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Routing Circular Mouldings for a Plinth
Question
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You asked about doing it with a router table, hence my response. But if you have a lathe with a big enough swing or the ability to turn outboard, you might find that to be the quicker, better way.
From contributor J: I would make it out of flexible rubber since it's going to be painted anyhow. From contributor U: Why are you trying to do this in one piece? You effectively have two sections there, both of which can be machined separately and then joined. Cut your two square sections and mark off their centres by scribing across the diagonals. Mark off the circumference(s) you need. Saw the rough circular shapes, then, using a special attachment on your router, use a straight fluted cutter to give you the good circle you need. Replace the cutter with the profile you want and a single run will get you to home base. Cut each of the pieces and, using the centre hole that located the router jig, glue and fix the sections together. Dead simple. About a half hour's work I reckon, and then without raising a sweat (so no excuse for stopping for a beer!). The advantage of that central locating hole is that it can be used to fix the new base centrally to the plinth, locating in a hole you'll have marked first in the square insert-base you'll have cut and then circled-up using the straight fluted cutter, glued and pinned into the base ready to drop over that locating pin. If the plinth can't be removed, then cut the moulded base as above, then move the straight cutter into the radius = plinth diameter and gradually plunge cut in increments to separate the moulded circle. Halve it and just push the two sections onto the base of the plinth, glue to the square base and plinth, with wood filler in the joints of the two halves to allow sanding to profile. But give it a little thought and you'll probably see that you could actually make that circular fixture yourself, or something like it that'll do the job just as well for a fraction of the cost.
From the original questioner: That is exactly what I was thinking. I made the plinth separately and will join the pieces. I figured that since the circle is small, the foot of the router would cover the center point and a jig would not work, hence the router table. From contributor U: Grief, didn't realize the plinth was so small! What's it a part of? You can actually make your own fixture for cutting circles out of MDF and use it to replace your existing base plate; it's just that you'd have to get the relative positions of the cutter/locating pin (just a screw will work) right if you want to replicate the dimensions exactly - otherwise near enough should be good enough if this is on its own, or you're making a pair. But you sound as if you've already got your head around it. Good luck. From contributor R: Sure - that's what bearings are for! From the original questioner: I am going to need a plunge router to start the cut, right? If not I am not sure how I would get it started. The plinth is 10" square; round base is 7" diameter. From contributor U: Not necessarily. I suppose my use of the word "plunge" was a bit careless. I'm always nervous of plunge routing unless the tool and the job are well located, which is why all five of my routers have screw-adjustment, allowing the depth to be set accurately to start with, then locking it in that position. Quite apart from the ability to set that depth of cut to the nearest thou, it also means I don't have to worry about keeping up that constant pressure to maintain a consistent depth of cut which, if you're working a long run of cut, can be a pain in the you-know-what. Using just the lightest pass at first, increase the depth of cut incrementally. I'd fix the circular piece down onto the bench, screwing through a spacer smaller than the intended finish diameter so that the cutter doesn't foul the bench. But you'll already have thought of that. Just one other point, make sure you have support for the router base around its external course of travel, which of course need only be rudimentary as long as it's the same level as the surface of the piece being cut, otherwise the tool could tilt as your weight shifts as you work around the piece, and you don't want any ridges in that cut. 7". Jeez, I see what you mean about the tightness of working on it. Trouble is, to make the job pay you can't really afford to mess around with elaborate jigs and fixtures, so I would definitely be inclined to fashion something myself out of MDF. The principle is straightforward enough once you've got the image of what you're trying to achieve in your mind's eye - just something to tie that router base down, a bit like nailing someone's foot to the floor - they're only going to be going one way!
From the original questioner: I still don't know how I would start the cut using a circular jig without a plunge router. I guess if the jig was nailed down in the center it could slide up and down. Only problem is at 7" diameter for my work, the foot of the router covers up that point. I have a feeling I will just use a standard router bit. From contributor U: Sorry - one hell of a senior moment there. Of course you're going to need to plunge. Apologies for confusing the issue. How else can you feed into the cut unless you have a circular template to bear against? You're working to replicate an existing item, which is actually not something I've had to do for some time. The last time I made this kind of item was for the bottom newel of a staircase and I used the lid off a tin of paint to guide the template cutter to make the circular template and then just routed around the circumference with a bearing guided cutter. Would you like to add information to this article? Interested in writing or submitting an article? Have a question about this article? Have you reviewed the related Knowledge Base areas below? 
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