Servo - July 2009

the big differences are.

Bottom line? With machining, etc.; a top beetle can run $1,000, while a thrifty builder can turn out a

competitive machine for under $500 total investment (not counting spare parts, chips and soda, first aid supplies, etc.). SV

Thanks to Thomas Kenney, Will Evans, and Dennis Beck, who provided information for this article. Graphics are from the websites of vendors mentioned throughout the article.

MAN UFACTUR IN G:

A Col r to Dye For

● by Pete Smith

UHMW (Ultra High Molecular Weight) polyethylene is an increasingly popular plastic for use in robotics in general, and combat robotics in particular. It is very tough and will not shatter or crack like polycarbonate does if it gets a big enough hit; it machines quite easily; and is very inexpensive. Its main drawback is a lack of rigidity which has to be taken into account when designing your chassis. It’s also only readily available in plain white or black.

Our 3 lb and 12 lb spinners make extensive use of UHMW for the chassis, armor, and roll-over bars (Figure 1). The expanse of white gets to be a little boring and since both are the prototypes for kits, it could make identification of your bot rather difficult if you draw a “clone” in a fight.

UHMW does not accept paint easily and its slippery surface makes even adding labels and stickers unreliable. It can, however, be colored using dyes.

The dye I use is RIT Navy Blue as seen in Figure 2. (I have also tried the golden yellow but with less success.) RIT dyes are readily available in any crafts store.

First, you need to choose a big enough dish or baking tray that will allow all the parts to float, without over-filling it. I use a sturdy turkey roasting pan (do NOT use one of the flimsy aluminum kinds often available around Thanksgiving), but an oven-proof glass one is fine if you only have smaller parts. (Note:

FIGURE 1. Plain UHMW.

FIGURE 2. RIT dye.

check with your better half before using anything that might get ruined.)

Fill the tray with just enough water to float all the parts (Figure 3), add all the dye from the packet, and add a tablespoon of salt and stir well.

WARNING: The dye will stain everything it touches, so be very careful not to spill it on the floor, worktop, or even on your hands.

Pre-heat the oven to 250°F (120°C), cover the tray with aluminum foil, and “bake” for

2-1/2 hours, turning the parts over every 45 minutes or so. Let the oven cool down with the parts still in it for about eight hours (I let it sit overnight).

The parts will now be dyed very close to the actual color of dye used (Figure 4). The dye is still very potent so be very careful disposing of it. Rinse everything with lots of clear water and dry with disposable paper towels.