Mini-Sumo Robot Ring Disaster

Warning: It turned out this was a tutorial on how not to make a Robot Sumo ring. See the successful instructions if you’re planning on making your own ring.

A miniature sumo robot differs from other sizes of Sumo robots in depth, width, and weight. Some competitions even reduce the maximum allowable voltage or motor torque.

Class Height Width Depth Weight Ring Diameter Ring Height
Japanese unlimited 20 cm 20 cm 3000 grams 154 cm 5 cm
Illinois unlimited 20 cm 20 cm 1500 grams 154 cm 5 cm
Mini unlimited 10 cm 10 cm 500 grams 77 cm 2.5 cm

I’m going to concentrate on building a mini-Sumo robot ring for two reasons:

  1. Its smaller size is less wieldy for an inexperienced carpenter
  2. It can fit into my Green Turbo Beetle

All the necessary materials can be obtained from a hardware store.

I chose 4-foot by 4-foot 1/8-inch solid pressed board.

Except for the lack of holes, solid pressed board is just like peg board, which is often used to hang tools above a workbench. It’s lightweight, very easy to work, inexpensive, and has a smooth side. Of course, it isn’t as durable as natural woods, linoleum, or ceramics.

To make the 77-centimeter circle, apply a method you should have learned in second-grade art class.

Matching pin and pencil set

Matching pin and pencil set

Tie one end of a wire to a tack. Ordinary string can be substituted for wire, but it stretches under tension, changing length. A push pin can be substituted for a tack, but its pin is a little too long and won’t hold firmly to the board. These were two of my mistakes. Actually, a rod or board should be using instead of anything flexible.

Measure 38.5 centimeters of the wire (using a ruler or tape measure) and mark the end, but don’t cut it. Tie the end of the wire around a pencil. Because you’ve only marked 38.5 centimeters rather than cutting, you’ve got some slack to loosen the knot and adjust the length after double-checking. You should now have a wire connecting a tack on one end to a pencil on the other end, 38.5 centimeters apart.

Push the tack into the center of the rough side of the board. Stretching the wire taunt, draw a circle around the tack. It should be very easy to draw a nice, round circle, exactly 38.5 centimeters in radius or 77 centimeters in diameter.

Imperfect circle drawn on board

Slightly imperfect circle drawn on board.

Because the circle is drawn in pencil on the rough side of the board, you can trace over it again and again to fix bumps or mistakes. Measure across the circle in multiple directions to verify the diameter.

After becoming confident in the circle, an ink marker can be used to enhance the contrast of the pencil circle, to aid in cutting.



To cut the board, I used a jigsaw.

Vermont American 30015 thin wood jigsaw blade

Vermont American 30015 thin wood jigsaw blade.

Use a scroll, soft wood blade. I used Vermont American 30015.

Cut the board with the circle facing you (rough side up). This makes a lot of sense, since otherwise you couldn’t see the circle you just drew. But, it also causes the smooth side of the board to cut cleanly, whereas the jigsaw frays the rough side’s edges.

Pressed board cut into the shape of the ring

Pressed board cut into the shape of the ring

Time to paint!

Actually, I should have sanded the edges smooth, first.

Place the ring-to-be somewhere where spray paint mist isn’t going to ruin something of value or choke the lungs of a loved one. I chose my asphalt driveway, which needs black topping anyway. I made sure our cars were pulled far away.

White and black spray paint

White and black spray paint

For a smooth, even finish, I’m painting with Rust-Oleum enamel spray paint. Gloss white (high-reflectivity) for the ring border and flat black (low reflectivity) for the ring itself. I followed the directions on the can.

Mishapen sumo circle painted black

Mishapen sumo circle painted black.

The spray paint is wonderfully even. Definitely flat reflectivity. A small bug and some hairs landed on it, but not bad!

Daylight inspection reveals a lot of grit landed in the paint due to being outside. Perhaps I should have painted in the garage?

Sand the edges first with rough-grit sandpaper to smooth the overall circle. Then sand with fine-grit sandpaper to smooth the edge itself. The pressed board sands easily and cleanly. Don’t forget to use a damp cloth to clean the dust off the board or else the paint won’t stick.

Paper mask for painting white border

Paper mask for painting white border

A 72 cm circle mask was created from newspaper using the same second-grade method. When centered on the board, this mask should leave a 2.5 cm border visible for spraying white. Unfortunately, variances in the pressed board circle and the paper circle cause the border to vary from 2.25 cm to 3.5 cm!

Semi-cicle mask for decreasing variances when painting the white border

Semi-cicle mask for decreasing variances when painting the white border

In Visio, a drawing application, I created a 2.5-cm thick, 77-cm circle segment. I then converted the file to Adobe Reader PDF format:

Arc PDF icon

After printing and cutting away the black area, it was reinforced with tape and heavy paper. This mask will be moved along the edge of the pressed board to decrease the variances while spray-painting the border.

White border painted with mask White border painted

White border painted with mask in place and after removing the mask.

The white border turned out poorly.

Horrible misty spary paint job for mini sumo circle boarder

Horrible misty spary paint job for mini sumo circle boarder.

Too much mist from the spray paint got under the newspaper mask. If I had to do it over again, I’d carefully mask the circle using masking tape, and then paint it with a brush.

Shikiri lines masked with tape Shikiri lines masked with tape and then covered with a paper mask

Shikiri lines masked with tape (left) and then covered with a paper mask (right)

In Visio, I created two 1-cm thick, 10-cm long lines separated by 10 cm, which I then converted to Adobe Reader PDF:

Shikiri PDF icon

This time, to avoid the white border disaster, I used the mask to stencil the outline of the lines in pencil directly on the board. Masking tape now surrounds the stenciled lines underneath the paper mask.

After spraying with some off-brand of brown paint found in the basement, I realized the paper mask didn’t cover enough of the board (see photo on right). Some tiny droplets of misted brown paint slightly contaminated the black portion of the board. The addition of newspaper covering the rest of the board avoided further contamination.

Shikiri line showing leakage

Shikiri line showing leakage

Dang! Remember to use fresh masking tape and rub all edges of the tape firmly against the surface or else the paint leaks under.

Ready to host a battle?

Ready to host a battle?

The mini-Sumo robot ring is done. And, well, it doesn’t look pretty. Click on the above image to see how truly awful it turned out.

To do...


Not every project turns out well. This is clearly a disaster.

I learned a lot from this experience. In fact, now let’s see how some truly beautiful mini sumo boards can be made...