Hand Tools Archive
A hundredth mouth will probably be significant to a lot of beginners because they won't get in any serious trouble with it. There will still be small tearout, but nothing large will lift.
I still remember the first piece of curly maple I planed. I fought the plane through it, and there was a little burl button or some grain that was about the size of a marble, and the half circle of it was in my board. It was interesting looking.
I horsed it with a coarse plane and the entire little half circle popped out in one piece. Terminal. That kind of significant thing is stopped, as is less significant.
BUT, it's still miserable planing with a plane if there is small tearout (like working a 1 hundredth mouth against the grain), and if you don't want to walk around your bench all the time, you're going to be going against the grain at some point, especially if you use the beginner's process of planing an X pattern.
The ability to plane and keep a plane in the cut with an even thickness shaving, even against the grain, without the jumpiness of a tearing cut ...it really can't be overstated. You just cannot accurately plane depth and thickness when one part of a board isn't tearing and another is.
It might sound strange that I'm making this claim about common pitch and a one hundredth mouth, but I set my third infill (a shepherd kit) to a 1 hundredth mouth, and never filed the front going away toward the front of the plane (it was just 90 degrees). You couldn't get the cap iron close or it ran into the "wear" in that plane. Once I learned to use the cap iron, I didn't open the mouth, but carefully filed it going away (like norris did) and reprofiled the cap iron a little bit so that it wasn't so blunt (as shepherd's slab type cap iron was obnoxiously blunt, because that was easy to make - a wonderful ward or norris cap would not have been).
The difference with that panel plane from that minor modification was night and day. Any direction on the wood, power through it (it was a heavy plane) and no jerkiness in the cut. You could do overlapping strokes from one side of a board to another, measure the depth change, and do the exact same thing over and over and the change was predictable (as was the expectation that if you were removing a certain amount of thickness, you'd do it from end to end and left to right coplanar).