Conditional statements (IF-THEN) in CNC Programming

Introduction

Conditional statements let the CNC machine decide what to do next based on a condition. This helps make the program smart and flexible.

Stop the machine if a safety limit is reached.

Choose different cutting speeds based on material type.

Skip a step if a tool is not available.

Change the path if a sensor detects a problem.

Syntax

CNC Programming

IF [condition] THEN
  [commands]
ENDIF

The condition is checked first. If true, the commands run.

Use ENDIF to close the IF block.

Examples

If variable #100 is greater than 50, stop the program with M30.

CNC Programming

IF #100 GT 50 THEN
  M30
ENDIF

If variable #101 equals 0, jump to line 100.

CNC Programming

IF #101 EQ 0 THEN
  GOTO 100
ENDIF

If variable #102 is less than 10, set spindle speed to 500.

CNC Programming

IF #102 LT 10 THEN
  S500
ENDIF

Sample Program

This program sets a variable #100 to 60. It checks if #100 is greater than 50. Since it is, the program stops with M30. If not, it would pause with M00.

CNC Programming

O1000 (Simple IF-THEN example)
#100=60 (Set variable #100 to 60)
IF #100 GT 50 THEN
  M30 (Stop program)
ENDIF
M00 (Pause program)

Important Notes

Always close IF blocks with ENDIF to avoid errors.

Conditions use operators like GT (greater than), LT (less than), EQ (equal).

Variables start with # and hold numbers for conditions.

Summary

IF-THEN lets the CNC program make choices based on conditions.

Use IF with a condition, then commands, and close with ENDIF.

This makes CNC programs smarter and safer.

Practice

1. What does an IF-THEN statement do in CNC programming?

easy

A. It repeats a set of commands multiple times.

B. It stops the machine immediately.

C. It defines the speed of the machine.

D. It makes the program choose actions based on conditions.

Solution

1. Step 1: Understand the purpose of IF-THEN

   IF-THEN checks a condition and runs commands only if the condition is true.

2. Step 2: Compare with other options

   Repeating commands is done by loops, speed is set by other commands, and stopping is a different command.

3. Final Answer:

   It makes the program choose actions based on conditions. -> Option D

4. Quick Check:

   IF-THEN controls flow based on conditions [OK]

Hint: IF-THEN means "if this, then do that" [OK]

Common Mistakes:

• Confusing IF-THEN with loops
• Thinking IF-THEN sets machine speed
• Assuming IF-THEN stops the machine

2. Which of the following is the correct syntax for an IF statement in CNC programming?

easy

A. WHEN condition DO
commands
END

B. IF (condition) {
commands
}

C. IF [condition] THEN
commands
ENDIF

D. if condition:
commands

Solution

1. Step 1: Identify CNC IF syntax

   CNC uses IF [condition] THEN ... ENDIF structure.

2. Step 2: Check other options

   Options B and C use programming languages like C or Python syntax, A is not standard CNC syntax.

3. Final Answer:

   IF [condition] THEN commands ENDIF -> Option C

4. Quick Check:

   CNC IF syntax uses THEN and ENDIF [OK]

Hint: Look for THEN and ENDIF keywords in CNC IF syntax [OK]

Common Mistakes:

• Using parentheses or braces like other languages
• Omitting THEN or ENDIF keywords
• Confusing IF with WHEN or DO

3. What will be the output of this CNC code snippet?

IF #100 GT 50 THEN
  #101 = 1
ELSE
  #101 = 0
ENDIF
PRINT #101

Assuming variable #100 is 60.

medium

A. 0

B. 1

C. Syntax Error

D. No output

Solution

1. Step 1: Evaluate the condition #100 GT 50

   #100 is 60, which is greater than 50, so condition is true.

2. Step 2: Determine the assigned value to #101

   Since condition is true, #101 is set to 1, then printed.

3. Final Answer:

   1 -> Option B

4. Quick Check:

   60 > 50 is true, so #101 = 1 [OK]

Hint: Check condition truth to find which branch runs [OK]

Common Mistakes:

• Confusing GT (greater than) with LT (less than)
• Ignoring ELSE branch
• Assuming syntax error due to ELSE

4. Identify the error in this CNC IF statement:

IF #200 EQ 10
  G01 X100 Y100
ENDIF

medium

A. Missing THEN keyword after condition

B. Incorrect variable name #200

C. Missing ENDIF keyword

D. G01 command is invalid inside IF

Solution

1. Step 1: Check IF statement syntax

   CNC IF requires THEN after the condition, missing here.

2. Step 2: Verify other parts

   Variable name is valid, ENDIF is present, G01 is valid command inside IF.

3. Final Answer:

   Missing THEN keyword after condition -> Option A

4. Quick Check:

   IF must have THEN before commands [OK]

Hint: IF condition must be followed by THEN [OK]

Common Mistakes:

• Forgetting THEN after IF condition
• Thinking ENDIF is missing
• Assuming commands can't be inside IF

5. You want the CNC machine to move to X=50 only if the tool number (#1) is 3 or higher. Which IF statement correctly does this?

hard

A. IF #1 GE 3 THEN
G01 X50
ENDIF

B. IF #1 GT 3 THEN
G01 X50
ENDIF

C. IF #1 LE 3 THEN
G01 X50
ENDIF

D. IF #1 EQ 3 THEN
G01 X50
ENDIF

Solution

1. Step 1: Understand the condition "3 or higher"

   This means #1 is greater than or equal to 3 (GE 3).

2. Step 2: Check each option's condition

   IF #1 GE 3 THEN G01 X50 ENDIF uses GE (greater or equal), B uses GT (greater than) which excludes 3, C uses LE (less or equal), D uses EQ (equal only).

3. Final Answer:

   IF #1 GE 3 THEN G01 X50 ENDIF -> Option A

4. Quick Check:

   GE means "3 or higher" [OK]

Hint: Use GE for "greater or equal" conditions [OK]

Common Mistakes:

• Using GT excludes the value 3
• Using LE or EQ does not cover all values 3 or higher
• Confusing GE with GT