I have used a flush trim bit to make an exact copy of some templates, and I think that I understand the mechanism there. It wasn't a router base, but it was a hook for attaching yard tools to french cleat bases - I used the template to repeat a batch of them with a flush trim bit.
I think that the advice I've received so far about using an inlay kit is the most relevant. JW2170
made a post on this thread with a link to RouterWorkshop
, which was helpful and clear. If I describe the inlay process, and then contrast it with my imagined project, it might be the clearest description.
The steps on RouterWorkshop appear to be Template / Cavity / Plug
- Begin with a template - a flat piece of material with a hole in the shape of the desired inlay. (The hole is a little bigger than the exact inlay shape, because of bushing tolerances.)
- Cavity - Equip with larger bushing and a spiral bit. Use the pattern to cut the inlay hole/cavity in a board. Either cut all the way through the board for a hole, or remove the material at depth for a smooth bottom cavity.
- Plug - Equip with smaller bushing and a spiral bit. Mount the template to a piece of plug material. Use the pattern to cut the perimeter of the plug piece.
- Do the Inlay - place the plug into the cavity, some tweaking may be required for corners and small radii curves but it should be a match.
For my project, I am starting with a specific Router base, and I want to recess it into a workbench. So the order of operation here seems to be Plug / Template / Cavity
- Begin with a plug (the router base).
- Equip with smaller bushing and spiral bit. Secure the router base to a large flat piece of hardboard, and make a plunge cut with the bushing against the edge and a spiral bit to trace the exact perimeter of the router base. This creates the template for making router base-sized cavities. It should be oversized compared to both cavity and router base.
- Equip with larger bushing and spiral bit. Secure the template to the workbench, and make a cavity only to the depth of the router base. This allows the router base to rest in the cavity like an inlay. Then route out the full hole that just fits the router chassis through the workbench surface, while leaving a supporting rabbet around the cavity perimeter.
I now think I have the order correct, and the sequence of using the different sizes of bushings. The mental geometry of lining up bushing and bit sizes and aligning them with the exact same vertical line (the plug/cavity edge) is the part that I'm slowly getting. Inlay kits appear to include Bits, Large Bushings, and Small Bushings that ratio correctly to one vertical line. (The Whiteside Inlay kit is 1/8" bit with 1/4" shank, 5/16" OD Bushing, and 9/16" OD collar. The Router Workshop post describes a 1/4" bit, a 7/16" OD guide bushing, and a 1/4" inlay bushing for an OD of 15/16".) This tells me that the difference in Large/Small OD of the bushing sizes should be double the diameter of the bit. The Stumpy Nubs video about complimentary curves supports the math, as he works with a 1/4" flush trim bit and then adds a 3/4" bearing for offset.
I think I can follow this formula to start with any object (my Pinewood Derby gold medal, a ceramic coaster, a powerstrip surge protector) and end up with a template that I can use to make a hole to recess the object into.