How do I flatten the back of a chisel or plane iron?

An edge is where two surfaces meet. Everyone concentrates on the bevel and forgets that the back is the other surface — and if the back is not flat and polished right at the edge, the two surfaces meet in a ragged line whatever you do to the bevel. This is why a tool that has been carefully honed can still refuse to get sharp.

What you'll need

• Coarse, medium and fine flat stones or plates
• A genuinely flat reference surface, if using sheet abrasive
• Marker pen
• Thin steel ruler (optional)
• Water or honing oil as your stones require

Step by step

1. Check the stone is flat first.You cannot flatten anything on a hollow stone. Flatten the stone before you start, and again during the job if it is a soft water stone.
2. Mark the back with permanent marker.Scribble across the back near the edge. When you start work, the ink is removed where the stone is contacting, which shows you immediately whether the blade is flat, hollow or twisted.
3. Work only the area that matters.The first 10 to 25 millimetres behind the cutting edge. Polishing the whole back of a chisel to a mirror is a great deal of work that changes nothing about how it cuts. Ignore anything a metal-polishing enthusiast tells you.
4. Start on a coarse stone with the back dead flat.Lay the back flat on the stone with fingers pressing down near the edge, and work back and forth or in figure-of-eights. Do not lift the far end, and do not let the blade rock — any lift puts a bevel on the back, which is precisely what you are trying to remove.
5. Keep going until the area behind the edge is uniform.Look at it under a bright light at an angle. Dull patches are low spots the stone has not reached, and while any of them touch the cutting edge, the tool cannot be properly sharp there. This first flattening on a neglected tool can genuinely take half an hour.
6. Work up through the grits.Medium then fine, until the band behind the edge is bright and uniformly polished. Each grit only needs to remove the previous grit's scratches from that band.
7. Use the ruler trick on a badly dished plane iron.Lay a thin steel ruler along the edge of the stone and rest the far end of the blade on it, so only a very narrow strip right at the cutting edge contacts the stone. That puts a tiny back bevel on — one or two degrees — and polishes only the strip that matters. It saves hours on an old iron and is entirely legitimate. Do not do this on a chisel, where the flat back is used as a reference surface against the work.
8. Note that Japanese blades are hollow ground on the back.They have a deliberate hollow with a flat rim around it, which is why they flatten so quickly. Do not try to remove the hollow — flatten only the rim, and stop before the hollow reaches the edge.
9. Record that it is done.Once a back is flat, it stays flat. All future sharpening is bevel work plus a few strokes with the back on the fine stone to remove the burr. This is a once-per-tool job.

Tips

• Sheet abrasive stuck to float glass is a cheap and very effective way to do the initial flattening on a badly out-of-flat tool, and you can throw the paper away when it clogs.
• The first flattening on a cheap new chisel can take longer than everything else you do to it. That is part of what you saved by buying it cheap.
• Do not use a hollow-worn oil stone for this. It will faithfully reproduce its own hollow in your blade.

If it doesn't work