Plane Articles

Single Irons vs. Double Irons
by Bill Clark & Larry Williams

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.

18th Century planes with typical hand-forged irregular surfaced thin irons.

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.

At some point someone noticed that a torn splinter during planing would could be broken by causing an additional steeper turn after it passed the cutting edge. So yes, a cap iron can help reduce tear out.

Our question would be what value does this really offer in practice? Before anyone writes and accuses us of re-inventing the wheel, we'd like them to include the mouth opening of their favorite smoother and how far back its cap iron is set from the edge.

Think about the thickness of any shaving that contains tear out. The spot containing any tear out will be substantially thicker than the rest of the shaving. If you are using a mouth opening tight enough to help control tear out, somewhere around .005" or a little more, we believe that your plane will stop the offending tear out before it can reach a chip breaker set at an uncommonly close 1/32". A 1/32" setting would be more than five times greater than the shaving aperture.

Even if you managed to shove that chip through the mouth, a chip breaker set at 1/32" will allow more tear out than you can remove in a couple passes with a plane. How are you going to plane the same area several times with out getting more tear out? It would require several passes to remove a divot who's depth was even a third its length. If planing wasn't successful on the first pass, why would it be better on several with the same plane?

There are several good ways to control tear out during FINAL SMOOTHING by our thinking:

  1. Keep your iron very sharp
  2. Take light shavings of about .001" to .002". A fine shaving won't have the beam strength to lift wood ahead of the iron.
  3. Use an appropriate angle of attack. The bed angle of the iron should match the characteristics of the wood being planed. Hardwoods that scrape well should be planed with a steeper bed angle. This actually uses the additional turning of the shaving you get from a cap iron but takes place at the edge rather than further up the iron. Some woods don't have the surface structural strength to resist the additional cutting resistance that results from a steep bed angle. Softwoods can require a bed angle of 45 to 55 degrees.
  4. Use a plane with a tight mouth. The tight mouth actually holds the wood being planed in place and does this immediately ahead of the irons edge. This additional support will go a long ways to reducing tear out.
  5. A thick iron is less likely to chatter causing irregular cutting and tear out. Proper bedding of the iron, even with thick irons, helps greatly in this.
  6. Use the plane at a skew. The shearing cut of skewing the plane can help at times.

All the above will also work with heavier roughing cuts, though not as well. Our experience is that single irons are, at a minimum, as effective as double irons.

We believe mass production plane makers benefited more from the idea that double irons stop tear out far more than woodworkers. Expecting double irons to control tear out, perhaps the least effective method, allowed manufacturers to make planes to lower tolerances.

Double irons also offer a couple drawbacks. There's the extra tuning required to keep errant shavings from getting under or catching on the cap iron. The biggest problem is that they limit the escarpment size at the most critical point. When you turn a fine shaving, with little beam strength, back on itself it will collapse. No longer will it slide out the escarpment instead it will fold up on itself and become packed in the between the wear (vertical part at the front of the mouth for metal planes) and choke the plane. The only way to clear this on these planes is to remove the iron and go through the process of setting the plane again. A single iron plane, however, will nearly always clear itself on the next pass in the much more rare event of choking.

We're simply not all that impressed with the supposed "improvement" of double irons. We feel that it's one of those oft-repeated things that is blindly accepted and repeated without regard for its accuracy.