A little history and theory:
The double iron plane was invented probably in the middle of the eighteenth century. Its purpose was to allow cabinetmakers of the day to plane ‘difficult’ woods. An ad by planemaker Samuel Caruthers in a Philadelphia paper in 1767 stated: “Also, double iron planes, of a late construction, far exceeding any toothed plane or uprights whatsoever, for cross grained or curled stuff”. Caruthers mentions 2 out of the 3 main methods to control tearout at the time; toothed planes and ‘uprights’. Uprights are single iron planes with a high pitch angle, usually 55 to 65°. Toothing planes cut the grain with an iron with little grooves in it. It has obvious disadvantages for subsequent cleanup. High angle planes will work, but of course are limited to that one pitch and have other drawbacks like pushing really hard and fast edge wear. The other method Caruthers didn’t mention was the use of a tight mouth. to stop tearout. I mainly want to focus on a tight mouth vs double irons to control tearout.
An ideal plane, in my opinion, would be a 40-42°, bevel down, single iron plane with an easily adjustable mouth. The low angle would allow for the easiest pushing, while the mouth could be closed up tight to stop all tearout. If only the mouth would stop all tearout. Unfortunately, it won’t . Why doesn’t that work? Well, we need to look at what is happening at the cutting edge.
When the plane’s cutting edge encounters downhill grain ( grain reversals ), its natural inclination is to follow that grain down. The grain is literally being levered up by the edge, and the edge wants to dive deeper to follow the grain. Now, if we close that mouth up tight, that should stop the chip from being levered up, holding it in place for the iron to cleanly sever it. However, don’t forget about the iron wanting to dive in to follow the grain down. The iron can only go so far down before it must come back up, and this will create the tearout. The mouth doesn’t exert enough pressure down to keep the chip from popping up. Also, the mouth doesn’t exert even pressure across its width, especially when planing the face of a board with overlapping plane tracks and uneven surfaces. That is why demonstrations of single iron planes on difficult woods are usually done on the edge of a board, so the plane is cutting 1 1/2″ of wood with a 2″ iron. The mouth then will have perfect registration on wood for maximum effectiveness. The other major issue with relying on the mouth to stop tearout is wear on the mouth right ahead of the blade. And I’m not just talking about wooden planes. Metal ones also wear quite fast in fron of the blade, which of course reduces the effectiveness even further.
Now let’s contrast that with what happens with a chipbreaker. I like to think of the chipbreaker as being somewhat like a closely set mouth, only above the cut. With a closely set chipbreaker, the blade will encounter the same downhill grain. Its naural reaction is to follow that grain down. However, it is also attached to the chipbreaker. So, when this blade dives down, the chipbreaker acts like a closely set mouth, and puts pressure on the wood right where the blade is cutting. As the blade wants to dive down, the pressure from the chipbreaker actually increases. The blade is then able to sever the fibers cleanly because they are being held down by the chipbreaker. Compare that to the tight mouth on the previous example. There, as the blade wants to dive down, the mouth becomes less effective at putting pressure on the cut. Here, as the blade dives in, the chipbreaker goes with it, and the pressure becomes more effective at stopping chips from lifting.
As you can imagine, the distance from the cutting edge to the chipbreaker is one of the main factors affecting its effectiveness. If it is too far, the chip has room to lift and tear before the chipbreaker can stop it. If set too close, it limits the thickness of the shaving and the plane is much harder to push. As a very general rule, the heavier the cut, the bigger the chipbreak distance and the lighter the cut, the closer the chipbreak distance. As previously stated, judging the right distance does require some experience, because there are multiple factors involved. Experimentation is the key to unlocking the usefulness of this great invention made over 200 years ago.