I added this auxiliary router base to my Porter Cable router almost at the moment I purchased it in 1983 and brought it home. I did this as at the same time I made a table for it, which has been seldom used BTW. Just for the record, this larger base has never been removed from my router, since that initial installation, and has never gotten in the way of everyday routing.
Now, on to circle cutting......look at the small drilled holes in that base (picture #2). I simply drilled a hole to match the radius needed for a specific bit that was installed.That becomes the fulcrum point, rotating on a small nail. The router shown is not a plunge style but works just fine by making multiple passes and lowering the bit with each additional pass. That router base was made from material on hand (scrap plexiglass), which was 1/4"thick and two layers were glued together for the thicker version.
Step 1: Routing Larger Diameter Circles
To make larger circles that were beyond the radius of the holes drilled into the aux. base, I made up a an extended detachable arm. You simply drill the necessary holes in the arm that match the radius you need.
The materials used were lengths of 1/2 X 1/8" flat steel stock and some 5/16 diameter bolts cut to length with some suitable nuts. The TEE bar that plugs into the router was welded together, but could have easily been a bolted connection. The Tee and the arm are shown rigidly attached, but the joint could have been a short length of threaded rod to give you more incremental adjustment.
Step 2: Ready to Rout Circles.
So there you have it....Load your bit and install the tethering arm. Install a pivot point (nail) and ease the router into your work, increase the depth of cut with each pass. I use a foot switch and the vise set up shown in the first picture for much of my everyday routing. I have found that a large table is not always needed.
Let me know if I have skipped over any details.