Making Perfect Circles With a Belt Sander
4 Steps

Step 3: Get Your Final Dimension

Now it's time to sand your circle to it's final dimension.  With your measuring device, determine how far over the desired dimension your circle is.  For me, my desired dimension was 1.45" and the smallest dimension of my rough circle was 1.48".  I would need to sand .030" inches off the diameter to meet my requirements.  By sanding off .015" (half the distance) all the way around, I should end up with a diameter that's closer to my desired dimension of 1.45".

Your circle won't be exact.  Measure your it to find out in which direction the smallest dimension is.  With the smallest dimension perpendicular to the sand paper, cautiously,using small increments, sand the circle down until you create a flat spot on one side.  Keep sanding and measuring until the distance across the flat spot to the opposite side of the circle matches your desired diameter.  Be sure you don't over-do it and go beyond your final dimension.  I've marked the flat spot on the disk I made to make it easier to see.  The peg poking out of my circle is off-center and matches the hole in the handle this cap is being made for.  It has nothing to do with this Instructable.

Now set up your tooling.  Any scrap you have lying around will do.  Lay a straight edge on your sander's stage, parallel with the sand paper.  Now, place your circle on the stage between the straight edge and the sand paper with the flat spot against the sandpaper. Move the stop forward until it stops, pinching the circle in the middle.  Clamp your scrap stop to the stage.

Next, take another piece of scrap and place it to the side of the circle (I'm left handed, so mine is on the left... If you're right handed, don't complain, just reverse it.  Us left handers have had to figure out reverse directions our entire lives:).  Clamp it in place and start the belt sander.

Use the small piece of scrap to keep the circle in place horizontally on the belt.  This will insure the same section of belt is sanding on your circle.  Rotate the circle, keeping slight sideways pressure against the small piece of scrap and backward pressure against the stop until you're able to rotate the circle freely with no grabbing.

Stop the belt sander and measure your piece.  I like to sand in smaller increments and sneak up on my dimension as opposed to throwing caution to the wind and grinding it all off at once.  Using .010, .015, .020, .040 & .050 shims placed between the stop and the circle, I can work down to the dimension I want without adjusting the clamps.

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papagun says: Jan 20, 2012. 8:31 PM
It would perhaps be faster to drill a hole the size of the "axle" or "center pin" in the "back support" board being used here. Flip the workpiece over and stick the extending "center pin" into the hole. Slowly move that board (with the work or disk attached to it with the "center pin" and free to rotate) towards the running sander until the radius desired is reached and clamp the board in place. Then pivot the work to make a true circle with the "axle" in the center.

I know it is mentioned that the center of the circle is not important in this instructable. Most of the time, the center is very important, however.

I also would not put my fingers into an "enclosed space" this small with the sander I own. Too dangerous.
bfk (author) says: Jan 21, 2012. 12:50 AM
Thanks for your insightful comment. The goal of this project was to quickly make a plug that locks a handle to a winch. The pin in the plug is .093 off-center (to line up with an off-center hole in the handle, not the center of the circle). I probably should have waited until I finished the circle to drill and mount the pin to keep the Instructable less confusing. It was quicker to accurately sand the edge of the circle to the face of the pin (a measurable dimension) than locate, mark and drill a hole .093 off-center of a located and marked centerpoint (all calculated dimensions).

Measuring and tooling across the diameter can be twice as accurate as the traditional way of measuring and tooling to the radius (errors are doubled when working to the radius). Plus, no centering, math, holes, pins, or plug-filling needs to be done. Just a parallel surface, spring clamped to the table and a few shims. Very simple, very fast and very satisfying.

Keep your scrap wood stops lower than your workpiece and your fingers will always be behind your work. I grabbed what I had lying around, but I also like to live dangerously:)