Tips From Tommy Mac: Staying Sharp with Diamond Stones

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Tommy Mac

Every woodworker knows how important it is to work with sharp tools. We also know there are plenty of opinions about how to get chisels and plane blades sharp and keep them that way. When I worked as a carpenter, I used oilstones. Later on, when I studied woodworking at the North Bennet Street School, I learned how to get razor-sharp edges using waterstones. But I also discovered how quickly waterstones become dished and require flattening. The first time I used a diamond stone, it wasn’t for sharpening, but for flattening my waterstones. Eventually, I got tired of flattening my waterstones and went straight to diamond stones made by DMT–a company right in my own neighborhood.

Today diamond abrasives are the mainstay of my sharpening system–effective not just for chisels and plane blades but also for scrapers, router bits, kitchen knives and just about any edge that needs to be sharp. Although diamond sharpening stones cost more than other types, they more than earn their keep in my shop. I like the fact that diamond stones deliver years of trouble-free performance–with no flattening or other maintenance required.


diamond stone

Tommy photo: Anthony Tieuli; Product photo: Larry Hamel-Lambert; Video stills courtesy of WGBH

How to use diamond sharpening stones

Start small

Diamond sharpening stones come in a wide variety of grits and sizes. There’s no need to buy a full range of stones if you’re just starting with diamond abrasives. Skip the extra coarse stones if you’ve got a grinder to reshape a badly damaged edge. I recommend starting with a couple of the finest stones because these are the ones you’ll reach for most frequently to restore an edge.

Keep it simple

In my shop, I store my most-frequently used diamond stones right next to my chisels and hand planes. Getting diamond stones set up for sharpening is pretty easy. The DMT stones I use can be placed on a nonskid pad or on a specially designed stand. Both methods keep the stone from shifting while you’re sharpening, which is important. I use a wet rag to put a few drops of water on the surface of the stone before I start sharpening. Keep a damp rag handy to wipe down the stone after use. Like any sharpening stone, a diamond version should be cleared of swarf (loose metal filings) once you’re finished sharpening.

Begin with the back

It’s always smart to make sure the back of the chisel or plane blade is flat before you work on the bevel. This is especially true with any chisel (new or secondhand) that you haven’t used before. I check the back of a new chisel or plane blade by rubbing the first 3⁄4" of the back along the edge of a fine diamond stone for half-a-dozen strokes or so. Then I examine the scratch pattern left by the diamond abrasives. A uniform scratch pattern indicates a flat surface. Hollow areas will be evident because they won’t show the same scratch pattern. To take out a hollow, rub the back over a coarser stone and repeat the scratch pattern test until you get uniform results. Then remove the coarse scratches by finishing the back on a super-fine stone.


diamond stone

Sharpen the bevel until you detect a burr

After finishing with the back of the chisel, sharpen the bevel. I always hollow-grind my chisels and plane irons, but if you use a micro bevel, that’s fine, too. Either way, sharpen up to the finest grit until you can detect a burr when running a finger up the back of the chisel. Remove the burr by going over the back and bevel lightly on your finest stone. Then reach for the paste, which I’ll explain ahead.

Watch, listen and feel

Diamonds cut faster and deeper than other abrasive particles. If you’re accustomed to oil or waterstones, you’ll be surprised at how quickly diamond stones remove metal–even in the finer grits. Pay attention to the look and feel of your blades as you sharpen, and even the sound they make as you move steel across the diamond-studded surface. Before long, you’ll be taking full advantage of this sharpening technology.

Don’t forget the paste

For the ultimate edge on a chisel or plane blade, I rely on specially formulated diamond paste that contains even finer diamond particles. The DMT paste I use comes in a hypodermic-type applicator that’s easy to use. I use my finger to spread the paste on a scrap of hardwood plywood or MDF. Then I run the blade over the coated surface, noting when the grey paste turns dark with microscopic metal particles. Once you see the extremely sharp results you get from this final step, you’ll want to do it every time you sharpen.

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