CNC Subprograms & Macros: M98, M99, G65, G66, G67 Explained with Real Examples

Subprograms and macros are essential tools for modular, efficient, and automated CNC programming.
They allow repeated toolpaths, logic-based machining, and parameterized operations without rewriting G-code.

This guide explains M98, M99, G65, G66, and G67 — the foundation of subroutines and parametric control in modern CNC systems.

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📌 1. Overview

Code	Function	Description
M98	Call subprogram	Executes another program (Oxxxx)
M99	Return or loop	Returns to main or restarts sub
G65	Non-modal macro call	Executes custom macro once
G66	Modal macro call	Executes macro continuously until G67
G67	Cancel modal macro	Stops G66 mode

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📌 2. M98 — Subprogram Call

M98 P1000

Calls subprogram O1000 and returns after completion.

You can also specify repeat count and location:

M98 P1000 L3

Calls subprogram O1000 three times.

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📌 3. M99 — Return or Loop

M99

• Inside a subprogram → returns to main program.
• In main program (at end) → loops back to program start.

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📌 4. Example: Simple Subprogram Call

Main Program (O0001):

%
O0001 (MAIN PROGRAM)
G90 G17 G21 G40 G80 G54
T01 M06
M98 P1000 L2
M30
%

Subprogram (O1000):

%
O1000 (DRILL HOLE)
G81 X50. Y50. Z-20. R2. F150
G80
M99
%

The hole cycle runs twice because of L2.

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📌 5. Nested Subprograms (Multi-Level Calls)

M98 P2000
...
O2000
M98 P3000
M99

Subprogram O2000 calls another subprogram O3000, forming a hierarchy.
Used in repetitive machining with different setups.

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📌 6. Local Subprogram (Inside Main Program)

You can define a local subprogram at the end of a main program:

%
O0100
M98 P9001 L5
M30
O9001
G81 X0 Y0 Z-10. R2. F200
G80
M99
%

Many CNCs allow internal subroutine definitions using O9xxx numbers.

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📌 7. G65 — Macro Call with Parameters

G65 executes a custom macro program (O9000–O9999) with parameter passing.
Parameters are defined using address letters (A–Z) and accessed via local variables (#1–#33) inside the macro.

Example:

G65 P9001 A50. B25.

Macro Program (O9001):

O9001
#1 = #1 (A parameter → #1)
#2 = #2 (B parameter → #2)
G00 X#1 Y#2
G81 Z-10. R2. F200
G80
M99

Parameter	Mapping
A → #1	B → #2

Passes variable coordinates or settings directly from the main program.

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📌 8. Example: Drilling Array Using G65

%
O0101 (ARRAY DRILLING)
#100 = 0
WHILE [#100 LT 3] DO1
G65 P9002 X[#100*50] Y0
#100 = [#100 + 1]
END1
M30

O9002
#1 = #24 (X)
#2 = #25 (Y)
G81 X#1 Y#2 Z-15. R2. F200
G80
M99
%

Automatically drills three holes spaced 50 mm apart — fully parametric.

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📌 9. G66 — Modal Macro Call

G66 P9003 A10. B20.

Runs macro O9003 automatically for every subsequent block until G67.

Example:

G66 P9003 A10. B5.
X0 Y0
X50 Y0
X50 Y50
G67

Macro (O9003):

O9003
#1 = #1 (A)
#2 = #2 (B)
G81 Z[-#1] R#2 F150
G80
M99

Great for drilling multiple holes with one setup command.

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📌 10. G67 — Cancel Modal Macro

G67

Ends modal macro mode — always use before tool change or new operation.

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📌 11. Fanuc Example — Hole Pattern

%
O0200 (MACRO HOLE PATTERN)
G90 G17 G21 G40 G80 G54
T05 M06
S1500 M03
G66 P9010 Z-20. F200.
X0 Y0
X50 Y0
X50 Y50
X0 Y50
G67
M30

O9010
G81 Z#26 R2. F#9
G80
M99
%

Simplifies drilling multiple locations using a single macro template.

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📌 12. Haas Example — Subprogram Call

M97 P10
...
N10 G81 Z-15. R2. F150
G80
M99

Haas uses M97 for local subroutine calls (label N within same program).

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📌 13. Siemens Example

CALL O1000
RET

Siemens supports structured subprogram syntax similar to Fanuc M98/M99.

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📌 14. Heidenhain Example

CALL LBL 1
LBL 1
CYCL DEF 200 DRILLING Q200=+2 Q201=-10 Q206=+200
LBL 0

Heidenhain uses LBL (Label) for subprograms — more readable and flexible.

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📌 15. Common Mistakes

Mistake	Result
Missing M99 in subprogram	CNC continues into next code block (crash risk)
Forgetting G67	Macro keeps running unintentionally
Wrong parameter mapping	Macro reads incorrect data
Missing # variable scope	Local/global confusion
Infinite recursion (M98 inside itself)	Loop freeze or alarm

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📌 16. Advanced Example — Automatic Bolt Circle Drilling

%
O0300 (BOLT CIRCLE MACRO)
#101 = 8     (Number of holes)
#102 = 40.   (Radius)
#103 = 0.    (Start angle)
#104 = 360./#101

#105 = 0
WHILE [#105 LT #101] DO1
#106 = [COS[#103 + #105*#104]*#102]
#107 = [SIN[#103 + #105*#104]*#102]
G81 X#106 Y#107 Z-15. R2. F200
G80
#105 = [#105 + 1]
END1
M30
%

Automatically drills 8 holes equally spaced on a 40 mm radius bolt circle — no CAM needed.

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📌 17. Future Trends (2025–2030)

• AI-driven macros: CNC interprets machining intent and generates macro logic automatically.
• Adaptive subprogram libraries: Machines share macros through networked databases.
• Digital twin verification: Simulates macro logic and parametric conditions before execution.

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✅ Conclusion

Subprograms (M98/M99) and macros (G65/G66/G67) are the backbone of intelligent, modular CNC programming.
They enable code reuse, automation, parameterized machining, and drastically reduce human error — essential for modern high-mix, low-volume production environments.