Skewing boards to create a shearing cut?

[Jeffrey Makiel]

I often hear folks talking about feeding stock on a slight angle to create a "shear cut". However, I don't think it's a shear cut. Rather, it's still a flat cut. That is, when the straight cutter hits the wood, the entire edge of the blade enters the wood at the same time. Therefore, a skew feed is still 'slapping' the wood.

To me, a shear cut is when one end of a cutter initially engages the wood, then the point of contact moves along the length of the blade as it rotates until it reaches the other end of the cutter. This can only happen when the edge of the blade is at an angle relative to the rotating axis of the cutterhead...not the angle of wood feed.

A Shelix is an example of a true shear cut. Its little cutterheads are set an angle to the rotating axis of the cutterhead. The Grizzly spiral cutterhead simulates this effect in a segmented fashion using several small straight cutters parallel to the rotational axis of the cutterhead but mounted in an overall spiral configuration.

Sometimes I try to skew feed boards into my little planer or jointer having straight knives. This has nearly insignificant results for me because I've never found a board that had perfectly straight grain. Aside from quartersawn, the grain usually shifts side to side, and up and down, thus changing the effective skew angle anyway.

If I ever upgrade my machinery, I will be going with a spiral or helical cutterhead. I hear many positive things about them regarding reduced tearout.

Perhaps I'm just petty. Maybe I should have a New Year's resolution to get out more often. :)

-Jeff :)

[glenn bradley]

I agree with your assessment/definition. I do skew certain boards through my non-shearing planer and get different results than when feeding straight through. Other boards I don't skew ;-).

[Jim Becker]

Jeff, I agree with you technically, but find that in practice, I get a much nicer surface, even on straight grained project components by feeding them skewed to the knives. I don't think that my Tersa knives are any different in that respect than "traditional" knives, although they tend to be nice and sharp as well as never out of alignment.

[Chip Lindley]

Your accessment of the skew vs. a spiral cut is correct. Skew still cuts across the width of the knife, but at a new angle of attack. Spiral cuts the wood at a different point all along the knife width.

Jointing on a skew is more practical on a wide jointer; 12"+. Only very narrow stock can be skewed to any great degree across a 6" or 8" cutter head.

My limited experimenting with skew-cutting tells me its not worth the trouble. A set of knives with a *back grind* are better at preventing chip-out in very hard, or wild grain woods. These knives tend to scrape the wood rather than shear it, lessening tear-out in difficult grain.

[Keith Westfall]

I would think that the skewing would help more if there was a nick in the blades. By "crossing" along the knives (wood being skewed) it would help clean the nick mark instead of having it in the same place all the time...

Maybe..., Oh well, we'll soon find out! :)

[Mike Heidrick]

Can you can see the difference in shavings on a planer or jointer if your dc is not hooked up? Maybe that would shed some light on teh difference in reality. On shorter pieces I am face jointing I sometimes skew the board through the cutterhead - 12" DJ30 with straight knives. Seems to work well.

[Steve Rozmiarek]

I've gotta disagree. It's the same principal as skewing a hand plane. The blade angle does not actually change, but the direction of the fibers in relation to the force imparted by the movement of the knives against the stock is more against the grain than directly with it, making tearout less likely. It's all about grain orientation in relation to cutter direction.

I could be wrong, but I also think a geometry expert could also make a case that skewing a cut changes the effective angle of attack for a knife. If I'm right there, it has big implications to the geometry of the cut as well.

I use the technique on a 16" jointer, and it works.

[Harvey Pascoe]

I use almost nothing but highly figured wood and find that skewing is a marked improvement when combined with feeding the right end. Fortunately I use mainly rather short stock so getting a large angle skew is possible.