This is a modification of an existing project, the Arduino Drawing Robot, that inspired me to modify, and create a set for my robotics class. These instructions have been created for my students to assemble and program their own Drawbots.

You will need the following tools:

Pliers
phillips head screwdriver
hair dryer or heat gun (optional)
drill (optional)

The following hardware is required: (it is possible these parts could be sourced elsewhere at lower cost and smaller quantity)

16x 8mm M3 screw 100 pack - $2.64
16x M3 hex nut 100 pack - $2.06
2x Rubber O-ring (1 7/8" inner diameter) 100 pack - $13.12
1x 5/8" Steel bearing 50 pack $7.65
Small diameter markers

The following electronics are required: (most of these can be found cheaply on ebay)

1x arduino compatible board, with short USB cable (12 in/ 30cm is ideal) (I used the uno r3)
1x arduino prototype shield
1x 100 uF electrolytic capacitor
22 gauge jumper wire
1x ULN2803 Motor driver
2x 5v 28BYJ-48 stepper motors
Male header pins, preferably longer ones like this: 17mm header pins
1 x SG90 servo motor - - this piece is optional, depending whether you want to be able to lift pen
1x 5v power bank - the designs currently fit the power banks that are 5/8" (16mm) or less in thickness and 2.83" (72mm) or less in width. This is an example of one that should work: Elivebuy Power bank

The following 3d printed components are required:

1x Chassis
2x motor holders
1x 5v battery mount
2x wheel
1x ball bearing/caster holder
1x pen collar - this piece is optional, depending whether you want to be able to lift pen
1x pen holder/servo mount - this piece is optional, depending whether you want to be able to lift pen

Step 1: Prep Motor Mounts

For this step you will need:

2x motor holders

You will notice that the holes on the side where the screws go through are a little messy after printing. Use the pliers to remove the excess plastic and clear it out. This will ensure the screws are able to fit through.

Step 2: Attach Motor Mounts to Chassis

For this step you will need:

1x chassis
2x motor mounts
4x M3 8mm screw
4x M3 hex nut

Attach the two motor mounts using the screw and hex nuts. You will notice that the motor mounts have holes that fit the hex nuts. You may want to press them in place using pliers.

As you secure the mounts in place, try to ensure edge of chassis and end of motor mount are aligned flush with each other.

Step 3: Prep Servo Mount

For this step you will need:

1x pen holder/servo mount
1x servo, with servo horn and screw

Prep servo mount:

Using pliers, break off the vertical support
Clean and hanging strands as shown in diagram, and clean any remains of vertical support

Attach servo horn - please follow instructions in the video to ensure it is attached correctly

Attach servo to servo mount

Push servo through back as shown in photo.
Tighten into place with the servo mounting screws. When tightening screws, be careful not to strip them as holes are tiny. If it is giving too much trouble, you could try opening them slightly using a drill.

Step 4: Attach Motors and Servo Mount

For this step you will need,

6x M3 Screw
6x M3 Hex Nut
2x stepper motor
chassis + motor mount assembly
servo + mount assembly

Attach motors to motor mounts.

As you attach the motors, if you have trouble fitting screw through opening, you may need to clean it out a bit more.
Make sure motors are flush against the mounts.
Notice that the wires for the motors point upwards.
You may want to use pliers to hold hex nut in place as you tighten screw.

Attach servo+mount assembly to the chassis. The servo should hang off to the back of the Drawbot.

Step 5: Attach Battery Mount

For this step you will need:

1x Battery mount
4x M3 Screw
4x M3 Hex Nut
Chassis Assembly

Attach Battery mount to the motor mounts. The hex nuts have fitted holes on the underside of the motor mount, you can press the hex nuts in place with the pliers.

Step 6: Prep Wheels and Bearing Holder

For this step you will need:

2x wheels
2x O-rings
1x bearing holder
1x bearing

Stretch O-rings to get them to fit into ridge on each wheel.

Push bearing and pop it into holder. Be careful not to snap PLA. If necessary, use a blow dryer or hot air gun to warm up plastic before popping bearing into place.

Step 7: Add Wheels and Bearing

For this step you will need:

2x wheels with O-rings
2x M3 Screw
2x M3 Hex Nut
Bearing holder
Chassis assembly

Attach the bearing mount to the underside of the chassis. The hex nuts have fitted holes on the underside of the bearing mount, you can press the hex nuts in place with the pliers.

Press the wheels onto each motor axle, until they are as tight as possible.

Step 8: Prep Header Pins and Capacitor

For this step you will need:

long header pins
1x100uF electrolytic capacitor

Use the pliers to break apart 2 sets of headers that are 5 pins long, and an additional set that is 3 pins long.

Trim the legs of the capacitor so that they are 1cm long.

Step 9: Assemble Electronics

For this step you will need:

Arduino Uno R3
Arduino prototype shield
1x 100 uF electrolytic capacitor
22 gauge jumper wire
1x ULN2803 motor driver
2x 5 pin male header pins
1x 3 pin male header pin

Attach breadboard to prototype shield, and attach prototype shield to Arduino Uno.

Add ULN2803 driver and jumper wires to breadboard as shown in diagram.

***IMPORTANT NOTE: On a breadboard, wires can be placed in any pin of the same row. Notice how in the image, the wires are placed in the same rows, but slightly different pins than the diagram.
If you connect the driver to slightly different pins on the arduino, it wont make a difference.
Make sure the three red wires on the left side of the diagram are in the correct spots, otherwise, something could potentially be damaged. Also, make sure the grey wire is connected to the ground pin.

Add header pins and capacitor as shown in the diagram.

Use the pliers to push the plastic part of the headers downward
Make sure the capacitor has the negative side (the side with the stripe) pointed downward.

Step 10: Add Electronics to Chassis

For this step you will need:

Drawbot Assembly
Electronics

Organize the wires as shown in the first image so they don't have as much excess wire.

Run each wire around the back to the opposite side it is attached to.

Attach arduino to battery mount.

It should slide from the top down into the arms sticking off the battery mount.
Move wires so they are not stuck beneath it.

Attach motors to arduino.

Notice that the top motor has the red wire facing upward, while the bottom motor has the red wire facing downward.
Based on this arrangement, the top motor is the left one, and the bottom motor is the right one. If you have wired it the same way as me, when you control it, the left motor is connected to pins 7-10, and the right motor is connected to pins 3-6.
Notice that for the servo, the yellow wire is facing upward and the brown (ground wire) is facing downward. The brown wire should connect to the same row as the negative side of the capacitor.
If wired the same as mine, the servo should be connected to pin 11 of the arduino.