How to centre a template guide?

[Dengue]

I have a 18mm OD template guide that screws to the base of the router, and there is room for it to move slightly, and a 4mm router bit. Can anyone please suggest the best way to make sure the template guide is fitted so that the OD of the guide and the bit are perfect concentric circles ?

[malb]

The usual method is to chuck a centering cone or a parallel centering pin into the collet (instead of a router bit), mount the template guide loosely, plunge the centering tool into the guide to centre it, then tighten the fastening to lock the guide in position. In your case, since the guide screws to the base, it might be difficult if the system uses countersunk screws as these will want to pull the template into a particular position, and no amount of trying will improve on the machining accuracy or inaccuracy of the production line.

In that case the next approach is to use an aftermarket base/rubstrip that is set up to accommodate guides, and to use mounting screws recessed below the working surface with oversize holes. You can then loosen the mounting screws to allow the base and template to float, and use the centering tool to align the guide to the collet axis and tighten the screws to hold position.

A centering cone is a tapered unit with a shank that the collet can grip. The size range of the taper must include the ID of the guide as the guide ID to flange transition rests on the taper to center. A centering pin is made to specific guide ID with a collet sized shank, is mounted in the collet, and interacts with the guide ID surface for centering.

Note that some routers, particularly early fixed base units, will loose concentricity as soon as the depth is changed because of manufacturing and design issues.

[Dengue]

Thanks Malb for this very comprehensive reply. I was not aware of a centering cone before, but did find an example of one here that will do up to 22mm ID template guides.

I like the idea of a centering pin, as a quick and cheap solution. The ID of the 18mm guide is just under 16mm. My son in law has some 16mm aluminium rod and a metal lathe, so will get him to make a centering pin, 12.7mm for half its length to go in the router collet, and the remainder machined to suit the ID of the template guide.