Why does my chisel keep digging in below the line?

A chisel does not go where you point it. It goes where its geometry sends it, and the geometry is dominated by one asymmetry: one face is flat and one is a wedge. Understanding which face to reference against the work turns the chisel from an unpredictable tool into a precise one.

Step by step

1. Look at which way the bevel is facing.If the bevel is against the timber, the deeper you push the more the wedge pushes the edge down. That is the tool telling you where it is going, and no amount of care will overcome it.
2. For paring, put the flat back on the surface you are cutting to.The back becomes the reference face. It rides on the timber, so the edge stays in the plane of that surface and takes a controlled slice. This is the standard orientation for cleaning out a recess, levelling a joint or trimming a proud dowel.
3. Knife the line before you cut anything.A scored knife line severs the surface fibres and gives the chisel a small wall to drop into, which stops the edge creeping over the line. A pencil line has width and no wall; a knife line has neither.
4. Chop inside the waste first, bevel facing the waste.Working from the waste side, take a series of vertical chops well inside the line, with the bevel facing into the waste so the wedge pushes the waste away from your line rather than pushing the chisel back across it.
5. Remove the waste in layers, not in one bite.Chop a series of shallow cuts and pare the chips out between them. A chisel driven deep in one blow is a wedge splitting the wood, and the split runs where the grain sends it, not where you wanted.
6. Come back to the line last, paring.With most of the waste gone, reverse the chisel, put the flat back against the knife line or on the surface, and pare the last fibres away. The chisel drops into the knife line and stops exactly there.
7. Watch the grain direction.Paring with the grain rising away from you splits ahead of the edge and takes out more than you intended. Work from both ends towards the middle if the grain reverses, and always take a lighter cut in the direction that wants to split.
8. Sharpen.A blunt chisel needs force, and force is what drives an edge off its line. Almost every complaint about chisel control is partly a sharpness complaint.
9. Check the result with a straightedge.A recess that rocks a hinge or a lock was pared with the bevel down somewhere. Check as you go rather than at the end.

Tips

• For a deep vertical cut, stand so you can sight along the chisel from behind and keep it plumb. It is far easier to keep a chisel vertical by eye from directly behind it than from the side.
• The one time the bevel deliberately goes down is when you want the edge to follow a curve — cleaning out a hollow, or easing a shoulder.
• Small controlled taps with a mallet give far more control than pushing hard by hand, because you can stop between blows and look.
• Clamp the work. Almost every slipped chisel involves a workpiece held with the other hand.

Common mistakes

• Paring with the bevel down — The bevel is a wedge that drives the edge into the timber, so the cut runs deeper the further you push and the surface ends up hollow.
• Cutting straight to a pencil line — A pencil line has width and gives the edge nothing to register against. A knife line is a physical wall the chisel drops into.
• Taking the full depth in one chop — The chisel acts as a splitting wedge and the split follows the grain, often below the line and outside the joint.
• Chopping with the bevel facing the finished face — The wedge then pushes the chisel back across your line, undercutting the face you are trying to keep.