As an edge becomes dull from use, steel is worn away from the surfaces that intersect to form the cutting edge. On this site I will refer to these surfaces as the face and the back relief surface.

• The face is the surface the shaving slides along as it leaves the throat of the plane. On a bevel-down plane, this corresponds to the back of the blade.
• The back relief surface travels just above the newly planed wood. On a bevel-down plane the back relief surface is on the bevel side of the blade.

Using my small microscope it hasn’t been possible to see the shape of the wear profile. The illustration above is based on several published images of wear profiles, and has been adjusted to match the angles I use on my smoothing plane blades. I’ve also compared it to electron microscope images of worn edges, and all of these sources show asymmetrical curves with the longer wear surface on the back relief side of the edge.

My microscope images of worn edges show a bright line on the back relief surface that grows as the blade becomes duller. The width of this bright line is a good measure of blade wear.

The width of this wear surface is used by Leitz, a major manufacturer of industrial blades, as an indicator for when a blade needs sharpening. For blades used in power equipment such as planers and shapers, Leitz recommends sharpening when the wear surface reaches .2 mm (200 microns) in width. This amount of wear is much greater than the amount acceptable for a plane blade. I consider a smoothing plane blade ready for sharpening when the wear area on the back relief surface reaches about 12 microns in width. In my images this equals 4 pixels.

While the bright line on the back relief surface is a good wear indicator, the much wider bright line that develops on the face is not. I believe this bright area is due to polishing by the shaving as it exits the cutting area, and does not correspond to an area of steel that is worn away to any significant degree. My evidence for this has to do with the amount the cutting edge recedes as the blade wears. As the illustration above suggests, the part of the blade that cuts through the wood fibers to create the shaving recedes as the blade wears. The total amount of this edge recession is about 15 microns (well less than .001”) when the blade is ready for sharpening. If the bright line on the face indicated significant wear, the amount of edge recession would be much greater. In my microscope images the bright line on the face is about 90 microns wide.

Here is the wear profile illustration with some dimensions added that represent a blade at the point it is no longer sharp enough for smooth planing:

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