Hand Tools Archive

Diamond scratch depth
Response To:
WD40 smudge ()

Wiley Horne
Hi David and all,

On the 3V iron, it appears to me that the prominent diagonal diamond scratches, each terminate in a ‘low place’—an incised place—at the edge. I can’t make out whether this also happens on the Tsunesaburo and V11 irons. So it may be, at least on the 3V iron, that the diamond is ploughing a furrow which carries away a piece of edge where it daylights.

If so, what is the relationship between the depth of scratch on the bevel, and the depth of incision at the edge? Ordinary geometry would say that,

Incised depth= furrow depth/tan(included angle). Or, for 33 degrees.
=furrow depth/0.65.

Well, this is pretty fanciful, but suggests at least a possibility that the effect at the edge is greater than the furrow depth itself. Meaning the diamond is creating toothiness at the edge, which may be greater than intuition would suggest.

So what? Well, what I wonder is: In a duration test, if two irons differed only in their toothiness, which edge would last the longest? Would the toothy iron, or the more ‘compact’ iron last longer. My guess would be that the more compact edge would outlast the toothy edge, because each grain of the compact edge has more support on either side (and above and below) than the toothy edge. The toothy edge is more vulnerable to attack, and would unravel faster.

But maybe it ain’t so. Maybe the toothy edge puts up a better fight against the wood fibers.

Your photographs add a lot! I’ll be curious to see how the more deeply scratched edges do versus the more compact edges.


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