The Theory of Frogs
A microscopic look at a shaving will reveal a series breaks across the grain. As you push your plane forwards, the blade acts as a wedge, lifting a tiny splinter of wood. The angle of the blade forces this splinter upwards until it breaks. The next splinter lifts then breaks and so on, forming a continuous ribbon. This is why a shaving curls.
Higher blade angles create shorter splinters. The breaks in the splinters occur closer to the timber’s surface, thus leaving a smoother finish. Lower angles can leave a coarser finish. Higher angles require greater effort to push the plane. Lower angles work much better on end grain. The lower angle helps to cleanly sever the ends of the timber’s fibres. Higher angles are better for timber with wild, interlocked grain. They produce a better finish as the action is slightly closer to scraping. It is a matter of finding the best compromise for the timber you use or the type of work you do.
A 40° frog provides a low cutting angle that is easier to push and leaves a cleaner finish on end-grain. The 45° frog offers a cutting angle that is suitable for most work. Also known as common pitch, it is the most frequently found bed angle on bench planes. The 55° frog provides a higher cutting angle. Sometimes referred to as middle pitch, it helps to minimise tear out in highly figured timber.