Most of you know that I am not a big fan of honing back bevels on plane irons. I believe that many inexperienced plane users allow themselves to be mislead, thinking that a back angle (bevel) is going to be some type of magic pill. But after a crafts person is sufficiently confident in their ability to sharpen and maintain appropriate cutting geometry, the use of a back bevel can be very beneficial.
Several days ago I was working in my friend, Les’, shop. He asked me if I’d take a look at his L-N #4 that he had set up with a York pitch (50°) frog. He wasn’t very happy with the surface he was getting on a piece of cherry stock. The stock was not particularly gnarly, but there was a grain direction change, right in the middle. The iron was the same that had been removed from the Common pitch (45°) frog. Les, who is always meticulous about the condition of edge tools, said that he believed the iron was sharp. But I, “doubting Thomas” that I am, suggested that we test the edge. Sure enough, the iron sliced a piece of unsupported paper easily. Next, it trimmed a neat bare patch on the back of my forearm. But upon closer inspection under a loop, it was clear that there were some tiny edge fractures present.
Cogitation began. Increasing the pitch angle of a bench plane (bevel down), supposedly increases the tool’s ability to work in very dense and/or highly figured stock. Obviously, either condition is a challenge to shearing tools. And an increase in pitch causes an increase in the amount of effort required to push the shearing edge through the material. Neither of us had ever experienced a similar problem when using an iron that was back beveled to effectively create a higher pitch. Could back beveling be better than increasing pitch? Turns out, yes. Maybe.
Take a look at the following diagrams (Please note that, for the purpose of illustrating the problem, I’ve used Common Pitch (45°) and Middle Pitch (55°) for the examples.)
When a back bevel in employed, the increased included angle strengthens the tip of the iron. The clearance (relief) angle is kept to a minimum, thereby providing the greatest possible amount of support to the iron.
When a conventionally prepared iron (25° primary bevel, 30° secondary bevel) is secured to a higher pitch bed (frog), the clearance angle significantly increases and the amount of unsupported surface significantly decreases. I believe that the tip section would be effectively weakened by these two factors.
It may be appropriate to increase both primary and secondary bevel angles in order to increase tip section strength and reduce clearance thereby increasing the amount of support surface. It is common for adjustable scraper planes (Stanley 12, 112, 212) to be ground at 45°. But these tools are rarely used at attack angles under 90°. We need to do some physical testing, as there seems to be very little, if any, information available on the matter. If anyone has any thoughts and/or experiences on this question, please share them with us.