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How I Set Tool Length Offsets on a Hobby CNC with a Z-Axis Tool Setter

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Disclosure: I work with AutomTool, a UK supplier of CNC tool setters. This post is a general how-to and applies to any Z-axis tool setter.

If you run a hobby or small-shop CNC router, setting the Z height after every tool change gets old fast. A paper shim works, but it is slow and the result changes depending on how firmly you feel the drag. A dedicated tool setter (also called a Z-axis probe) removes that guesswork.

What a tool setter actually does

A tool setter is a fixed switch with a known height. The controller lowers the spindle until the tool tip touches the setter and the switch triggers. The controller then records that position and works out the tool length from it. You do this once per tool, and your offsets stay consistent from job to job.

Three things worth checking before you buy one

  1. Repeatability. How closely the switch triggers at the same point each time. For fine engraving or PCB work, look for repeatability in the low microns. The setter I use is rated at under 0.002 mm.
  2. Over-travel protection. If a probe routine overshoots, the spindle can crush the setter or snap a small end mill. A 4-wire design with a normally closed (NC) over-travel circuit acts as a second switch, so the controller can stop the machine before anything breaks.
  3. Controller support. Make sure it wires cleanly into what you run. Mach3, LinuxCNC and GRBL all support probe inputs, and each has a basic tool-length probing routine.

Basic workflow

  • Wire the probe signal to your controller's probe input, and the NC over-travel line to an e-stop or limit input.
  • Measure the setter's height once and enter it in your probing macro.
  • After each tool change, jog over the setter and run the probe routine. The controller updates the tool length offset for you.
  • Run a test cut on scrap to confirm.

Coolant and chips

Shop environments are messy. An IP67-rated setter can handle coolant splash and chip dust, so you can leave it mounted on the bed for good.

The setter I use is from AutomTool, based in Sheffield, UK. If you're setting one up on Mach3, LinuxCNC or GRBL and have questions, ask in the comments.

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