Guide to Building a DIY Scara Robot for PCB Laser Engraving Using GRBL Firmware and Stepper Motors
Building a DIY SCARA Robot for PCB Laser Engraving
A step-by-step guide using GRBL, stepper motors, and precision mechanics for reliable PCB prototyping
What You’ll Build: A dual-arm SCARA robot capable of precise X–Y positioning (±0.05 mm) with laser engraving capabilities, all controlled by GRBL—no 3D printer or expensive kits required.
Why SCARA for PCB Engraving?
SCARA (Selective Compliance Articulated Arm Robot Arm) robots excel at horizontal plane tasks—exactly what PCB laser engraving demands. With two parallel rotary joints and rigid vertical compliance, they maintain accuracy across the working area, unlike Cartesian or delta designs.
By pairing GRBL (the same firmware that powers cheap laser cutters) with NEMA 17 stepper motors and timing belts, you get industrial-grade repeatability at a fraction of the cost. This guide walks you through hardware, firmware, and calibration—everything you need for home-grade precision.
1. Core Components & Tools
Don’t over-engineer the foundation. Start here:
- NEMA 17 stepper motors × 2 (1.7A, 1.8° step angle)
- GT2 6mm width timing belts × 2 (length depends on frame size)
- GT2 20-tooth pulleys × 4 (or 16T for higher speed)
- Aluminum extrusion (2020 or 2040) or laser-cut acrylic frame
- Arduino Uno or RAMPS 1.4 + A4988 stepper drivers
- 5V laser diode module (300–500 mW for PCB)
- 3D printer or CNC mill for custom brackets
- Digital calipers (0.01 mm accuracy)
- Stepper motor torque wrench (for belt tensioning)
- USBasp or ISP programmer (for GRBL upload)
- Laser safety goggles (OD4+ for 450nm blue lasers)
2. Mechanical Assembly
Pro Tip: Design the arms with a “Z-axis” offset—position the second motor 5–10 mm behind the first to reduce collision risk during rapid moves and simplify belt routing.
Step 1: Frame Construction
Cut extrusion to match your target work area (e.g., 250 × 250 mm). Mount first motor at shoulder (base), second at elbow. Ensure both arms rotate freely on low-friction bearings (e.g., 608ZZ). Use 3D-printed or machined hubs for belt tensioning.
Step 2: Belt Drive Setup
Attach pulleys directly to motor shafts. Route belts in a crossed configuration: the first belt connects shoulder pulley to elbow pulley, while the second belt loops back to the tool head. This mimics real SCARA kinematics—parallel motion without twisting.
Step 3: Tool Head Integration
Mount a lightweight aluminum or acrylic tool plate at the end effector. Include a bracket for your laser module, aligned with the second arm’s pivot point. Add a microswitch or limit switch to detect tool height (for Z-zero). A spring-loaded carriage helps compensate for slight belt stretch.
Mismatched belt pitch or pulley tooth counts introduce cumulative error. Double-check tooth counts and match pulley ratios precisely—1:1 is safest for beginners. Measure belt tension with a phone app (e.g., Belt Tension Meter); optimal frequency: 100–120 Hz.
3. Electronics & GRBL Setup
GRBL 1.1+ supports 2-axis motion with minimal tweaks. For SCARA, you’ll use a hybrid mode: one motor controls the base (X), the other controls the forearm (Y), but the actual X–Y workspace is computed in software.
Wiring Overview
| Component | Arduino Pin | Function |
|---|---|---|
| Stepper 1 (Shoulder) | X (D2, D3) | Step/Dir |
| Stepper 2 (Elbow) | Y (D4, D5) | Step/Dir |
| Laser Enable | Z (D6) | PWM / Enable |
| Endstops | Xmin, Ymin | Home switches |
Why Z = Laser? GRBL repurposes the Z signal for laser PWM on pins D6/D7 when using $32=1 (laser mode). This simplifies control without extra hardware.
Flashing GRBL
Use GRBL 1.1f (lightweight and stable). Flash via Arduino IDE:
- Download GRBL source and open
grblfolder in Arduino IDE. - Select “Arduino Uno” board and correct port.
- Upload sketch (disable serial monitor to avoid conflicts).
Next, configure GRBL via GRBL Control (Windows/macOS) or bCNC.
