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How to Use a CNC Touch Probe

Guide · Updated October 28, 2026

A spindle touch probe locates edges, bores and bosses and writes the result into a work offset. It is accurate only after calibration: length, stylus runout and effective ball diameter.

Step by step

1. Check stylus runout: Indicate the ball and adjust to near zero runout.
2. Calibrate length: Touch a known surface or tool setter.
3. Calibrate ball diameter: Probe a ring gage of known size.
4. Probe the part: Run edge, bore or boss cycles.
5. Write the offset: Store the result in the work offset.
6. Verify: Check with an indicator or a test feature.
7. Recalibrate: After stylus changes or crashes.

Worked calibration example

A 1.0000 in ring gage and a 4 mm (0.1575 in) stylus ball. A bore cycle measures diameter as center-to-center travel plus the ball diameter it has stored. With the nominal 0.1575 in stored, the probe reports 1.0004 in, so the ball acts 0.0004 in smaller than nominal because of trigger pre-travel. Effective ball diameter = 0.1575 − 0.0004 = 0.1571 in. Store that value (most controls do this automatically in the ring-gage calibration cycle) and re-probe: the ring should read 1.0000 in.

Edges vs bores

A bore cycle finds center from several points and ignores ball size; an edge cycle needs the calibrated ball radius.

Speeds

Probe at the maker's calibrated feed; a different speed changes the trigger point.

Worked example: probing a bore center

A part has a nominal 2.000 in bore at X3.000 Y2.000 from the corner. Probe the bore; the cycle returns center X3.0012 Y1.9995 and diameter 2.0003 in. Write X and Y to the work offset so the bore becomes the datum, or compare to nominal: the bore is 0.0012 in off in X and 0.0005 in off in Y. Probe twice; if the two results differ by more than about 0.0002 in, clean the stylus and bore and recheck stylus runout.

Checking calibration quickly

Probe a gage block stack of known width with an edge-to-edge (web) cycle. A 1.0000 in stack should read 1.0000 in; an error of the same amount on every size means the effective ball diameter is off.

Batteries and signal

Low battery or a blocked optical or radio signal causes missed triggers and alarms. Replace batteries on schedule.

Common problems

Wrong sizes: uncalibrated ball. Wrong centers: stylus runout. False triggers: chips or coolant.

Tips

• Offsets: work offsets.
• Tools: tool length offsets.
• Rings: ring gages.

FAQ

Why calibrate?

So size and position are accurate.

What do I calibrate on?

A ring gage and a known surface.

Does a bore cycle need ball size?

Center does not, diameter does.

How often?

After stylus changes or crashes.

Why use the same feed?

The trigger point depends on speed.

Related guides and calculators

• Work Offset Basics (G54–G59)
• Tool Length Offset Basics
• Ring Gage Basics
• How to Use a Tool Presetter
• Edge Finder Basics
• All free machinist calculators

General shop information only — follow your machine and tooling manufacturer's instructions and your shop's safety rules.