How do I sharpen a chisel or plane blade?

Flattening the back once, honing a secondary bevel, and the guide that makes it repeatable.

Difficulty: intermediate
Time: 45 min
Read: 4 min
Safety: warning

Short answer

Flatten the back first — once, properly — then hone a small secondary bevel at about 30 degrees on progressively finer stones and strop it. A honing guide makes it repeatable, and a blunt chisel is more dangerous than a sharp one because it needs force.

Sharpening is the skill that separates woodworking that works from woodworking that fights you, and it is mostly a matter of understanding that only a tiny part of the blade actually needs attention.

Safety

A properly sharpened chisel cuts skin without being felt. Always cut away from your body, keep both hands behind the edge, and never hold work in the hand you are cutting towards. Store chisels with edge guards — reaching into a drawer of loose chisels is a common injury.

Step by step

Flatten the back first, once.An edge is where two flat surfaces meet, so a hollow or rough back cannot be sharp however much you work the bevel. Rub the back flat on a coarse stone until the first 20mm is uniformly polished. It only ever needs doing once per chisel.
Understand the two bevels.The primary bevel is ground at about 25 degrees and does the bulk removal. The secondary or micro-bevel is honed at about 30 degrees and is the bit that actually cuts. Honing only the small secondary bevel is what makes sharpening quick.
Use a honing guide.An inexpensive roller guide holds the angle consistently. Freehand is a real skill and a guide removes the main reason people give up — inconsistent angles that never quite reach an edge.
Work through the grits.Coarse to reshape, medium to establish, fine to hone. Roughly 1000, 3000 and 8000 grit on waterstones, or equivalent diamond plates. Skipping grits means the next one is doing work it is not designed for.
Feel for the burr.Work the bevel until a fine wire edge can be felt along the whole back of the blade. That burr is the proof the two surfaces have met. Without it you have not reached the edge, whatever it looks like.
Remove the burr on the back.A few flat strokes on the same stone, back down. Then back to the bevel on the next grit. Alternate until the burr becomes too fine to feel.
Strop to finish.A leather strop with honing compound, edge trailing. It removes the last of the burr and polishes the edge, and it makes a noticeable difference to how the tool cuts.
Keep waterstones flat.They dish in the middle with use, and a dished stone rounds the bevel. Flatten with a diamond plate or on wet-and-dry paper on glass, regularly.
Test it properly.It should shave end grain cleanly, or pare a curl from a pine offcut. The paper test tells you it is sharp; the end grain test tells you it is sharp enough for work.
Touch up often rather than rarely.Thirty seconds on the fine stone every so often keeps it sharp indefinitely. Waiting until it will not cut means starting from the coarse stone again.
Regrind only when the secondary bevel gets wide.After many honings the micro-bevel grows and honing gets slow. That is the point to regrind the primary bevel back to 25 degrees, not before.
Do not overheat on a grinder.Bluing the steel destroys the temper permanently and that part of the blade will never hold an edge again. Light passes, frequent dips in water, or a slow wet grinder.

Tips

Flatten the back properly the first time. It is tedious and it is the reason a chisel that has never been sharp becomes sharp.
The burr is the whole test. If you cannot feel a wire edge along the back, you have not reached the edge yet.
A honing guide and three stones is a modest outlay that transforms every edge tool you own.

Common mistakes

Working only the bevel and never the back — An edge is the meeting of two flat surfaces. However well the bevel is polished, a rough back means there is no edge to reach.
Overheating the blade on a bench grinder — Blue steel has lost its temper and will not hold an edge again. The damaged section has to be ground away entirely.

If it doesn't work

Never seems to get sharp

Cause: Back not flattened, or inconsistent angle — Fix: Flatten the first 20mm of the back on a coarse stone until uniformly polished, then use a honing guide.

Edge chips in use

Cause: Bevel angle too shallow for the work, or damaged steel — Fix: Hone the secondary bevel a little steeper — 30 to 35 degrees for a chisel taking mallet blows.

Sharpening takes a long time each session

Cause: Secondary bevel has grown wide — Fix: Regrind the primary bevel to about 25 degrees so the micro-bevel is small again.

Edge rounds over rather than getting sharper

Cause: Dished stone, or rocking freehand — Fix: Flatten the stone and use a honing guide.

Questions people ask

What angle should I sharpen a chisel to?

About 25 degrees for the primary bevel and about 30 for the honed secondary bevel — steeper, to 35, for a chisel taking mallet blows. A honing guide keeps it consistent.

How do I know when a chisel is sharp?

Work the bevel until you can feel a fine burr along the whole back, then remove it on the finer grits and strop. It should pare end grain on a pine offcut cleanly.