Crash Course FRC – Materials

When building a robot, there are tangible objects that go into the robot, not just the sweat, tears, and colas of build seasons past.  Of course, we’re talking building supplies and where to get them.  Let’s take a look at the goods and services that teams have benefited from for years, and that your team might just learn something from.


Most folks in robotics know of metals and plastics, but there are specific materials within those categories that folks may not be aware of how to best use, let’s dive into it.

Aluminum

This metal is absolutely universal, making appearances in nearly all parts of your life.  Though best known for soda cans, it’s used in everything from laptop bottoms, to car parts, it’s strong, light, and relatively easy to manufacture.  Almost every FRC bot every made has made use of aluminum plate, sheet, tube, or hex shaft. 

Aluminum isn’t all perfect though, as overloading it, like most metals in FRC, will result in it shearing, snapping or bending.  Additionally, due to it’s strength if properly sized, you generally need some tooling to work effectively with it.  While a hand or coping saw will work, you’ll likely want to get a band saw with a metal cutting blade, or use a CNC capable of working with the material.

Best Practices

  • Keep track of the alloy, as certain ones are better than others.  (For example, 6063 is relatively easy to drill, but gums up when machined, while 6061 is relatively easy to both drill and machine).
  • Vacuum up!  Aluminum chips not being ferromagnetic makes it that much harder to get out of hair and skin if caught or embedded.
  • Oil it like a meal!  When cutting, drilling or tapping using a lubricant or tapping solution will preserve the life of your tool! 
  • Pocket here, pocket there!  Aluminum is a strong material, and removing material to make is lighter is an option.  That being said, removing too much may result in parts that break!  As such, some teams do FEA (finite element analysis) to empirically tread the line.

Steel

This strong, ubiquitous metal made possible the high-rises of New York City, but also serves as an invaluable material for certain FRC parts.  Commonly used in high-load gears and shafts, it’s a better choice than aluminum parts in certain situations.  That being said, if your team has incredible sheet metal manufacturing, you may want to go with that in areas where other teams can’t and won’t go.

https://frcdesigns.files.wordpress.com/2018/12/c6d06-28157462_1480211988771263_2413875567150497792_n.jpg?w=748&h=748
Team 5817’s 2018 Robot.  Made using primarily sheet steel. Photo Source: Michael Stokes

The most common use case for most teams using steel in modern robot design is in Versaplanetary stages.  That being said, the material at that size and high load is still prone to breaking, as transmitting several hundred watts of power through a small gear tends to not go well when it’s hit by another robot.  Though not as common, teams also use steel for high load shafts, parts designed to wear rather than break, ratchet (and prawl) mechanisms, and other specialized parts.

That being said, steel has also found it’s way into modern robots… as ballast.  In the event a team’s robot is too light, they might put steel bar or bricks in the bottom to make it not top-heavy, but this is less and less of a problem by the year.

Best Practices

  • Keep track of the alloy, as there are differences between different alloys, same as aluminum. 
  • Vacuum it up, and clean the vacuum!  Steel is ferromagnetic, meaning it can be pulled out of your finger with a magnet in a pinch, but it’ll get caught in a vacuum with a massive electromagnet (most of them).
  • Oil it like a meal!  When cutting, drilling or tapping using a lubricant or tapping solution will preserve the life of your tool! 
  • Pocket here, pocket there!  Steel is a strong material, and removing material to make is lighter is encouraged.  That being said, some teams do FEA (finite element analysis) to empirically tread the line between parts being too heavy and being too fragile.

Polycarbonate

While often confused for acrylic, the two are incredibly distinct.  Polycarbonate, colloquially known as polycarb or as a product named “Lexan”, is a flexible thermoplastic commonly used in robot mechanisms.  When you need a light, impact-survivable wall, hopper, intake material, or something entirely new, polycarbonate is there for you.

It can be bent and manufactured into various shapes, cut on entry-level CNC routers, comes in bar, tube, box tubing, and sheet form, and can be obtained from a variety of manufacturers.  Truly a ubiquitous material.

One notable example of polycarbonate use in robots is 1678’s 2014 intake arms, which were both lightweight and effective, surviving many impacts during this high-contact game.

https://www.chiefdelphi.com/uploads/default/original/3X/6/2/62fd31c0e58c608a75cb98100be5a1e4d7a1ac62.jpeg
Team 1678’s 2014 robot, “Beca”.  Photo Source: “cavalier” | Chief Delphi

I did specify the difference between lexan and acrylic, and for good reason.  While acrylic is relatively easy to laser cut, it also suffers an immense brittleness which makes it not the right choice for FRC bots.  Confusing the two could result in broken mechanisms and lots of time-consuming re-manufacturing.

