI the planer has a square head with four blades, with a diagonal distance, point-to-point, a smidgen over 5 inches.

A.
If you are using a square head, the safety factor must be considered. Square heads have not been widely used for over 30 years at least.

I would suggest that you convert to a cutterhead that uses either a standard planer blade or one with carbide inserts. If you go with carbide inserts, the noise will be less, the finish improved due to the spiral of the teeth, and the length of run will be very good.

If you go with a standard planer head then use a hook angle of 10 to 12 degrees. The hook angle is the angle built into the cutterhead that creates the cutting action. With hardwoods, a scraping action is needed. The low hook angle provides that. With softwoods, a slicing action is needed, and a hook of 20 to 30 degrees in the head would be needed.

The angle of 49 degrees is the angle of the cut, including the hook angle (head angle) and the back clearance angle (ground onto the knife).

Be sure to have your tools as sharp as possible.
Dave Rankin, forum moderator



From another WOODWEB expert:
I would add that the moisture content (MC) of the lumber is a major factor in tear-out. Check the white oak for overdrying.

Unfortunately, a pin meter is not accurate under 7 percent MC, so if the wood is 5 percent, if it was dried at too high a temperature, or even if it was overdried and then the MC increased, it will still be behave as though it is very brittle.
Gene Wengert, Sawing And Drying Forum moderator



From the original questioner:
This is entirely possible. He used a solar kiln after a year of air-drying, and didn't measure MC; he simply figured that when there was no more condensation in the kiln for a few days it was finished. I have noticed that some of the thin strips I have cut off when straight-lining seemed rather brittle. This could be it.

By the way, I have found some reference material on WOODWEB on blade angles and all of the different terms.

I have found a few inconsistencies. It seems that the terms "back-bevel" and "front-bevel" mean the same thing, only that wood people use the term "back-bevel" the same way machine people use "front-bevel." You have introduced me to two new terms: "hook angle" and "back clearence angle."

Forgive me if I seem confused, but these blades were ground with only one angle. There is no front-bevel, relief bevel, or face bevel.

From what I have gathered, from the materials here on WOODWEB, the cutting angle may be too sharp for the hardwood we are planing. The illustrations and data imply that a front bevel will create more of a scraping cut, which will be less likely to tear-out.



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  • KnowledgeBase: Wood Engineering: Wood Properties

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