The Shape of Bench Plane Irons (the truth revealed)

The modern woodworking community seems to be obsessed with jigs and fixtures and that’s probably a good thing.  Accuracy of construction has continued to improve over the millenia.  But, as with so many things in life,  there are occasions when “less is more”.  The wholesale use of sharpening jigs may well be one of these things.  True, they do assist the sharpener in creating perfectly flat surfaces and the intersection of two perfectly flat surfaces, forming a perfect angle, is, certainly, the classic definition of “sharpness”.  They also make it simpler to create a perfectly straight edge which is square to the length of the iron.  And “straight and square” are absolutely necessary when sharpening irons for tools like rabbet and shoulder planes (and most chisels).  But when dealing with the class of tools known as bench planes, straight and square is not always the most advantageous condition.

Rabbet, shoulder, molding and other types of specialty planes are used to create structual or ornamental details and, in some cases, used to remove miniscule amounts of material in situations like the final fitting process.  Bench planes are used to create true and finished surfaces to a given dimension.  As this process may require the removal of significant amounts of material, the use of tools that have been “fettled” to remove very small amounts of material with a high level of accuracy may not provide optimal productivity.  In other words, when you’re “rough” planing a board to dimension, you probably want to do as quickly as possible – especially if you’re getting paid by the job, not by the hour. 

Roman Jack plane found in Pompei excavations - "shod" in wrought iron on both sole and top of body - single iron

Planes, as we know them, have been in use for several thousand years.  When looking at the irons of antique planes, modern woodworkers are often struck by the fact that most of them are not “straight across”.  In fact many have a significant arc to the cutting edge.  The exceptions are usually smoothing, panel and jointer planes.

Try planes (called fore planes in North America), scrub planes and jack planes are tools used to establish the “A” or primary data (a reference surface from which other angles and measurements are based) and/or dimensions.  The surface condition left by these planes looks and feels “wavy”, to a greater or lesser degree, depending on the width of the iron and the arc height of the crown (curve of the cutting edge).  Most hand built antique furniture will have unseen surfaces that are, in most cases, left in this condition.

Working with a set of winding sticks, the craftsman will determine whether the first surface to be trued has any cup, warp or twist that must be dealt with.  Affected areas will be marked and trimmed with the Try or Scrub plane.  After the “A” data has been planed flat and true, the craftsman will use a thickness marking gauge and mark the required dimension.  If the amount to be removed is minimal, the craftsman may elect to plane the “B” data to thickness with the Try plane (as it takes wider cuts than the other crowned planes).  However if there is a fair amount of stock to be removed, a Scrub plane will be employed.  When using any crowned (radiused) plane, the craftsman will move the plane diagonally or completely cross grain, as a heavily radiused plane can lift and tear fiber bundles and create voids in the surface that are below the desired elevation.

The radiuses employed are determined by the experience of the individual craftsman.  The type of work, material and its condition, are just a few factors that must be considered when “fettling” planes.  The goal, of course, is to achieve a product of acceptable quality with the minimum output of energy.  Radiuses are described in terms of arc heighth and they may range from as little as a 64th to as much as a 1/4″.  Again, only the user can determine what works best for him.  Below is a comparison of  typical radiuses found on older “working” handplanes (those used by professional woodworkers).

Typical plane iron radiuses

 We’ll talk more about how to properly sharpen a radiused iron and the proper sequence of planing stock in upcoming installments. 

Remember, making shavings is cheap therapy.

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One Comment on “The Shape of Bench Plane Irons (the truth revealed)”

  1. Hi, I think you are right that the modern woodworking community seems to be obsessed with jigs and fixtures and that’s probably a good thing because it gives accuracy to dimensions.

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