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Tool Changes & Offsets

Changing tools mid-job allows you to use different end mills (e.g., roughing, finishing, v-carving) in a single project. grblHAL supports standard G-code tool change commands (M6) and length offsets (G43).

The Basic Tool Change (M6)​

By default, when grblHAL encounters an M6 T<n> command:

  1. It stops the spindle (M5).
  2. It pauses the program (Feed Hold).
  3. It waits for you to physically change the tool.
  4. It waits for you to press Cycle Start to resume.

The Problem: The new tool is likely a different length than the old one. If you just resume, your Z-zero will be wrong, and you'll cut air or crash into the bed.

Solving the Length Problem​

You have two main strategies:

Strategy 1: Re-Zero Z (Simple)​

After changing the tool, you manually re-probe or re-zero the Z-axis to the top of the stock.

  1. Job Pauses (M6).
  2. Change Tool.
  3. Jog to a clear spot.
  4. Probe Z / Set Z Zero.
  5. Resume Job.

Pros: Simple, works with any sender. Cons: Risk of losing X/Y position if you move the machine too much; requires accessible stock surface.

Strategy 2: Tool Length Offsets (Professional)​

This method uses G43 to compensate for the difference in length between tools.

Concept:

  • Reference Tool (T1): The tool you set Z-zero with. Length Offset = 0.
  • New Tool (T2): Longer or shorter than T1.
  • Offset (H2): The difference in length.

Workflow:

  1. Measure T1: Touch off T1 on a fixed sensor (tool setter). Store this position.
  2. Run Job with T1.
  3. Change to T2.
  4. Measure T2: Touch off T2 on the same fixed sensor.
  5. Calculate Difference: Offset = T2_Position - T1_Position.
  6. Apply Offset: G43 H2 automatically adjusts Z.

Configuring grblHAL for Tool Changes​

You typically handle tool changes via your Sender Software (ioSender, UGS, etc.) using macros.

Example Macro: Semi-Auto Tool Change​

A typical macro for a machine with a fixed tool setter:

; --- Tool Change Detected ---
M5              ; Stop Spindle
G53 G0 Z0       ; Lift Z to safe height
G53 G0 X0 Y0    ; Move to tool change position (front-left)
M0              ; Pause for user to change tool

; --- User Changes Tool & Presses Start ---
G53 G0 X200 Y200 ; Move to Tool Setter location
G38.2 Z-100 F300 ; Fast probe down
G91 G0 Z2       ; Retract 2mm
G38.2 Z-5 F50   ; Slow probe for accuracy
G10 L20 P1 Z... ; Update Z offset (Logic depends on sender script)
G53 G0 Z0       ; Retract safely

Note: Advanced senders like ioSender have built-in "Tool Change" tabs where you simply enter the position of your tool setter, and it handles the math automatically.

Settings Used​

  • $341: Tool Change Mode (if enabled in firmware).
  • $342: Tool Change Probing Distance.
  • $343: Tool Change Locate Feed Rate.

Automatic Tool Changers (ATC)​

For machines with an automatic spindle (ISO20/30, BT30) and a tool rack:

  1. Requires specialized hardware (pneumatic drawbar, tool forks, sensors).
  2. Requires Macros: Custom M6 macros to handle the pick-and-place logic.
  3. Requires I/O: Inputs/Outputs for Drawbar Open/Close, Dust Shoe Open/Close.

grblHAL is fully capable of driving ATCs, but it requires compiling a custom build with the ATC Plugin or writing complex macros.

Relevant Settings:

  • $675: Macro ATC Options (Enable M6 T0, enforce tc.macro presence).
  • $683: ATCi Configuration (Rack monitoring and keepout zones).

Best Practices​

  1. Always use a "Safe Z" height before moving to the tool change position.
  2. Clean the collet every change. Dust adds runout.
  3. Measure twice. If using offsets, verify the first tool change carefully.

Next: Explore specialized spindle and laser capabilities: Spindles & Lasers Setup