Making parts
When building your own robot you should make custom parts to get a tight design
Below is a list of parts that I have built for the robot and I present in steps how they where made and the techniques used. For some of the parts I have used a 3D printer and a CNC to get good quality parts and because it´s fun to use machines.
Part list
Chassis
The goal was to design a simple robot chassis to create a sturdy base to handle uneven ground but still keep the weight low. The chassis consist of a 2.0mm thick aluminium plate that I bent on two sides for mounting the DC motor gearbox driving the big front wheels.
Build steps
Aluminium plate
I started with a aluminium plate that was 270 x 190 x 2 mm
Plate bending
Along the two 190mm sides I bended 35 mm from the edge to create chassis sides for mounting the DC motors for the front wheels.
Caster wheel
The back caster wheel I selected for the robot is 70mm wide and 65mm high.
Plastic spacer
I added two plastic spacers to the back wheel plate to get the chassis in level to the ground. Each plastic spacer is 40x15x11mm.
Back wheel plate
The back wheel plate is a aluminim plate with the dimensions 115x50x2mm and mounted with four screws to the main plate.
Back wheel rotation
When mounting the back caster wheel make sure that you can rotate the wheel 360 degrees without it hitting the main plate or the cutting knifes.
Front motor mount
The DC motor gearbox is mounted with four screws and here you also see the axel mount for the wheel.
Front wheel mount
The two front wheels are mountied to the DC motor 6mm shaft with these Pololu universal aluminium mounting hub for 6mm shaft with M3 holes.
Final chassis
Final assembled robot chassis.
Body
The robot body makes up the robots character and I personally like a simple stream lined and compact design. This body is 3D printed PLA plastic and I printed it in multiple parts to fit them on my Creality Ender 3 printer.
Build steps
First print test
First i tried to print as big parts of the robot body as possible flat on the print bed which got the slicer software to add alot of printing support structure and the time to print was up to 10 hours. When running these relatively big prints the risk of a print failure is quite high. You also use alot of print filament in the support structure and it takes long time removing the print support when the print is done.
Smaller parts
I then changed the printing strategy and divided the body in to smaller parts that could all be printed standing up without the need of print support. This should lower the risk of print failuers and should lower the total printing time and the use of print filament.
Second print test
Even with these smaller and standing parts I had the problem with print layers that was cracking when running the prints in my small hobby room in the winter with low room temperatures. This was solved by putting a room heater in front of the 3D printer to keep the part warm while printing. This was a 1000W heater and I set the thermostat to 35 degrees celsius (about 95 degrees fahrenheit) which probably kept the plastic at about 40 degrees celsius.
All parts printed
I got all the 14 parts printed in about 3 days and I was quite happy with the print quality. There is always some warping of bigger parts without print support.
All parts printed
I glued the parts together with 2 part epoxy glue.
Spray paint
I then hand sanded the hole body with first 180 grit and then 240 grit sandpaper and then sprayed the body with matte black paint.
Body mounts
Then I 3D printed these four orange body mounts to hold the body in place on the robot chassis.
Cutter
The cutter is built from a round stainless steel disc that is 13cm in diameter (about 5 inches) that you can get from a tart pan, for example this one at Amazon. The disc is powered by a 18V DC motor and is mounted directly on the motor shaft. The cutting blades are bought from Amazon.
Build steps
Motor mount
The 18V DC cutting motor is mounted in the middle of the chassis base plate. First I drilled a 15mm hole for the shaft mount (the actual size of the shaft mount on the DC motor is 13mm) and then I drilled two holes for the two M3 screws for mounting the motor to the chassis.
Shaft mount
The cut disc is mounted to the motor shaft with the same mounting hub as for the two front wheels. It´s super important to get the hub mount dead centered to avoid shaking when running the cutter. And the disc also needs to be perfectly balanced so it´s important to have the holes for the blades perfecly placed 120 degrees apart from each other and with exactly the same distance from the disc edge. Use locknuts to make sure that the knifes don´t get loose and fly away.
Final assembly
Here is the final assembly of the cutting disc and the disc is about 35 mm from the ground. For the mounting hub i used locknuts to make sure that the cutting disc don´t get loose and fly away.
Front plate
The front plate is cut out of 3mm clear acrylic and it is used to mount the two perimeter sensors and to stop cut grass from entering the robot body. I bought rolled acrylic because it´s cheaper but be aware that cast acrylic is better for cutting out shapes since it is less prone to crack while cutting.
I then used my Sainsmart Genmitsu 3018 CNC router to cut out the front plane shape.
Build steps
End mill
I used a single flute 3.175mm (1/8 inch) end mill. It´s recommended to use single flute mill with acrylic to prevent the plastic from melting on the mill bit. You can find these mill bits on Amazon.
CNC router
I use my Sainsmart Genmitsu 3018 CNC router to cut out the front plane. This CNC router can be bought on Amazon. First I mount a piece of wood with double sided tejp to the CNC working table and then I mount the acrylic to the wood with more tejp. I try to avoid tejp where the CNC will cut.
CNC CAD software
I use the Easel web software to run the CNC cutting. There is a free version that has the functionality to cut simple shapes. To cut through the acrylic in this free version of Easel I use a simple trick and set the acrylic thickness to 3.1mm instead of 3.0mm.
Cut settings
Below are the cut settings I use to cut acrylic without plastic melting on the end mill bit. It might be possible to mill faster but you have to try with your own settings and be sure to wear protective glasses as the end mill bit might break.
Start position
Before starting the cut place the end mill cut point in the lower left corner of the acrylic piece very close to the surface. This is then used by Easel as the reference point and also the start and end point for the cut. You move the end mill with the control panel in Easel that you open by clicking the arrow button next to the "Carve..."-button in the upper right corner.
