Combat Robot for Under $250




This instructable details how I built my 12 pound combat robot, Spider, for less than 250 dollars. Now keep in mind that if this robot were entered into an event, it would probably not pass safety inspection. Even if it did, it would be the underdog. That being said, it is still a fully functional combat robot which is a) fun to build, b) can thrash boxes and beer cans in the garage as your friends watch in envy, and c) a great way to unleash your inner mad scientist and learn about combat robotics. Also, for a couple hundred more, it can easily be upgraded to a competition worthy robot.

I wanted to write this instructable because I feel that it is a great place to start for someone who is wanting to get into combat robots. Only basic tools are needed, the parts are readily available, and it can all be done for a couple hundred bucks.

Parts list:

Part -------------------------------------------- Cost --------------------------- Source

Wheels x 2 ------------------------------------- $5 -------------------- Used medical walker

HF drill x 2 ------------------------------------- $46 ------------------------ Harbor Freight

Used cordless circular saw

(or anything with a similar motor) -------- $10 -------------------Goodwill/Garage sale

Futaba standard servo ---------------------- $13 ------------------------------- Online

FlySky transmitter/receiver ---------------- $43 -------------------------------- Ebay

Scorpion 2x12 ESC ------------------------- $65 --------------------- Robot Market Place

Frame/Weapon/Armor --------------------- $15 ---------------------- Local scrapyard

Hardware Home depot --------------------- $20 --------------------------- Home Depot


Tools needed:

Cordless drill, wire strippers/crimpers, hacksaw or jigsaw, table vise, hammer/screwdrivers/wrenches/etc.

Step 1: Hacking for Parts

The drills, circular saw, and walker aren’t very useful in their intended forms for this project. You should only need a screw driver and wrenches to take the wheels off of the walker and to get the motor and gearbox out of the saw. The drills can be a little trickier, but luckily there are plenty of tutorials online that will help you get the motors out. Once you are done, you should have all of the parts you need to begin building. (The picture also shows a receiver battery pack [light green]. This is not needed for this project because the speed controllers have a built in BEC).

Step 2: Servo Switch

In order to keep costs down, a servo can be used to make a switch that will turn the weapon on and off (this is the part that I suspect wouldn’t pass inspection in an event). I used some of the contact leads from the drills for the switch contacts but anything that is conductive will work. The basic idea is to attach one lead to the servo so that when an RC signal is sent to the servo, it will rotate and touch the contacts together, completing the weapon motor/battery circuit.

Step 3: Attaching Parts to the Baseplate

Now that the switch is done, it’s time to start putting everything together. The quickest way is to attach everything to a baseplate. I used piece of scrap aluminum for Spider, but anything that can be easily drilled through will work. The arrangement of your parts will largely depend on your weapon design. For Spider’s overhead spinning weapon, I wanted everything to be as compact as possible within the weapon radius.

Hose clamps and/or metal straps are a nice cheap way to hold the drive motors in place. Zip ties should be sufficient enough to hold the battery and electronics in place for this project. Tape comes in handy as well for some of the lighter components.

The weapon motor will probably be the heaviest part, and since it will also be spinning metal around at high RPMs, it should be supported a bit better than the other parts. I used 1”x1/8” aluminum bars to build support brackets. The bars are easy to bend using the table vise and a hammer. Once they are bent to the correct lengths, simply bolt them onto the motor and the base. The gearbox on the weapon motor should already have holes in it that can help with mounting.

Step 4: Electronics

For Spider, I used one of the drill batteries to power both the drive motors and the weapon. Hooking up both drill batteries in parallel would allow the robot to run twice as long, but would take up space and add weight. Each system could also be run by its own battery. Pick whichever setup works best for you. Ring terminals can be used to connect the weapon motor and ESC to the battery. (Note: Don’t connect at least one of the ring terminals directly to the battery. Put some type of connector in between the battery lead and the ring terminal so the whole system can be easily disconnected without having to take the ring terminals apart). Crimp connectors are suitable for all other connections. The main things you want to be careful of here are a) not to short circuit the battery and b) make sure to connect the positive and negative battery leads to the proper terminals on the ESC. If the battery is connected to the ESC backwards, it will instantly destroy the most expensive part of the robot. Once the ring terminals are fastened together, wrap some electric tape around them to avoid any shorts. The following visuals should help ensure the wiring is done properly.

