In the early 1990s, I began my woodworking journey with a couple of vintage Stanley bench planes and a Fine Woodworking book on hand tools. I dutifully followed the book’s instructions on setting up and sharpening my new planes, and everything was going pretty well until I came to the section on setting the cap iron (also known as the chipbreaker). According to the author, for difficult hardwoods I was supposed to set the edge of the cap iron “as close as possible” to the cutting edge. So I did, and disaster ensued. I could barely push the plane; it shuddered, shook, and quickly came to an unceremonious halt, the mouth hopelessly clogged with balled-up shavings. I moved the cap iron to a safe distance (about 1/16" - 1/8") from the cutting edge, and there it would stay for nearly two decades.
After that initial experience, I readily embraced the late 20th-century conventional wisdom that cap irons were a con game foisted on a gullible public, and that their real purpose was to make the planemaker’s job easier, and perhaps to stiffen the cutting iron, allowing for the use of thinner, cheaper irons. If you wanted to stop tear-out, forget the cap iron: You needed a single-iron plane with a tight mouth, a thick iron, and a high cutting angle of 55° or higher. The ubiquitous Stanley/Bailey plane with a cap iron, otherwise known as a double-iron plane, was for rough carpentry, not fine furniture. And the instructions I had read in that Fine Woodworking book, along with similar instructions in texts dating back nearly 200 years, were just plain wrong.
In 2012, I began to see discussions about cap irons popping up on the internet. Spurred by a Japanese film of a cap iron on a planing machine, a number of woodworkers taught themselves to use the cap iron the way it had been described in all those earlier sources. The results they obtained, and in some cases documented, were dramatic, and by following their detailed descriptions, I was soon able to achieve equally spectacular results. The double iron stopped tear-out more effectively than any other method, required less physical effort, and left a better surface finish. Those old texts weren’t wrong; they just weren’t detailed enough. The double iron is a finicky instrument, and if it’s not set up just right, it won’t work at anywhere near its tremendous potential.
–Steve Voigt, excerpt from “Cutting-Edge Technology: Discovering the Double-iron Plane,” in Issue Six