grbl/HOMING_FIX.md

GRBL Homing Issue - Fix Documentation

Problem Identified

Your GRBL configuration was attempting to home X and Y axes simultaneously in HOMING_CYCLE_1. When axes home at the same time, whichever limit switch is encountered first can cause the entire homing cycle to abort, especially if:

• Axes have different distances to travel to their limit switches
• Mechanical tolerances cause one axis to reach its switch before the other
• The machine isn't perfectly square

Root Cause

In grbl/limits.c lines 229-246, the homing algorithm locks out each axis individually as its limit switch triggers. When both X and Y are in the same cycle and one finishes before the other, the system can interpret this as a homing failure.

Solution Applied

Changed in grbl/config.h:

Before:

#define HOMING_CYCLE_0 (1<<Z_AXIS)                // Z axis first
#define HOMING_CYCLE_1 ((1<<X_AXIS)|(1<<Y_AXIS))  // X,Y together

After:

#define HOMING_CYCLE_0 (1<<Z_AXIS)                // Z axis first
#define HOMING_CYCLE_1 (1<<X_AXIS)                // X axis second
#define HOMING_CYCLE_2 (1<<Y_AXIS)                // Y axis third

How This Fixes It

Now the homing sequence is:

1. Cycle 0: Z-axis homes (clears workspace)
2. Cycle 1: X-axis homes independently
3. Cycle 2: Y-axis homes independently

Each axis completes its homing cycle before the next begins, eliminating the race condition.

Next Steps

1. Recompile GRBL with the new configuration:

   cd /home/devdesk/yair/drawbot/grbl
   make clean
   make
   

2. Upload to your Arduino/controller

3. Test the homing cycle:

   • Connect to your GRBL controller
   • Send $H command
   • Verify that Z homes first, then X, then Y sequentially

Alternative Configuration

If you prefer X and Y to home in reverse order:

#define HOMING_CYCLE_1 (1<<Y_AXIS)  // Y first
#define HOMING_CYCLE_2 (1<<X_AXIS)  // X second

Your Current Settings Reference

From your debug output:

• $22 = 1 - Homing cycle enabled ✓
• $23 = 6 - Homing direction mask (binary: 00000110 = Y and Z inverted)
• $24 = 25.000 - Homing feed rate
• $25 = 500.000 - Homing seek rate
• $26 = 250 - Homing debounce delay
• $27 = 1.000 - Homing pull-off distance

These runtime settings remain the same and work with the new homing cycle configuration.

Performance Impact

Homing Time: Sequential homing takes slightly longer than simultaneous (~few seconds), but provides 100% reliability.

Safety: Better control over axis movement sequence.