BattleBots & Choosing the Right Materials

What Makes Material Selection So Important?

Creating a robot capable of handling the immense loads robots face in the BattleBox makes materials selection one of the most important aspects to designing a BattleBot. But selecting materials isn’t easy: no single material can solve every design problem. Many builders face compromises with many of their material choices due to the constraints of the build. Factors like material cost, weight, long-term durability, and ease of repair all can tie into material selection for BattleBots.

Choosing materials have different implications throughout many different stages of a robot’s development, affecting the build process, in-fight performance, and long-term performance over the season. For example, a medium-carbon steel strikes a good balance of toughness and strength that makes it perfect for armor. However, due to the 250lb weight limit, heavy steel cannot be used to protect every portion of the robot. If you focus on armoring up, then you won’t have enough weight for the rest of your robot, like the weapon or drivetrain. If you instead choose a stiff but lightweight material for armor such as titanium, it will come at a high price. Some teams use large blocks of UHMW as armor, which prioritizes toughness over strength and stiffness to slow opponent weapons down and “gum them up” when it gets hit.

On the weapon side, the ideal kinetic weapon has a high hardness so it can effectively bite into the armor of other robots. But the hardest materials are often very brittle and will shatter on impact. So, finding a material (and a temper) that balances hardness and toughness may make a weapon less effective, but also less likely to break over multiple hits, and even multiple fights over the course of the season.

Each material choice in a robot’s design faces a similar compromise of multiple factors, not just when it comes to material properties in-battle. These can include cost, machinability (how much time and effort will it take to work with?), repairability (is it easy to weld together for a quick fix on-site?), and even accessibility (how quickly can we source and acquire the material?).

Every builder faces these decisions when designing a BattleBot and must weigh the constraints unique to their design and budget. Online Metals provides access to a wide range of certified materials that can be cut and shipped the next day, making sourcing materials a simple and efficient process during the build season.

Common Materials for Robot Combat

Scrutinizing the material choice for each element in a fighting robot means you can select the material that best matches the design’s performance requirements. Below are some of the most common materials used in robot fighting, and where they can make the most impact:

Aluminum

Aluminum is one of the most used materials in the industry, from consumer products to airplanes and automation equipment. Known for being machinable and lightweight, it is commonly used for internal structural components such as frame elements and gearbox housings in combat robots. Since aluminum is a softer metal, it is typically not used as armor unless it is thick enough that another robot’s weapon cannot cut all the way through.

The most used alloys are 2024, 5052, 6061, & 7075

Titanium

Titanium 6Al-4V Grade 5 is less than half the weight of aluminum but provides three times the strength. Since Titanium is so light compared to steel, it can be used as armor if you are nearing your weight limit. However, since it is more brittle than steel it tends to fracture instead of bend, which makes it less useful against kinetic weapons. Valkyrie utilizes titanium as its armor in areas we know we are less likely to get hit.

4130 Steel

4130 steels, also known as chromoly, are weldable, easy to machine, and due to its high strength, it is commonly used in automotive and aerospace industries. Since 4130 can be heat-treated to a wide range of different tempers, its properties can be modified for a wide range of applications after it is machined. In robot combat, it is commonly used for shafts and axles, where high precision and high strength are critical.

AR500 Steel

AR500 steel is an incredibly durable material used in snowplows, tanks and rock crushing machines. It is roughly six to seven times stronger and harder than mild steel but has the same density. Combat robots tend to utilize AR500 as their choice for weapon material or armor because it strikes the perfect balance of toughness and hardness.

S7 Tool Steel

S7 Tool Steel is typically used in a variety of machine tooling, and it can be oil-hardened to increase its hardness and toughness. In robot combat, hardened S7 is sometimes used for smaller weapon teeth or tooth inserts because it can punch through materials such as AR500, but because it becomes more brittle the harder it is, it can snap easily if not heat-treated and quenched evenly.

UHMW

Ultra-high molecular weight polyethylene (UHMW) is a tough, abrasion resistant material that has recently become more popular in robot combat. Because of its properties, it can be used as a “buffer panel” for armor, slowing down and “gumming up” opponent weapons so that they can’t reach the core part of a robot’s frame. Its low-friction, impact-resistant nature also makes UHMW useful in other ways for robotics, since it can be used as a skid-surface for bushings, balancers, or skid-plates.

How Team Valkyrie Evaluates Materials

Over the past four seasons competing with Valkyrie, we have gone through multiple iterations of almost every single part of the robot. This iterative design process has shifted our material priorities and at times pushed materials to their limits. By taking Valkyrie from concept to tournament every single season and getting a hands-on understanding of our failures and how we might overcome them, Valkyrie has evolved to have a more robust design year after year.

