For round sub-base: “truing the sub-base”.
This is the second part of a series of two so if you have not read “Amateur techniques to align sub-base” please do so before continuing.
You will want to do this if you ever use the sub-base against an edge as a guide.
In the last posting we aligned the bit using the template guide to the sub-base. Now that they are concentric we will attempt to make the outside of the sub-base also concentric to the bit.
Using the same set-up as before, when we did the first alignment procedure, a ¼” shank mounted in the router, and the disk in the template guide.
Select a piece of scrap wood maybe a 1” X” 4 X 12”or similar. Draw a line down the center of the piece full 12” in this example. Measure the Diameter of your sub-base, for example it might be 6” across. Now measure half the diameter + 1 inch down from one end of the scrap wood piece (in our example this would be 4”).
Mark that location on the centered line then drill a ¼” hole through the wood at this location. This hole must be ¼” and as vertical as you can get it.
Take the router and put the shaft into the hole. You want the routers sub-base sitting against the scarp wood, if need be adjust things so they are sitting flat. Now take a sharp pencil and hold it near the sub-base where the wood extends about 1” past the sub-base. This location should be on the centered line. Very slowly rotate the router about the shaft and watch for the location where the sub-base is nearest the pencil. When you have found that spot use the pencil against the sub-base and make a mark across the centered line.
Using the same method locate the spot on the sub-base where it is at the greatest distance from the mark you made. Mark this location.
Lift the router off the scrap wood and carefully cut the wood a so the line closes to the hole is gone (1/32” is good). This cut is perpendicular to the centered line you made and the resulting wood will be about 11” long in this example.
Put the router shaft back in the hole. The sub-base will now extend beyond the edge of the wood where the cut was made. Slowly rotate the router to make sure the sub-base extend over the wood all the way around.
Now depending on the material of your sub-base you can sand, file or scrape the edge of the sub-base using the fresh cut end of the wood as a guide. Remove material of the sub-base by slowly turning the router until the edge of the sub-base and the wood match.
The sub-base should now be concentric with the router bit.