Advanced G-Code Programming: Macros, Subprograms & Conditional Logic

Advanced CNC programming goes beyond simple motion commands. It incorporates macros, subprograms, and conditional logic, allowing for intelligent automation, reusable cycles, and powerful parametric programming.

This guide explores the deepest layers of G-code programming using real-world examples, across systems like Fanuc, Haas, Siemens, and others.

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📘 Table of Contents

1. Introduction to Macro Programming
2. Subprograms (M98/M99)
3. Macros with G65/G66/G67
4. System Variables (#100+)
5. Conditional Logic (IF, GOTO, WHILE, DO/END)
6. Realtime Decisions in CNC
7. Practical Applications
8. Safety & Debugging Techniques
9. Brand-Specific Notes
10. Final Tips

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1. 🧠 What is Macro Programming?

Macro programming allows you to define parameterized blocks of code that can be reused, customized, and evaluated at runtime. Think of it like CNC’s version of “functions” in programming languages.

Benefits:

• Reduces code duplication
• Enables smart behavior (like decision trees)
• Can react to sensor inputs or process data
• Highly customizable machining cycles

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2. 🔁 Subprograms – M98 / M99

A subprogram is a separate block of G-code saved under a different program number that can be called multiple times.

🔹 Format

O1000 (Main Program)
M98 P2000 L3 ; Call subprogram O2000 three times
M30

O2000 (Subprogram)
G1 Z-1.0 F100.
G1 Z1.0
M99

• M98 calls the subprogram.
• M99 returns to the main program.
• L is repeat count.

🔹 Use Cases

• Drilling multiple holes
• Repeating motion at multiple coordinates
• Tool checks or safe positioning

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3. ⚙️ G65/G66 – Calling Macros

🔸 G65 – Non-modal macro call

G65 P1000 A2.5 B4.0

• P1000 = O1000 program
• A and B = arguments passed as #1, #2, etc.

🔸 G66 – Modal macro call

G66 P1001 A2.5 B4.0
G1 X50 Y50
G1 X100 Y0
G67 ; Cancel modal macro

• Runs the macro every time a motion block is executed.
• Useful for probing or on-the-fly measurements.

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4. 🔢 Variables in G-Code

CNC machines support variable usage via system variable ranges.

Range	Description
#1–#33	Local variables (macro arguments)
#100–#199	User-defined general vars
#500–#999	Persistent across reboots
#1000+	System status values

🔹 Example: Using Variables

#100 = 0
WHILE [#100 LT 5] DO1
    G1 X[#100 * 10] F100.
    #100 = #100 + 1
END1

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5. 🧮 Conditional Logic – IF, WHILE, GOTO

You can make decisions based on variable values:

🔸 IF Statement

IF [#100 EQ 0] THEN
    (MSG, Tool Not Detected)
    M30
END

🔸 GOTO + Labels

IF [#101 GT 5] GOTO 50
...
N50 G1 Z50

🔸 WHILE Loop

#1 = 1
WHILE [#1 LE 10] DO1
    G0 X[#1*10]
    #1 = #1 + 1
END1

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6. 🧭 Real-Time CNC Decision Making

Macros enable decision-making based on:

• Probe data
• Tool offsets
• Alarm states
• Timer values
• Sensor or I/O signals

🔹 Probe Example (Fanuc)

#505 = 0 ; Clear variable
G65 P9810 Z-10.0 F100. ; Run probe macro
IF [#505 GT 0.5] THEN #3000 = 1 (Part too high)

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7. 🛠️ Real Applications

Application	Tool Used	Description
Tool breakage detect	IF / G65 / Macro	Compare load + tool time
Multi-part machining	Subprograms	Process same part N times
Adaptive roughing	Variables + IF	Change feedrate based on depth
Pallet shuttles	Macro + M-code	Conditional start sequence
Smart probing	G66 + G65	Auto-offset measurement

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8. 🧪 Safety, Debugging & Simulation

• Use dry-run (M01, block skip) on macro-heavy programs
• Monitor variables on the control screen
• Use M0 or MSG to pause or display debug info
• In Fanuc, use #3000=1 to trigger custom alarms

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9. 🏭 Brand-Specific Macro Notes

🟨 Fanuc

• Supports full macro logic
• Use G65, G66, IF, WHILE
• Persistent variables: #500–#999

🟩 Haas

• Supports G65, IF, WHILE
• Macros optional (needs macro option enabled)
• Persistent memory: #500–#699

🟦 Siemens

• Logic via R, IF, TRANS, FRAME
• Cycles created using Cycle 800+ or user cycles
• Supports inline math

🟥 Heidenhain

• Uses Q parameters and logical IF
• Structured conversation format
• Requires level 3 access for full logic

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10. 💡 Final Tips

✅ Use macros for repeatability
✅ Use subprograms to modularize code
✅ Use conditional logic to avoid collisions and tool errors
✅ Never assume a variable value — always initialize it
✅ Comment generously for maintenance

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📌 Summary

Advanced G-code programming with macros, subprograms, and conditional logic gives you the power to:

• Optimize cycle time
• Automate safety procedures
• Build reusable machining blocks
• Enable real-time intelligence in machining

🚀 Master these tools to elevate your CNC skills to engineering level.