The two drive motors connect to the ESC just like the battery. It doesn’t really matter which leads from the motor connect to the A and B slots. If the motor turns the wrong direction, you can either switch the leads or reverse the channel in the transmitter setup screen.

The two RC cables from the ESC plug into channels 1 and 2 in the receiver. Again, you’ll just have to guess at first and switch them around if the robot doesn’t respond the way it should. The RC cable from the servo can plug into whatever channel you want. I like to use the left stick (channel 3) to activate the weapon but any of the buttons/sticks/switches on the transmitter can be used.

Time for a test run! First, make sure the battery is disconnected and mount the robot so the wheels aren’t touching the ground. Turn on the transmitter first and then connect the battery. Play with the sticks to see how the motors respond. This is where you may have to switch the receiver cables around or reverse the channels with your transmitter. If either of the drive motors slowly turn on their own, the trim function on the transmitter will fix that.

Step 5: Weapon

All of the parts are mounted, the electronics are hooked up, the robot has been tested and the motors all work as planned… time to put the weapon on! This step depends on the design of your robot. The cheapest way to mount a weapon is to attach it directly to the motor, using the same mechanism that attached whatever the motor was originally meant to spin. This will shorten the life of the motor, but will work for now. With Spider, I attached the overhead weapon the in the same manner that the saw blade was attached (MAKE SURE that the motor is disconnected from the battery when attaching the weapon!). Also, you will want to be sure to that the weapon spins in the same direction that the original saw blade was meant to spin. This will prevent the bolt that holds the weapon in place from becoming loose (be sure to check that the bolt is tight before and after each robot carnage session). Once securely mounted, check that the servo switch is in the “off” position, turn on the transmitter (always power transmitter before powering the receiver), and connect the battery to the motors/ESC. Now you’re ready to rock. Drive the robot around for a little bit to get a feel for the controls. Once you get the feel of it, activate the servo motor. The weapon should spin up to full speed. Now smack some cans/boxes/toys around!

Step 6: Additional Notes


Combat robots are dangerous! Make sure to take proper safety measures when tackling a project like this. Wear safety glasses at all times and be very mindful of your circuitry (especially if it involves your weapon). It doesn't take much to accidentally bump a wire and complete a circuit that will activate the weapon before you are ready to. Whenever possible, keep tape, foam, or anything else non-conductive on any exposed leads.


There are a few major upgrades that would make this a much better overall combat robot.

Batteries: The batteries from the HF drills are pretty much bottom of the barrel. There are a ton of batteries out there that would plug right in and deliver more power as well as have more capacity.

Weapon motor: There are some pretty good motors out there that are used in combat robotics. They can get quite spendy, but some are pretty reasonable. Any of them would be better than an old cordless saw motor.

Weapon belt drive: Putting the weapon on a shaft and driving it with a belt and pulley system will decrease the wear on the motor. It will also allow for better control of the speed and torque by adding a reduction stage.

Weapon ESC: Adding an ESC for the weapon would replace the cheap servo switch. This would make it much safer and add the ability to control the speed and direction of the weapon.

Armor: If you are fighting another robot, you'll obviously want to protect yours the best you can. Pick a material based on your weight/strength needs and bolt plates of it around your vulnerable components.


The pictured version of Spider weighs in around 7-8 pounds. This leaves 4-5 pounds to increase the weight of the weapon and add armor in order to compete in the 12 pound hobbyweight class.



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    3 years ago

    Wow! That blade looks very fast and powerful!