How to Build a GRBL Arduino CNC Plotter with Servo Control

An electronic leadscrew replaces traditional mechanical change gears in CNC lathes by using a stepper motor controlled by a microcontroller to synchronize the lead screw with the spindle, eliminating the need for manual gear changes and providing flexible, programmable thread cutting capabilities.

The Z-axis uses a 16mm pitch, 10mm lead screw with 20mm aluminum linear guide. Initial builds use linear guides (pistons), while upgraded machines benefit from superior linear guide systems. Motor selection requires NEMA 34 stepper motors (100gf, 34oz) for X and Y axes, with the Z-axis requiring a more powerful DM8600 driver. Proper motor configuration prevents step loss. Belt tensioning uses aluminum brackets, with pulleys (20 teeth) and tensioning pulleys maintaining proper tension and eliminating play.

This video demonstrates how to build a 3D printed CNC plotter machine using NEMA 17 stepper motors for X and Y axes, a mini servo motor for Z-axis pen lifting, and an Atmega-based custom PCB for control; the project involves assembling 3D-printed parts, wiring electronics with limit switches, installing GRBL firmware for G-code control, and using Inkscape with a Forex Draw extension to convert images to G-code for automated drawing.

This segment provides a detailed comparison between stepper and servo motors for the electronic lead screw application. The presenter swaps the NEMA 24 stepper for a NEMA 23 servo motor and adjusts the drive to a 3:1 belt ratio (24-tooth to 72-tooth pulleys). The presenter notes that the servo makes more noise than the stepper despite having a DSP driver, but runs cooler despite being smaller with less surface area for heat dissipation. The presenter explains that servo motors run with lower current when not needing to maintain synchronization, only peaking current when necessary. The presenter demonstrates that the servo handles the same material removal operations (cutting half-13 threads in steel) effectively, with the presenter expressing satisfaction with the servo's performance and smaller size.

When upgrading to dual Z-axis, the original lead screw (36.5 cm) is replaced with a longer one (38.1 cm). The process involves placing something below the screw to hold it in place when unscrewing, then removing the original screw. The new screw must be rotated into place completely to the bottom of the step hole before tightening the bolt. Both upper and lower bolts must be tightened securely to prevent loosening during printing.

The $32 GRBL setting controls whether your laser operates in 'laser mode' or continuous mode. When $32 = 0, the laser stays on continuously, creating connecting lines and burn spots. When $32 = 1, the laser turns off between passes, producing clean, separated engravings. To access this setting, you need GRBL firmware version 1.1f or higher - older versions (like 0.9) don't display these settings. You can modify $32 by typing $32=1 in Laser GRBL or LightBurn's console. This fundamental setting enables proper laser operation for desktop engraving machines.

GRBL 1.1 introduces three new parameters for laser mode: Parameter 30 sets maximum spindle speed (maximum laser power), Parameter 31 sets minimum spindle speed (minimum laser power), and Parameter 32 enables/disables the laser module. The PWM output on pin 11 varies from 0 to 5 volts, directly controlling laser power proportionally to spindle speed settings. The M3 command activates the laser at constant power regardless of machine movement, maintaining the last configured spindle speed setting. The M4 command activates the laser with dynamic power adjustment based on machine velocity, automatically increasing power as the machine accelerates and decreasing as it decelerates. The laser only activates during cutting movements (G1, G2, G3) and not during rapid positioning (G0). To configure GRBL 1.1 for laser mode, access the configuration file (config.h) and ensure the spindle PWM variable is assigned to pin 11 (analog output) rather than pin 12 (digital output). After uploading GRBL to the Arduino, use the Serial Monitor to activate the laser module by sending 'G21=1'. To measure PWM output, connect a multimeter to pin 11 and ground, set to DC voltage measurement. The multimeter displays voltage corresponding to current laser power: 5 volts at maximum spindle speed (1000 RPM), 2.5 volts at half speed (500 RPM), and 0 volts at minimum speed. The M5 command always disables the laser. Commands G0 (rapid positioning), G80 (cancel cycle), and G38 (test move) will always disable the laser if active. The machine enters a safety mode and stops movement when laser state changes occur. Users should test both M3 and M4 modes with their specific materials to determine which produces better results for their particular application.

GRBL firmware, originally designed for CNC milling machines, includes a laser mode ($32) that enables the laser to turn off during travel movements. If the laser doesn't turn off during travel, check the $32 setting by typing $$ in the console. The value should be 1 for laser mode. If it returns 0, enter $32=1 into the console and verify with another $$ command to confirm the change.

Laser GRBL software requires installing the CH340 driver for first-time connections and enabling Z-axis controls in settings. Critical configuration involves matching the GRBL $30 setting to your test file's S-value (typically 1000) to ensure accurate power delivery. Custom buttons simplify operations including low-power focusing mode, full-power activation, and mode switching between router ($32=0) and laser ($32=1) modes.

GRBL is a CNC software originally designed for milling machines that can be adapted for laser cutting and engraving by converting rotational speed settings to laser power percentages; to configure laser power, users must find their machine's maximum RPM setting in the GRBL configuration file (e.g., 10,000 RPM for a 10W laser module), then set the desired power percentage by dividing the target RPM by the maximum RPM (e.g., 4000 RPM represents 40% power when maximum is 10,000 RPM), with different modes available for engraving (line-by-line tracking, matrix, horizontal) and cutting (vector mode), and the software supports simultaneous cutting and engraving operations with adjustable speed, power, and pass settings.

CNC machine accuracy depends on properly configuring the steps per millimeter (steps/mm) parameters ($100 for X-axis, $101 for Y-axis, $102 for Z-axis) in GRBL firmware, which can be calibrated by measuring actual travel distance against commanded distance using painters tape and vernier gauge, then iteratively adjusting these values until the machine achieves precise positioning within acceptable tolerances for the intended application.

Configure steps per unit: X and Y are 100, Z is 400 (for 4mm lead screws), and E is 140 (for Mark 3 extruder at 16x microstepping). Acceleration and feed rates can be left at defaults and tuned later. Classic jerk is preferred over junction deviation for better linear advance compatibility.

Parameters 100 (G100), 101 (G101), and 102 (G102) control steps per millimeter for X, Y, and Z axes respectively. This is calculated as: (steps per revolution × microstepping) / (millimeters per revolution). For a typical motor with 200 steps per revolution and 16 microstepping, the formula is (200 × 16) / (millimeters per revolution). Each axis requires its own calculation based on its lead screw pitch.

This section covers the critical process of tuning machine axes for optimal performance. Steps per millimeter calculation depends on motor resolution, lead screw pitch, and gear ratio—for example, a 200-step motor with 1mm lead screw yields 1000 steps/mm. Maximum speed (mm/minute) and acceleration (mm/second²) settings determine how quickly axes can move. Users start with standard values (500 mm/min, 10 mm/s²) and incrementally increase them to find maximum limits without step loss. The tuning process involves moving axes to known positions, issuing movement commands, and observing whether targets are reached. Open loop control is preferred for stepper motors as it cannot detect missed steps, requiring careful parameter tuning.

The correct step/mm values can be discovered empirically through testing rather than calculation. The process involves: (1) Setting a known distance (e.g., 10mm or 1cm) in the CNC software, (2) Commanding the machine to move that distance, (3) Measuring the actual distance traveled using a ruler or caliper, (4) Adjusting the step/mm value based on the difference between commanded and actual distance. This method works because step/mm values depend on motor characteristics, lead screw pitch, and gear ratios that vary between machines.