Choosing the Right Materials
This tutorial goes over the different materials you should use when building your bot. Modified from the work of previous BattleBots leads and aspects of the BattleBots Bible (RioBotz Combot Tutorial 2.0).
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Step 1: Getting Started
Choosing the right materials is a crucial element in creating your robot. It's essential to have a basic understanding of the properties, usages, and manufacturing methods associated with these materials. In this tutorial, we’ll be talking about different types of materials you might use for your robot.
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Step 2: Aluminum
Aluminum is probably going to be the most commonly used metal in your bots. It can be used in almost anywhere in your robot.
Aluminum can be used anywhere from walls to electronic mounts. A good standard for thickness, in terms of walls, would be to keep them at at least a quarter of an inch.
We generally work with a few different alloys in BattleBots. These include 6061, 2024, and 7075:
- 6061: most common structural aluminum alloy; softer than 2024 and 7075; more inexpensive; weldable
- 2024 and 7075: useful for structures that demand high strength-to-weight ratio; harder and stronger than 6061 (and 7075 is stronger than 2024); more expensive; not weldable
In terms of manufacturing, aluminum is usually highly machinable and easy to make things out of when it comes to using a mill, lathe, or waterjet.
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Step 3: Steel
Steel is another metal that is also used in BattleBots. When compared to aluminum, steel is a lot harder, but it's not used everywhere because it weighs a lot more.
Steel is used for specific parts that we don't want to break. This includes weapon blades or inserts, and various shafts that tend to take large loads on impact. It's a good idea to use steel sparingly for key parts.
Steel alloys commonly used in BattleBots include 1018, 4130, and AerMet 100. Tool steel (S7) is also used in combat bots:
- 1018: low strength; easily machined and welded; least expensive; easily bendable, so avoid using in spinning weapon components that need to be well-balanced
- 4130: low strength (but a bit stronger than 1018); easily weldable and machineable
- AerMet 100: special class of high strength steels with high nickel and cobalt content; probably the best metal when it comes to strength and fracture toughness; extremely expensive
- tool steel: commonly used for weapon inserts; extremely hard, but also very brittle (low impact toughness)
Just like aluminum, steel needs to be machined on a mill, lathe, or waterjet. Unlike aluminum, however, steel has various ranges of machinability. Some steels are more machinable than others and it’s important to know which ones you are using when you are actually working with them, in case you break a tool. If you aren’t certain if it is machineable with the tool you're using, ask someone that knows!
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Step 4: Plastics (3D Printing)
A wide variety of plastics can be used for 3D printing. The most common processes for printing plastics are SLA and FDM.
SLA prints with photosensitive resin, this resin is generally less durable than the plastics used with FDM printers, but can have greater detail.
For FDM printers, the most common filaments are either ABS or PLA. PLA has a higher tensile strength but is more brittle than ABS. This means that PLA will be more likely to shatter when subjected to an impact, while ABS will bend. We'll most likely use ABS for all of your 3D-printed parts.
Other materials can be added to filament to enhance its properties. These include, but are not limited to, fiberglass, wood, and carbon fiber.
All three materials are available at UCLA, however ABS and PLA are much easier to find.
Keep in mind that the biggest weakness in 3D printing - especially with FDM - is the joints between the layers. The best way to combat this weakness is to design the geometry such that the forces on the layer joints are minimized (see figure below).
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Step 5: Plastics (Machineable)
There are also a lot of machinable plastics that can be used in a robot. Keep in mind that plastics are weaker, weigh less, and will deform more before breaking than metals.
Some plastics have other useful properties, such as being low friction and ease of bonding.
A common plastic used in combat robotics is polyoxymethylene (Delrin). It is very durable and very smooth, but is hard to bond to.
Another common plastic is polycarbonate (Lexan), which is pretty durable and a good alternative for sheets of metal in a top/bottom plate of a robot.
Finally, HDPE is also popular due to its durability and impact resistance.
Avoid using acrylic; it has the same density as polycarbonate, but 20-35 times less impact toughness, so opt for polycarbonate instead.
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Step 6: Other Materials
For this project, you'll mainly use a combination of aluminum, steel, and plastics in your bot. However, there are a few other materials commonly used in combat robotics that are worth mentioning:
Titanium is lighter than aluminum and stronger than (some) steel, but not as hard: the stuff of dreams. One of the only drawbacks of titanium is its enormous financial cost; it is also very difficult to weld. We would all love to build titanium robots, but due to the costs, it can only be used very sparingly, and most often, not at all.
Copper alloys are much heavier than steel and have much lower strength, so using them for the weapon/structure of your bot is a poor choice. However, alloys like bronze can be used effectively for other parts, like bushings or threaded inserts. (We would recommend using some if you're going to use screws to attach anything, like armored plates, to your 3D-printed chassis.)
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Step 7: Next Steps
Now that you have a better idea of what materials to use for your bot, start applying these changes to your CAD model/purchase list!
If you haven't made changes to your CAD model yet, be sure to change the Material Properties of all of your parts to the correct materials. Then, ensure your bot's center of mass isn't too far out of place, and double check the weight of your bot to make sure it adheres to the 3-lb. limit.
When it comes to purchasing metals, we'd recommend looking at Online Metals, Midwest Steel Supply, and McMaster Carr, although some materials can only be purchased from specific vendors. When adding materials to your purchase list, be sure to specify the alloy needed and its dimensions.
