Wooden Plane Throat Geometry

I love old wooden hand planes. There’s something about tuning up a 150 year old woodie and putting it back into working order that is very satisfying to me. The thick laminated irons, 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 my iron planes.  I’m not saying that I dislike using my iron planes. I just like using the woodies more.

From the rear, a 2 ¼" x 8" x 47 ½ degree double iron smooth plane, a 2" x 7 ¾" x 45 degree double iron smooth plane, and a 6 ½" x 1 ¾" x 55 degree single iron smooth plane.
From the rear, a 2 ¼” x 8″ x 47 ½ degree double iron smooth plane, a 2″ x 7 ¾” x 45 degree double iron smooth plane, and a 6 ½” x 1 ¾” x 55 degree single iron smooth plane.

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 re-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.

Throat geometries of single iron and double iron wooden planes.
Throat geometries of single iron and double iron wooden planes.

The lower front of the throat on a wooden plane is called the wear. 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 so obviously that’s not possible. But there’s more to consider.

Observe the two drawings above, representing the throat geometry of a single iron and double iron plane. Notice how it’s fairly straight forward to create a nice tight mouth on the single iron plane. The wear in a single iron plane typically angles back over the bed at an angle of about 15 degrees greater than the bed angle. This provides for about 10 degrees for the wedge and another 5 degrees to provide for a bit more clearance for the shaving to be ejected. So in the drawing on the left above, the iron (red outline) is bedded at 50 degrees and the wear is cut at about 65 degrees.  In this drawing, notice how the front of the mouth is easily made right up against the cutting edge of the iron.

Now look at the drawing of the double iron plane to the right. Notice how the presence of the chipbreaker impacts the angle of the wear.  In this drawing, in order to create the same tight mouth that the single iron plane has, it was necessary to increase the wear angle to almost vertical. With a wear angle any less than the illustrated 80 degrees (a full 30 degrees more than the bed angle), the chip breaker would prevent the iron from being able to be extended through the mouth because it would contact the wear.

This is kind of interesting, because when we look at most double iron wooden planes, the wear angle is typically around 70-75 degrees. But based upon the drawing above, we’ve already established that if the wear angle is any less than 80 degrees, then the chip breaker would contact the wear and wouldn’t allow the iron to extend through the mouth (and if, by chance, it did, the shaving would get trapped between the wear and chip breaker). That is, unless the mouth was opened slightly.

IMG_0163
By opening the mouth up slightly, the wear angle can be kept lower.

As shown in the drawing above, 19th century makers of planes with double irons made a compromise. Most didn’t want to use such a steep angle on the wear because routine maintenance would result in a mouth that opened up very rapidly. So to slow the opening of the mouth, they chose to keep a slightly lower (70-75 degree) wear angle, albeit not quite as low as those found on single iron planes. However, in order to let the double iron through, and permit the shavings to pass over the chipbreaker, they chose to make planes with mouths that weren’t quite as tight as those on a single iron plane. Seems kind of like robbing Peter to pay Paul, doesn’t it? You open the mouth a bit right from the beginning in order to keep it from opening as fast later on.

Some would criticize double iron planes because of this.  However, proper use of the double iron more or less eliminated the need for the tight mouth that was so necessary in single iron planes. With the edge of the chipbreaker set very close to the edge of the cutting iron, the chip was broken before it had a chance to tear out, so the mouth could be slightly wider without any loss in performance. Unfortunately, this allowed wooden planes to be manufactured by less skilled workers as the 19th century progressed (many American wooden planes were made by prison labor later in the 19th century).  This eventually led to the demise of the wooden plane as the quality of these tools declined to the point that they were completely replaced by the modern iron hand planes.

The typical mouth opening of a "new" double iron wooden plane.
The typical mouth opening of a “new” double iron wooden plane. This plane has the tightest mouth of all of my wooden bench planes.

