Skewing: The No. 7 Way to Reduce Tear-out

I keep a short list in my head of what I like to call “The Woodworking Mysteries” – things that I pretend to understand but are really just outside my grasp.

One of the mysteries is how a tree can pump water and nutrients from its deepest roots to the furthest reaches of its branches. We have many clues as to how it works, but a complete picture eludes me at least. Another mystery is about how yellow glue (polyvinyl acetate) actually works. Again, I’ve never read a satisfying explanation.

A third mystery relates to handplanes and basic geometry. One common strategy for reducing tear-out in a board is to skew the plane as you make the cut. This strategy was beaten into my head by all my teachers dead and living. It’s repeated on the Internet by people I deeply respect and trust. And I do it myself in my work.

But if you do the math, you will quickly see how this strategy doesn’t make much sense on its face.

Let’s start with a fact that we do know: The higher the angle of attack when you plane a board, the less likely you are to experience tear-out. This is an almost immutable truth. It’s why we have high-pitch planes and scrapers in our arsenals.

Now for another fact: Skewing a plane in use reduces your angle of attack. Mike Dunbar, the founder of The Windsor Institute and a personal hero, explained this to me in the clearest way possible. When a shaving encounters a plane iron, the angle of attack is like a hill that the shaving has to walk up. If you walk straight up that 45° hill, that’s a lot of work. When you skew the tool, it’s like the shaving is walking up the hill at a lower angle. Or put another way, it’s a bit like building a road up a steep mountain. You don’t make the road go straight up the mountain, you build switchbacks so the vehicles can actually make it up the incline. Skewing reduces the amount of work required – both to plane a board and to climb a hill.

How much does skewing reduce your angle of attack? John Economaki, the founder of Bridge City Tools, published a brilliant chart that explains this on his web site page that promotes his variable-pitch plane. You can see the chart in full here (scroll down to the section titled “Skewing the Plane.”) You can look at this chart and see immediately that skewing a 45°-pitch handplane by 30° will reduce your effective angle of attack to 40.9°.

So here’s the problem: If high planing angles reduce tear-out, and skewing a plane reduces your angle of attack, then how can skewing the plane reduce tear-out?

Here’s a hint: The answer is in the branches.

Planing with no skew resulted in no tear-out on this ash board.

To explore this seeming contradiction, I did a little experiment in the shop on Saturday. I took a short piece of ash with pronounced grain direction – that is, there was no question about which way the grain was traveling in the board.

I cleaned up one face with a smoothing plane and then turned the board around so that I planed against the grain, which is when you are more likely to encounter tear-out. Then I planed the board with a bevel-up block plane, the Lie-Nielsen 102. This plane is bedded at 12° and the iron is sharpened with a 35° micro-bevel, so its angle of attack is 47°. The mouth on the plane is wide open, so it’s not much of a factor. The tool is set to take a shaving that is about .002” thick.

Planing with a 30° skew created this ugly patch of torn grain.

First I planed the board against the grain without skewing the tool. This cleaned up the board just fine with no tear-out. Then I skewed the tool by 30° (which lowered my effective planing angle to about 43°) and did the same operation. I tried skewing both to the left and to the right. Two areas of the board tore out grotesquely.

Then I cleaned up the board again and tried skewing the plane at 20°. Tear-out occurred at the same two places but not as badly. So I tried skewing the plane at a variety of angles. And without fail, the more I skewed the plane, the more tear-out occurred.

So how can skewing reduce tear-out?

You have to remember that trees are not manufactured items. They are giant cones made of fibers that grow in different directions as the tree responds to its environment: a hill, a disease, a wind storm. Then we slice them up into shapes suitable for building things, regardless of how the fibers are traveling through the tree.

In some boards, grain can change directions on you a couple times. And the grain can be at odd angles – you cannot assume that all your boards will have grain running from one end to the other – the grain may be traveling at a 20° direction along the face of the board and 10° along the edge. And the grain might be in the shape of a shallow wave.

So there are times when skewing the plane puts the edge in the right position at the right time to deal with that patch of grain.

Planing at a 20° skew created a little tear-out.

In my example board above, the two places where the tear-out occurred were at places where the grain rose quickly. So how did I deal with this board? As I encountered the areas that tore out, I straightened out the tool – no skew. When I worked the areas that didn’t tear out, I skewed the tool to reduce the effort required for planing.

So the trick with skewing takes us back to the No. 1 way to reduce tear-out: The best strategy is to select the best woods possible and learn how to read the grain so you can begin to predict how your tools will behave. Sometimes, the best strategy is to not skew the tool.

Or put another way: Because grain is irregular, sometimes skewing the plane allows the blade to encounter the grain at a non-skewed angle – and to therefore plane it without tear-out.

This is the end of my series on planing. I hope that some part of it was helpful. Next week, we’ll probably return to the topic of (surprise!) workbenches.

— Christopher Schwarz