In the past 30 years sharpening with diamond stones has gotten more and more popular. Teachers like Paul Sellers have been huge advocates of the technology, and more folks are using diamonds for some of all or their hand tool sharpening needs. The largest US maker of diamond stones for sharpening, and one of the best is DMT and lately we have started to stock most of their entire range. This is the first of two blogs on diamond sharpening, this entry being about the issues involved, and the next part about with my personal experiences with the stuff.
Thirty years ago the major complaints about diamond stones were that the stones weren't flat enough for precision sharpening, the stones didn't last, and you could not get a finished edge from one. The major positives about the technology were that the rise of exotic alloys in woodworking tools such as A2 and D2 increased the demand for a sharpening media that could handle these tough materials with speed. Also diamond stones don't need lubrication or flattening.
Another popular application of diamonds in sharpening is to charge a plate with diamond paste, which turns the plate into a fine abrasive stone. This works great, especially for very fine grits. This is an old method of sharpening that has been applied to woodworking tools for the past ten or twenty years.
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In addition to directly sharpening tools, another very popular use of diamond stones is to flatten waterstones. It's a quick method and works great with one major problem. The problem is that the way you make a diamond stone is by taking a flat piece of steel, sprinkling diamond dust of a specific grade on the plate, and then nickel plating the entire plate, rocks and all, to cover the stone. The plating bonds the diamonds to the plate. When you use a diamond stone to flatten a waterstone, the water stone particles are abrasive and wear away the the plating that keeps the diamonds on the stone - so the diamond stones work slower and slower. In the photo below (taken with my inexpensive not very sharp USB microscope) you can see the plating surrounding the diamond particles like irregular halos.
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When sharpening, diamonds produce a coarser edge than does the same grit waterstone. In order to understand why, we need to get a little material-sciencey here. Bear with me.
Diamonds, like most abrasives have a nominal grit or "mesh" (I could write an entire entry on the technical difference between "grit" and "mesh," but for the purposes of this discussion they're essentially interchangeable) assigned to the stone. The mesh numbers—220, 600, 1200 etc.—essentially correspond with the maximum size of the diamond particle.
This is the same with all abrasive stones - there is a nominal abrasive grade and an actual variance on the particles. Lower quality stones will have a greater grit variance but all diamond stones have some variance.
With regular waterstones the second you start sharpening, any large grit particle shatters, and all the particles start to round over and wear. So very quickly you get an even scratch pattern that we associate with the grit of stone.
Diamonds on the other hand, which cut fast, don't shatter (very much) and the larger diamonds on plate scratch the edge deeper, and don't get worn down. The end result is that for the same grit stone, the diamond scratch pattern is a fair bit coarser. But, because the diamonds don't break down, very fine diamond pastes can sharpen quickly and for a long time.
Pre-diamond revolution, my basic sharpening sequence for waterstones was as follows: Hollow grind on a grinder, use a 1000 grit stone to create the wire edge, then chase it with first a 4000 or 5000 stone, followed by an 8000 grit or better finishing stone. For harder Japanese tools I stop there, for Western steels which are typically softer I follow with a plain, untreated leather strop.
So the question is: How does one port this traditional progression over to diamond sharpening? What's an appropriate sequence of diamond stones? Do I need as many? Of what size and style? It might confuse a newcomer to see that DMT makes two basic styles of diamond sharpeners: the original DuoSharp with a polka-dotted sheet of diamond surface over plastic substrate, and the DiaSharp which is a continuous diamond surface on a precisely flattened steel substrate.
For woodworking tools the latter, the steel plate, is the way to go - it's what they were designed for. They're more expensive than the earlier stones, are overkill for knives and other non-precision tools, and weight a ton; I like large stones, but the weight of the steel stones has kept me on the 8" size, not the 10". A fair number of customers like the 6" length because it's a lot less expensive, and much lighter. To keep the stones from slipping around a bench we stock the non-skid mat and the absolutely fab magnetic holders that comes with 12" stones and have the advantage of lifting the stone off the bench for more clearance.
DMT does manufacturer, and we offer, an 8000 grit (extra-extra-fine) DiaSharp, but as I am chock full of finishing stones I haven't tested it.
In the next part of this series I will take a closer look at diamonds and start getting into practical experiences. I'll cover at least two things: 1) How can I get an edge that is the equal to or better then the edges I get using oil or water stones? And 2) What's the fastest way of getting there?
Those of you in the NYC region:
If you'd like to learn this in person, I'm teaching a free sharpening class this Saturday, July 15th, 12 noon at our shop in Brooklyn. Details are here.
This "Tools & Craft" section is provided courtesy of Joel Moskowitz, founder of Tools for Working Wood, the Brooklyn-based catalog retailer of everything from hand tools to Festool; check out their online shop here. Joel also founded Gramercy Tools, the award-winning boutique manufacturer of hand tools made the old-fashioned way: Built to work and built to last.