Build your own robot lawn mower: open-source, wire-free, fully designed

Lawn planner: how would the mower cut your lawn?

Pick an example, or draw your own: click to place the corners of the lawn, then add flowerbeds, trees or a pond as no-go areas. The planner adds two loops round the edge and one round each no-go area, then tries every stripe angle and keeps the quickest.

Times include turning on the spot at stripe ends. Coverage is computed on a 5 cm grid from the planned path; the closed-loop simulation in the Python package adds RTK noise, wheel slip and fix losses (mowerbot simulate).

What the design checks say

Every figure here comes from the project's own models, run on the default design. They are simulation and estimates under stated assumptions, not measurements on a built mower.

Manufacture it: download the fab package

One parametric CAD model generates everything a fabricator needs: laser-cut DXF files (flat 5052 aluminium, true millimetres, no bends or tapped holes), STL files for the printed ASA parts (each fits a 256 mm printer bed), STEP files for any CAD or CAM tool, a cut list, a full bill of materials with every fastener, and assembly drawings. Every build checks each part against every other at all seven cut heights, with the spinning parts given running clearance and the shell pushed 8 mm each way, and checks that every printed part's bolt holes land on holes in the plate it bolts to.

Download the mower package (zip, 1.4 MB), the dock package (zip, 0.1 MB), or browse the files (GitHub shows the STLs in 3D).

Design v0.1, not yet built: the first build will find things to fix. Change any dimension and regenerate with python -m orm_cad mower (parametric CAD source).

Build one

• The mower: chassis, drive, cutting system, height adjustment, floating shell and sensors.
• Navigation: RTK base station and rover, mapping your lawn, and the perimeter-wire alternative.
• Charging dock, electronics and pin map, ESP32 firmware, and safety.
• Bills of materials: mower and dock and RTK base (rough estimates).
• Software: pip install "git+https://github.com/duncan-haywood/oss-monorepo#subdirectory=ventures/open-robot-mower", then mowerbot plan --example backyard.

Building a ball-collecting rover instead? The Open Tennis Robots collector uses the same drive, firmware pattern and CAD generator.

Questions

Can I build my own robot lawn mower?

Yes. This design uses two 24 V gearmotors driving the rear wheels, two front casters, an outrunner motor spinning a 200 mm disc with three pivoting razor blades, a Raspberry Pi with an RTK GNSS receiver for navigation and an ESP32 for the motors and every safety input. The laser-cut, printed and STEP files, the bill of materials, wiring, firmware and planning software are all in the repository under the MIT licence. It is a v0.1 design that has been checked in CAD and simulation, not yet built and tested.

Does a robot mower need a boundary wire?

Not this one. It navigates by RTK GNSS: a fixed base station near the lawn sends corrections, and the mower knows its position to a couple of centimetres. You walk or draw the lawn boundary and the no-go areas once, and the planner does the rest. If you would rather bury a perimeter wire, the navigation notes describe that option too; it needs a signal generator and two coil sensors instead of the base station.

How does the mower plan its route across the lawn?

It cuts two loops round the edge, one loop round each flowerbed or tree, then fills the rest with parallel stripes. It tries every stripe angle in 5 degree steps and keeps the quickest, counting both distance and the time spent turning at stripe ends. Moves between stripes that would cross a flowerbed are routed round it. The planner above runs the same code as the robot.

How long does it mow on one charge?

With a 126 Wh pack (7S, 5 Ah) the power model estimates about 33 W while cutting at 0.35 m/s on flat grass, or roughly three hours, but every coefficient in it is an assumption until someone measures a real build. When the battery runs low part-way through a lawn, the scheduler sends the mower home to charge and back to where it stopped.

Is a DIY robot mower safe?

Only if it is built and tested carefully. The blade tips move at about 34 m/s and the disc stores about 50 joules. The design puts a guard ring round the blade that reaches below the blade plane, keeps the blade at least 100 mm inside the shell edge, and stops the blade when the shell is lifted or bumped, when it tilts past 20 degrees, when the stop button is pressed, when it rains or when the controller goes quiet. The model predicts the disc stops in about half a second with short braking. You are still responsible for building, testing and running your mower safely, and for keeping children and pets away while it works.

How steep a lawn can it mow?

The statics model gives about 17 degrees (30%) on dry grass, limited by the rear wheels' grip when climbing, and only about 5 degrees on wet grass. Those friction figures are assumptions. It will not tip sideways before it slides. Steep lawns need more weight over the drive wheels, better tyres, or all-wheel drive.

How much does it cost to build?

The bills of materials add up to roughly 1,050 US dollars for the mower and dock, plus about 300 for an RTK base station if no correction network covers your area. These are rough planning estimates, not quotes. The RTK receivers and the battery are the largest items. Check current prices before you buy.