Fanuc Probing Basics: Measuring, Setting, and Verifying

Probing on a CNC isn’t just a “luxury automation feature.” It’s a way to turn your machine into a measuring device — one that can find features, set offsets, and verify dimensions automatically. On Fanuc controls, probing is particularly powerful because the control integrates probing cycles directly into its logic and supports advanced routines for work and tool setup.

But there’s a learning curve: how do you measure, how do you set values, and how do you verify them in a way that actually improves accuracy and saves time? Let’s walk through it with real shop insights.

Why Probing Matters on Fanuc Controls

On a Fanuc CNC, probes are not just “point and stop” devices — they’re tools that interact with your CNC’s motion control logic. You send a probing command, the machine moves the probe until contact is detected, and the controller records the exact position where that contact happened.

This transforms Fanuc machines from cutters to measuring machines capable of:

• Setting work offsets (e.g., G54/G55),
• Measuring tool length and diameter,
• Locating features like bores and edges,
• Verifying geometry before machining begins,
• And—even in some setups—performing in-process inspection.

A well-tuned probing routine reduces setup time, boosts repeatability, and cuts scrap — because the machine isn’t guessing where edges or surfaces are; it’s measuring them for you.

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Measuring with Fanuc Probes: The Fundamentals

Fanuc supports several probing approaches — from simple skip commands to full macro cycles:

G31 – The Skip (Basic Stop on Contact)

This is the simplest form of probing available on Fanuc:

• You command movement (e.g., G1) with the addition of G31,
• Motion continues until the probe hits the surface,
• The controller automatically stops the move and logs the position.

In practice, this looks like:

G91 G31 Z-50. F100

This means: in incremental mode, move Z down until the probe triggers. When contact happens, the motion stops immediately and the controller records the position.

For basic edge or surface detection, this is often all you need — but this doesn’t automatically set offsets yet.

G65 – Calling a Probing Macro

For real work offset setting and standard tasks (like edge finding or center finding), most Fanuc machines use probing macros called with G65. These macros wrap the logic you need — approach, contact point capture, offset math, and storage.

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Setting Work Offsets (X/Y/Z) on Fanuc

A typical use case: finding the part surface and setting your work coordinate system accurately so your machining references are true.

Here’s the general logic:

• Call the probe tool (T number set to the probe),
• Move to an initial approach position near where you want to measure,
• Execute a probing cycle (often via G65 macros),
• The controller records the touch point and computes the offset.

Fanuc stores the value in a work offset like G54.

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Don’t Just Measure — Verify

Measuring once and assuming the result is correct can be risky — especially on critical work where fixturing may shift slightly or raw stock isn’t uniform.

Verifying involves one of these approaches:

• Double-Touch Method: Touch the surface, retract a bit, then re-touch the same surface. If the two touches record the same point, the probe measurement is consistent.
• Check Multiple Surfaces: For edges and corners, touching multiple sides can help verify part orientation and fixturing alignment.

This deeper intuition — measuring to verify, not just measure once and trust blindly — is what separates robust probing routines from fragile setups.

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Fanuc Tool Setting — One More Piece of the Puzzle

Tool length offsets are just as critical as work offsets. On Fanuc systems, tool probing often uses routines like G76 for tool measurement — telling the machine to measure the tool tip position relative to a known surface and updating the tool offset register.

Unlike the part work offsets (G54, G55, etc.), which are stored in work coordinate registers, tool measurements populate the tool offset registers (H values). Making sure these values are measured and verified before machining starts ensures consistent depth of cut and part accuracy.

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Verification — The Step Most Shops Skip

Setting offsets is half the battle — verifying them is the rest.

Good fans of probing routines don’t just say “touch and set.” They say:

• Touch once, record, retract, and touch again,
• Compare against expected geometry,
• And if unexpected variation appears, halt or alert the operator.

This transforms probing from a one-time measurement into a confidence check of your machining foundation.

It’s the difference between “I think the origin is right” and “The control knows the origin is right.”

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When Probing Beats Edge Finders on Fanuc

The old manual method – edge finder or dial indicator – relies on operator feel and interpretation. It’s effective in hands of experienced machinists, but it carries:

• Human variability,
• Manual offset entry errors,
• Lack of repeatability between operators.

Probing on Fanuc gives you:

• Automatic coordinate capture,
• Elimination of manual number entry,
• Lower variability between operators,
• Faster setup and verification cycles.

What you lose in tactile “feel,” you gain in data and confidence — and modern production environments value data over feel every time.

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A Deep Shop Insight: Thinking Like a Measurer

Here’s the mental shift that separates average probing usage from advanced use:

Probing is not “move until you feel contact” — it’s “collect precise coordinate data and use that to inform cutting decisions.”

On Fanuc, that means using:

• G31 skip moves for single touches,
• Macro cycles (G65 calls) for stored, reusable routines,
• Offset registers to retain and use the measurements,
• Verification moves to confirm accuracy.

That’s what real metrology-driven machining looks like — not just hardcoded G code.

Closing Thought

Fanuc probing isn’t just a checkbox in a manual — it’s a toolbox for precision preparation. Measuring without setting and verifying is like taking a measurement but not recording it — the value is lost.

By integrating probing routines that measure, set, and verify:

• Your setups become faster,
• Your results become more consistent,
• Your scrap goes down,
• And your confidence goes up.

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