Purchasing Materials for FTC Teams
(Recommendations by Dan Niemitalo, Coach, FTC team #4150)
Basic Kit of Parts
Every team gets the Lego Mindstorms kit and the Tetrix Base kit through a Rockwell Collins grant. That includes the basic things that your team must have to build a complete robot: Lego NXT brick, battery, motors, motor controllers, wheels, gears, hubs, axles, structural pieces, wires, etc. See the contents of the basic Tetrix kit here. A “Samantha” wireless communication module is also provided to each team in addition to the Lego and Tetrix kits.
It is possible to build a legal, functioning robot while adding practically no parts beyond the basic kit of parts. However, if you want a competitive robot you should consider adding some carefully chosen extra parts. This document discusses some of your options and will hopefully help your team decide where to put some of its resources.
Limitations of the Base Kit
Here are some reasons why it would be difficult to compete in the top, say, 25% of the field with only the basic kit.
Tetrix Resource Kit
Each team gets one Tetrix Resource Kit along with their kit of parts through the Rockwell Collins new team grants. Additional resource kits can also be purchased in the FTC store for $200; the kit provides nearly double that amount in value, according to the FTC store prices.
Note: the value of the resource kits diminishes as you accumulate more parts, because it is not especially useful to have too many extras of certain parts. If you want a second robot for practice and/or software testing, on the other hand, you might get more value from these additional parts.
The resource kit is missing certain useful parts (especially motor controllers and sprockets & chain), so do not assume that multiple resource kits will provide everything you need.
Tetrix DC Motors
After you get one (or two) resource kits, you’ll want to increase the possible number of DC motors that you can put on your robot. This is expensive, but it opens up many possible robot manipulators. It also makes it possible to power your robot’s drive with four motors instead of two, which can be a big advantage in certain FTC games.
Since you already have a resource kit, your next purchases should be one DC Motor Controller ($56) and two Tetrix DC motor wires ($1). At that point you could put four DC motors on your robot.
If you want to further upgrade to 6 or 8 DC motors, then the most economical way is to purchase one or two “Tetrix DC Motor Packs,” ($105) which provides all of the following at a discount of about 20% as compared to buying the parts separately: two Tetrix DC motors, one DC motor controller (each controller runs two motors), two wires, two mounts, and two hubs.
Note: Have at least one or two spare DC motors! They can burn out pretty easily if you try to run them for a few seconds while they’re stalled (smells like burning plastic). It’s nice to have spares of everything, but the DC motors are more prone to failure than most other FTC parts.
Note: Normal motor wires are cheap ($1), but you can also get fused protected wires ($10) with thermal breakers to protect your motors from smoking. Those breakers automatically reset, so the wire still functions after it trips to protect the motor. Not only is the wire cheaper than replacing a motor, but avoiding a motor replacement at a tournament can prevent some stress and loss of robot function.
The initial kit provides enough materials to equip a robot with two servo motors. That is enough for most robots.
Even so, it’s not such a bad idea to increase that number to four, because it is not a very expensive upgrade; it costs $17 per servo and $7 for a two pack of single servo brackets. Servo extension cables ($1) are very handy - definitely get a few if you’re using servos. No additional pricey servo controllers are required, because the controller in your kit can run up to six servo motors. If you buy two additional servos, you will probably have all you need even if you design a fairly complex robot. More importantly, you’ll have spares if you build a robot that uses two servos.
An FTC robot needs two types of batteries: an NXT battery (or 6 AA batteries) for the Lego NXT brick, and a Tetrix 12V battery to run everything else.
NXT Batteries: You get one of the $50 rechargeable batteries in the kit of parts plus a charger. You want a spare NXT battery for competitions, but I suggest getting six AA batteries as your spare since they are much cheaper. But if you like the convenience of a single battery, buy as you like. Don’t throw away the battery cover for the AA batteries, even though it isn’t required with the NXT battery.
AA Batteries: AA batteries are inexpensive (48 for $20) compared to NXT batteries ($50 + $30 for charger). Rechargeable AA’s are only 1.2V, which can cause connection issues with the field control system.
Tetrix Batteries: You really want to have a spare Tetrix battery ($35). At competitions, you can drain one in a pretty short time if you do a lot of testing between matches. You don’t want to delay your testing while you wait for a battery to charge, and you really don’t want to run low on battery power during a match. Note that if you make it into the elimination rounds, you might play up to six matches in a relatively short time. Also note that DC motors are more likely to fail as battery voltage drops.
