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Sumo rings... how to build

I've helped out occasionally at a school with LEGO robotics, and the teacher asked me today for instructions on how to make her own Sumo ring. After typing them up... I realized I don't think I'd ever put them out there "in the wild" so to speak, for others to use. So, here's a copy of the instructions for those who would like to make a reasonably cheap but professional-looking LEGO sumo ring... and if you make it out of melamine, maybe even a more interesting "test environment" as well. Share and enjoy.

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> Do you happen to have a list of supplies needed?

Nope... so let's make up a recipe and parts list on the spot :)

A lot of people make a sumo ring out of plywood (often 1/4" or more) supported off the floor with 2x4's or something similar. That's really heavy, and really needs sanding to produce a smooth surface, and can warp when wet, needs a lot of paint, etc. So here's my version.

Materials:
• A 1/4" thick, 4x4 sheet of MDF board (also called "Hardboard"). This is inexpensive, and already has smooth, finished surfaces. It's also not terribly strong and would not support itself well (a good thing). See possible alternative at the end.
• A square of 4x4, 4" thick styrofoam insulation sheeting. Very light, fairly cheap, and can be cut with a hot-wire cutter (can be picked up at a craft store for a few bucks) nice & smooth.
• Flat white & flat black spray-paint.

Tools:
• Saw to cut MDF board (jigsaw works well, but you have to keep the board supported when cutting it)
• Hot-wire cutter (to cut foam board)
• Painter's masking tape (wide, 2" wide if possible)
• String.

Procedure:

1) Stretch string from corner to corner on 4x4 MDF board, marking where it falls across the middle of the board (do that twice, once from top-left to bottom-right, once from top-right to bottom-left). Where the two diagonal lines cross is the middle of the 4x4 sheet.

2) Pin or hold one end of the string to this center point, and tightly hold or wrap a pencil in the strong 2' away. You now have a great big compass to draw a 4' diameter circle. Note: the lower stretch the string is, the better. Upholstery thread is actually really good for this if you have some (stuff is strong and nearly zero stretch). A survey tape works too (eh, I'm a caver, I actually have this stuff lying around).

3) Use the makeshift compass to mark out a really big circle, and cut it out. You might want to sand the cut edges of the board to round them just a bit and take off and rough edges.

4) Lay the 4' diameter circle in the middle of the 4x4 foam board, and trace the edges. You now have a marked circle of foam board you can cut out (using the hot-wire cutter for the final shaping) that can be slid under the ring to raise it a uniform 4", but is really really light so it's easy to transport. Also, since it is rigid and supports the entire surface of the ring, the somewhat floppy MDF board isn't a problem... in fact if it warps a little, just laying it flat usually solves the problem (the ring I have is about 6 years old, and has been sitting on edge in my basement most of that time... but is nice and flat when I lay it down).

5) Spray-paint the outer 3" or so of one side of the MDF ring flat white. Don't worry about the inner edge being irregular, just make sure the outer 2" at least is a nice uniform flat white. MDF board takes paint well, and probably a couple of coats is all you need.

6) Once the white outer edge is completely dry, mask off the outer 2" completely with painter's tape. If the tape is 2" wide, this is really easy to do with overlapping pieces. Make sure the tape seals down to the white along the inner edge.

7) With the outer 2" of already white ring completely protected, spray-paint the entire inside of the ring flat black (again, it might take a couple of coats, but MDF takes paint well). Let this dry completely.

8) Remove the masking tape. You should have a 2" white boarder around an otherwise black circle.

Notes:

• For reasons I've never ever understood, most on-line version of these instructions I've encountered suggests you paint the black first, then mask off and try to paint the edge. I've no idea why, since that means a heck of a lot more white paint to try to completely cover the black paint at the edge you are overspraying, or a rather non-uniform surface. The method above works, and takes a minimum of paint and effort... and it's a lot easier to make a uniform painters tape edge when masking the outside of a curve.

• Since both MDF/Hardboard and the foam insulation usually come in 4x8 sheets, it's actually normally better to make two of these rings at the same time - you're likely to end up with the materials for it anyway. And storing two rings really doesn't take up much more space than one (both rings together are just under 9" thick... you can tuck them behind a file cabinet or along a wall).

• I originally used spray glue to glue the MDF ring to the foam. Don't. The glue eventually fails, and if it's not glued then if & when the foam gets too abused, replacing it is just cutting another piece. The glue takes away flexibility (see below for an idea perfect for teaching settings, that requires "flipping the board"), fails eventually, and really doesn't make it any easier to set up. This also means that if you don't like your robots falling the catastrophic 4" to the floor when they are pushed off the ring... you just slide the foam out. Or, if you are a glutton for punishment, slide a couple of more pieces in there so that the loosing robot takes a 1' fall :)

• Instead of MDF, a possible alternative I've thought of but not tried is white melamine board. It's the same stuff, with a plastic coating on one side for dry erase markers, and not much different in price. If you paint the "back" side into a sumo ring, you still have the big round "front" melamine side that you can draw on with dry-erase markers. It would be good for simple line following courses, or making robots that can draw with dry-erase markers (a lot of fun, actually... one robot can draw a line, while another tries to follow it at the same time for example. The accumulation of trails starts working like an ant colony, and has some interesting "emergent" behavior aspects). My original thought was to just use the white side to start the sumo ring, in place of a painted white edge, but I would not recommend that. The black paint is unlikely to stick well to the melamine, and the friction between the smooth untreated melamine surface and the black spray-painted surface would be significantly different, which isn't something you want either... not to mention what you'd save on not using white paint you'd probably more than outspend on the multiple layers of black painted needed to "black out" all that white melamine.

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Brian Davis

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