In part 1 of this discussion, a comparison of leading edge and trailing edge honing strokes was illustrated with examples from the Chosera 1k and the Shapton 16k hones. As described in The Honing Progression, edge leading strokes on those particular hones leads to a stable triangular apex with finite edge width and that geometry is maintained with continued honing. The examples in part 1 demonstrated that edge trailing (stropping) strokes on those hones leads to a reduced edge width (increased keenness) however, continued honing leads to the formation of a foil-edge burr.
The effect of stropping on a compressible substrate such as leather or fabric is illustrated with the following example. A carbon steel straight razor blade was honed edge-leading on a Shapton GS 8k, followed by a stropping progression of 100 laps on a chromium oxide loaded hanging horsehide strop, followed by 100 laps on 250nm cubic boron nitride (CBN) on Nanocloth, and finishing with 30 laps on 100nm CBN on Nanocloth. Blade cross-sections are shown before and after stropping at the same magnification for comparison.
Measurement of the edge geometry shows that the increased keenness (reduced edge width) is achieved by increasing the near-apex angle by approximately 2 degrees (inclusive). A concurrent increase of 200nm in the apex width at 3 microns is observed. In terms of our definitions of keen and sharp, keenness is improved at the expense of sharpness.
This particular stropping progression has achieved three results; the bevel is polished to a mirror finish, the edge width is reduced, and the angle near the apex has increased. Pasted stropping is often disparaged due to the irrational fear of apex convexity. The images below shows how little convexity is introduced by a effective stropping progression.
The example above was chosen to illustrate the goal of pasted stropping; to increase keenness (reduced edge width) by introducing convexity to the the last few microns of the apex. This effect is distinct from producing a convex bevel; in this case the bevel was shown to remain triangular other than those last few microns. I will use the term “micro-convexity” to describe this effect. The progression described here is not unique; a similar effect can be achieved with a variety of substrates and abrasives. Future parts of this discussion will investigate the effect of the substrate, the abrasive type and size.
Compared to stropping on a incompressible hone, where a burr forms readily, a correctly designed stropping progression will avoid the formation of a foil-edge burr through the introduction of micro-convexity.