Router Circle Jig: Build Your Own Step-by-Step

This guide shows you how to build a simple, adjustable router circle jig that lets you cut perfect circles and arcs in wood, plywood, and MDF. You’ll make a pivoting arm that attaches to your router base, so you can set any radius from a few inches up to a couple of feet. This is a weekend shop project that costs very little if you have scrap materials.

What You’ll Need

• 1/2-inch or 3/4-inch plywood or MDF (at least 12″ x 24″)
• Router with a fixed base (a plunge base works too, but fixed is easier)
• Router bit with a bottom bearing (straight or spiral bit)
• Drill with a 1/4-inch bit and a countersink bit
• Two or three 1/4-inch bolts with washers and wing nuts (or T-nuts and knobs)
• One 1/4-inch or 3/16-inch pin or nail (for the pivot point)
• Wood glue and a few 1-inch screws
• Measuring tape or ruler
• Pencil and straightedge
• Safety glasses and hearing protection

Step-by-Step

1. Cut the base plate. Cut a piece of 1/2-inch plywood or MDF to about 10 inches by 14 inches. This will be the main board that the router sits on. The exact size is flexible, but it must be large enough to support your router base.
2. Locate the router mount holes. Place your router (without the bit) upside down on the board. Center it so the collet area is roughly in the middle of the board’s width. Trace around the router base with a pencil.
3. Mark the mounting screw holes. Most router bases have three or four holes for the screws that hold the sub-base. Use a pencil to mark those hole positions through the router base onto the plywood.
4. Drill and countersink the mount holes. Remove the router. Drill 1/4-inch holes at the marked positions. Then use a countersink bit so the screw heads sit flush with the top surface of the board.
5. Attach the router. Remove the original plastic sub-base from your router. Place the router over the holes and secure it to the plywood using the same screws that held the sub-base, or slightly longer ones if needed. Make sure the router is firmly attached and the collet is accessible from below.
6. Cut the pivot slot. On the board, draw a line from the center of the router collet down the length of the board. This line is your radius reference. Drill a 1/4-inch hole at the far end of this line, about 1 inch from the edge. Then, using a router or a jigsaw, cut a slot from that hole toward the center of the board. The slot should be about 1/4-inch wide and at least 6 inches long. This slot is where the pivot pin will slide to adjust the radius.
7. Make the pivot pin assembly. Take a 1/4-inch bolt about 1.5 inches long. Insert it through the slot from the bottom of the board. On top of the board, add a washer and a wing nut. This bolt will act as the pivot point. The distance from the center of the router bit to the center of this bolt is your cutting radius.
8. Create the pivot point on the workpiece. To use the jig, you need a small hole in the center of your workpiece. Use a nail or a 1/8-inch drill bit to make a shallow hole. The pivot pin (the bolt) will sit in this hole.
9. Set the radius. Loosen the wing nut, slide the bolt along the slot until the distance from the router bit center to the bolt center equals your desired radius. Measure carefully with a tape measure. Tighten the wing nut firmly.
10. Test the setup. Clamp your workpiece to a workbench. Place the pivot bolt into the center hole. Lower the router so the bit just touches the surface. Turn on the router and slowly rotate the jig around the pivot point in a clockwise direction. Cut in several passes, lowering the bit a little each time, until you reach the full depth.

Common Mistakes

• Measuring from the wrong point. The radius is measured from the center of the router bit, not the edge of the router base. Always measure from the bit’s center to the pivot bolt’s center.
• Loose pivot pin. If the wing nut is not tight, the pivot point will shift during the cut, giving you an oval or wavy circle. Make sure the bolt is clamped firmly in the slot.
• Routing too deep in one pass. Trying to cut the full depth at once puts huge stress on the bit and can cause the router to kick. Make multiple shallow passes, no more than 1/4-inch deep each.
• Router rotation direction. Always move the router counterclockwise around the outside of the circle (or clockwise on the inside of a hole). This gives you a clean cut and prevents the bit from grabbing.
• Not using a bearing bit. A bit without a bottom bearing relies entirely on the pivot pin. A bearing bit gives you a smoother finish and reduces friction.

Tips

• Use a scrap piece of wood to test the setup before cutting your final piece. This lets you verify the radius and depth without ruining good material.
• Add a small piece of sandpaper or a rubber pad between the router base and the plywood to prevent slipping.
• For very large circles (over 24 inches), extend the board by screwing a longer piece of plywood onto the end. The slot can be cut into the extension.
• Mark the radius on the jig with a pencil and label it. That way you can quickly set common sizes without measuring each time.
• Use a spiral upcut bit for cleaner cuts in plywood, as it pulls chips up and out of the cut.
• Clamp a straight edge to the workpiece as a guide for the first pass if you are cutting a circle that is larger than the jig’s range.

Troubleshooting

• The circle is not round. This usually means the pivot pin moved during the cut. Tighten the wing nut more, or replace the bolt with a T-nut and knob that locks more securely. Also check that the workpiece is clamped firmly and the jig is not flexing.
• The cut is rough or burned. You are likely moving too slowly or taking too deep a pass. Increase the router speed slightly, move faster, and take shallower cuts.
• The router bit is cutting a spiral instead of a circle. The pivot pin is not seated in the center hole, or the hole is too large. Use a pin that fits snugly in the hole.
• The jig lifts off the workpiece. The router is pulling upward due to the bit’s rotation. Make sure the pivot pin is long enough to stay in the hole, and use a bit with a bearing that rides on the surface.
• The radius is slightly off. Double-check your measurement from the bit center to the pin center. Even a 1/16-inch error shows up as a 1/8-inch difference in the circle’s diameter.
• The slot is too tight for the bolt. Widen the slot slightly with a file or sandpaper, or use a slightly smaller bolt.

Frequently Asked Questions

Can I use this jig with a plunge router?

Yes, you can. The process is the same, but you will need to adjust the plunge depth between passes. The jig works best with a fixed base because the depth stays consistent.

What is the smallest circle I can cut?

The smallest circle is limited by the distance from the router bit to the edge of the jig’s pivot slot. With a typical setup, you can get down to about a 2-inch radius (4-inch diameter). For smaller circles, use a different method.

What is the largest circle I can cut?

The largest circle is limited by the length of your board. A 14-inch board will give you about a 12-inch radius (24-inch diameter). You can extend the board with a longer piece of plywood to make much larger circles.

Do I need a special router bit?

No, but a straight bit with a bottom bearing works best. The bearing rides on the workpiece surface and helps keep the cut clean. A plain straight bit works too, but you need to be more careful about depth.

Can I cut circles in thick hardwood?

Yes, but take very shallow passes (1/8-inch or less) and use a sharp bit. Hardwoods like oak or maple are harder on the router, so go slowly and let the bit do the work.

How do I cut a circle that is larger than my jig?

You can extend the jig by screwing a longer board onto the end of the base plate. Alternatively, use a trammel point attached to a straight router guide, or build a larger dedicated circle jig.

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