Why our diamond plates are not bonded to steel.
Most diamond sharpening plates on the market are bonded to a steel substrate. Ours are not. Our diamond stones, including the extra-large 200-grit plate, hold the diamonds in a thin brass layer over an aluminum base. Our ZenSharp cards carry the diamonds on a PET sheet, with a thin steel backing under the PET that lets the card sit magnetically on a precision-milled aluminum base. The substrate that holds the diamonds is soft in both products. The structural piece beneath is aluminum.
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A hard substrate registers nothing when the user is doing something wrong. The user can press on a single point, lean a bevel onto its corner, drift the angle a few degrees off across the stroke, and the plate will not show it. The abrasive will appear to do its work. What is happening, out of view, is that the diamonds and the blade are both losing material to the uneven contact. The next session, or the one after, the edge is duller than expected, or the plate has a stripe where a row of diamonds is gone.
A soft substrate has a small amount of give, microns at most, and the give shows up as a record of how the user is loading the plate. Press too hard at a single point on a ZenSharp card and the PET tears. Lean a diamond stone past its edge and the silver overlay marks. The diamonds underneath are intact and the working surface stays flat, but the marks are visible, and they let the user see what they were doing.
The give has costs. Because the substrate yields slightly under the work, the bevel comes off mildly convex at the scale of microns. We have measured it. It does not matter at any normal grit. Severe corner-loading on a ZenSharp card, repeated, can shear the PET layer off the steel backing of the card. The card is finished. The base it was sitting on is unaffected, and a new card goes onto it.
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The other reason for not using steel is flatness. Steel deforms under machining and under heat treatment. A steel substrate that holds true flatness to a few microns has to be ground after heat treatment, matched to a separately prepared abrasive layer, then assembled in a way that preserves the flatness through the bonding step. Most makers of steel-backed diamond plates do not run that process. The flatness numbers on the box are what the substrate would hold if those steps were taken; the plates that ship deviate beyond them.
Aluminum, after stress-relieving passes, holds tighter tolerances and is more stable over time. We mill aluminum bases for both products. The diamond layer is prepared apart from the base and joined to it once the base is finished: a brass film carrying the diamonds for the diamond stones, a PET sheet bonded to a thin steel backing for the ZenSharp cards. In both products the bevel meets a soft substrate, the soft substrate sits on aluminum, and the working flatness comes from the aluminum.
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Diamond plates retire two ways.
A steel-backed plate, in normal use, often retires by losing diamonds in a stripe, or by going dull faster than expected after pressure that exceeded what the abrasive could handle. The plate is replaced before the diamonds are spent.
A soft-backed plate retires by wearing. The diamond points round gradually under correct use, and the cutting rate drops slowly enough that the user can feel it coming. For a ZenSharp card the next step is a new card. For the diamond stones it is the end of that grit.
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The substrate holds the diamonds. It is also where the user reads how the work is going.
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