Hand Tools Archive
Bill Tindall, E.Tn.
Summary- They never explicitly recommended any angle
Professor Kato sent me all the papers he published on the topic of planing. With one exception all these papers involve the effect of various variables on the rate of dulling. No recommendation was ever made on a cap iron edge angle in any publication I have.
In the paper we had translated in detail the authors investigated cap iron angles of 40, 50 and 60 degrees on dulling and the related phenomena of “self sharpening”, the process where the combination of wear from the cap iron working and abrasion from planing work together to preserve a pointy cutting edge. In another paper they reported the effect on cutting force of various variables. Again the cap iron angles investigated were 40,50 and 60 degrees. In yet another paper they only used a cap iron at 40 degrees. In a paper focused on the discharge of the shaving they investigated shaving guide angles of 50,70,90 and 110 and a cap iron angle of 50 degrees. Professors Yamashita and Inoue also studied the effect of the cap iron. Both used a cap iron angle of 50 degrees.
The only place where a cap iron angle of 80 degrees is mentioned is in the video. Indeed, in the video, under the conditions of the experiment a cap iron angle of 80 degrees worked better than the far removed angle of 50 degrees at mitigating tear-out. In this experiment the clearance angle is 10 degrees and the blade is sharpened at 30 degrees. We don’t know from two far removed data points what angle was optimum under these specific conditions. Professor Kato explicitly stated the conclusion that in this case 80 was better than 50 at mitigating tear-out in a note he sent to me accompanying the video. However, this is not a global conclusion for setting the cap iron angle for general planing either with a planing machine or hand plane. Many other plane and planing variables could affect the optimum cap iron angle.
In the student tutorial on positioning the cap iron an angle of 50 degrees is used on the cap iron. Other than this inference there is no published recommendation for grinding the cap iron angle. Indeed, the optimum could well depend on what, how, and with what, you are planing. When in doubt try 50 degrees, but be sure we are all talking about measuring this angle form the same reference. Actually, when in doubt consult an old Stanley plane. Stanley probably knew what they were doing when they designed the cap iron.
As an aside, Kato found M2, M41 and D2 to be the best steel for plane blades with edge life and initial sharpness as criteria for best, but that research is described in another paper.