So Many Circles!
Circles are found in dozens of mechanisms including cams, eccentrics, cranks, pin wheels, collars, followers, and flywheels. Circles may also serve as the foundation for gear blanks, pulleys, or animated parts of an automaton. But how can you obtain this vital shape in wood?
The caterpillar automaton (above) by Timberkits is almost entirely made up of circles.
Circles the Easy Way
One option is to buy pre-made wooden discs and wheels. These are typically turned from hardwood and come in a variety of sizes and styles. You can find them at most hobby and craft stores or woodworking specialty stores. Small discs can also be sawn off the ends of large dowels. While easy, the circles produced are limited to the size of the dowels you can find.
When Buying Circles Won’t Do
Sometimes you need a circle of a specific size or of a certain wood. Fortunately, there are countless ways to make wooden circles. A wood-turning lathe is the ultimate circle-making tool. Don’t own a lathe? Have no fear. More common tools can do the job!
Using a Fly Cutter in a Pillar Drill
A fly cutter (often called a ‘circle-cutter’) can cut discs of various sizes from different thicknesses of wood. Mine will cut circles from 1″ to 6″ in diameter. The fly cutter must be used in a drill press; never attempt to use it in a hand drill. Position the cutter away from the middle of its pilot bit a distance that is one half the diameter of the circle you wish to produce. Mark the center of the future circle. Place a scrap board under the piece to be cut and secure it to the pillar drill (drill press) table with strong clamps. Using a light pressure, lower the spinning cutter into the wood. As more of the cutter makes contact with the wood, lower the tool even more slowly. I recommend cutting half way through the piece, then flipping it over to finish. This can help limit the friction one encounters with a single deep cut.
Using a Bandsaw or Scroll Saw
No drill press? With practice, you can cut circles freehand with a bandsaw or power scroll saw. On the bandsaw, the smallest circle you can produce will be limited by your blade width. A 1/4 inch blade will allow you to cut almost any size you might need. Using a compass, draw a circle on the wood, then cut to that line. A circle-cutting jig allows you to rotate a wood blank into the blade while keeping the center a precise distance from the blade. The result: a perfect circle. This clever circle cutting jig from Fine Woodworking Magazine, #180 allows you to cut a wide range of sizes.
Cutting Circles with a Hole Saw
If all you have is a power hand drill, a hole saw is your best option. A hole saw is a cylinder of steel with saw teeth on the bottom. Although you can only cut one size of circle with any given hole saw, they are sold in many sizes. Start by drawing a circle on the piece of wood to be cut. Clamp the wood to a work surface with a piece of scrap wood underneath. Drill a hole through the center of the circle with a drill bit that is the same size as the hole saw’s pilot bit. With the hole saw in the drill, hold the the drill at a right angle to the surface of the wood, start the drill, slowly sawing. Once the all of the teeth have reached a depth of about 1/4 inch, you can increase pressure on the drill. Keep the drill straight to avoid binding the saw! Drill halfway through the workpiece, flip it over, and saw the rest of the way through. This will leave some of the wood plug (your circle) sticking out of the hole saw, allowing you to retrieve it. It will also help ensure clean cuts on both faces. Incidentally, hole saws work well in a pillar drill too.
By Hand with a Coping Saw
No power saws or hole saw? Break out a coping or fretsaw to cut circles the old fashioned way! These saws have a thin blade that can turn without binding. The saw teeth point down toward the handle, so cutting is done on the downward strokes. Draw your circle on a piece of wood, and clamp it to a workbench with the edge to be cut hanging over. Better yet, cut a “V”-shape out of a piece of scrap plywood and mount it to the workbench’s edge. Place the wood on the V-board, keeping the blade in the open area of the V. Hold the saw vertically with the teeth pointing away from you and saw into the edge of the wood. Use steady strokes, take your time, and cut just outside of the line you made. Rotate the workpiece as you go until the circle is completed. To refine a hand-cut circle, sand to the pencil line with sandpaper.
Making Circles with a Benchtop Sander
Have a benchtop belt or disc sander? You can turn a rough circle into a perfectly round one. Make a jig out of a flat board with a hole drilled into it. Mount the circle blank to the board via a shaft around which the blank can spin. The circle blank should hang over the edge of the board a little. The entire assembly is clamped to the sander’s table with the edge of the circle just touching the abrasive. With the sander running, rotate the circle by hand around its center shaft and into the sander. After several turns, advance the jig toward the belt and rotate the circle some more. Repeat this process until you have sanded precisely to your pencil line.
Hopefully, your toolkit will allow you to use one or more of the techniques described. Regardless of the method you use, be sure to read and follow the instructions that come with your tools. Happy cutting!
Dug’s Automata Tips, Techniques and Tricks
A quarterly column by automata-maker and enthusiast Dug North
Copyright 2010 Dug North
Warning: The topics covered in this column include the use of tools and materials that have the potential to cause damage to property and/or bodily injury. Your safety is important and it is your sole responsibility. Always read and follow the safety instructions that come with tools and materials you use. Wear safety glasses, use guards and other forms of safety equipment, follow safety precautions, and use good judgment. Seek the guidance of experienced outside sources whenever required.