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Tool Length Offset Basics
Guide · Updated October 27, 2026
A tool length offset tells the control how long each tool is, so programmed Z positions land at the right depth. A wrong offset is one of the most common causes of crashes.
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Step by step
- Choose a reference: Use the spindle gage line, a master tool, or the part top.
- Touch off: Use a tool setter, or a gage block on the part: jog down until the block just slides under the tool.
- Enter the value: Store the Z reading, minus the block height if you used one, in the tool's H register.
- Call it: Use G43 H(n) with the tool number in the program.
- Verify: Run the first move high above the part, or single-block.
Tips
- Subtract the gage block height from the touch-off value.
- Re-touch after any tool change in the holder.
- Store a spare 1.000 in block at the machine.
Related: edge finders, milling speeds and feeds.
Quick shop check
Example: the master tool touches the Z setter at Z−10.0000 in machine coordinates and a drill touches at Z−8.2500 in. Its length offset is 1.7500 in longer than the master. If the control uses gage-line lengths, measure each tool from the spindle gage line instead. A wrong offset sign is the classic crash: run the first part with a single block and 25% rapid override.
Worked example: touching off a tool with a gage block
With the part top as Z zero, load the tool and jog down over the part with the spindle stopped. Slide a gage block of known height under the tool, and creep down in small steps until the block just slides under with a light drag. Take the Z reading, subtract the block height, and store the result in that tool's H register. In the program, call G43 with the matching H number. Before cutting, run the first Z move high above the part in single block and check that the distance to go matches what you expect.
FAQ
What is a tool length offset?
A stored length value, the H register, that the control uses with G43 to correct Z for each tool.
How do I set it with a gage block?
Jog down until the block just slides under the tool, then subtract the block height. Build any reference size free with the Gage Block Calculator at gageblockcalc.com.
What causes tool length crashes?
A wrong H number, the block height not subtracted, or a missing G43.
How do I verify it?
Single-block the first Z move with a high clearance.
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General shop information only — follow your gage manufacturer's instructions and your quality system.