Fuses for Tetrix Batteries: Buy spares 20 amp mini blade fuses for your Tetrix batteries. Yes, it is possible to pull enough current to blow that fuse if you stall some Tetrix motors. You can find the fuses at auto parts stores, and here is an online source ($3). If you blow the fuse at a tournament, you’ll be glad you have a spare.
Sprockets & Chain (highly recommended)
Gears are included in the basic kit of parts, but sprockets are not. Sprockets have a key advantage over gears: they can transmit power over a longer distance of your choice, whereas a pair of gears must be an exact distance apart. Sprockets can be useful in many types of situations, including the following:
The Tetrix motors are able to rotate in their mounting brackets, and that provides a really convenient way of tensioning your chains properly. Normally chain tensioning is more of a hassle than that.
Every team should get some sprockets and chain. The “Tetrix Sprockets and Chain” pack for $50 is a pretty good place to start (about $10 less than purchasing separately). If you buy the pack, consider adding a pair of small sprockets to that set ($12), because then you’d have four sets of small-to-large sprocket drives in two different ratios. Usually the driving sprocket is a small sprocket.
If you want to use sprockets to make a 4 wheel drive robot, you will need some additional small sprockets. That can be done in several ways, requiring between four and eight sprockets.
#25 chain can be purchased outside the FTC store if desired. Not all chain is equally reliable, so it can be useful to experiment with different brands.
In addition to regular connecting links (which come with the chain on the FTC store), half links are legal. Half links are used to properly tension a chain in cases when it is too loose, but not loose enough to shorten it by a link. That comes up reasonably often, but keep in mind that in many cases it can be avoided by rotating a Tetrix motor. As with normal connecting links, half links are a weak link in a chain. Regardless, it is nice to have some ($1.50 each). They are available from www.andymark.com, part AM-0682. AndyMark also sells regular connecting links and a few other, mostly field related, FTC items.
You will need a chain tool to use sprockets and chain. At a minimum, you need a way to break the chain into smaller lengths so you can make a loop using a connecting link (a.k.a. master link). Chain breakers work by pushing a pin out of the roller chain. You can buy the $11 chain breaker tool from the FTC store (somewhat flimsy), but you are better off ordering a “Dark Soul Chain Tool” for ~$30. Here are two places to find that tool. The Dark Soul chain tool allows you to not only break the chain, but also connect pieces together without using connecting links. That has some big advantages: 1) eliminates the weak link in the chain, 2) allows you to make chains even if you don’t have any connecting links left, 3) allows you to connect smaller pieces of chain together to form a longer piece. It is pretty common for people to break a chain one link too short, and that is frustrating and wasteful if you don’t have the connecting tool.
Tread & Conveyor Belts
You can buy a tank tread kit at the FTC store. Some teams have had success using the tread kits as both drive treads and conveyor belting. It costs $70 to get the parts for it at the Tetrix store. Add the paddle pieces to make a conveyor, and add extra tread links and rubber inserts if you are planning giant sized tank treads for your drive. Note that in some cases a 4 wheel drive is a better solution than adding treads, but of course it depends on the game and your strategy. Admittedly, my opinion is that a tank tread drive is inferior to a good 4WD or 6WD drive train for several reasons (cost, performance, reliability risk).
Fasteners (screws / bolts)
Tetrix screws are #6-32. That is, #6 diameter (0.138”) with 32 threads per inch. For #6 screws, 32 threads per inch is “coarse thread.” Do not mix coarse thread and fine thread screws/nuts, because they will not fit together. Most Tetrix screws are socket head cap screws (SHCS), which means they have a cylindrical cap with a hex hole, requiring an Allen (hex) wrench to tighten them. While you can easily find 6-32 screws with phillips or slotted heads in local stores, it is not as easy to find socket head 6-32 screws (Fastenal and Grainger have them). The kit also includes some button head cap screws (BHCS), which have a smaller, curved cap with a smaller hex hole. They strip out easier, but they also protrude less, so they are handy when you don’t want the screw to snag on anything.
The nuts included in the Tetrix kit are Kep nuts, also known as K-nuts. They have an attached, free spinning tooth washer that helps to lock the nut in place when it is tightened. They are a good choice for FTC robots.
