When sharpening, the choice to apply a multi-step progression from coarse to fine abrasive is predicated on the belief that there is a limitation in keenness or sharpness or uniformity for a particular grit that is overcome by progressing to a finer grit. This belief follows intuitively from the common experience of increasing smoothness of wood sanded with progressively finer (or higher grit) papers. This approach generally assumes that scratch depth correlates with edge width (keenness) and that scratch depth is proportional to grit size. Within this model, observations of the increasingly mirror-like quality of a blade bevel during a honing progression are used to predict an improvement in keenness. To be clear, there is no correlation between keenness and the polish of a blade bevel – a keen blade can be muted by drawing the edge across a stone with no effect on the level of polish on blade bevel. Also, keenness can vary dramatically between edge leading and edge trailing honing strokes where identical levels of bevel polish occur.
In context of sanding wood, “too big of a jump” refers to the idea that it should take longer to sand out 60 grit scratches with 320 grit paper than if a 100 grit sanding is performed between the two steps. Unlike hones, fine sandpaper wears out quickly, making “large jumps” uneconomical. A more salient question is whether we achieve the same result, setting aside expediency.
Contrary to the tenets of the honing progression, blades can be made shaving-sharp with relatively coarse abrasives. As was demonstrated in the diamond plate progression an excellent beard-shaving blade can be produced with a 325 grit diamond plate (yes, it was an extremely smooth and comfortable shave). One explanation for low-grit sharpness is the so-called “toothy edge.” Although there is no consensus on the meaning of this term, there is usually an implied analogy to a serrated blade and that the non-uniformity or “teeth” enhance cutting ability and perceived sharpness. In the coarse diamond plate example, the edge was shown to be highly refined with no evidence of “teeth.” To determine whether the diamond plate result was an anomaly, we investigate a different, more traditional coarse hone.
In this article, a series of images are presented from a carbon steel straight razor blade (native bevel angle 16.5 degrees, inclusive) honed on a Shapton Pro 320. The blade was first sharpened with back-and-forth strokes with progressively smaller sets per side and then finished with 10 edge-trailing strokes after rinsing the hone. The blade was maintained perpendicular to the stroke direction to achieve a scratch pattern normal to the edge.
The blade was then honed on a Shapton 16k glass stone with the blade approximately 30 degrees off normal to facilitate the identification of scratches from each hone. The blade was honed with 10 edge-leading laps and imaged, then honed another 90 laps, for a total of 100, and imaged again. In these examples, we follow a particularly large chip in the edge, allowing a comparison of the same area of the blade.
In these images, the deviations in the linearity of the edge are not the result of scratches. Although micro-chips account for some of the observed non-linearity, this effect is also due to variations in the near-apex bevel angle; the edge is longer where foil-burrs are present and shorter where the apex is burr-free.
The depth of the chip was measured at 23 microns prior to Shapton 16k honing. After 100 laps, the depth was 14 microns – 9 microns of the blade was removed with 100 return laps. From the blade geometry, this corresponds to 1.3 microns of steel removed from each bevel face – a removal rate of 13 nanometers per 15 centimeter long lap. At this rate, it would require another 150 laps to reach the bottom of the large chip. Performing 250 alternating strokes is substantial and tedious; however, the same result can be achieved rapidly with back-and-forth strokes, ending with a handful of alternating edge-leading strokes to ensure a keen, burr-free apex.
For reference, edge views of two other blades are shown following honing on the Shapton Pro 320. The first, honed only with edge-leading strokes (alternating sides) displays a coarse, broken edge – essentially continuous, overlapping microchips. The edge width varies from about 1 micron to near flats more than 5 microns wide. The second, honed with only edge-trailing strokes (alternating sides) displays a massive foil edge burr. Beneath the burr, there is a keen, linear and chip-free edge. These two blades, and the one described above, have identical scratch patterns, but wildly different apex geometries. In these blades, there is no correlation between scratch patterns and keenness. The scratches do not typically end in microchips. The scratch pattern does not correlate with the presence of “teeth.” The edge non-linearity is not correlated with stria on the bevel face.