Woodworking Tip of the Day: Flattening Your Sharpening Stones: Pencil Grid, Lapping Plate and How Often

Not for nuthin'…. But….. the sharpest-looking set of stones in a lot of shops is also the reason the chisels won't pare a clean shaving. Waterstones wear every time you use them, and they wear the most right in the middle. Nobody notices until the plane iron comes off the stone with a rounded edge and a back that won't lap flat. I've got a dory's worth of bevels, gains and frames to cut this winter, so I went back to the stone makers' own instructions and the Fine Woodworking and Popular Woodworking articles and wrote down how I keep mine flat.

Why it matters: A waterstone cuts fast because its binder lets go and keeps exposing fresh abrasive. The price is that it dishes. Shapton says it plainly: the blade hits the middle of the stone more often, so the stone goes concave, and that makes for uneven sharpening. Tom Caspar wrote in Popular Woodworking that the wear "produces an undesirable curved edge on chisels and plane irons." It's worse on the back of a blade. Lay a flat back on a hollow stone and it only touches the high ends, so lapping it there rounds it over instead of flattening it. Paul Sellers' answer to that problem is to lap backs on something that stays flat, like a diamond plate or thick glass. A sharp edge is where two faces meet, and a dished stone spoils both of them.

Oilstones vs. waterstones: Norton says its oilstones (benchstones) "typically do not require flattening with normal use" because they're hard, but after years of steady use they can dish in the spots where you bear down the most. Waterstones are the other way around: they dish easily, but they're quick to bring back. Same job, different schedule.

Materials

• Your sharpening stones, every grit (not just the coarse one)
• A soft pencil
• Water, plus a tub or a sink without a garbage disposal (DMT says the slurry is hard on disposal bearings)
• Rags and a stiff nylon brush for cleanup

Tools: pick one flattener

• Diamond lapping plate. The DMT Dia-Flat (DMT is out of Marlborough, Mass.) is 10" × 4", ground flat to ±0.0005", and comes in 120 micron for stones of 1000 grit and coarser or 95 micron for finer stones. An Atoma plate is 140x or 400x diamond on a 10 mm aluminum block, flat to ±0.04 mm (about 0.0016"). Lee Valley suggests the 400x for general truing and the 140x when you need to take off a lot fast.
• Coarse diamond stone. Lee Valley says a coarse (about 400x) metal-backed diamond plate makes a good flattener and doubles for grinding a primary bevel. There's a catch, though. DMT doesn't recommend its regular sharpening plates for lapping stones, only the ones marked Dia-Flat, and Deneb Puchalski (Fine Woodworking) warns that some diamond stones wear out of flat fast. If you use one, keep it just for lapping.
• Silicon carbide grit on glass. Lee Valley's lapping kit says to true stones with 90x or 180x and finish with 280x, on plate glass, with water as the only lubricant for waterstones. Their guide calls for glass at least 1/4" thick, supported on flat plywood or MDF so it can't flex. A plastic laminate sheet on the glass keeps the grit from dishing the glass itself. (Don't use loose grit on a steel plate for waterstones. Lee Valley says the oxides from a wet ferrous lap will ruin the stone.)
• Wet/dry sandpaper on glass or granite. It's the cheapest way in. Norton says 100 grit, Charlesworth used 180 or 240 on 1/2" float glass, and Caspar used 220 on plate glass 1/4" or thicker. Lee Valley also lists coarse aluminum oxide sheet (over 100 micron) on glass or granite. Sellers' advice for a flat granite tile is to take a straightedge to the store and pick through them until you find a flat one.
• Grooved silicon carbide flattening stone. Norton recommends one. Soak it and the waterstone, then rub them together, and the grooves carry the slurry away.
• Shapton's Diamond Glass Lapping Plate is only for stones of 500 grit (30 micron) and finer. Don't use it on coarse stones, don't put loose grit on it and don't run hot water over it.
• A good straightedge