The kit comes with five different types of #6-32 screws:
5/16” SHCS (100)
3/8" BHCS (50)
1/2" SHCS (100, plus 100 in resource kit)
1.25” SHCS (a few, for motor mounts and gear spacers)
1.5” SHCS (a few, for motor mounts)
Tetrix also uses aluminum hex standoffs (¼” hex, 6-32 threads, female to female). It would be a good idea to purchase some extra standoffs, possibly including a variety of lengths. They have a some interesting uses.
Buying Additional Fasteners
Miscellaneous Useful Parts in Tetrix Store
You will discover what parts are most useful for your team soon enough, but here are a few other items from the Tetrix store to keep in mind.
Sensors are especially useful during the autonomous period - the brief period at the start of a match when the robot can use only its pre-programmed commands (no driver input). Sensors provide information about the environment that can allow the robot to reliably accomplish its autonomous strategy. Sensors can also be used to help automate or speed up certain functions while drivers control the robots.
The basic kit includes three types of sensors (touch, light, and ultrasonic), and they can all be useful in the right situations. A variety of other sensors are available from www.hitechnic.com.
Some Optional Sensors:
Each of these sensors costs $55 except the magnet sensors, which are $16 in FTC store.
Most teams soon learn that Lego parts are not sturdy enough to withstand the abuse they will experience in FTC games. However, it is interesting to note that practically any Lego part is legal in FTC, including the myriad of parts not included in a Mindstorms kit. That includes Lego pneumatic parts and some different Lego motors. There are probably a few hidden gems in the Lego catalogue. If your team likes the standard Mindstorms parts and wants more of the pieces, there is a Mindstorms Resource Kit.
Non-Tetrix / Non-Lego Materials
In addition to Lego and Tetrix parts, FTC robots can also use a variety of other materials, such as plastic and metal sheets. The list of additional components allowed is in the game manual for the most recent FTC game. Browse to this page by going to www.usfirst.org, click FTC at the top, click “Game and Season Info” at the side. Find the list in the “Robot Rules” section. New in 2012-2013: the raw materials rules have been greatly relaxed; teams can now use any widely available raw material.
Using the additional materials to make custom parts has two key advantages: the raw materials are typically cheaper than buying Tetrix parts (provided you look for a decent price on the materials) and you can shape them any way you want. In turn, most custom parts require additional time, tools, and expertise to fabricate.
Some key additional materials allowed on FTC robots
In the kit, you get one set of Allen L-keys and a small screwdriver. Every team on any budget will need some other basic tools. Here are the absolute essentials:
More Essential Tools
If you have the resources to acquire or borrow additional tools, here are some more tools that you will want access to.
If you have access to a metal shop, you will already have access to these tools. If not, and you have a space to put some larger tools, here are a few that would be really useful. In particular, these tools are great for making custom parts out of sheet materials.
Your team will eventually need to decide whether it will disassemble this season’s robot or keep it for future use. If you keep it together, you can use it for demonstrations and for scrimmaging, possibly with a future robot from your team. If you disassemble it, you will already have many of the parts to build next year’s robot. Many teams disassemble their old robots.
A related question is whether your team should ever build a second robot for other purposes. Having a basic driving robot with a couple of sensors for your software team to program and test is very useful. It is also nice to have a second robot for driving practice to make the practice more like a real FTC game.
It costs money to put together a second robot, obviously. Probably the best way to do it is to purchase a second complete Mindstorms + Tetrix + kit in your second year as an FTC team ($650). In the past, the kit of parts has been discounted by around $200 when purchased as your FTC kit of parts as compared to buying same kit through the Lego Education store. You can only purchase one kit of parts per year at this price. If you buy a second kit, also purchase a second Samantha wireless module ($50) at the same time (can only buy one per year). That will get you a second drivable robot plus many useful spare parts; you may also want another resource kit ($200), depending on your plans for the second robot. For reference, it costs $450-$500 to buy the parts for a very simple “Ranger” robot, which is probably the simplest possible drivable robot. If you eventually want the ability to build a second robot, it is well worth it to spend the extra $200 it takes to buy a full kit of parts instead of the bare essentials for a simple driving robot.
Starting in the 2013-2014 season, FTC teams in North America will have access to the Matrix Robotics building system. I recommend that new teams in North America stick with Tetrix since availability of local help resources will be better for Tetrix. Matrix uses a different set of electronics (motors, motor controllers, batteries, servos). If you are confident enough to use the system despite the lower availability of expert help, here are some potential advantages:
Linn-Mar High School
FTC Team 4150
FTC Team 4324
FRC Team 967