What's inside
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A convex timber edge is high in the middle and low at both ends. It is usually caused by cupping, uneven drying, or previous planing, and it can make a board difficult to joint, fit, or mark accurately. The cure is not to attack the high spot with long, heavy strokes. Remove the waste in measured stages, check the edge often, and shorten the plane’s effective cut as the surface comes flat.
Assess the edge before cutting
Start by placing the timber on a stable bench with the edge facing up. Clamp it firmly, but do not distort a thin board by overtightening the clamps. Sight along the edge from both ends, then check it with a straightedge. Mark the high area with a pencil or cabinetmaker’s pencil. A few diagonal lines across the edge make it easy to see where the plane has cut.
Measure the amount of convexity at its worst point. A 1 mm hollow under a straightedge calls for a different approach from a 4 mm bow. Also check whether the edge is square to the face. If the face is already finished, protect it with thin card under the plane or use a plane with a low, clean sole.
Look at the grain direction before setting the blade. On a straight-grained board, plane in the direction that gives the least tear-out. If the grain changes around knots or reverses, take lighter cuts and consider a sharp low-angle jack plane or a finely set smoother for the final passes.
Choose the plane and setting
A jack plane is the most useful general choice for correcting a convex edge. Its moderate length bridges small irregularities without being as awkward as a long jointer plane. A jointer plane is useful when the board is long and the edge needs to become straight over its full length, but it can remove material slowly if the convexity is pronounced.
| Tool | Best use | Trade-off |
|---|---|---|
| Jack plane | Initial correction and general flattening | Good balance, but not as fast as a heavily set fore plane |
| Fore plane | Removing several millimetres of waste | Fast, but leaves deeper tracks and needs careful finishing |
| Jointer plane | Long edges and final straightening | Accurate, but heavy and slower on a badly bowed edge |
| Block plane | Short edges, end grain, and spot corrections | Too short for establishing a long straight edge |
Set the blade for a controlled cut. For the first passes, about 0.3 to 0.5 mm of projection is usually enough on softwood; 0.2 to 0.3 mm is safer on hardwood or figured timber. Open the mouth slightly for rougher work, then close it for the final cut. A cambered blade is helpful because its corners do not dig into the edge. If you are buying one plane for occasional repairs, a decent No. 5 jack plane is generally better value than buying separate roughing and smoothing planes.
Use controlled passes, not full-length strokes
With a convex edge, begin by planing only the high middle section. Keep the plane centred over the crown and take strokes roughly one-third to one-half of the edge length. Do not run from end to end yet. The short strokes remove the hump without immediately lowering the ends, which are already below the centre.
After every three or four strokes, brush away the shavings and inspect the pencil marks. Re-mark the surface when the lines disappear. Work evenly from both faces of the edge so you do not create a diagonal bevel. Keep the plane level across the edge; pressure on one side will quickly produce a skewed edge.
As the crown reduces, extend the strokes gradually. A useful sequence is centre-only strokes, then strokes covering the middle two-thirds, then full-length passes. On the longer passes, begin with the blade over the timber and finish with the blade still supported. Lifting or twisting at the ends creates a concave edge, the opposite problem to the one you started with.
Use a modest forward pressure behind the plane and light pressure at the front. As the plane leaves the timber, move your pressure toward the front handle to prevent the sole from dropping and taking a deeper cut at the far end. Take a fresh shaving across the full edge only when the straightedge shows that the crown has nearly disappeared.
Check with tools, not eyesight alone
A straightedge is the main check. Hold it against the edge in several positions: near each face, through the centre, and diagonally if the timber is wide. For a joint that must close tightly, aim for no visible light over the working length. For general framing or a trim piece, a small gap may be acceptable if the mating part covers it, but do not rely on filler to correct a badly twisted edge.
Use a woodworking marking gauge to establish a reference line when the finished width matters. Scribe from the best face, leaving perhaps 0.5 mm of waste for the final plane strokes. A marking knife gives a cleaner shoulder, while a pencil is quicker and easier to see during rough correction.
Check square with a small engineer’s square or combination square. If one corner is low, take narrow passes on the high side rather than leaning the whole plane. Stop frequently: correcting a 0.2 mm error is simple; correcting an edge that is now hollow may require removing the entire surface again.
Avoid common failure modes
Chatter usually comes from a dull blade, excessive projection, a loose cap iron, or pushing too hard. Tear-out comes from planing against the grain or taking too thick a shaving. Reduce the cut, sharpen the blade, and reverse direction if the grain permits. On stubborn reversing grain, a high cutting angle or a finely set smoother can help, but it will not compensate for poor support or a blunt edge.
Do not try to correct a severely twisted board edge while the timber rocks on the bench. Pack under the low corner or clamp the piece to a flat carrier. If the edge is more than about 4 or 5 mm out over a short board, a handsaw, scrub plane, or power tool may remove the bulk more efficiently; use the hand plane for control and the final fit.
Finish with light, full passes
When the straightedge shows a continuous line, reduce the blade projection and take two or three light full-length passes. Plane from the end with the friendlier grain direction, then inspect the surface under raking light. Ease the sharp arrises with a single light pass or a few strokes of a small block plane. The result should be straight, square where required, and free of deep hollows—not merely smooth to the touch.