CNC Variables & Logic Control: # Parameters, IF, WHILE, DO, GOTO Explained with Real Code Examples

CNC variables and logic control bring true programming intelligence to your G-code.
Instead of running static programs, you can create dynamic, self-adaptive machining cycles using conditional logic — just like in modern programming languages.

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📌 1. Introduction

Parametric programming in CNC is based on variables (#100, #500, etc.) and logical control commands like IF, WHILE, and GOTO.
This allows your program to make decisions, loop, and adapt in real time — without editing the code manually.

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📌 2. Types of Variables

Range	Type	Description
#1–#33	Local	Temporary, used inside macros
#100–#199	Common	User-defined, shared between programs
#500–#999	Permanent	Saved even after power off
#1000+	System	Machine parameters (axes, alarms, etc.)

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📌 3. Basic Example — Using Variables

#100 = 50.      (X POSITION)
#101 = 25.      (Y POSITION)
#102 = -10.     (Z DEPTH)
G00 X#100 Y#101
G81 Z#102 R2. F200
G80

The machine drills a hole at (50,25) to depth -10 using variable values.

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📌 4. Arithmetic Operations

Symbol	Operation	Example
+	Addition	#100 = #101 + 10
-	Subtraction	#102 = #101 - 5
*	Multiplication	#103 = #101 * 2
/	Division	#104 = #101 / 3
[]	Parentheses	#105 = [#100 + #101] * 2

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📌 5. Conditional Logic — IF Statements

#100 = 25
IF [#100 GT 20] THEN #101 = 1
IF [#100 LT 20] THEN #101 = 0

Operator	Meaning
EQ	Equal (=)
NE	Not equal (≠)
GT	Greater than (>)
LT	Less than (<)
GE	Greater or equal (≥)
LE	Less or equal (≤)

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📌 6. GOTO — Conditional Branching

#100 = 10
IF [#100 GT 5] GOTO 100
M30
N100 G81 Z-15. R2. F200
M99

If #100 > 5, program jumps to line N100 and executes the drilling cycle.

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📌 7. WHILE / DO / END Looping

Loops allow repeating a block until a condition is false.

#100 = 0
WHILE [#100 LT 5] DO1
G81 X[#100*20] Y0 Z-10. R2. F200
G80
#100 = [#100 + 1]
END1

Drills 5 holes spaced 20 mm apart — fully automated using logic.

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📌 8. Nested Loops Example

#101 = 0
WHILE [#101 LT 3] DO1
#102 = 0
WHILE [#102 LT 3] DO2
G81 X[#101*30] Y[#102*30] Z-10. R2. F200
G80
#102 = [#102 + 1]
END2
#101 = [#101 + 1]
END1

Creates a 3×3 grid of drilled holes using nested logic — no manual coordinates required.

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📌 9. DO Loops with Conditional Exit

#100 = 0
DO1
#100 = [#100 + 1]
IF [#100 GT 10] GOTO 500
G04 P1.
END1
N500 M30

Runs a 1-second pause 10 times, then jumps to program end.

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📌 10. Using System Variables (Fanuc)

Variable	Description
#3000 = n (message)	Alarm message
#3006 = n (pause)	Operator message
#4001–#4009	Active motion modes
#5001–#5003	Current machine position (X/Y/Z)
#149	Current tool number

Example:

IF [#5003 LT -50.] THEN #3000 = 1 (TOOL BELOW SAFE LIMIT)

Automatically stops if Z position is below -50 mm.

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📌 11. Advanced Example — Auto Drilling Pattern with Variables

%
O1200 (AUTO DRILLING)
#101 = 6      (NUMBER OF HOLES)
#102 = 40.    (CIRCLE 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 6 equally spaced holes along a circular bolt pattern — pure parametric power.

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📌 12. Haas Example — Simplified Conditional Logic

#100 = 15
IF [#100 LT 10] THEN GOTO 50
G04 P2.
N50 M30

Haas follows the same logic as Fanuc but supports inline condition evaluation.

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📌 13. Siemens Example

IF EQ 1 THEN
  G81 Z-10 F150
ELSE
  G83 Z-30 Q5 F120
ENDIF

Siemens uses structured IF/ELSE/ENDIF blocks for cleaner macro logic.

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📌 14. Heidenhain Example

LBL 1
Q1 = +10
IF +Q1 GT +5
  CYCL DEF 200 DRILLING Q201=-10 Q206=200
ENDIF
LBL 0

Heidenhain macros use Q variables and IF/ENDIF syntax, similar in logic but more readable.

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📌 15. Real-World Application — Tool Life Counter

#500 = [#500 + 1]
IF [#500 GT 100] THEN #3006 = 1 (TOOL CHANGE REQUIRED)

Automatically alerts the operator after 100 tool uses — no PLC needed.

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📌 16. Real-World Application — Smart Safe-Z Logic

IF [#5003 LT -80.] THEN #3000 = 5 (Z LIMIT EXCEEDED)

Monitors Z-axis position and raises an alarm if tool goes below safe level.

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📌 17. Combining Conditions

#100 = 25
#101 = 50
IF [[#100 GT 20] AND [#101 LT 60]] THEN G65 P9001

Executes macro only if both conditions are true.

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📌 18. Future Trends (2025–2030)

• AI macro optimization: Machine automatically adjusts parameters (# variables) based on cutting force sensors.
• Self-learning condition monitoring: WHILE loops adapt to real-time vibration or temperature input.
• Cloud-synced macro libraries: Share logic routines across connected CNC machines.
• Smart alarms: CNC predicts error states before they occur using conditional thresholds.

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✅ Conclusion

Parametric programming transforms G-code into a real programming language.
By mastering variables, conditions, and loops, CNC machinists can build intelligent, modular, and fully automated machining strategies — capable of adapting to any geometry or condition without manual intervention.