The plane pictured above is my 8″ long smooth plane with 2 ¼” wide double iron bedded at 47 ½ degrees. This plane was virtually unused when I bought it several years ago, and has seen very little maintenance flattening since then. As you can see, even though very little of the wood on the sole has been removed since the plane was made, the mouth is still much wider than what most would consider acceptable for a smooth plane today. However, with the chipbreaker set properly, I haven’t had any problems smoothing any of the woods that I typically use. In fact, I’ve never had a problem with any of my other wooden bench planes either and they all have even wider mouths than the smooth plane pictured above.

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 limited benefit. But for the typical work that most of us do, I just don’t think that mouth opening on a double iron plane is all that important. A sharp blade and a close set chipbreaker are much more important in a double iron plane than a tight mouth.

In the coming weeks I will be making two new smooth planes using some old irons I have.  One will be a double iron plane, using the 2″ wide double iron from the middle plane pictured above (I bought the plane just for the iron). The other will be a single iron plane made using an old 1 ¾” iron given to me by a friend (not the one in the plane pictured above).  I’ll do my best to document the builds here, so that you can get an idea of the process in case you want to give it a try for yourself. Who knows, maybe I’ll even film the process. Stay tuned.

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6 thoughts on “Wooden Plane Throat Geometry

  1. Hey Bob,
    A nice breakdown of the geometry of planes. I think there might be another explanation though, for why 19th century American planemakers used such wide mouths.
    When we look at the double iron planes in the Seaton chest, we see tight mouths and nearly vertical wears. I’ve seen photos of a couple other early English planes that are made the same way, and my own experience is that this design affords the best shaving escapement. But to make a plane this way, the abutments have to be stopped cuts that end in the middle of the wear. It takes a lot of labor-intensive hand work.
    On the later wide-mouthed planes, however, the abutment is a through-cut that comes out of the mouth. This can be done by machine, which obviously speeds up production a lot. And I think that’s the main reason for the later design. Now, if the mouth is really wide, the wear needs to be at a lower angle, otherwise the opening at the top of the plane will be unusually large; on a small smoother, it would not leave much room to grab the front of the plane.
    Anyway, that’s my theory. I like your new(ish) wordpress site and am glad to see you blogging more frequently these days.

    – Steve

    • Thanks for chiming in Steve! This is exactly how my largest smoother (the one whose mouth is pictured above) was done. The wear on that plane is 80 degrees and the abutments dead end into the wear, just as you describe. It is an English common pitch (47 ½ degree) plane. When I got it the mouth was about half as wide as it is now (i.e. about 1/16″). However, because the wear is so steep, it has opened up about 1/16″ with just a minor amount of routine maintenance. I’m not sure the mouth on this plane was ever any tighter than 1/16″. However, even with a ⅛” wide mouth, it still works fine.

      I agree with you on the later planes as well. The English tended to resist industrialization and hold on to the quality hand work longer than Americans did. But as industrialization took hold the tools were modified to make them easier to mass produce by machine. Shame really.

      I’ve yet to decide how I’ll handle the double iron smoother I’m going to make. I’ll probably raise the wear to 80 degrees like the Seaton planes and my English double iron smoother. My shop is climate controlled so my plane soles don’t require re-truing very often. If they did I might decide on a lower wear angle. For the single iron I’ll likely try about 62 degrees for the wear like my little single iron English smoother has.

        • I don’t know who made it. The plane is unmarked. The iron is marked Aaron Hildick, Diamic (the same “Diamic” inside of a diamond mark used by Henry Taylor today). My guess is that it’s from the late 1800s. There were pencil marks on the bed and back of the wedge when I got it. I think they were the maker’s original # marks to keep the body and wedge together. Other than that there are no other marks on the body.

  2. Beautiful article Bob!! I hope you will film the whole process about building a smoother plane.I ‘m looking for beech , maple and oak in order to build my future planes…. so i need your instructions.

  3. Great insights! Now all I have to do is get to work on that lump of red beech I have sitting in the shop, only I haven’t really decided whether to do classic german or english – maybe even a Krenow. I guess the drawing board will be first stop. Keep it up!

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