Sourcing a Build That Has No Parts List

"A problem well stated is a problem half solved." — Charles Kettering

You find a robot design you like online. The description says something like this: "In addition to the printed parts, you will need the following components: 5010 brushless motor, brushless ESC of at least 30 amps, 38mm diameter wheels that are approximately 10mm wide, 2 N20 DC gearmotors, receiver, 2s or 3s Lipo, power switch of some kind."

If some of that reads like code, here's the translation. An ESC is the speed controller, the little board that tells a motor how fast to go. The receiver is the radio that picks up your controller. A LiPo is the rechargeable battery, and "2s or 3s" is how many cells it has. The 5010 and the N20 are motor sizes, which we'll get to.

And that's the whole parts list. Seven parts, no part numbers, no links, and one of them describes a wheel you can't actually buy.

This is completely normal. Most published robot designs give you the shape of the robot and very little about what goes inside it, and that gap is where a lot of first builds stall out. It's rarely the printing or the soldering that stops people. It's the weeks of reading forum threads trying to figure out what "approximately 10mm wide" means while you're sitting at a checkout page. You can skip most of those weeks, though, and it starts with a file most people never open.

Start with the CAD file

Almost every published design comes with a CAD file, usually a .step or .3mf, alongside the files you print. CAD is the 3D model the designer actually drew, and STEP is the common format for sharing it. Most people ignore it, because the STL (the simpler file your printer uses) is all you need to print.

That CAD file is the real parts list, though.

Everything the description left vague is sitting in there as a number you can measure: hole sizes, pocket depths, shaft diameters, and the gaps that decide whether a part fits or not.

On one of our builds, the description just said the drive motors were "N20 gearmotors." The CAD showed the motor pocket was 10.10mm by 12.10mm. That's the body of an N20 motor with a tenth of a millimeter to spare, and it told us something the description never could. The pocket was cut for that exact motor, and nothing bigger was ever going in without redesigning the frame. A question we'd been going back and forth on for days turned into a number in about ten minutes.

If you pick up one habit from this whole course, make it this one. It gets you answers in one sitting that would otherwise take a week of forum threads, and you're reading a measurement instead of guessing.

You don't need to know CAD to do it. Free browser tools will open a STEP file and let you click on a surface and read its size. You're only measuring, which is a much easier job than designing anything.

When the size doesn't match anything you can buy

Of everything on this list, this one ate the most of our time. That same design asked for "38mm diameter wheels, approximately 10mm wide," and mentioned that "the included hub is 25.4mm." If you convert those, they're all clean inch measurements: 1.5 inches across, a 1 inch hole in the middle, 0.4 inches wide. So the designer clearly had a real wheel in hand. He just never says which one, and when somebody asked in the comments, they never got an answer.

We spent hours hunting for a wheel with a 1 inch hole. What finally clicked was that the hub (the piece connecting the wheel to the motor) is a printed part, so the wheel was never the thing that had to change. You shrink the printed hub to fit a wheel you can actually buy, and the problem goes away.

So when a spec and a real product don't agree, change the part you're printing. It's easy to forget that option exists when you're staring at a shopping site.

Which parts are interchangeable

About half of what you'll buy is interchangeable and the other half isn't. If you treat them the same, you'll either overpay on the first half or break the build with the second.

Interchangeable. Bearings, screws, connectors, wire and heat shrink. A 683ZZ bearing is 3mm through the middle, 7mm across and 3mm thick with metal shields, and it's the same from every seller in the world. The brand doesn't matter, and if a link dies you just search the number.

Not interchangeable. Motors, speed controllers (ESCs), wheels and hubs. These have specific sizes, mounting holes and performance that the frame was designed around, so the exact part matters and any substitute needs checking.

The easy way to tell them apart is whether the part has a standard name. 683ZZ, M3, XT30 and 6-32 each describe a part completely. "5010" tells you the size of a motor but not its speed, weight or shaft, so it needs a closer look.

Using the same test to save money

Once your first build has taught you the parts, the standard-name test is also how you save real money.

