While in the process of building a round table, I found a quick, inexpensive way to make a beam compass and router trammel using 1⁄2"-dia. electrical conduit. For the beam compass, I used a plastic cable clamp to hold a pencil. And for the trammel, I made a custom baseplate for my router that fastens to the coupling.
You can find screws at your hardware store to fit the threads in the couplings. For the trammel “centerpoint,” I used a longer screw, cut off the head, and ground it to a point. You’ll need to modify the couplings a bit so they’ll slide completely over the conduit. I did this by filing out the ridge on the inside of the coupling. With the internal ridge removed, you can easily adjust the position of the coupling along the length of the conduit and tighten the screw to keep it in position.
