|Tight Mouth, Small Throat = Fine Shavings|
During the scrub plane demonstration I did at our last Sin City Woodworkers Meeting I mentioned that the scrub plane has a very large mouth or throat opening. The is to allow the thick shavings to pass through the plane. You can see a short video of the scrub plane in action if you click on the SCWW link. You may remember that on the initial cuts with the scrub plane it had a "clogged throat" and there was a picture there to illustrate on a previous blog. The plane above is a Lie-Nielsen Bronze # 4 1/2 smooth plane. It features a very small mouth and the throat opening can be adjusted as shown here:
The beauty of this set up is that the frog can be moved forwards and backwards without having to remove the blade. The screwdriver is pointing to a locking screw, there's another one on the other side. After you loosen them the frog can be adjusted with the large screw in the center. Why do this? The smaller the opening (mouth) on the bottom of the plane is the, more support and pressure there is on the board. Since the plane is pushing down on the surface around the cut, there is less tendency for the grain of the wood to pull up and tear. You can see by the shavings that came through the throat, these are some pretty fine shavings from that piece of African Mahogany I've been working with.
This adjustable mouth is a nice feature to have on your plane. With the smooth plane it's the frog that's being moved back and forth to adjust the mouth opening. On block planes the section of the sole in front of the blade can be moved with an eccentric lever to change the mouth opening. There was a little confusion during my demonstration about which is the mouth and which is the throat. I thought it was pretty clear in my own mind until we started talking about it! I remember it this way; the mouth is the opening while looking at the bottom of the plane and the throat is where the shaving travels through. It's not a major point but just keep in mind that the small mouth will only allow thin shavings through the throat. By contrast, a large mouth will allow huge shavings through the throat.
Here's a comparison between the smooth plane and the scrub plane: