How to Make a Circle Cutter

Router bits used to cut specific profiles with a router saw.

A circle cutter is a type of jig or assembly used to guide a cutting device in a circle so that a perfect circle shape can be cut. Circle cutting jigs can be purchased for many types of cutting devices such as mat board cutters for framing photos inside circles, or plastic rotary cutters used for cutting fabric circles. These cutters are more specialized or less common than the circle cutter jig used for cutting wood. Circular pieces of wood are used for all kinds of purposes, making the wood circle cutter a common tool accessory in most wood shops. Many jigs are sold for this purpose but it is not difficult to make your own so that your router can cut circles exactly the sizes you need for your projects.

Things You'll Need

• 1/2-inch plywood
• Tape measure
• Table saw
• Pencil
• Compass

Instructions

1. Cut 1/2-inch plywood to 6-by-24 inches using a table saw. This is the material for the arm of the jig. Cut a second piece 6-by-15 inches. This is the material for the base and pivot plate.

2. Measure three inches across the width of the arm. Draw a straight line along the length of the wood and label this line A. Measure 1 1/2 inches above and below the center line A and draw two lines parallel with A. Label the two lines B and C. Measure two inches away from the right side end of the board and mark on line A. Label this mark D. Measure 18 inches from mark D along line A and mark. Label this mark E.

3. Drill a 1/2-inch hole at marks D and E. Measure 1 1/4 inch from the left side end of the board. Draw a vertical line across the short (six-inch) width of the board. Label this line F. Measure one inch from the top of the board along line F and mark. Label this mark G. Measure one inch from the bottom of the board along line F and mark. Label this mark H.

4. Drill a 1/4-inch diameter hole at marks G and H. Measure one inch from the left side end of the board and draw a vertical line across the short width of the board. Label this line I. Measure three inches from the left side and draw another vertical line labeled J. Draw a three- inch radius circle that touches the center of line J and curves toward the left side end of the board. Draw a 3 1/2-inch radius circle that touches the center of line I and curves toward the left side of the board. The two curves will create a crescent-like shape on the left side of the board.

5. Route the waste from mark D to mark E along the center line A. This waste removal cut should be 1/2-inch wide and should go all the way through the board. This creates a slot for adjusting the arm. Turn the wood over. Measure three inches from the top across the width. Draw your center line A on this side of the board. The slot should be in the center of this line. Measure 1/2 inch from the line above and below and draw these lines parallel to center line A. Label these lines K and L. Set your router to 1/4-inch depth and route along lines K and L. This will create a T shaped slot in your arm from one side to the other.

6. Turn the arm over. Cut from the right side of the board along lines B and C until you reach the first curved radius. Follow the curve from line B toward the top edge of the board. Follow the curve from line C toward the bottom edge of the board. Cut the entire curve of the 3 1/2-inch radius. This finishes the arm.

7. Position your second board so that the 15-inch side is horizontal and top. Measure three inches from the top edge. Mark and draw a line along the 15-inch length of the board. Label this center line A. Measure three inches from the left side along center line A and mark B. Measure 2 1/2 inches from mark B and mark C. Measure three inches from the right side along the center line A and mark D.

8. Draw three circles on the plywood using a compass set at a three-inch radius. Center a circle at marks B, C and D. Circles B and C should overlap. Draw a second circle using mark B as the center point. Set your compass radius to 1 1/2 inches. Place your arm over your second board so that the two drilled holes in the crescent are positioned over circle C. Place a pencil through each hole so that the holes will line up. Set your arm board aside.

9. Drill a shallow counter bore hole using a 7/8-diameter Forstner bit, 1/8-inch deep at each of your pencil marked hole spots. Use a 5/16-inch drill bit all the way through the center of each hole. Drill or cut out the three-inch hole at mark B. This is your router hole so sand the sides smooth. Drill a 1/4-inch diameter hole at mark D. Cut the six-inch circle at mark D. This is the pivot plate. Cut the one piece rectangle with curved ends shape at marks B and C. This is the base plate.

10. Select hardwood stock that is 5/8-by-1-by-12-inches long. Place the wood so that the one-inch surface is down. Drill a 1/4-inch diameter hole centered in the one inch. This hole is for your pivot pin assembly. Measure two inches from the center of your hole, along the center line and mark. Drill a shallow counter bore hole using a 7/8-diameter Forstner bit, 1/8-inch deep at your mark. Use a 5/16-inch drill bit all the way through the center of the hole.

11. Turn your stock over so that the counter bore hole is down. Mark 3/8-inch from the top and draw a line along each 5/8-inch side. Mark 1/4-inch from each edge of the one-inch face and draw two lines. Rabbet the waste to leave a T-shaped 12-inch board. This will fit into the slot in the arm. Measure 1/4 inch from your 1/4-inch diameter hole and cut crosswise. Measure three inches from this end and cut crosswise. Install a 1/4-inch T-nut into the counter bore hole. This completes your pivot pin block.

12. Select 1-by-1 1/2-inch hardwood stock at least six inches long to make it easier to work with. Position the stock with the one-inch side down. Measure 1/2-inch from the end of the stock and mark. Measure 1/2-inch from the mark and mark again. Measure 1/2-inch from your second mark and mark a third time Draw three parallel lines at the marks across the one-inch face You should have three even 1/2-inch sections. Measure 1/8-inch along the 1/2-inch side and mark. Remove the waste along the center 1/2-inch section to 1/8-inch deep. Drill a 1/4-inch diameter hole centered in the dado cut. Cut along your third line or 1 1/2 inches from the edge. This is your pinch block.

13. Insert a 1/4-inch star knob with one-inch stud through a 1/4-inch washer. Slide the knob through the pinch block, through the arm, into the pivot pin block with the T-nut facing down. Screw the stud into the T-nut. Cut off the end from a standard 1/4-inch hex bolt. Add a bit of epoxy glue to the threads and screw the threaded end into a 1/4-inch threaded knob. Taper the pin end to make it easier to insert. Insert the pin through the arm, through the pivot pin block and into the center hole of the plate.

14. Remove your router base plate. Place the base plate over the wood base and mark the screw holes. Drill holes through the wood base plate the same size as those on your metal base plate. Install a 1/4-inch T-nut into the counter bore hole in the bottom of your base plate. Insert a 1/4-inch star knob with one-inch stud through a 1/4-inch washer. Slide the knob through the arm, into the base and into the T-nut with the T-nut facing down. This finishes your jig.

Tips & Warnings

• You can build this same jig with a longer arm for larger scale projects. You can also mount the jig using clamps by screwing into some waste and clamping the waste to the project board.

References

• Woodcraft: Circle Cutter Jig
• South Point Hawaii: Router Jig Plan