How To Make A Circle Cutting Guide Homemade Machines & Jigs

I’ve started working on my new table saw and have reached the point where I need to make accurate trunnions, and I figure the best way to do that is with the router and a circle cutting jig. I already have one of those, but it’s really designed for bigger circles and it uses my full size router. While I could do it with that one, it won’t be easy or convenient because of the size.
So I decided that I should make something more compact that works with my cordless trim router that I can use for smaller circles. And the benefit of using the battery power tool is there’s no cord to get in the way while swinging it around making the cut.

I looked the router over trying to figure out the best way to attach it to an arm, and saw the knob screwed into the base. That is for accessories that work with this tool, and the irony is that there might’ve been something among those accessories that would work perfectly well is a circle cutting guide:

Of course, I either threw those away or put them in a place where I’d never find them. Typical!

So I removed the knob and thought I could just replace it with a threaded rod that would have an adjustable pivot block. Problem is, the threading on this knob is metric, and I don’t have any metric threaded rod. I do, however, have a 1/4″ threaded rod that is very close to the size of the bolt that is on the knob. So I used a tap to re-thread the whole for that quarter inch threaded rod:

After screwing it in, I threaded on a nut to lock in place to the base of the router, and got to work on the pivot block:

The pivot block is just a piece of solid hardwood that I cut into this L-shape and drill two holes in, one for the threaded rod, and one for the pivot.

And for the pivot I drilled a 1/8″ hole and then cut about 1/2″ off the end of the drill bit and epoxied that in.

The base had this rectangular notch and the pivot can extend into it for very small circles. The plastic base can be notched even deeper, if I need smaller circles.

The pivot block itself is held in position by two nuts and can be adjusted along the full length of the rod, and locked in place:

After the epoxy set, I did a sample cut to see how it works:

It was much easier to handle than the bigger one, especially not having the cord to deal with.

I went on to cut the trunnions for the saw using this jig:

And the guide worked perfectly.

I made a video of the build: