How do I use a combination square?

A combination square is a sliding rule in a head, and the sliding is the point. Locked at various positions it becomes a try square, a mitre square, a depth gauge, a marking gauge and a height setter. Most people use it for the first of those and buy separate tools for the rest.

What you'll need

• Combination square
• Marking knife or a sharp pencil

Step by step

1. Mark a square line across a board.Slide the blade so it just crosses the board, lock it, hold the stock hard against the face edge, and knife along the blade. Keep pressure pushing the stock into the edge, not just downwards, or it drifts as you mark.
2. Mark a 45 degree mitre.Turn the head so the 45 degree face bears on the edge instead of the 90. Same technique. This is the fastest way to mark a mitre on trim, and it is more reliable than trusting a mitre saw's detent on a corner that is not square.
3. Gauge a depth.Loosen the head, stand it on the surface with the blade dropping into a rebate, groove or hole, push the blade to the bottom and lock it. Read the depth off the rule, or carry it straight to the next piece as a physical setting.
4. Scribe a line parallel to an edge.Set the blade to project by the distance you want, hold a pencil against the end of the blade, and run the stock along the edge with the pencil trailing. It marks a line parallel to the edge without any measuring — this is the trick that replaces a marking gauge for rough work.
5. Set a height or a projection.Set the blade to the dimension and use it as a physical stop — setting a saw blade height, a router cutter projection, a plane's depth of cut, or the reveal on an architrave. Setting from a physical dimension beats reading a scale on a machine.
6. Find the centre of a board edge.Set the blade to a little under half the thickness, scribe from one face, then flip and scribe from the other. Two lines that straddle the centre; adjust and repeat until they merge. Two passes usually gets it.
7. Check the head is tight before each set of marks.The locking bolt works loose with use and vibration, and a head that has crept a millimetre gives you consistent, confident, wrong lines. Nip it and give the blade a push to confirm.
8. Clean the blade slot and the stock face.One chip of sawdust in the slot tilts the whole blade, and one under the stock lifts it off the edge. Both are invisible and both are large compared with the accuracy you are trying to achieve.

Tips

• Take the head off and you have a hardened steel rule with a machined edge — a genuinely good short straightedge for checking a plane sole or a shooting board.
• Set the blade so only a little projects beyond the work. A long overhang is a lever, and any sideways force on it lifts the stock off the edge.
• For repeated marks at the same setting, mark all the pieces before you change anything. Resetting the blade between pieces reintroduces exactly the variation you were avoiding.
• The scriber stored in the handle of many heads is for marking metal. On timber it tears the fibres; use a knife instead.

Common mistakes

• Letting the stock rock on a sawn edge — The reference edge must be straight and clean. Marking off a rough sawn edge transfers its irregularity into your line, and the tool gets blamed for it.
• Marking with a blunt pencil — It defeats a tool designed to give a precise reference. A knife or a chisel-sharpened pencil is the point of using a square this good.
• Not re-checking the lock after knocking the tool — A blade that has crept is undetectable in the finished mark, and it applies to every piece marked after the knock.

Questions people ask