How to Cut Perfect Circles with a Router
Cutting a perfect circle with a router is one of the most satisfying operations in the shop. The result – a smooth, clean, perfectly round edge – is nearly impossible to achieve with a jigsaw and very difficult to sand perfectly round after cutting. A router trammel (a simple beam compass that pivots around a center pin while swinging the router in an arc) makes this trivial. This guide covers building a shop-made trammel, setting the radius, and making clean, chip-free cuts in one or more passes.
How the Router Trammel Works
The trammel is a strip of material (typically 1/4-inch plywood or 1/8-inch hardboard) with the router attached at one end and a pivot point (a small finish nail or a turned wooden pin) at a measured distance along its length. When the pivot sits in a center hole in the workpiece and the router swings around it, the bit describes a perfect circle of the set radius. The radius is measured from the center of the bit to the center of the pivot pin.
Building the Trammel
Cut a strip of 1/4-inch plywood approximately 4 inches wide and 24 to 36 inches long (longer for larger circles). At one end, attach the router using the router’s base plate holes – drill through the trammel to match the base plate bolt pattern. At the other end, drill a hole sized to accept a 1/4-inch finish nail snugly. Drill additional holes along the trammel’s length at 1-inch intervals, starting from the center of the router bit and continuing outward. Each hole position represents the corresponding circle radius.
Setting the Radius
The radius is measured from the center of the router bit to the pivot point. For a 12-inch diameter circle, set the pivot 6 inches from the center of the bit. Mark the trammel clearly – the router bit center is the zero reference, not the end of the trammel or the edge of the router. Use a rule to measure from bit center to each pivot hole position and verify before cutting.
Making the Cut
Drill a small center hole in the workpiece at the exact center of your intended circle. Insert the pivot nail into the trammel and push it into the center hole. Turn on the router and slowly swing it in an arc. Take multiple passes – 1/4 inch of depth per pass for most bits in most materials. Make the final pass just shy of full depth, then take a final light climb cut (moving the router against the bit’s rotation direction for just the final 1/64 inch) to eliminate any tiny scallop marks left by the standard cut direction.
Pro Tips
- Back up the workpiece with a piece of sacrificial MDF so the bit doesn’t blow out the exit hole at the bottom of the last pass.
- For circles that go all the way through the workpiece, leave a small bridge (1/16-inch uncut section) until the final pass to keep the disc from rotating while you’re cutting. Cut through the bridge with a chisel after the routing is complete.
- For large circles that exceed your trammel’s length, use a longer strip of hardboard or a piece of T-track as the trammel arm.
FAQ
What’s the smallest circle I can cut with a trammel?
Limited by the router bit diameter plus clearance – the pivot must be outside the cutting path of the bit. With a 1/2-inch bit, the minimum useful radius is about 1.5 inches, giving a 3-inch diameter circle. Smaller circles are better cut with a hole saw or a drill.
Can I use a trammel for inside curves?
Yes. The trammel cuts the edge of the disc from the outside; removing the disc from the sheet gives you both the disc and the hole. If you want just the hole (like a round tabletop hole for a grommet), cut from the inside of the intended opening, being careful that the disc you’re removing won’t damage the router or the workpiece when it breaks free.