How to Sharpen a Plane Iron

Everything in how to sharpen a chisel applies here: flatten the back, hone the bevel, find the burr, remove the burr. Read that first if you have not.

A plane iron adds one thing a chisel does not have, and it is the difference between a plane that works and a plane that leaves tracks.

The extra thing: camber

A chisel is sharpened straight across. A plane iron usually should not be.

The reason is geometric. A plane iron is wider than the shaving it should take, and if the edge is perfectly straight then the corners are cutting at exactly the same depth as the middle. Those corners dig in, leave a step at each edge of the cut, and produce the parallel ridges people call plane tracks.

Camber is a slight curve across the width of the edge, so the middle cuts deeper than the corners and the corners cut nothing at all. The shaving comes off with feathered edges and the surface has no steps.

How much camber

It depends entirely on the plane's job, and this is the part that is worth getting right.

Jack plane (No. 5), roughing:pronounced camber, easily visible — on the order of a few hundredths of an inch of curve across the width. That heavy curve is what lets a jack take a thick shaving down the middle without the corners tearing the work. It is the single biggest reason a jack feels wrong when someone sharpens it straight.

Smoothing plane (No. 4), finishing:very slight camber, barely detectable — just enough that the corners do not touch. A few thousandths across the width. The shaving should still be full width; you are only relieving the corners.

Jointer (No. 7), edge jointing: essentially straight, with the corners relieved. A cambered jointer iron cannot produce the flat edge that is the entire point of the tool.

Block plane: straight, with corners relieved. Block planes are for fitting and end grain, where camber would work against you.

The sizes and their jobs are covered on jack vs smoother vs jointer.

How to produce camber in a guide

A honing guide holds the blade straight, so camber is created by varying pressure across the width during the stroke rather than by the jig.

The standard method: hone normally for a while with even pressure. Then do a set of strokes with finger pressure concentrated over the left corner, and an equal set with pressure over the right corner. More metal comes off at the corners than the middle, and the edge acquires a gentle curve.

For a heavy jack camber, do this at a coarse grit and repeat until you can see the curve against a straightedge. For a smoother, a handful of pressure strokes per corner at the finishing grit is enough. Count strokes and keep the two sides equal, or the camber ends up asymmetric and the plane will cut deeper on one side no matter how you set the lateral lever.

The bevel angle

Bevel-down bench planes: 25 degrees primary, optionally a 30-degree micro-bevel. The bed angle sets the cutting angle, so the bevel angle only affects durability.

Bevel-up planes: the bevel is part of the cutting angle, so this is a real decision. 25 for end grain, 33 for general work, 38 or more for figured grain. This is why a honing guide matters so much more on bevel-up planes. See bevel up versus bevel down.

The chipbreaker

One thing to check while the iron is out. The leading edge of the chipbreaker must sit dead flat against the back of the iron with no gap at all. A shaving that finds its way into a gap will jam the plane solid mid-stroke.

If there is a gap, hone the chipbreaker's leading edge on your stone until it seats. It takes two minutes and it eliminates the most baffling plane problem there is.

How often

A plane iron dulls faster than a chisel because it is in continuous contact with wood rather than making discrete cuts. Signs: the surface starts to look dull rather than reflective, the plane needs more force, or the shavings break up instead of coming off in one piece.

With a micro-bevel, a touch-up is a minute or two. Stropping between sharpenings extends the interval considerably and costs thirty seconds.