Kato/Kawai For Dummies - my interpretation
I posted this over at WoodCentral where Kees started the same thread.
In a nutshell, what we are achieving with the combination of a close set chip breaker and the angle at its leading edge is a bending of the chip close to the edge of the blade. This we all know.
We also know that there is a relationship between the distance to the blade edge and the angle at the edge of the chip breaker. Borrowing from aerodynamics, we can understand this relationship as follows ...
Think of a sports car being tested for "slipperiness" in a wind tunnel. First we have a vintage sports car ...
http://blog.mercedes-benz-passion.co...08DIG29432.jpg
What we see here are sudden uplifts of airflow when hitting the vertical sections (such as the grill and the windscreen). One would expect that this extra drag would slow down the car. Where the airflow passes over the more horizontal sections of the body (such as the hood and the roof), there is little interruption, with greater speed and/or efficiency expected.
Here is a more modern sportscar with more advanced aerodynamics ...
http://www.ferrarituningmag.com/wp-c...d-Tunnel-3.jpg
Another way of looking at the way the chip breaker interacts with the shaving is as an air break or wing flap on an aeroplane ...
http://www.parakaiairfield.co.nz/images/DaFlapDiag.jpg
As the air hits the flap, it is slowed down. Similarly, as the shaving hits the leading edge of the chip breaker, it is slowed down and forced upward, and forces the blade down (equal and opposite reactions).
There are three corollaries that follow:
1. the closer the chip breaker is to the edge of the blade, the greater the down force at the blade edge, and the greater the effort to push the plane.
Greater down force should also lead to increased blade wear.
2. the higher the angle at the leading edge, the greater the "braking effect" ... aka greater "bending effect".
K &K recommended an 80 degree leading edge on a 40 degree bed as optimum. It follows that the leading edge for a 45 degree bed smoother would be 75 degrees, for a 50 degree bed smoother it would be 70 degrees, and for a 55 degree smoother it would be 65 degrees.
3. a lower angle at the leading edge will reduce the bending effect, and do so progressively the further from the edge it is placed. It will also reduce the effort to push the plane.
The aim of the chip breaker effect on a bevel down plane is identical to that of a bevel up plane with a high included angle: the creation of a sudden bending effect, which is also known as a Type III chip formation ...
http://upload.wikimedia.org/wikipedi...ype_II.svg.png
It is the suddenness of the bend that reduces tear out. Tear out is created when wood fibres are allowed to bend slowly and progressively, breaking ahead of the blade's edge. This is also known as a Type I chip formation ...
http://upload.wikimedia.org/wikipedi...Type_I.svg.png
While some woods tolerate the slow and progressive bend, namely those with evenly formed and upward running grain, those woods with uneven and erratic grain require a plane that bends the shaving in a sudden action.
Regards from Perth
Derek