1. UniFeatherBot Chassis

1.1. Parts List




3mmx50mm Dowel Pin 304


Sleeve Bearing 3x5x5mm


18650 Battery Clip


MG90S Servo


28BYJ48 Stepper motor


M2x5mm self tapping screw


M3x10 machine screw


M3x5 flat head machine screw


M3 hex nut


M3 lock nut


0.4x10x20mm steel spring


1/2” slingshot ammo ball


M3x15 (or 12) machine screw


3D Printed chassis


3D Printed wheel


3D Printed caster shell


3D Printed caster plug


3D Printed pen holder


3D printed pen cap

Optional Parts for AA Battery Power




AA battery holders


M3x5mm machine screw


M3 lock nut

1.2. 3d Printed Parts


1.2.1. 3d Printed Parts


The shape of the wheel spokes were obtained from a Font Awesome icon under the Font Awesome Free License. It is available from https://fontawesome.com/icons/biohazard?style=solid






1.2.2. Printing Instructions

Print one each of the Chassis.stl, Caster.stl, PenCap.stl, Penholder.stl and two of the Wheel.stl files. When printed in their default orientation none of the parts should require supports when printed in a reasonably calibrated 3D printer. I printed mine with a 0.20 layer height on a WANHAO Duplicator I3 using Hatchbox PLA from Amazon.com.

1.3. Assembly Instructions

1.3.1. Step 1: Assemble the caster

Assemble the caster from the caster shell, 1/2” slingshot ball and caster plug by placing the slingshot ball on top of the plug then insert the plug into the caster shell.


1.3.2. Step 2: Insert the pen lift dowels

Insert the two 3x50mm dowels into the two 3mm holes in the front of the chassis. The dowels are a tight press fit that may require the use of a hammer.


1.3.3. Step 3: Mount the stepper motors

Use two M3x15 (or M3x12) machine screws and two M3 lock nuts to mount each of the 28byj-48 to the chassis as shown.


1.3.4. Step 4a: Mount the 18650 battery holder


Skip this step if you are planning to use four AA batteries for power.

Run the battery holder wires through the holes in the bottom of the chassis as shown.


Use a M3x5mm flat head machine screw and M3 lock nut to mount the 18650 battery holder to the bottom of the chassis as shown.


1.3.5. Step 4b: Mount the AA battery holders


Skip this step if you are planning to use a 18650 battery for power.

Use two M3x5mm flat head screws and M3 lock nuts to mount each of the AA battery holders to the bottom of the chassis as shown.


1.3.6. Step 5: Mount the caster

If you are using AA batteries for power use two M3x5mm flat head machine screws and M3 lock nuts to mount the caster to the bottom of the chassis between the AA battery holders as shown.


If you are using a 18650 battery for power use two M3x5mm flat head machine screws and M3 lock nuts to mount the caster to the bottom of the chassis as shown.


1.3.7. Step 6: Assemble the wheels

Insert two M3 hex nuts into the pockets in the wheel hubs as shown.


Inset two M3x10 machine screws into the M3 hex nuts in the wheel hubs a shown. The screws should not protrude into the motor shaft hole.


Fit one the two O-Ring’s around each of the wheels as shown.


1.3.8. Step 7: Assemble the pen holder

Insert two of the 3x5x5mm sleeve bearings in each side of the pen holder as shown. The sleeve bearing should be flush with the bottom and top of the pen holder. They are a tight press fit that will require quite a bit of pressure to insert. Inserting the bearings one at a time and pressing the pen holder down into a table with force works well. One bearing on each side should be inserted from the bottom and the other from the top. It’s not recommended that you use one bearing to drive the other.


Insert a M3 hex nut into the pocket in the pen holder as shown.


Place a M3 lock nut into the M3x15 machine screw and run the nut the length of the machine screw as shown.


Insert the M3x15 machine screw through the M3 hex nut in the pen holder as shown.


1.3.9. Step 8: Mount the wheels

Place one of the 0.4x10x20mm steel springs over the stepper motor shaft as shown. The spring should fit around the flange surrounding the shaft.


Place the wheel onto the stepper motor shaft being careful to align the flat cutouts in the shaft with the cutouts in the wheel hub. The wheels are a press fit to the shaft that may require wiggling the wheel back and forth slightly to get them to slide all the way into the shaft. Lightly tighten the M3 screws on each side of the wheels to lock the wheel in place. You can use the screws to make small adjustments to the alignment of the wheels. Be careful not to overtighten the screw of you can crack the wheel hub.


1.3.10. Step 9: Install the pen lift servo

Attach the small single sided servo horn onto the MG90S servo with the provided screw. The servo horn should be placed so it can move 180 degrees without hitting the chassis when mounted to the chassis.

Insert the MG90S servo into the slots in the front of the chassis as shown. The center of the servo’s shaft should be in the center of the pen hole in the chassis.


Use the two small self threading screws provided with the MG90S servo to hold the servo in place as shown.


1.3.11. Step 10: Install the pen holder

Place the pen holder onto the chassis as shown. The pen holder should be centered over the pen hole in the chassis with the M3 set screw facing forward.


The pen holder should slide up and down the dowels freely. Initially there may be some resistance and it may not move easily enough to drop when raised and let go. A dab of light PTFE lubricant will help and it should loosen up if you work the pen holder up and down the dowels for a little while. If it does not loosen up you should verify the dowels are parallel to each other. You can use light pressure to coax them into alignment.


1.3.12. Congratulations

Congratulations your UniFeatherBot chassis is assembled.