The Double Iron, Part II

Diagram of a double iron from Turning and Mechanical Manipulation by Charles Holtzapffel (1843)

In Part 1 of this series, I presented evidence that 18th century woodworkers viewed the double iron as a revolutionary technology that was more effective in combating tearout than any other method. An interesting feature of the 18th century writings I quoted is that they give no specifics: No measurements, no angles, nothing. They just describe how the cap iron works in very general terms.


In the 19th century, writers start attempting to describe the operation of the cap iron in more precise terms. The most important of these is surely Turning and Mechanical Manipulation by Charles Holtzapffel (1843). After describing how a narrow mouth opening can reduce tearout, Holtzapffel writes:

The same effects are obtained in a much superior manner in the planes with double irons … the top iron is not intended to cut, but to present a more nearly perpendicular wall for the ascent of the shavings, the top iron more effectually breaks the shavings, and is thence sometimes called the break iron … the top iron has a true edge, which is also moderately sharp … the top iron is placed from one-sixteenth to one-fiftieth of an inch from the edge of the cutter.



Holtzapffel adds two critical facts that were missing in earlier descriptions. First, the cap iron should actually be sharp. Second, the cap iron should present a more nearly perpendicular wall for the ascent of the shavings, so it should make a roughly 45° angle to the blade at the tip; when this is added to the 45° bed angle, we get the 90 degree "perpendicular wall."


So far, so good. But on the crucial question of distance from the cutting edge, Holtzapffel misses the mark. One-fiftieth of an inch, or .020", is off by a factor of two or three. The big payoff with a cap iron, in terms of tearout reduction, happens when it's set .010" or a less from the cutting edge. Anyone following Holtzapffel's instructions to the letter would have to conclude that cap irons weren't very effective. I'll come back to this, but let's continue on…

Many other 19th century writers discuss the double iron, but the range of measurements given is wide. James Lukin, in Our Workshop: Being a Practical Guide to the Amateur in the Art of Carpentry and Joinery (1879), writes:

The break iron of the jack planes is generally fixed, so that its edge…is one-sixteenth behind the edge of the cutter. If the break iron be set back, say one-eighth of an inch, it will not bend the shavings sufficiently, and the planing will very probably be rough. The nearer the edges of the irons are to each other…the smoother will be the work produced, but the labour of driving the plane will be much increased.

As an aside, Lukin also writes that


Planes having single irons are much less laborious to handle, but the work executed by them is neither so smooth nor so truthful as that of the double-ironed description. The shavings escape in long curls or ribands from a single iron, and the surface of the work is left somewhat rough. It is a good plan to remove the first exterior and dirt from a plank with a single-ironed plane, after which the jack and smoothing planes will work pleasantly, and retain their edges for a longer time.

Francis Young, author of Every Man his Own Mechanic (1882) writes:

… when the jack plane is required for heavy work… the edge of the break iron should be about 1/8" from the edge of the cutter, but for finer work it should not be more than 1/20" from the latter; and in the smoothing plane the distance between the edges of the two irons should be less than this, indeed so slight as to be perceptible, but nothing more.

One other 19th century source worth mentioning is The Art and Craft of Cabinetmaking by David Denning (1891). Not so much for his description of the double iron, which is familiar and rather cursory, but rather for his comparison of double vs. single irons:

For coarse, rough work… the mouth may be set comparatively wide, and the edge of the back iron be set one-eighth of an inch from that of the other; while for fine work… the mouth is narrow and the cutting edge only very slightly in advance of the other. In any double iron plane, the nearer the two edges are together the finer will be the shaving, but the labour of planing will be increased. From this it will be seen that the relative positions of the two irons is of considerable importance, and that within certain limits, the planes can be regulated to suit the work on hand. As jack, trying and smoothing planes are made with both double and single irons, it should be said that the latter, though cheaper, are not suitable for cabinet working.

Writers from the first half of the 20th century continue in a similar vein; two excerpts will suffice to illustrate. The prolific author Charles Hayward wrote about the double iron repeatedly throughout his long career; a search of the recently released The Woodworker: The Charles H. Hayward Years turns up more than a dozen explanations of how to set the cap iron. Here's a representative example:

Its [the cap iron's] purpose is to break the shaving as it is raised, and so minimize any tendency of the grain to tear out. The closer to the edge it is set the more effective it becomes, but the greater the resistance it offers. For the jack plane it can be set about 1/16" from the edge; the trying plane (when set fine) about 1/32", the smoothing plane 1/32" or less.

Elsewhere, Hayward gives a diagram showing incorrect (Figure 10) and correct (Figure 11) shapes for the cap iron:

Finally, the book Planecraft, originally published in 1934 by C.W. Hampton, gives the following recommendation:

For rough work, [set the] cap iron 1/32" to 1/16" from edge. For finishing work… 1/64" to 1/16". For hardwoods with irregular grain… as close as you can get it to the cutting edge.

This last excerpt is quite interesting, because it's the only one that accurately conveys how the cap iron should be set: up to 1/64" from the edge for normal circumstances, and (implicitly) closer than 1/64" for hardwoods with difficult grain.

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So far in this series, I've presented excerpts spanning nearly 200 years. I think we can conclude three things about the authors of these writings: one, they believed that double irons were superior to single irons or any other method of controlling tearout; two, they agreed that the cap iron must be set close to the cutting edge; and three, they were (excluding the author of Planecraft) terrible at conveying just how close the cap iron needed to be set. So why did these guys fail so badly at this task?


I think there are two reasons. First, the earliest writers were not writing the kind of how-to manual for amateurs that modern readers are used to. In the 18th century, nobody learned woodworking from reading a book--the only way to learn the craft was as an apprentice. Nicholson and Salivet were describing, not instructing. it would not have occurred to a craftsman like Peter Nicholson to give a precise description, with exact measurements, of how to set a cap iron. If he had wanted to show someone how, he would have simply demonstrated it.


Later writers like Lukin, Young, and Denning were writing for amateurs, but I think they fell short for a different reason. A clue may be found in the book The Tool Chest of Benjamin Seaton. As Jay Gaynor and Peter Ross explain:


The workmen who made these tools as well as those who used them often shaped or fitted their work by sight or touch … rather than by inches and fractions. Rules were divided only to 1/8". Micrometers were in their earliest stages of development. On a workaday basis, most metal- and woodworkers didn't think in terms of smaller mathematical dimensions. They were unimportant as long as the result looked right and worked well.


In the 19th century, smaller fractional divisions gradually became more common, but divisions above 1/32" or at most 1/64" were rare. Today, we take it for granted that we can buy cheap dial calipers that accurately read in thousandths of an inch; we can roll down to the auto parts store and buy feeler gauges that give us fixed references for any distance between .001" and up, or we can used mass produced goods like paper (.003") or playing cards (.012") to create a frame of reference. But in the 19th century they didn't have cheap calipers, or mass-produced and standardized goods. In short, the authors I've quoted weren't bad at measuring, or describing; they simply lacked the language and frames of reference to accurately convey tiny dimensions in writing.


This point turns out to have very important implications for the use of the double iron. In the apprenticeship culture of the 18th century, where everything was learned by direct demonstration, it was easy to transmit the workings of the double iron from master to apprentice, or father to son. But from the 1880s until today, an ever-increasing number of hand-tool woodworkers have been amateurs learning from books and magazines. And let's face it, it would be extremely difficult to teach yourself the use of the double iron from any of the books I've quoted in this post. As a result, there was a gradual forgetting of the original purpose and function of the double iron, a forgetting that culminated in the misguided notion that the double iron was not only incapable of controlling tearout, but had never been intended to do so. But that is a subject for Part 3.