What's inside
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What a square peg hole requires
A square peg hole is usually a mortise: a straight-sided recess cut to fit a tenon or square-ended peg. The chisel does the cutting, but accurate layout does most of the work. A hole that is square in plan can still be too wide, too deep, or tilted, so check the fit and the walls as you go.
Start with the actual peg or tenon, not a nominal measurement. Mark its width and length with a sharp pencil or marking knife, then use a square to carry the lines across the face. If the peg needs clearance, allow only what the joint calls for; a loose hole cannot be tightened by chiseling later. For a snug hand-fit in a typical furniture joint, aim for a gap small enough that the peg enters with light mallet taps, not a forceful drive.
Tools and setup
Use a chisel narrower than the hole’s short dimension, so it can reach the corners without scraping both walls at once. A 6 mm (1/4-inch) chisel suits small holes; a 12 mm (1/2-inch) chisel is more useful for larger work. A wider chisel can remove waste quickly, but it is harder to control in a narrow mortise. A sharp bevel-edge bench chisel is a practical general choice; a dedicated mortise chisel is tougher but not necessary for occasional shallow work.
Clamp the work securely at a comfortable height, with the waste supported close to the cut. A bench hook may be enough for a small board, but a clamp is safer when striking with a mallet. Wear eye protection, keep your free hand behind the cutting edge, and never brace the work against your body. For a basic set, compare a bevel-edge bench chisel set with a single chisel sized for the job. A modest set is fine if the edges are properly sharpened; extra chisels do not compensate for dull ones.
Mark the hole and set depth
Score the outline with a marking knife if the surface is finished or the fit needs to be precise. The knife line severs surface fibers and gives the chisel a clean boundary. Otherwise, a sharp pencil is adequate for rough framing or utility work. Darken the waste area so you do not confuse the cut with the finished edge.
Set a depth gauge or mark the depth on the chisel with tape. For a blind hole, measure the required depth from the face, allowing for any shoulder or bottom clearance in the joint. A depth stop is useful when making repeated holes; for one hole, a tape mark or a simple gauge block is usually enough. Do not rely on counting mallet blows: wood density and chisel sharpness change how much each stroke removes.
Chisel the walls first
With the bevel facing the waste, hold the chisel vertically on one layout line and make a light scoring cut. Repeat on the opposite end and both sides. These shallow cuts establish the boundaries. Keep the chisel square to the face; leaning it inward makes the opening narrow at the surface and wider below, while leaning outward undercuts the edge and weakens the fit.
Now remove waste in the middle, working toward the scored walls rather than starting directly on them. Take slices roughly 3–5 mm deep at a time. For a deeper mortise, work from each end toward the center, then pare the remaining ridges. Do not lever hard against the walls: that can split the grain beyond the outline. If the chisel binds, withdraw it straight and clear the chips instead of twisting it sideways.
When nearing the final depth, use lighter taps and check the bottom with a rule or depth gauge. Pare the walls in thin cuts, keeping the bevel oriented so it supports the edge rather than digging into it. A bench chisel with a short, controlled stroke is easier to manage for this finishing work than a heavy mortise chisel.
Square the corners and check the fit
Most chisels leave a slight radius in the corners. To sharpen them, place the chisel on the layout line and pare straight down, taking a fine shaving from each corner. A very narrow chisel reaches tight corners, but it is easier to bruise the edge or drift off the line. Work slowly and check the opening with a small square or the peg itself.
Test the fit before making the hole deeper or wider. If the peg stops at one point, look for a high spot with pencil marks or a light rub of chalk on the peg. Remove only that spot. Forcing the peg can split the workpiece, especially near an end or in thin stock. If the hole is slightly undersized, pare the marked wall; if it is oversized, the usual repair is a fitted shim or a new part, not more chiseling.
Tool choices for the job
| Tool | Best use | Trade-off |
|---|---|---|
| Bevel-edge bench chisel | Occasional holes and accurate paring | Versatile and affordable, but avoid heavy levering |
| Mortise chisel | Repeated or deeper mortises | Strong under mallet blows, but bulkier and less nimble for fine trimming |
| Marking knife and square | Clean, repeatable layout | More precise than pencil marks, but needs careful setup |
If you are cutting several identical holes, a mortise chisel can take repeated mallet work better than a thin bench chisel. For one shallow hole, a sharp bench chisel is generally the better buy. A marking knife is worthwhile when crisp edges matter; a pencil and square remain perfectly serviceable for rough work.
Common mistakes to avoid
The most common error is cutting to the line too early. If you remove the boundary before the center waste is cleared, the edge can crumble or split. Another is using a dull chisel: it needs more force, wanders more easily, and leaves a ragged bottom. Hone the bevel and flatten the back enough to make a clean cutting edge before starting.
Also avoid trying to remove the full depth in one blow. Deep cuts trap chips, increase the chance of splitting, and make it harder to keep the walls vertical. Clear waste often, check depth in small increments, and stop when the peg seats correctly. A square hole comes from controlled cuts and frequent checks, not from hitting harder.