Wooden Plane Throat Geometry

I love a good wooden hand plane. The thick laminated iron, the feel of wood on wood, and the simplicity of having no moving parts has a special appeal that I just don’t get when I use an iron plane. I’m not saying that I dislike using my iron planes. I just like using the woodies more. And they’re just so darn pretty. It’s unfortunate [for me] that my current shop environment isn’t so wooden plane friendly.

But have you ever wondered why most old wooden planes found in the wild have mouths that you could drive a Stanley #8 through? This condition is typically attributed to wearing down of the sole from use and routine flattening. And while it’s true that routine maintenance of a wooden plane will open up the mouth, there’s a little more to the story than just routine maintenance and wear.

If we could look inside the throats of the three wooden coffin smoothers pictured above, perhaps we could shed some light on the mystery of wooden plane mouth openings. But I’m not about to take these planes to the bandsaw just to show you what their insides look like. To do so would be criminal (well, at least for the two English ones – the Sandusky was junk when I got it). Not to mention I sold them (the English ones) some time ago. So instead, lets cut them open with the aid of modern technology (a.k.a. Sketchup) and take a virtual look inside of these planes.

Pictured above is a drawing of a single iron wooden plane that has been cut in half (this is what the front plane in the photo above looks like inside). The bottom part of the throat, called the wear, is what we’re really interested in looking at here. This area of the throat angles over the iron for about an inch before angling back in the opposite direction to form the upper throat where the shavings can be removed. The closer the angle of the wear is to the bed angle, the less the mouth will open as material is removed from the sole during flattening. In an ideal situation the wear angle and bed angle would be the same, but the wedge adds about 10 degrees, and about another 5-10 degrees are needed to allow clearance for the shaving to escape, so obviously making the wear the same angle as the bed isn’t really possible.

In a single iron configuration, the wedge abutments can be sawn out with a small saw (like a flush cut saw) or an edge float. This has a great benefit for the planemaker as it makes the process of creating and fitting the wedge mortise much faster and more controllable. Because both sides of the wedge mortise (the bed side and the abutment side) exit the mouth of the plane, the saw and float can pass through the mouth and it is quick and easy to saw and float the sides of the wedge mortise to the proper fit. Also, because there is no chipbreaker, the front of the mouth can be closed up right to the edge of the iron, making for a plane with an extremely tight mouth.

Now let’s look at what the inside of the Sandusky (middle) plane pictured above looks like. This is what you will find inside of the typical, American factory made double iron plane. In most American factory made double iron planes, both sides of the wedge mortise were still typically sawn out to make the labor easier to perform by less skilled workers (typically prison inmates). However, the addition of the chipbreaker causes a couple of problems with this configuration.

First, notice how the presence of the chipbreaker impacts the mouth opening. In this drawing, if the mouth were closed up any more than it is, the chipbreaker would contact the wear. In fact, the mouth in this illustration is actually narrower than it should be because in the drawn configuration, there’s no room for the wedge tips to extend over top of the chipbreaker. So in actuality, the mouth opening in the drawing above should be even wider because some additional room needs to be opened up for the wedge. Also, keeping the wear angle around the 65-70 degrees, similar to the wear angle in a single iron plane, means that the mouth needs to be more open to prevent the chipbreaker from contacting the wear and preventing shavings from traveling up the throat.

For most of the American factory made planes, the double iron was apparently considered enough of an improvement that a tight mouth was not warranted and cheaper labor by less skilled workers prevailed. But there was a way to have ones cake (tight mouth) and eat it too (double iron), albeit with a bit of a compromise.

Before the American prison factories industrialized the production of the wooden hand plane by less skilled labor, English planes, made by very skilled planemakers, used a slightly different throat geometry to allow the use of a double iron while still providing for a tight mouth. The illustration above, which is the geometry of the rear plane in the photo at the top, shows the differences.

First, in order to address the issue of the wear hitting the chipbreaker, the makers increased the wear’s angle from about 65-70 degrees (typical for a single iron plane) to about 80 degrees. This steeper angle prevented the chipbreaker from contacting the wear, though it also resulted in the mouth opening faster with flattening of the sole (there’s always a catch). Second, in order to still allow room for the wedge to slide over top of the chipbreaker, the abutment was modified to end about half way up the wear rather than at the front of the mouth of the plane. These two modifications allowed for a double iron plane to be constructed with a tight mouth.

There was just one problem. The front of the wedge mortise (the abutment) could no longer be sawn through the mouth. Because of this, making planes with this geometry required a lot more precision work with chisels. Hence, as the 19th century progressed, and industrialization took hold, the “refinements” discussed above for the Sandusky plane became more common, and the quality, hand made planes of the previous generations slowly disappeared to be replaced by a cheaper, easier to make product.

Some would criticize wooden double iron planes because of the concessions made by the American factories. However, while the changes made by the American factories were likely implemented in order to allow a less expensive product to be made by less skilled workers, I’m not sure that having a double iron plane with a more open mouth is a huge disadvantage. At least one, very comprehensive, recent study has shown that proper use of the double iron more or less eliminates the need for the extremely tight mouth that was so necessary in single iron planes. With the chipbreaker properly tuned and set very close to the edge of the cutting iron, the shaving is broken before it has a chance to tear out. So, according to the study, using a properly set chipbreaker, one can even plane against the grain with little to no tearout, even with a plane that has no front sole at all. Of course this is in controlled laboratory conditions, but still, the study makes a very strong case for the double iron, and history has shown that the double iron was preferred after its introduction.

My experience using old wooden planes has more or less supported this finding. The plane pictured above is my old 8″ long smooth plane with 2 ¼” wide double iron bedded at 47 ½ degrees (the rear plane from the top photo). This plane was virtually unused when I bought it. Over the years I had flattened the sole a few times, and although very little of the wood on the sole had been removed since the plane was made, the mouth was still much wider than what most would consider acceptable for a smooth plane today. However, with the chipbreaker set properly, I never had any problems smoothing any of the woods that I typically use.

To me, this suggests that mouth opening on a double iron plane is less important than most people make it out to be, for the majority of situations. Sure, for some of the really squirrely grained boards that we occasionally run into, a really tight mouth might provide some additional benefit, but in all likelihood, so would stopping to sharpen the blade. For the typical work that most of us do, however, I just don’t think that mouth opening on a double iron plane is all that important.