A circle-cutting jig turns a router into a compass: pivot at the center, radius set to the bit’s inside cutting edge, and several shallow passes.
Most circle cuts go wrong before the router ever touches wood — at the pin, the radius, or the depth of the first pass. The gap between a wobbly oval and a true circle usually comes down to how you use a router circle cutting jig: where the pivot sits, how far the bit’s edge is from that pivot, and how deep each cut goes.
The steps below track the manufacturer manuals for the POWERTEC 71637, the Kreg KRTBP100, and the Northern Tool router jigs.
Router Circle Jig Setup: Mounting, Pivot Pins, and Radius
Setup runs in a fixed order: kill the power, bolt the jig to the router base, fit the bit, pick the pivot pin, then set the radius from the pivot center to the bit’s inside cutting edge.
- Unplug the router before any mounting, bit change, or pivot swap.
- Attach the jig. Remove the stock base plate where the jig replaces it, line the jig’s holes up with the router base, and run the machine screws down. Some jigs reuse the router’s own fasteners; Kreg’s manual covers centering the router on its base plate.
- Install the bit per the router maker’s instructions. A plunge router gives controlled entry and easier depth passes.
- Choose the pivot pin. The POWERTEC 71637 manual assigns Pin A to cuts made from the bottom surface or through a predrilled 1/8-inch hole, and Pin B to cuts made from the top surface. Its pins carry left-hand threads, so they tighten the opposite way you’d expect. POWERTEC’s 71637 jig manual spells out both pin assignments.
- Set the radius. Slide the pin along the jig slot until the distance from the pin’s center to the bit’s inside cutting edge matches your radius. A 14-inch finished circle needs 7 inches.
- Mark, clamp, and dry-run. Mark the center, pin it, and clamp the board to a solid bench. With the router still unplugged, swing the jig through a full circle — the bit should trace your line without the cord, clamps, or base hitting anything.
Making the Cut: Depth, Direction, and Control
Run the router counterclockwise around the pivot and take the circle in several shallow passes. The POWERTEC 71637 manual specifies counterclockwise feed for that jig, and the Northern Tool manual directs users to remove small increments rather than plunging to full depth.
Set a partial depth, start the router, and ease the bit in with the base flat. Let the jig arm carry the radius instead of steering the machine by hand. Work one lap, stop, drop the bit slightly, and go again.
On the last pass, shut the router off and leave it in place until the bit stops spinning. A groove that traced your pencil line all the way around, plus a motionless bit, means the circle is finished.
Why Does the Circle Come Out the Wrong Size?
A circle misses its size most often because the radius was measured to the bit’s shank or center rather than its inside cutting edge. The other frequent cause is a board that moved, since an unclamped workpiece shifts under the cut and drags the pivot with it.
| What You See | Usual Cause | Correction |
|---|---|---|
| Circle off by a steady amount | Radius measured to shank or bit center | Re-measure to the inside cutting edge |
| Circle drifts into an oval | Board or jig shifted mid-cut | Clamp down and recheck the pivot |
| Scorched, chipped rim | Too much depth in one pass | Smaller increments, sharp bit |
| Router fights the cut | Full-depth pass or dull cutter | Shallow passes only |
| Bit wanders off the line | Clearance never checked | Swing the jig by hand before powering on |
| Center hole wallows out | Pin mismatched to the entry surface | Follow the jig’s pin-specific instructions |
“Fits most routers” is not a fit guarantee. Check the mounting-hole pattern, screw type, base opening, bit capacity, and plunge clearance against your own machine before ordering, and our tested wood router circle jig roundup sorts the options by base pattern. Wear dust protection; machining wood throws fine particles.
Common Questions
Can a Plunge Router Cut Circles With These Jigs?
Yes, and it is usually the better choice. A plunge router lets you set a partial depth and lower the bit into the work with the base already flat, which makes repeated shallow passes simple. Fixed-base routers still work, though entering the wood cleanly takes more care because bit height is locked in before the cut starts.
How Do I Set the Radius for a Finished Diameter?
Halve the diameter. A 20-inch circle needs a 10-inch radius, measured from the pivot pin’s center to the bit’s inside cutting edge. Run a test circle on scrap first — bit diameter, jig scale markings, and small measuring errors all show up as a finished circle that is slightly large or slightly small.
Will Any Circle Jig Fit My Router?
No. A jig mounts through a specific hole pattern, so the jig’s holes, screw size, base opening, and bit capacity all have to match your router base. Kreg’s KRTBP100, KRTBP150, and KRTCG100 plates and the POWERTEC 71637 each list their own mounting requirements, and comparing those to your machine is the only way to know before ordering.
Sources
- POWERTEC. “Model 71637 Router Circle Cutting Jig Manual.” Supports pivot pin A and B assignments, left-hand threads, and counterclockwise feed direction.
- Northern Tool. “Router Circle Jig Product Manual.” Supports the instruction to cut in several shallow passes of small increments.
- Kreg Tool. “KRTBP100 Router Base Plate Manual.” Supports base plate centering and mounting-hole alignment steps.