Single Irons vs. Double Irons

Historically, the first reported double irons appear in a 1767 advertisement by Philadelphia plane maker Samuel Caruthers. There's strong evidence these were imported irons from England. The history of double irons undoubtedly predates the Caruthers ad.

One of the most obvious characteristics of early irons is that they are very thin. One of the working characteristics of thin irons is that they are prone to chatter. One requirement for eliminating harmonic resonance or chatter in planes with thin irons is the labor intensive mating of the bed, iron and wedge. Remember, these were hand forged irons rather than the precisely machined irons you would find today. Fitting the wooden plane body and wedge to the irregular surfaces of these irons takes time and skill.

A thicker iron reduces the chance for harmonic resonance from incomplete support. Thick irons simply don't require as much support. But in the 1700's tool steel was expensive and labor was less so. Then someone came up with the ideal of double irons and was able, we think, to double the effective thickness of the irons without adding additional high carbon steel.

Not only was there the cost of the thick irons as a deterrent, there was also maintenance. Thick irons required a parallel increase in other technologies. Abrasives would be one area to look at. As most woodworkers can attest, maintaining thick irons takes more time. 18th Century woodworkers didn't have grinders, carborundum, silicon carbide and other sharpening mediums available today so sharpening time would have increased greatly with an increase in iron thickness. Yet these woodworkers were professionals and, like the plane makers of the day, were paid by piece work. Time and labor savings were important to both trades. The double iron, we think, offered and increase in the effective thickness of the iron without the negative increase in sharpening time.