How do I stop tearout when cutting plywood and MDF?

Tearout is not a sign of a bad saw. It is what happens when a tooth pushes through the last fibres of a veneer with nothing behind them to hold them down. Every technique for preventing it does one of two things: reduce how hard each tooth pushes, or put something behind the fibres.

Safety

Step by step

1. Know which face your saw tears.The teeth enter one face and exit the other, and it is the exit face that splinters. A hand-held circular saw and a jigsaw cut upwards, so they tear the upper face. A table saw and most track saws tear the underside.
2. Orient the sheet so the good face is protected.With a circular saw, cut with the good face down. On a table saw, good face up. This single decision does more than everything else combined.
3. Use a blade with more teeth and less aggressive geometry.A general-purpose blade with few large teeth is designed to remove material fast, which is exactly what tears veneer. A fine-toothed blade intended for sheet materials takes a smaller bite per tooth.
4. Score the cut line first.A sharp knife along a straightedge, or a shallow scoring pass with the saw set to a millimetre or two of depth. Severing the surface fibres before the full cut means there is nothing left to lift.
5. Back the cut with a sacrificial piece.Clamp scrap material tight underneath along the cut line so the teeth exit into that instead of into air. This supports the fibres exactly where they would otherwise break out.
6. Use masking tape over the cut line.A strip of tape on the exit face holds the surface fibres down while the blade passes. It is not as good as backing the cut but it is quick and helps noticeably.
7. Support the whole sheet so it cannot flex or pinch.A sheet sagging between two trestles closes on the blade, which burns, wanders and tears. Full support underneath, with the offcut supported too, improves the cut and is safer.
8. Keep the speed steady and let the blade cut.Forcing the saw increases the bite per tooth. Going too slowly burns and can wander. A steady feed with a sharp blade is what a clean edge needs.

Tips

• A track saw or a clamped straightedge gives a straight cut, which matters because a wandering cut tears even with everything else right.
• For a visible edge on veneered board, cutting slightly oversize and trimming the last millimetre with a router or plane gives the cleanest result.

Common mistakes

• Cutting veneered board good-face-up with a circular saw. — That is the face the teeth exit, so it is the face that splinters. Turning the sheet over solves most of it.
• Using a fast rip blade for sheet goods. — Fewer, larger teeth take a bigger bite. Speed of cut is bought with quality of edge.

Questions people ask