So how do planes actually work...?

[Leif]

I know this sounds remedial, but I've never quite been able to find a satisfying answer. Let me explain:

A plane is essentially a flat surface (sole), with a cutter protruding through the bottom a few millimeters. Well it's that protrusion, beyond the flat, that gets me.

The toe rides on the wood to be cut, while the heel rides on the just-exposed wood beneath the cut. Those surface are not coplanar (off by the thickness of the shaving), so the sole should only have 2 points of contact (the back of the heel, and just in front of the throat), right? Then the accuracy of the plane is determined by how parallel the rear edge of the sole is with the leading edge of the throat.

Or basically, if you work in the middle of the board, you'll end up digging out a bowl. If you work from middle to edge, you end up cutting a downhill.

So it would seem that the only way to actually make use of a flat, level sole is to have the blade edge on the same plane as the sole, with the toe slightly above. That way the sole behind the blade defines the plane, the blade knocks down bumps, and the toe limits the depth of cut. When the surface of the board is all coplanar with the sole, the blade stops cutting - the board is flat!

And that's how power jointers work!

What am I missing here?

[mattvan]

It's all smoke and mirrors lol! Just kidding. I have to admit I've wrestled with this too But after having an old No 5-1/2, which was obviously messed up by someone prior to me owning it, with the toe about 1/32" higher than the blade I assure you it doesn't.

From what I understand, a lot of the work happens because the fibers of the wood are being bent and deflected during the cut. Once the blade removes the shaving, which is typically much much thinner than a power jointer's shavings, the fibers return to their original shape but now any dips or hills have been leveled out.

Especially with hardwoods this notion of bending the fibers doesn't seem possible, but with any blade (muscle powered or otherwise) the cutting action involves the wood fibers being stretched and pushed to their final limits and ultimately being torn from their original position. The final result is whether it's a clean break or a nasty tearing.

I'm sure there's more I'm missing, but that's my understanding in a nutshell.

[Bob Rozaieski]

Western planes work by shifting the reference plane from front to rear during the stroke. At the beginning, only the area in front of the iron is in contact with the wood. Once the area behind the iron becomes more involved in the cut (due to transfer of pressure and additional weight behind the iron), the toe lifts up and ceases to contact the wood (I'm talking western planes here, not Japanese, which work differently). So at this point, the points of contact are the area of the sole immediately in front of the iron, and the heel. In essence, the plane is at a slight angle with the cutting edge of the iron acting as the fulcrum.

[mattvan]

I'm copying and pasting Bob's response into my notes for future reference lol.

[wilburpan]

Western planes work by shifting the reference plane from front to rear during the stroke. At the beginning, only the area in front of the iron is in contact with the wood. Once the area behind the iron becomes more involved in the cut (due to transfer of pressure and additional weight behind the iron), the toe lifts up and ceases to contact the wood (I'm talking western planes here, not Japanese, which work differently). So at this point, the points of contact are the area of the sole immediately in front of the iron, and the heel. In essence, the plane is at a slight angle with the cutting edge of the iron acting as the fulcrum.

There's a great article in the upcoming April issue of Popular Woodworking that has an awesome photo that illustrates this.

[Bob Rozaieski]

There's a great article in the upcoming April issue of Popular Woodworking that has an awesome photo that illustrates this.

[Roger T]

I am not a believer in the "Plane sole has to be ground flat to the .000001 crowd" But, I am a believer in having the area just in front of the mouth, and the trailing edge of the plane in the "Same Plane". While these areas during a planing stroke, will not be co-planar, out from being coplanar by an almost unmeasurable amount, you may as well just call them co-planar.

Here is an experiment using your favorite smoother that you can do in your shop that will demonstrate how important the area in front of the mouth is. Assuming that the area in front of the mouth, and the trailing edge are coplanar. Using a freshly sharp iron, set your frog up so you will have as narrow opening in the mouth that you can get without the plane clogging, or being unable to use the lateral adjuster. Now take a board and plane the surface, trying to get light shavings, less than .002. Closely examine the surface that is left on the board now, if you have some sort of magnification, that will help tons. Now, remember what this surface looks like, and either set it aside, or flip it over for part 2 of the experiment. Take your plane, and move the frog back as far as it will go and still be able to install the blade assembly without it touching the back part of the mouth. Now using the same board, try planing again, trying to get that same .002 shaving or less. Compare surface A to surface B.

At this point, you should find that the surface of the "B" side of the board is different from the "A" side. What you should be seeing is that the surface of the "B" side is rougher (relatively speaking) has possible tearout, and in general is not as nice of a surface as the "A" side. The mechanics of how a chip/shaving is made come into play here.

As the plane blade is pushed into the wood, the fibers begin to fracture ahead of the cutting edge of the blade. With the blade set up to the rear of the mouth, and not having the counter force of the front of the mouth opening controlling the amount of pre-edge fracturing you will get a rougher, more unfinished surface. Now with the blade set in its most forward position, as close to the mouth edge as feasible, the leading edge of the mouth now acts as a compression to the fracturing of the chip ahead of the blade edge. The more we can control this fracturing ahead of the cutting edge, the better the final surface of the board will be.

Dan, the OP said:

So it would seem that the only way to actually make use of a flat, level sole is to have the blade edge on the same plane as the sole, with the toe slightly above. That way the sole behind the blade defines the plane, the blade knocks down bumps, and the toe limits the depth of cut. When the surface of the board is all coplanar with the sole, the blade stops cutting - the board is flat!

And that's how power jointers work!

What am I missing here?

I know I've probably not answered your question, but in your scenario, going over the hills and bumps, you lose the front of the mouth. And also, using the plane as you've described above, the same as a power jointer, you can plane your board into nothingness. Also with a plane as I've described above, the same thing. The trick is learning when to stop planing, so that you don't get into making a hollow, or rounded end. This can't really be taught, but rather, needs to be learned from experience.

HTH,

Roger

[Vic]

It's just magic to me. I don't need to understand the how and why, just the what happens. I know it works with practice.

[sandvike]

To paraphrase Robert Wearing in his book. You are right, it is theoretically impossible to plane a surface flat without a power planer type setup. However the longer the plane the greater the radius of the circle you are planing is. Think of the camber on a blade if the edges are 1/32nd above the middle its a very large radius. The radius of the circle on a jointer plane will be very large and the segment nearly flat.

We also take advantage of this property when doing things like edge jointing. We plane a hollow which is possible because the plane describes the radius of a circle then plane full strokes until its flat. This is because in practice there is a tendency to plane a hump.

What it comes down to is we aren't machines. We are bringing the tool to the wood and not the other way around. Unlike a power jointer we will not plane exactly the same all the way through the stroke nor will each stroke be exactly the same. At the same time we can observe and adjust what we do based on the results of our previous strokes which is also something we can do that a machine cannot.

So we drive a plane like we drive a car. We may not be perfectly straight but we can constantly and subtlety correct so that the result is a straight line.

[baok]

Leonard Lee's book "The Complete Guide To Sharpening" has a good description of what's happening down where the blade meets the wood. I checked it out from the library but found it useful enough to buy.