Anything with a standard name can be ordered from the slow overseas marketplaces for a fraction of the specialist price, because a 683ZZ bearing or an XT30 connector or a bag of M3 screws is the same thing no matter which factory made it. Bearings, connectors, heat shrink, silicone wire, pin headers and screws all fall into this group. On a typical small robot that's about a third of the items on your list, at roughly a fifth of the price you'd pay domestically. It comes with two catches, though.

Timing. Shipping from overseas takes two to six weeks. Order these parts the same week you sign up for an event, or pay the domestic price and have them by Thursday. A cheap bearing that shows up after the event cost you more than the expensive one would have.

Quantity. At those prices, quality control is hit and miss, so don't buy exactly as many as you need. There's a reason bearings come in packs of ten. Buy the ten and plan on eight good ones, and you still come out well ahead.

For a first build, everything without a standard name is worth buying from the combat robot shops: motors, speed controllers, hubs and wheels. Their catalogs are picked so the parts already work together, and one avoided wrong purchase covers the price difference. So our advice is specialist shops for robot number one, and then move the interchangeable stuff to the cheap suppliers on every build after that, once you know exactly what you're looking at.

Some parts only come as a set

There's one more catch hiding in the interchangeable rule, and it can leave you with a box of parts that each looked right on their own.

Wheels, hubs, and the little keys and washers that hold them together are usually a matched set from one maker. The foam wheel's center hole is sized for that maker's hub, and the hub's center hole is sized for one shaft. None of it mixes and matches, so a wheel from one set won't stretch onto a hub from another, no matter how close the sizes look on the listing.

Say a designer used hobby RC wheels with a printed hub, and the parts list says 38mm wheels. A shop sells great 38mm foam wheels made to go on its own aluminum hubs. Same size, same robot, but the two wheel systems don't share a single part. If you buy the foam wheels and print the designer's hub, you end up with half of two different answers.

So buy the whole set instead of one piece. Decide whose wheels you're using, buy the wheel, hub and hardware from that one maker, and then check the last connection, the hub's center hole against your motor shaft, before you order. It's the same chain check described further down, except the middle links already come matched.

Read the comments

The description gets written once. The comments get written over years by people who actually built the thing, so they end up holding a lot of what the description should have said.

On one design, the comments had the designer's actual electronics list, his print temperature, how solid he printed the parts, a prep step for competition that doesn't appear anywhere else, and the fact that he makes two versions of the frame for two different motors. None of that was in the description, and all of it changed what we bought.

The comments also tell you what broke. A line like "At 100 percent power, the blade shattered" is worth more than a spec sheet, because it tells you how the part fails and roughly where its limit is.

Specs hiding in part names

Here's a small example of a habit that pays off everywhere. A design called for an "F2822/17" weapon motor, and nobody in the comments ever said how fast it spins. Motor speed is usually given as a KV number, which is how many RPM the motor turns for every volt you give it. The /17 looks like a version number, but it's the number of wire turns inside the motor, and on that motor it works out to 1100KV. We confirmed it three ways: two other sellers list the same motor as 1100KV, and the manufacturer has since renamed the part using KV instead of turns, which is also why the original link now goes nowhere.

Part numbers usually carry their own specs: 683ZZ, 2822/17, 5010, M3x30. When a description seems to be missing a number, check whether it's already sitting inside a part name you have.

Which rulebook counts

This one's worth spelling out, because mixing the two up leads you to ask questions that are already answered.

Events usually publish two documents. A construction spec covers how to build, and a ruleset covers what's legal. They're not the same, and only the ruleset has the final word.

Working from the construction spec left us with three open questions about whether certain metal parts were allowed. The actual ruleset answered all three in one sentence, by listing screws and bearings as parts that can be any material. We spent days unsure about something a linked document had answered the whole time.

So find the ruleset and read the section for your class before you start buying. Reading the Rules Before You Buy Anything goes through it in more detail.

Checking a chain of parts end to end

Before you order, pick one chain of parts and follow it end to end, checking that every connection fits. This catches almost every mismatch.