Best Practices

  • Mark your polycarbonate, don’t confuse it for acrylic.
  • Be careful using loctite near it!  Additives to certain varieties of Loctite and other thread-lockers causes the polycarbonate to become brittle and shatter.
  • Don’t be afraid to use a heatgun on it!  the stuff bends, just have a level bending surface and you’ll be set.
  • Don’t bother pocketing it unless you need to.  Given the rather large difference in material density between aluminum and polycarbonate  (2.7 g/cm³ versus 1.22 g/cm3 respectively), you can often get away without pocketing it. 

Delrin / Acetal

Delrin is yet another thermoplastic used in FRC for it’s ease of manufacturing, but also sheer versatility.  It’s easy to manufacture, and comes in nearly as many forms as polycarbonate.

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Team 340’s 2017 transmission.  Photo Source: Zeb340 | Chief Delphi

Some teams use it to laser cut custom gears and pulleys for mechanisms, others for gearboxes cut on a CNC, shown above.  The use cases are limited by your creativity… and the material’s properties of course.

Best Practices

  • Don’t be afraid to use a heatgun on it!  the stuff bends, just have a level bending surface and you’ll be set.
  • Don’t bother pocketing it unless you need to.  Given the rather large difference in material density between aluminum and delrin  (2.7 g/cm³ versus 1.41 g/cm³g/cm3 respectively), you can often get away without pocketing it. 
  • If you’re laser-cutting it, ensure you’re aware of some of the gotchas.

Woods

Wood, the age old enemy of The Doctor.  But in all seriousness, this stuff is amazing.  Easy to cut, relatively inexpensive, easy to find and order in person, and above all else, well known.  People know how wood reacts, how to treat it, and how to make it look good.  All teams are required to work with wood as part of robot bumpers, so as long as the game needs them, but wood can be used in plenty of other use cases.

Team 3847’s 2018 Prototype Intake.  Photo Source: Allen Gregory | Team 3847 Blog

Wood is relatively easy to manufacture, being easy to CNC and especially quick to laser cut thin sheets of it.  Several teams build prototype mechanisms using essentially only laser-cut wood, and quite a few make it to the competition field.  Use of thin baltic birches and other common prototyping woods are ubiquitous, and Makerspaces are a prime candidate for help manufacturing woods.  Because baltic birch tends to not splinter and cuts more evenly both on laser and CNC, it’s often preferred to run-of-the-mill plywood.

Team 1899’s 2013 Robot.  Photo Source: Allen Gregory

That being said, not all woods are created equal.  Using a composite like MDF may result in a less than ideal break at the worst moment.

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Team 3647’s 2017 robot, with a broken MDF bellypan. Photo Credit: Juju | 3647

Best Practices

  • Get wood blades for cutting on hand-tools, so as not to dull your metal blades, and vice versa.
  • If laser-cutting, be sure to watch for smoke damage / catching fire.
  • Be wary of wood type and grade, as not all are equal (see above photo.)
  • Use drill sizes incrementally to avoid splitting the wood.

Printed Plastics

The modern evolution of 3D printers has made them more accessible to FRC teams than ever.  A wide varieties of plastics with acronyms so plentiful your head will spin power the printing revolution, so let’s break them down a bit more. 

ABS filament is based on oils, and is actually the same plastic many keyboard key tops are made of (commonly known as keycaps), as well as the material of LEGO bricks.  Despite it’s versatility, it behaves inconsistently, and can cause health problems if fumes are inhaled.

PLA filament is based on sugarcane, emitting a smell not unlike fresh pancakes doused in syrup (speaking from experience).  This plastic is inexpensive, safer to use, and generally “glossier” in finish. 

PLA vs a post-processed ABS print respectively.  Photo Source: AllThat3D

The one property notably absent from this discussion is strength, but I’m not qualified to attest in either direction, so I’ll let AllThat3D take over:

ABS is a strong plastic if printed at sufficient temperature. It has a decent amount of flexibility and tends to bend rather than snap under pressure. In short, ABS is impact resistance.

Ultimately, 3D printing is an incredibly useful technology for robotics teams, and using it can work to your advantage.  Team 3187 printed major parts of their robot for the 2018 season with minor issue, as well as some teams printing entire swerve drives.

Other types of printer filament have made their way onto robots recently though, with teams such as 125 and 971 making use of carbon-fiber reinforced 3d prints.

Best Practices

Best practices for 3D printing are very particular to the printer, but also generally covered in AllThat3D‘s article on PLA vs ABS, so give that a read.  Additionally, this Chief Delphi thread covers a lot of uses and examples of 3D prints on robots. 

Conclusion

This list is nowhere near exhaustive of all the things your team may or can use, but hopefully addresses some of the major benefits and drawbacks of relatively safe and common materials used in the FIRST Robotics Competition.  As the hobbyist manufacturing industry expands further, who knows what we might see make it onto robots in the future!


Thank you to the competent and supportive readers on Chief Delphi who provided feedback to make this article better in later iterations.  It’s much appreciated!

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