Programming Guide

Unlocking the Beast: Why Fanuc Custom Macro B is the Ultimate Level-Up for CNC Geeks

A practical guide to Fanuc Custom Macro B for CNC machinists: variables, arithmetic, and machine logic for parametric and intelligent machining.

Published March 10, 2026 · Lukas Knop

If you have been in CNC long enough, you know the difference between a parts changer and a machinist. If you want to move into top-tier manufacturing, there is one gatekeeper you eventually face: Fanuc Custom Macro B.

To most operators, a G-code program is a static list of coordinates. To a macro programmer, it becomes a living algorithm that can react, calculate, and decide.

What exactly is a custom macro?

Think of Macro B as a hybrid between standard G-code and a programming language mindset.

Standard G-code says:

• Go to X10.0
• Drill at Z-12.0
• Repeat fixed cycle

Macro B lets you say:

• Read measurement data
• Calculate correction values
• Apply offsets conditionally
• Re-run a pass only when needed

The foundation from the Fanuc documentation is built on three pillars: variables, arithmetic, and logic.
Reference: Fanuc Custom Macros documentation

The three pillars of macro mastery

1) Variables: the memory banks

In standard code, numbers are fixed. In macros, values live in variables like #100 or #500.

• Local variables (#1 to #33) are used inside a macro call for temporary calculations.
• System variables (#1000+) connect to machine state such as position, offsets, timers, and controller data.

This is what makes adaptive logic possible.

2) Arithmetic: let the machine do the math

Macros can evaluate math directly, including trig and square roots.
Example:

#101 = #102 * COS[45]

Instead of hand-calculating coordinates and risking transcription errors, the control computes values at runtime.

3) Logic (IF, GOTO, WHILE): the machine brain

Logic is where Macro B becomes a true process tool:

• IF for condition-based branching
• WHILE loops for repetition with controlled increments
• alarms and condition checks for safer automation

Example use case: if a condition indicates a broken tool, trigger alarm or swap to a sister tool strategy before continuing.

Real-world applications for hobbyists and pros

Why spend time on macro syntax? Because it unlocks practical advantages:

• Parametric programming: one master program for part families, only a few variables change.
• Custom probing cycles: build checks tailored to your parts and setup strategy.
• Error-proofing (poka-yoke): verify setup conditions before material removal starts.

How to get started without crashing confidence

• Start in MDI: set a variable like #100=10, then execute G0 X#100.
• Use a simulator first: test IF/THEN and loops safely before running near hardware limits.
• Think in loops: when repeating patterned operations, write one loop with controlled increments instead of copy-paste code.

Bottom line

Learning Fanuc Macro B is not just coding for coding’s sake. It changes how you think about machining: from writing lines to designing behavior.

It takes you from using the machine to architecting process logic.