You should be very aware of the weight limit when building your robot. You should try to squeeze in the most power, the thickest armor, and the biggest weapon you can, while staying below the weight limit. One way to do this is by using the highest performance materials available.
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At Season 4 of BattleBots, BioHazard weighed in at 220 pounds. Following is a list of
raw materials, and how much of each I used in the robot:
The remaining 107 pounds is made up of the following components:
"TS" is tensile strength. It is a basic measure of a materials strength. It
is listed as thousands of pounds per square inch. For example: a one inch square bar of
the highest performance alloy (AerMet) has a breaking strength of 300,000 pounds. It can
be used to lift about 80 Cadillac Eldorados! "E" is elongation. It is one
measure of a materials ability to absorb an impact and bend rather than break. The higher
the number, the less brittle it is. The best materials have high numbers for tensile
strength, AND elongation. "D" is density. It is the materials weight in pounds
per cubic inch. The price is how much lighter your wallet will be for each pound of the
material you purchase. These are prices I paid. Your mileage may vary.
Materials Facts and FAQs:
Check out the "Robot Parts" section of RobotBooks.com for a link to a great source for small quantities of metals and plastics for building robots.
Try these guys too:
The aluminum alloys that can be heat-treated are the ones that start with 20, 60, or 70. Common examples of these alloys would be 2024-very strong and resistant to cracking, 6061-general purpose, strong alloy, and 7075-very hard. The last two digits in the alloy specification tell you if and how the metal has been heat-treated. Look for 2024-T3 or T4, 6061-T6, or 7075-T6. T6 is heat treatment alone; T3 is heat treatment plus cold working.
If you weld T6 material, you will probably lose some strength in the heat-affected zone, but you could probably re-heat treat. If you weld T3 material, you could re-heat treat, but cold working would be impossible, so you will only regain part of the original strength. If you heat treat your frame after welding, It will probably warp. You should probably anneal it first, straighten it, and then heat treat it.
If you want to use aluminum, then 2024-T3 or T4 is probably your best bet, if you can find it in the shape you need. If you want to use steel for the chassis, you should probably go with 4130, 4140, 4340, or some other "chromoly" alloy. This is available in many different sizes of tubing which have good strength to weight ratio.
For high-strength steel, a tool steel known as S-7 is probably your best bet. This steel is also used in chisels, so it is shock resisting, and holds an edge well. You probably wouldn't want to use it as framing material, but for highly stressed parts and weapon blades and spikes, it works very well. You machine it in its soft state, then you have to take it to a heat-treater to harden it to about 53-57 Rockwell. AerMet 100 is the ultimate in high-strength steel, but it is not very available in anything but round bars. It is very difficult to machine, and it also requires heat treatment after machining.
Power electronics and motors can easily be damaged by heat. It is a good idea to mount these items on a material that will conduct that heat away quickly, and keep your expensive components cooler. The best material for your heat sink is pure silver. If you can't afford that, aluminum and magnesium work very well. Copper is about twice as effective as aluminum, but it is also very heavy. Silver is about 50% more effective than copper.
In BioHazard, the Vantec electronic speed controllers are bolted directly to a magnesium body panel. I made sure to use the side of the Vantec that all the transistors are on as the mounting surface. This is a pain because of all the little screw heads on that side. I drilled a little clearance hole in the body panel for each of the 48 screw heads. This keeps about 90% of the surface area for heat transfer. I also give it a thin coat of heat-sink compound to increase the rate of heat transfer. Vantec makes controllers that already have extruded heat sinks, but they just wouldn't fit into the 3" space that I had to work with.
The confusion here comes from the fact that rubber has a very unusual property. The more lightly it is loaded, the higher its apparent coefficient of friction. I used this fact to my advantage with my Sumo Robot. It has tank treads made from pure gum rubber. The rubber touching the ground makes up about half of its total footprint. I can tilt the ring up at an angle of about 80 degrees, and the robot can still cruise around it. With its internal vacuum turned on, I can tilt the ring past vertical. It's a pretty amazing sight!