Key Settings
Calibrate steps/mm for your belts and pulleys:
$0=40 (step pulse, µs) $1=25 (step idle delay, ms) $2=0 (step port invert) $3=6 (direction invert: 6 = X and Y reverse) $4=0 (enable invert) $5=0 (limit pin invert) $6=0 (probe invert) $10=1 (report in mm) $11=0.010 (junction deviation, mm) $12=0.002 (arc tolerance, mm) $13=0 (report in absolute) $20=0 (soft limits off by default) $21=1 (laser mode) $22=0 (homing force-override) $23=0 (homing direction invert) $24=25.000 (homing feed, mm/min) $25=500.000 (homing seek, mm/min) $26=250 (homing debounce, ms) $27=1.000 (homing pull-off, mm) $100=80.000 (X steps/mm) — see calculation below $101=80.000 (Y steps/mm) $102=0 (Z steps/mm) $110=1000.000 (X max rate, mm/min) $111=1000.000 (Y max rate) $120=10.000 (X acceleration, mm/s²) $121=10.000 (Y acceleration) $130=250.000 (X max travel, mm) $131=250.000 (Y max travel) $132=0 (Z max travel)
How to Calculate Steps/mm:
Steps/mm = (steps per revolution × microstepping) / (pulley teeth × belt pitch)
Example: 200 steps/rev × 16 microsteps ÷ (20 teeth × 2 mm pitch) = 160 steps/mm for each axis. Adjust $100 and $101 accordingly.
Warning: Always verify $100/$101 with a dial indicator. If you misplace the decimal (e.g., entering 8 instead of 80), your laser will engrave at 1/10th the intended scale.
4. Kinematics & Calibration
GRBL doesn’t natively interpret SCARA joint angles. Instead, you’ll simulate Cartesian motion by treating the joint coordinates as a virtual workspace—then use a lookup table or small script to convert G-code positions to motor steps.
Option 1: Direct Mapping (Quick Test)
Homing sequence moves both arms to home positions (0,0). After calibration, $100 and $101 define your linear scale. For simple flat PCB work, this works—if you accept minor non-linearity at extreme angles.
Option 2: Forward Kinematics (Recommended)
A small Python or Arduino sketch computes X/Y from joint angles. Example Arduino code snippet:
// Forward Kinematics (in mm)
float L1 = 120.0; // Length of first arm
float L2 = 120.0; // Length of second arm
void computeXY(float theta1, float theta2, float &x, float &y) {
// Angles in radians
x = L1 * cos(theta1) + L2 * cos(theta1 + theta2);
y = L1 * sin(theta1) + L2 * sin(theta1 + theta2);
}
Store this in your GRBL controller or use a post-processor in Easel, LightBurn, or Fusion 360 to precompute moves.
Calibration Workflow
- Homing: Engage endstops at $130/$131 limits.
- Grid Test: Laser-etch a 10 × 10 mm grid. Measure with calipers or microscope.
- Backlash Check: Engage each axis, move 5 mm forward/back, note displacement.
- Offset: Add $133.5 (backlash compensation) if needed.
Goal: Maintain ≤0.05 mm error over the full bed for PCB trace work (target: 10 mil / 0.25 mm lines).
Laser Safety First: Always test power at low duty cycles (e.g., S500). Never leave the engraver unattended—set up a smoke detector or fireproof mat beneath the PCB.
5. Workflow Integration
Once built, integrate into your PCB process:
- Design: Export Gerber files from KiCad or Eagle. Convert to G-code using gerbvor or LightBurn’s Gerber plugin.
- Fixturing: Use double-sided tape or vacuum hold-down to prevent PCB movement during engraving.
- Engrave: Set feed rates to 40–60 mm/min and laser power to 10–25% for standard FR-4 boards. For fine traces, use two passes.
- Inspect: Check with a magnifier. Adjust $32 (laser mode) to match pulse width.
Pro Tip: Add a “safety pause” via $100 = 1 (door switch) to halt the laser when the cover opens—critical for DIY setups.
6. Maintenance & Optimization
Longevity comes from simple habits:
- Monthly: Re-tension belts (check frequency resonance).
- Weekly: Clean lens and optics with isopropyl alcohol.
- Every 100 hours: Lubricate linear rails or bushings (dry PTFE only).
Update GRBL periodically—newer versions improve acceleration blending and reduce stutter on complex paths. Back up your settings first with $x export.
Final Thoughts
This project marries accessibility with performance. While industrial SCARAs cost $20,000+, your DIY version—built for under $350—delivers the accuracy and control needed for professional-grade PCB work. The real magic isn’t just moving arms; it’s in the precision of each step, calibrated, tested, and trusted.
Now go engrave something bold.
Comments
Post a Comment