Steps

1. Wet everything. Soak your soaking stones the way the maker says. Shapton says to give its stones 2 to 3 minutes before lapping and to let other brands soak up all the water they can hold. Set the plate or glass on something solid and flat, and splash it with water.
2. Draw a pencil grid across the whole face of the stone. Fine Woodworking, Lee Valley and Shapton all start this way. The pencil shows you what the stone is doing.
3. Lap with long figure-8 strokes. DMT says to use long, overlapping figure-8s with light to moderate pressure and the whole plate, and to finish lighter so you leave fewer scratches. Shapton's way, forward and back and then both diagonals, works just as well. Turn the stone end for end now and then. Keep it wet, and rinse the slurry out of the plate's grooves as it builds up.
4. Read the grid. The lines go first on the high spots. On a dished stone that's the ends and edges, and the middle hangs onto its lines the longest. When every line is gone, the stone is flat. Lee Valley says not to fuss if it doesn't quite reach the corners, since that's the part of the stone that matters least.
5. Check it with a straightedge. Lay it along the length, the width and both diagonals and hold the stone up to the light. If any light shows under it, go back to the plate.
6. Chamfer the edges. Caspar sands a 45° bevel on every edge of the stone "to prevent flaking." Charlesworth eased the soft edges so they wouldn't crumble. Touch them up after each flattening, since lapping leaves a sharp corner again.
7. Do every stone. The 1000, the 4000 and the 8000 all wear, just at different rates. Rinse and brush each stone before you go to the next grit so coarse grit doesn't ride along and scratch up your finish stone. If you'd rather lap stone against stone, use three and keep rotating them. Lee Valley warns that rubbing just two together can leave one concave and the other convex.
8. Clean up the flattener. DMT says to clean and dry the Dia-Flat after every use, and to scrub any diamond plate now and then with a nylon brush and a mild cleanser. Lee Valley says to wipe the swarf out of an Atoma's channels. Put a straightedge across your plate or glass once in a while too, because a flattener that isn't flat just copies its own error into every stone.

How often? More often than you'd think. Lee Valley says to lap waterstones every 3 to 5 minutes of use. Shapton says to lap every time you sharpen a blade. Charlesworth flattened his 800 after about 100 strokes of back work, and Caspar flattened his three stones before every session, which he said took a minute or so. Plenty of folks flatten at the end of each session so the stones are ready the next time. Oilstones are on a much longer clock: check them with the straightedge now and then and flatten when they show a dish. The rule is little and often. A stone that's barely off comes back in a minute, and a badly dished one costs you a lot of stone.

One for the New England shop: Charlesworth warns not to store waterstones in water if the shop freezes at night, "because the stones will shatter." Come December in an unheated Marblehead shop, my soakers come in the house or get stored dry.

MOAM's shop note: The mistake I see most is flattening the 1000 and calling it done. The finish stone dishes too, just slower, and it's the last stone that touches your edge. The second most common is the fella who flattens only when he can see the hollow. By the time you can see it, you've already rounded a few irons. Draw the grid, lap it out, knock off the edges, and keep your flattener clean and flat. A few minutes, and every stone on the bench is flat.

Sources: DMT Dia-Flat product page and DMT FAQ, Lee Valley's Atoma Diamond Plates page, Lapping Grits instructions, "Getting Started with Japanese Water Stones," "Care and Maintenance of Ceramic and Water Stones" and Sharpening by Hand guide, Shapton Diamond Glass Lapping Plate instructions and FAQ, Norton "A Guide to Waterstones" and "A Guide to Benchstones," David Charlesworth, "A User's Guide to Waterstones" (Fine Woodworking #169), Deneb Puchalski, "Flatten Waterstones with a Coarse Diamond Stone" (Fine Woodworking #224), Hendrik Varju, "Pencil and Paper" (Fine Woodworking), Tom Caspar, "Waterstones" (Popular Woodworking / American Woodworker), and Paul Sellers, "Sharpening on hollow stones."

MOAM Out.