Probing Guide

Probing is the process of using a specialized sensor (a probe) to automatically find the position of your workpiece or tool. It turns your CNC machine into a measuring device, allowing you to set Work Zero offsets with extreme accuracy.

How it Works

A probe acts like a switch. When the tool touches the probe (or the workpiece), the circuit closes, and grblHAL captures the exact position.

Common Probe Types

  1. Touch Plate (Z-Probe): A metal plate of known thickness used to find the top of the material.
  2. XYZ Probe / Corner Finder: A block that allows finding X, Y, and Z zero simultaneously at a corner.
  3. Tool Setter: A fixed switch on the table used to measure tool lengths.
  4. 3D Touch Probe: A precision stylus mounted in the spindle for digitizing surfaces or finding edges.

Configuration

1. Wiring

  • Signal Pin: Connect to the Probe input on your controller.
  • Ground: Connect to the machine ground (often the spindle body or a clip attached to the end mill).
  • Test: Use a multimeter to ensure continuity when the tool touches the plate.

2. Inverting Input ($6)

  • Use $6 to invert the probe input if it reports "Triggered" when not touching.
  • Check status with ? status reports. Pn:P indicates the probe is triggered.

The G38 Probing Commands

grblHAL supports standard G-code probing cycles:

Command Description Stop Condition Error on Fail?
G38.2 Probe toward target Contact Made Yes (Alarm)
G38.3 Probe toward target Contact Made No
G38.4 Probe away from target Contact Lost Yes (Alarm)
G38.5 Probe away from target Contact Lost No

Most common usage: G38.2 (Move until touch).

Syntax

G38.2 [Axis][Target] F[Rate]

Example:

G38.2 Z-25 F100

Meaning: Move Z down toward -25mm at 100mm/min. Stop immediately upon contact.

Practical Examples

1. Simple Z-Zero (Touch Plate)

Suppose you have a 10mm thick touch plate.

  1. Jog the tool roughly 5-10mm above the plate.
  2. Attach the magnet clip to the collet/tool.
  3. Run Command:
    G38.2 Z-25 F50    ; Probe down slowly
G10 L20 P1 Z10    ; Set Z Work Offset to plate thickness (10mm)
G91 G0 Z5         ; Retract 5mm relative
    Result: Z0 is now exactly set to the material surface.

2. Finding a Corner (XYZ)

For a corner finder plate (let's say 10mm offset for X/Y/Z).

  1. Position tool inside the corner area.
  2. Probe Z:
    G38.2 Z-25 F50
G10 L20 P1 Z10
G91 G0 Z5
  3. Probe X:
    G91 G0 X20        ; Move X away
G0 Z-10           ; Move Z down
G38.2 X-25 F50    ; Probe X back toward material
G10 L20 P1 X10    ; Set X offset
G0 X5             ; Back off
  4. Probe Y: (Repeat logic for Y axis)

Safety & Troubleshooting

Always Test Connectivity First!

Before running a probe cycle, touch the clip to the plate manually and check the status report (?). If it doesn't show the probe pin active (Pn:P), DO NOT PROBE. You will crash the tool.

Use Two-Stage Probing

For best accuracy:

  1. Probe fast (F300) to find the approximate location.
  2. Retract slightly (1-2mm).
  3. Probe slow (F25) for the precise measurement.

Probe Protection

If you have a specialized probe (like a Renishaw), ensure it's wired Normally Closed (NC) if possible, so a disconnect stops the machine.

Next: Learn how to handle tool changes efficiently: Automatic Tool Changer