What's inside
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Cutting a lock mortise is mostly controlled paring, not heavy chopping. For a typical interior door, the lock case is around 150–170 mm (6–6¾ in) long and the faceplate is often about 20–25 mm wide. A sharp, properly sized chisel makes it easier to get a flat recess without bruising the door face or splitting the narrow strip beside the mortise.
What to look for in a lock-mortise chisel
A 19 mm (¾ in) bench chisel is a useful starting size for most interior-door faceplates. A 12 mm (½ in) chisel is better for trimming the mortise ends and working around the lock case; a 25 mm (1 in) chisel can remove waste quickly from a wider pocket but is less nimble in tight corners. If buying only one, choose a 19 mm chisel with a comfortable handle and a blade that holds an edge.
For occasional work, an ordinary bevel-edge bench chisel is enough. It can pare the faceplate recess and clean the lock-case mortise, provided it is sharp. A firmer or mortising chisel has a thicker blade and tolerates harder mallet blows, but its extra strength is rarely necessary in a softwood or light hardwood interior door. A dedicated mortise chisel is generally too bulky for the shallow, narrow faceplate recess.
Look for a blade long enough to guide accurately, but not so long that it feels awkward inside a deep lock pocket. A comfortable handle matters: repeated mallet taps can loosen or damage a handle that is not intended for striking. If you need a set for general carpentry as well as this job, compare bevel-edge bench chisel sets. A single quality chisel is often a better buy than a large set of blades you will rarely use.
Which chisel suits the job?
| Type | Best use on a lock mortise | Trade-off |
|---|---|---|
| Bevel-edge bench chisel, 12–25 mm | Faceplate recess, corners and general paring | Versatile, but avoid hard blows that can damage a light handle |
| Firmer chisel, 19–25 mm | Removing waste from the lock-case pocket | Stronger blade, but bulkier for fine corner work |
| Mortising chisel | Deep, narrow mortises in heavier joinery | Usually unnecessary and awkward for a door-lock faceplate |
| Budget bench chisel | Occasional fitting in softwood or light hardwood | Fine if sharpened; may need more frequent honing |
What is worth paying for?
For one or two doors, a budget chisel can do the job. The main limitation is usually its edge: it may arrive with a rough bevel, need flattening at the back, or lose sharpness sooner than a better-made tool. Those are manageable drawbacks if you already have sharpening stones and do not mind touching up the edge.
For regular fitting work, buy a chisel with a blade that sharpens predictably and a handle designed for mallet use. A 19 mm bevel-edge bench chisel covers much of the faceplate work, while a 12 mm one complements it for the ends and lock-case corners. There is no need to pay extra for a full premium set if these are the only sizes you use.
Do not choose solely by advertised hardness or a polished finish. A chisel that fits your hand and can be sharpened to a clean edge is more useful than an expensive one kept dull. If you are buying a mallet as well, a woodworking mallet gives controlled taps without the harsh impact of a steel hammer. A small nylon or dead-blow mallet is also suitable. Keep heavy striking for work that actually needs it.
Cut the mortise without damaging the door
Mark the lock height and backset according to the lock instructions. Use a sharp knife or marking gauge to score the faceplate outline, and extend the marks only as far as needed. The faceplate is commonly around 2–3 mm thick, but measure yours rather than assuming. Set the chisel depth to that thickness; cutting too deep leaves the plate recessed, while a shallow pocket makes it stand proud and can prevent the door from closing cleanly.
For the faceplate, hold the chisel bevel toward the waste and pare in thin slices. Work from the scored outline toward the center of the recess. Do not lever out a large chip against the finished edge: that can tear fibers past the line, especially in softwood. For the deeper lock-case pocket, remove material in stages, checking depth and width against the lock body. Keep the chisel square to the door edge and take light mallet cuts rather than trying to reach full depth in one blow.
On a hollow-core door, first confirm that the lock is suitable and that the pocket will remain within the solid edge block. Cutting beyond that block can break through into the hollow skin and leave little material to support the lock. On a solid door, watch for grain runout: a cut that follows the grain can split farther than intended. If the wood starts to split, stop and pare from the opposite direction.
Sharpness matters more than force
A dull edge crushes fibers, wanders off the mark and encourages you to strike harder. Before starting, hone the bevel and remove the wire edge from the back. A practical honing angle is about 25–30 degrees for a bench chisel; use the existing bevel if it is already sound. Check the edge on scrap wood. It should shave cleanly rather than burnish or tear the surface.
Protect the finished door face with masking tape or a thin sacrificial board, and clamp the door securely at a comfortable working height. Keep both hands behind the cutting edge, and never use a chisel as a pry bar. After fitting, wipe away dust and check that the faceplate sits flush, the lock case enters without force, and the latch operates before reinstalling or adjusting the hardware.