Bench joinery
Router guide-bush offset
- where
- offset (mm)
- bush outside diameter (mm)
- cutter diameter (mm)
Explain this equation
Gives the offset between a template edge and the routed cut when using a guide bush.
Db is the outside diameter of the guide bush and dc the diameter of the cutter, both in millimetres. o is the radial offset: how far the cut lies from the template edge. The calculator also shows twice that, which is the overall size difference across an opening or shape.
The bush rubs against the template and the cutter runs concentrically inside it. The gap between the bush edge and the cutter edge is half the difference in their diameters.
A 30 mm bush with a 12 mm cutter gives a 9 mm offset. An opening routed from a template would be 18 mm smaller overall.
Watch out: inside and outside templates need the offset in opposite directions. Check that the bush is properly centred.
Calculate the radial offset between guide bush and cutter.
Enter your values to see the result.
Good to know
Use actual compatible diameters and a concentric bush. Internal and external templates require opposite offsets; check against the drawing.
Sources
- Geometry / arithmetic
This is a calculation aid, not a structural design or compliance certificate. Every default is an example, not a specification for your project.