Throughout our many seasons of competing, we have stuck with 4 core design tenets. Firstly, we want to make Valkyrie adaptable to any opponent. Second, Valkyrie should be easy to repair or improve so we can fix the robot quickly during competition. Third, we heavily focus on overall system reliability to endure in any match. And lastly, we want to put on entertaining fights!

These design goals heavily influence our design and our material selection. Valkyrie is unique in that we have configurable armor for each match. It allows us to shift the weight and strength of certain panels around the robot depending upon who we fight. For example, if we are fighting a hammer robot, we can swap our top panel out for a thicker, shock-absorbing top plate and use lighter material around the sides. If we fight a horizontal spinner, we might use thicker side armor with lighter armor on top. Leveraging different combinations of steel, aluminum, and titanium, we optimize our armor configuration for our opponents while keeping it below the weight limit. This allows us to think critically about which materials will perform best against which weapon types. If you watch our recent fights against vertical spinners, we add thick UHMW “cheeks” to the sides of the robot to keep spinners from biting into our core frame. Our modularity in this regard also allows us to constantly test different materials and designs to see how they perform.

Designing for MRO (maintenance, repair, and operations) is another huge part of how we evaluate designs and materials. We were inspired to create a durable, competitive machine that could be disassembled and reassembled like a car in a NASCAR pit. When it comes to material selection, we heavily consider how different materials can be reworked in the case that we need to modify something during the event. For example, we use 6061 Aluminum for our frame and baseplate. Although 7075 Aluminum is stronger, it is more difficult to machine, it cannot be welded well, and it is more likely to fracture rather than deform when loaded. This means that although our 6061 Aluminum components might end up slightly worse for wear after a fight, we can easily get them back into shape and ready for whatever comes next.

Lastly, we focus heavily on improving the system reliability of the machine to decrease maintenance overall. Since so much of designing a BattleBot revolves around dissipating the shock of large hits so that they don’t impact the robot’s functionality, we pay careful attention to how the internals of the robot are designed to ensure Valkyrie will hold up not just in one 3-minute fight but lasting through however many rounds we need to in order to get to the championship. This involves selecting materials that strike the right balance of strength and toughness: each component should be able to take a beating and continue to function, even if it comes out a little deformed on the other side. Critical components like our drive shafts are made from 4130 steel and heat-treated to strike this balance, meaning that Valkyrie can be dropped from 20 feet in the air again and again and still keep on driving. Materials such as UHMW we use for shock-absorbers, skid-pads, and internal support elements because it dissipates much of the energy we see in battle and prevents the heaviest shock loads from going directly into the motors or electronics. Each of the materials we select plays a critical role in the design of Valkyrie, as we strive to increase its durability and make it easier to operate and maintain year after year.

Combat Robotics vs General Robotics

One of the key insights many BattleBots competitors have after building a combat robot is that they have improved as an engineer or fabricator by building and competing on the show. BattleBots provides an environment that demands higher durability than most real-world applications. Consumer products more often utilize aluminum, mild steel, a variety of polymers like ABS and polyethylene, and silicone. Additionally, many of the electrical connector’s teams select for use are used in go karts, drones, forklifts, and industrial robots which have similar power and maintenance demands as a BattleBot.

“As a professional design engineer who has worked at multiple robotics companies, many of the skills picked up in BattleBots I have been able to apply to my career, especially when it comes to materials selection.” – Leanne Cushing, Mechanical Design Engineer on Team Valkyrie

Assessing the environment and performance requirements of a robot in operation is the first step toward selecting materials. For example, a food environment will require food safe handling materials like stainless steel and HDPE whereas an outdoor environment may need to focus more on dust control like selecting polymer bearings to avoid binding and powder coated steel to avoid corrosion. The robot’s responsibilities and weight will help designate the appropriate materials needed for cycle life and strength required to repeat its given task.

While the design and safety constraints we work with in BattleBots come from its rulebook, industry requirements stem from a robot’s applications and regulations. Any in-production robot or other electro-mechanical assembly must follow an array of certifications based on its environment (a couple examples: FDA for medical, UL for automation, CE mark for consumer products), but those constraints aren’t required for in-house prototypes. Other regulations such as power, weight, factors of safety, and machine speed are also scrutinized, especially if these robots are operating around humans. These all affect a robot’s overall power consumption, its speed to cycle a task, and the overall system cost.

Selecting materials that comply with these standards in production and meet the requirements of the application is a key part of the design process. Just like in BattleBots, it all comes down to weighing the different factors at play in the design. Whether you’re designing a robot, an automobile, or a piece of consumer electronics, think critically about what the part will experience. What will the part need to hold up to? Is it expecting heavy shock loads, or will it experience constant forces over long periods of time? Where will the forces be coming from, and how will they distribute? Will the part need to operate in different climates, or be exposed to different chemicals or solvents? At Team Valkyrie, we ask ourselves questions like these to guide us down the path to the right material. Questioning the environment and loading conditions of the parts of your next design will help lead you to the right materials and successful implementation.