For a drive wheel, that chain is: motor shaft → hub center hole → hub outside → wheel center hole → wheel outside → the pocket in the frame. That's six connections, and if any one of them is wrong you end up with parts that don't go together.

A wheel once made it onto our parts list because its outside size was perfect. Its center hole was 10mm, and the hub it was supposed to sit on was 25.4mm. We'd only checked one connection, and walking the whole chain is how you catch that before it ships.

Do this for each part of the robot before you spend anything: the drive, the weapon and the power. It can take you some time, but it's the difference between a working robot and a drawer full of parts that “almost” fit.

Tools that make this quick

You don't have to install anything to open and measure a STEP file. Onshape runs in your browser and is free for personal use, and it'll open a STEP file and let you click a surface and read its size. Fusion 360 is free on a personal license and does the same thing on your computer. Either one takes about ten minutes to set up, and you'll use them for measuring long before you ever design anything.

For finding a design in the first place, GrabCAD's combat robot library has designs across the weight classes, and awesome-robot-combat collects build diaries, part files and suppliers from all over the hobby.

As for the parts themselves, our first robot came from four places. We didn't shop around, because we had no idea what we were doing and a curated catalog was worth more to us than saving a dollar. That was the right call then, and it's probably the right call for you too. The list is here so you can shop around later, once you know what you're looking at.

FingerTech, Repeat Robotics and Just 'Cuz Robotics cover most of what the table doesn't. Their catalogs overlap and the same part is often a few dollars different between them, so check two before you check out.

On a first build it's worth paying a little extra at a combat robot shop. Their parts are already known to work together, and in this hobby the expensive mistake is buying parts that don't fit.

What a first robot costs, bucket by bucket, is in the cost breakdown. This piece is about how to find the parts, and that one is about the budget.

Your robot will mix inch and metric screws

This cost us a week, and it's easy to avoid if you know it's coming.

Combat robot parts in North America use inch sizes. FingerTech's drive hardware uses 2-56, 4-40 and 6-32 screws, because that's the standard in the hobby here. The motors that spin the weapons use metric. Motors like the 2822 and 5010 come from the drone world, so their mounting holes are M2 and M3.

Almost every small combat robot ends up with both kinds on it, which means you now own two kinds of screws, two sets of drivers and two bins of hardware.

It gets more confusing on a real robot. On one of ours, the wheel motors use inch screws and the weapon motor uses metric. On the other, every screw is metric but the hex keys are inch sizes. None of that is easy to keep straight at a pit table at two in the afternoon.

🚨 Watch out: 6-32 and M3 screws look almost the same. A 6-32 is 3.51mm across and an M3 is 3.00mm. If you drop an M3 into a 6-32 standoff (a threaded spacer post), it'll start, turn about two times, and then strip the threads. It feels almost right the whole way in, and that's why it's such an expensive mistake.

Here's what helps, starting with what helps most:

1. Standardize what you can. Standoffs, frame screws and spare hardware are interchangeable, so buy them all metric or all inch.
2. Accept what you can't. The motor decides its own screw size.
3. Label bins by thread. "M3" and "6-32," not "small" and "medium."
4. Keep the hex key sets apart. A different pouch or a different color.

The first one does the most, because every screw you don't have to think about is one you can't mix up. The motors will always force a second system on you, so the labels and the separate key sets are there to make the mix-up hard when your hands are moving faster than your head.

You won't get this perfectly clean on a first build, and we didn't either. Just knowing about it gets you most of the way there, because the alternative is finding a stripped thread in a part you can't easily replace on the day of the event.

Ask before you decide

The one thing this piece can't give you is someone who has built the exact design you're looking at.

The Combat Robotics and Antweight Robot Combat Facebook groups usually answer questions about whether a part fits within a few hours, and your event almost certainly has a Discord where the people running your weight class hang out. Something that would take you four days of measuring takes one of them about nine seconds to answer, and people in this hobby are genuinely happy to help.