How do I use a router template and a guide bush?

Safety

What you'll need

• Router with a guide bush plate, or a template bit
• Guide bush of a known outside diameter (optional)
• Template material — MDF or plywood
• Clamps or double-sided tape
• Vernier caliper or accurate rule
• Scrap for a test cut

Step by step

1. Decide between a guide bush and a bearing-guided bit.A bearing-guided template bit follows the pattern exactly, so the template is the same size as the finished part — simple, but the bit has to reach past the template and the bearing rides on the pattern edge. A guide bush works with any straight bit, handles deeper templates and is essential for jigs like hinge and lock templates, at the cost of an offset.
2. Measure the bush and the bit, do not assume.Measure the guide bush's actual outside diameter and the cutter's actual diameter with a caliper. Nominal sizes are close but not exact, and this calculation is the whole job.
3. Work out the offset.Offset equals the bush's outside diameter minus the cutter diameter, divided by two. That is how far the cut sits from the template edge, on every side.
4. Make the template the right size, which is the wrong size.For a bush running on the outside of a template, the finished part comes out smaller than the template by the offset all round — so make the template larger by the offset on every edge. For a bush running inside an aperture, the routed hole comes out smaller than the aperture, so cut the aperture oversize by the offset all round.
5. Make the template well.12mm MDF or good plywood, cut and sanded until the edge is genuinely smooth. Every imperfection in the template is copied into every part you make from it. Take the time here; you only do it once.
6. Fix the template so it cannot move.Clamps outside the router's path, double-sided tape, or a jig with fences and stops. A template that shifts a millimetre halfway round ruins the part and can snatch the router.
7. Remove the bulk of the waste first.Cut the workpiece to within a few millimetres of the line with a jigsaw or a bandsaw before routing. Asking the router to remove 20mm of waste is what breaks bits and burns edges. The router should be finishing, not shaping.
8. Rout in light passes with the correct feed direction.Anticlockwise around the outside of the template, clockwise inside an aperture. Take the depth in steps, keep the bush pressed against the template throughout, and do not stop mid-pass.
9. Test on scrap before touching the real workpiece.One test cut confirms the offset arithmetic, the depth and the direction. It is the cheapest insurance in routing.
10. Deal with the corners.A bush and bit combination cannot cut an internal corner tighter than the bush radius. Square internal corners are finished with a chisel, or the design is adjusted to accept a radius.

Tips

• Hinge and lock jigs sold for routers are pre-calculated for a specific bush and bit combination. Use exactly the pair specified or the recess will be the wrong size.
• Keep a written note of the offset for each bush-and-bit pair you use, taped inside the router case. You will not remember it next time.
• Double-sided carpet tape holds a template surprisingly well and leaves the surface completely clear for the router base — far better than clamps on a small part.
• For repeat parts, make the template from a material tough enough to survive being handled and stored. MDF templates dent easily on the corners.

Common mistakes

• Making the template the size of the finished part when using a guide bush — The part will come out undersize by twice the offset — a fit that was designed to be tight will be loose, and vice versa.
• Trusting the printed sizes on the bush and bit — Both are nominal. Half a millimetre of error in the offset is a millimetre of error in the part.
• Routing away the full waste rather than sawing it off first — It overloads the cutter, burns the edge, and puts the whole load on a bit that only needed to take a light finishing cut.
• Using a template with a rough edge — The router copies it perfectly, including the roughness, into every part made from it.

Questions people ask