Robot Arm

Robot Arm

4 pack of MG996R

5 pack of SG90

1 Nema 17

ELEGOO UNO R3 Board

2 pack PCA9685 servo driver

5V 4A Power Adapter

Servo Extension Cable pack

800pcs Screws M1/M1.2/M1.4/M1.6/M1.7

750PCS M3 Screws Assortment Kit

Rough arm dimensions

Suggested starting sizes:

1. Base height: 40–60 mm
2. Upper arm: 100–130 mm
3. Forearm: 100–120 mm
4. Wrist stack: 40–60 mm total

Sketch this on paper first.

1. Create a new Component for each joint
2. Base
3. Shoulder bracket
4. Upper arm
5. Elbow bracket
6. Forearm
7. Wrist housing(s)

Do not model everything as one body.

Find datasheets for:

1. MG996R
2. SG90

Model accurate servo bodies, including:

1. Mounting ears
2. Shaft position
3. Screw holes

1. Cylindrical or hex base
2. Servo mounted vertically
3. Output shaft pointing up
4. Add:
5. Bearing or bushing pocket (optional but nice)
6. Cable exit slot
7. M3 mounting holes

Constraint idea:

1. Servo = grounded component
2. Base plate rotates around servo axis

Shoulder joint (MG996R)

1. Servo mounted sideways
2. Arm bracket attaches to horn
3. Use double-side support (servo horn sandwiched between plates)

IMPORTANT:

1. Avoid single-sided cantilever mounts
2. Add fillets (3–6 mm) to reduce stress

Upper arm link

1. Hollow box or I-beam shape
2. Internal cable channel
3. M3 screw holes at both ends

Elbow joint (MG996R)

1. Same sandwich concept as shoulder
2. Servo body fixed to upper arm
3. Forearm rotates on horn

Check:

1. Full ±90° clearance
2. No self-collisions

Wrist joints (SG90)

Keep these compact

Stack pitch + roll using two SG90s

Make wrist modules replaceable

Torque is low keep lengths short!

Assembly and joints in Fusion

Use:

1. Revolute joints
2. Correct rotation limits (usually ±90°)

Then:

1. Animate the arm
2. Look for collisions
3. Check reach envelope

Screws & hardware strategy

Use:

1. M3 for all structural joints
2. M1.6–M2 only for servo mounting if needed

Add:

1. Heat-set insert pockets (HIGHLY recommended)
2. Or nyloc nut traps

Print orientation

1. Arms: print flat, layers along length
2. Servo mounts: print so layers resist torque
3. Material:
4. PLA+ (ok)
5. PETG (better)

Power (VERY IMPORTANT)

DO NOT power servos from Arduino.

Correct setup:

5V 4A → PCA9685 V+

Common ground:

Power supply GND

PCA9685 GND

Arduino GND

You have two PCA9685s → plenty of channels.

15. Wiring

For each servo:

Brown/Black → GND

Red → 5V (from PCA board)

Yellow/Orange → Signal

Arduino → PCA9685:

SDA → A4

SCL → A5

VCC → Arduino 5V

GND → GND

FIRST MOVEMENT TEST

Bench test servos first

Before installing:

1. Connect ONE servo
2. Sweep slowly
3. Confirm:
4. Direction
5. Range
6. No jitter

Mount in neutral position

1. Set all servos to 90°
2. Attach arms straight
3. This saves your sanity later

Incremental assembly test

After each joint:

1. Mount
2. Test motion
3. Check for binding
4. Only then continue

CONTROL LOGIC (starter level)

Simple control approach

Start with:

1. Manual angle control
2. One joint at a time

Then upgrade to:

1. Coordinated movement
2. Inverse kinematics

Future upgrades

Once it works:

1. Add end effector
2. Add encoders
3. Add ROS
4. Add vision
5. Lose weekends forever