Divide A Circle With A Yardstick or Meter Stick.

Circle Dividing Jig Click Photo To Enlarge	The circle dividing jig divides a circle by temporarily fastening the disc to be marked on the large rotating wheel of the jig.  A 1/16" steel cable is connected to a slide at one end, and a weight at the other end (a pound or two).  The cable fits in a groove in the edge of the wheel and rotates the wheel as the slide is moved along the yardstick.  A straight edge goes over the wheel and disc and the other end is fastened to the board to prevent moving.  The division marks are drawn along the straight edge.  The yardstick needs to be positioned at a tangent to the wheel so the cable stays parallel to the yardstick as the slider is moved.
Click To Enlarge	The axle for the wheel is a 1/4" hex bolt with the head cut off.  The threaded end goes through board and has a nut on either side to hold the axle in place.  The nut on the front of the jig is countersunk to allow the wheel to be flush against the board.  The wheel and disk are a snug fit on the un-threaded portion of the bolt.  The wheel and disk should easily be able to rotate without the cable slipping.  Attach the disc to the wheel temporarily using hot glue in a few small spots along the edge of the disk.
Click To Enlarge	I made a perfectly round plywood wheel by bandsawing the circle roughly to size, drilled the center on drill press, then sanded to size with a benchtop disk sander rotating the disk on its center hole mounted on an axle attached to the sander table.  I used a ball-bearing guided slot cutter - actually an old adjustable finger joint bit for a router to cut the groove into edge of the plywood wheel with my router table.  It is very important that the bottom of the groove is concentric with the rest, so I went around the wheel twice with the bit to insure that the pilot bearing of the bit touched the wheel all the way around.  It's probably not exactly safe to use the bit this way with the extra added washers possibly causing it to be out of balance, but I used a speed control to slow down the router motor.  I centered the groove by flipping the disk over and making another few passes with the slot cutter.  The tapered sides keep the 1/16" steel cable on track nicely.
Click To Enlarge	The slide is two pieces of wood with a rabbet to fit the thickness of the board that clamp onto the board by tightening the wing-nut.  The cable end is attached by using an eye screw.  There is not much tension on the cable, so I just crimped the loop closed with a short piece of copper tubing I found in my parts bin.  The yardstick is attached to the board parallel to the edge that the slide rides on.  The weight on the other end of the cable is attached the same way.
Click To Enlarge	The vernier scale multiplies the accuracy of the yardstick by a factor of 10 (if the marks are clear enough, and you have good vision). The marks accurately determine how far past the last 1/16" mark the left end of the scale is.  The mark on the vernier that lines up best with the marks on the yardstick tells how many tenths of a sixteenth (1/160") to add. The measurement is read starting at the left end of the scale.  In this example the measurement would be 13 5/8" + 8/160 = 13.675" or 13 + 10.8 sixteenths
Click To Enlarge	Before using the jig to draw dividing marks, you must first measure the circumference of the wheel that the cable rides in.  To do this, simply draw a mark on the wheel at the straight edge after moving the slider to the starting position - I use one inch as my starting point.  Move the slider until the wheel makes one full revolution, and the mark is back exactly where it started.  Read the measurement and subtract the one inch starting point to get the circumference.   Here is a disc I marked with the jig.  I scanned the disk and then imported it into Autocad to draw 15 perfectly spaced (red) lines on top of it.  As you can see, it is very close.  Later, this disc will become a gear.  The darker area is a film of wood glue smeared on the surface to help prevent chipping as the gear is cut.