How do I use a mitre box or mitre saw for neat corners?

Mitres are unforgiving in a way most cuts are not, because the gap at the joint shows twice the error of the cut. Almost all of the difficulty is that walls are not square and everyone assumes they are.

Safety

Step by step

1. Measure the actual corner first.A sliding bevel or an angle finder in the corner. Almost no corner in a house is 90 degrees, and cutting two 45s into a 92-degree corner leaves a gap you cannot fill invisibly.
2. Halve the measured angle.Each piece is cut at half the corner angle. A 92-degree corner means two 46-degree cuts. This single step is what separates mitres that close from mitres that do not.
3. Check the saw's own accuracy.Cut two offcuts at 45 and put them together against a square. If they do not make 90, the saw's detent is out. Adjust it before doing anything that matters.
4. Cut long and creep up on it.Leave a millimetre or two, offer it up, and take slivers off until it closes. Cutting to the exact measurement first time works occasionally and wastes material the rest of the time.
5. Support the whole length.Roller stands or an offcut at the same height as the saw table. An unsupported length sags, which changes the angle through the cut and is a common cause of mitres that look right and do not fit.
6. Hold the piece against the fence firmly.Any movement during the cut ruins the angle. Clamp it if the saw has a clamp, and never hold anything short by hand on a powered saw.
7. Mark the face and which way up it goes.Cutting a mitre the wrong way round is the most common error, especially on skirting and coving. Pencil an arrow on the face before every cut.
8. Cut skirting and coving in the orientation they will be fitted.Skirting laid flat on the saw table cuts differently from skirting stood against the fence. Coving in particular must be held at its fitted spring angle or the mitre is wrong.
9. Use a fine-tooth blade.60 to 80 teeth for trim work. A coarse blade tears the face of the cut, and on a mitre the torn edge sits at the most visible point of the joint.
10. Let the blade do the work.Full speed, then down through the cut steadily. Forcing it deflects the blade, which changes the angle across the thickness of the piece.
11. Scribe internal corners on skirting instead.Internal corners in skirting are better scribed than mitred — a mitre opens as the timber shrinks. Mitre external corners only.
12. Use a mitre box properly if you have no saw.Clamp the box to the bench and clamp the work in it. A mitre box held by hand gives the saw room to wander and produces exactly the errors it exists to prevent.
13. Fine-tune with a block plane or sandpaper on a flat block.A mitre that is very slightly out can be corrected by hand rather than recut. Faster and it saves the length.
14. Fill the last hairline only after fitting.Decorator's caulk on painted trim, a wax filler on stained. Fill after both pieces are fixed, since the joint closes slightly as it is nailed.

Tips

• Measure the corner, halve it. Assuming 45 degrees is the reason most mitres do not close, and it takes ten seconds to check.
• Cut long and sneak up on it. You can always take more off and you can never put it back.
• Mark the face and the up direction on every piece before cutting. Reversed mitres waste more material than bad angles do.

Common mistakes

• Assuming corners are 90 degrees — Almost none are. Two 45-degree cuts into a 92-degree corner leave a wedge-shaped gap that cannot be filled invisibly.
• Mitring internal corners on skirting — The mitre opens as the timber shrinks and filler cracks out. Internal corners should be scribed; only mitre external ones.

If it doesn't work

Questions people ask