Sumo 3K - Post 03 - Hardware Build

This post discusses chassis material comparison, the welding decision, and the staged mechanical assembly with all six build stages.

The chassis is the spine of a combat robot. Three materials were on the table: 3D-printed plastic, aluminium extrusion framing, and laser-cut metal sheet. The decision came down to three things — rigidity under impact, mass contribution, and cost of fabrication.

















Plastic: Cheap and customisable but cracks under sustained impact force. Also too light — in sumo, mass is a weapon.

Aluminium extrusion: Lightweight and modular but more expensive and less rigid than a solid sheet.

Laser-cut metal sheet: High rigidity, contributes meaningful mass, and custom geometry is achievable through laser cutting services.


Metal won easily. A fabricator used laser cutting to produce both the base plate and the top cover from a single sheet design. The custom geometry included slots for cable routing, mounting hole patterns, and angled corners to reduce weight without sacrificing structural integrity.


Motor holders were welded to the chassis rather than bolted. Bolted joints loosen under repeated vibration and impact which means in a competitive environment, that's a failure waiting to happen. Welding makes the motor mount as strong as the chassis itself.


The assembly was staged. Here's the sequence:


Stage 1 — Chassis base and cover

The laser-cut metal base plate arrived with the slot patterns and mounting holes already cut. The matching top cover was cut from the same sheet. 


Stage 2 — Welded motor holders

Four motor holders were positioned at the corners of the base plate and welded in place by a professional welder. The holders use a circular through-hole that the DC motor shaft passes through, clamping the motor body securely.


Stage 3 — Motors mounted

The four DC Johnson 12V 500 RPM motors were pressed into the welded holders and secured. With all four motors in place, the robot already felt substantial — the combination of metal chassis and four solid motors added significant mass before any electronics.


Stage 4 — Tyres fitted

High-tread rubber tyres were pressed onto the motor shafts. These tyres have an aggressive tread pattern, not unlike a small off-road vehicle, chosen specifically for maximum grip on the arena surface.


Stage 5 — Battery and motor driver

The 7.4V 18650 lithium-ion battery (4000 mAh) was secured to the chassis centre using zip ties: a simple but effective solution that keeps the mass central and low. The L298N motor driver was positioned alongside it.


Stage 6 — Microcontroller

The ESP32 was positioned and connected. With all electronics in place, the top cover was fitted and the robot took its final form.


Motor speed - 500 RPM

Motor voltage - 12V DC

Battery - 7.4V 4Ah

Final weight - 2.35 kg

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