How do I cut a rebate or a groove with a router?

Safety

What you'll need

• Router
• Rebate bit with bearing, or straight bit
• Parallel fence or clamped straightedge
• Clamps
• Scrap of the same material for test cuts
• Eye, ear and dust protection

Step by step

1. Decide which method the cut needs.A rebate along an edge: a bearing-guided rebate bit is simplest, because the bearing references the edge and no setting up is required. A groove parallel to and near an edge: the router's parallel fence. A groove anywhere else, or across a wide panel: a straightedge clamped to the work.
2. Choose the bit by the width you need.A groove cut in a single pass is exactly the bit's diameter, so the cleanest results come from a bit that matches the piece going into the groove. Where no bit matches — sheet material is almost never its nominal thickness — cut narrow and widen with a second pass rather than trying to force a fit.
3. Set the depth against the workpiece, not against the scale.Router depth scales are approximate. Set the bit against the actual workpiece or against a piece of the material that will sit in the groove, lock it, and confirm with a test cut.
4. Set the fence offset by test cut.With a parallel fence, the distance from the fence to the near side of the cutter is what matters and it is easier to establish by cutting scrap and measuring than by measuring the setup. Cut, measure, adjust, cut again. Two minutes here saves the workpiece.
5. Clamp the workpiece and clear the router's path.Clamps have to be outside the line the router base will travel. Check by running the unplugged router along the whole path before switching on.
6. Cut in passes.Set the first pass shallow, run the full length, then lower and repeat. Do not move the fence between passes — only the depth changes.
7. Feed in the correct direction.With the fence against the edge nearest you and the router moving away from your body, the standard direction applies: the timber should be fed into the cutter's rotation, not with it. Check by turning the bit by hand before switching on.
8. Support the base at the ends of the cut.As the router leaves the workpiece, half its base is unsupported and it tips, which gouges the end of the cut. Clamp scrap of the same thickness at both ends so the base always has something under it.
9. For a stopped groove, mark and use the plunge.A groove that stops short of the end — for a shelf housing that does not show at the front — is plunged in at the start mark, run to the end mark, and plunged out. Mark both stop positions on the workpiece and on the router base so you can see them from above.
10. Square the ends if the groove is stopped.A plunged groove has a round end matching the bit radius. Either square it with a chisel, or round over the end of the piece going into it — for a shelf housing, rounding the shelf is much quicker.

Tips

• For a housing to take sheet material, measure the actual board — 18mm ply is often 17.4mm — and cut the housing in two passes to that measured width. A joint that has to be forced is a joint that will split the sides.
• Cutting a rebate for a cabinet back is one of the jobs a router does better than anything else, because the depth is exact and the corners are crisp.
• If the groove has to be a long way from any edge, screw or clamp a batten to the workpiece as a guide, and set its position from a test cut on scrap.
• Cutting a groove across the grain leaves a small blow-out where the cutter exits. Clamp a sacrificial block against the exit edge.

Common mistakes

• Setting the width from the bit's marked diameter — The nominal size and the actual cut differ, and sheet material is never its nominal thickness. Cut a test slot in scrap and measure.
• Cutting to full depth in one pass — The bit deflects, the walls come out bowed rather than square, the cut burns and the router snatches.
• Letting the router tip off the end of the work — It gouges the last few centimetres of an otherwise perfect cut. Support both ends with scrap of the same thickness.
• Moving the fence between passes to widen the groove and losing the reference — Once the reference is lost, the two halves of the groove will not line up. Complete all depth passes at one setting, then reset once for the width.

Questions people ask