Subprogram call (M98) and return (M99) in CNC Programming - Mini Project: Build & Apply

Subprogram call (M98) and return (M99) in CNC Programming

📖 Scenario: You are programming a CNC machine to perform a drilling operation on multiple holes. To avoid repeating the same drilling commands, you will use subprogram calls with M98 and returns with M99.

🎯 Goal: Learn how to create a main CNC program that calls a drilling subprogram using M98 and returns from it using M99. This will help you write cleaner and more efficient CNC code.

📋 What You'll Learn

Create a main program that calls a subprogram using M98 P1000

Create a subprogram labeled O1000 that performs drilling commands

Use M99 in the subprogram to return to the main program

Print the program lines as output to simulate CNC code

💡 Why This Matters

🌍 Real World

Using subprogram calls and returns in CNC programming helps reduce code repetition and makes programs easier to maintain and modify.

💼 Career

CNC programmers and machinists use subprogram calls (M98) and returns (M99) daily to write efficient and modular CNC code for manufacturing parts.

1

Create the main CNC program header

Write the first three lines of the main CNC program exactly as follows: O0001 as the program number, G21 to set units to millimeters, and G90 for absolute positioning.

CNC Programming

# Write the main program header lines
# Your code here

Hint

Start your CNC program with a program number line like O0001. Then set units and positioning modes.

2

Add a subprogram call using M98

Add a line to the main program that calls the subprogram O1000 using M98 P1000. Then add a program end line M30.

CNC Programming

O0001
G21
G90
# Call subprogram O1000 using M98
# Your code here

Hint

Use M98 P1000 to call subprogram O1000. End the main program with M30.

3

Create the drilling subprogram with M99 return

Write the subprogram O1000 that drills two holes at coordinates X10 Y10 and X20 Y20 using G81. End the subprogram with M99 to return to the main program.

CNC Programming

O0001
G21
G90
M98 P1000
M30
# Write subprogram O1000 with drilling commands and M99
# Your code here

Hint

Start the subprogram with O1000. Use G81 drilling cycles for each hole. End with M99.

4

Print the complete CNC program

Print the entire CNC program stored in a variable called cnc_program to simulate sending it to the machine.

CNC Programming

cnc_program = '''O0001
G21
G90
M98 P1000
M30

O1000
G81 X10 Y10 Z-5 R1 F100
G81 X20 Y20 Z-5 R1 F100
M99'''
# Print the cnc_program variable
# Your code here

Hint

Store the whole program as a string in cnc_program and print it.

Practice

1. What does the M98 command do in CNC programming?

easy

A. Stops the machine immediately

B. Ends the current subprogram and returns to the main program

C. Starts the spindle rotation

D. Calls a subprogram by its number, optionally repeating it

Solution

1. Step 1: Understand the role of M98

   The M98 command is used to call a subprogram by its number and can repeat it multiple times if specified.

2. Step 2: Differentiate from M99

   M99 is used to end a subprogram and return to the main program, not to call it.

3. Final Answer:

   Calls a subprogram by its number, optionally repeating it -> Option D

4. Quick Check:

   M98 calls subprogram [OK]

Hint: M98 calls subprogram; M99 returns from it [OK]

Common Mistakes:

• Confusing M98 with M99
• Thinking M98 stops the machine
• Assuming M98 starts spindle

2. Which of the following is the correct syntax to call subprogram number 100 with 3 repetitions?

easy

A. M100 P98 L3

B. M98 P100 L3

C. M98 L3 P100

D. M99 P100 L3

Solution

1. Step 1: Identify correct M98 syntax

   The correct syntax to call a subprogram is M98 followed by P (program number) and optionally L (repeat count), so M98 P100 L3 is correct.

2. Step 2: Check other options for errors

   M99 is for return, not call; order matters; M100 is not a command.

3. Final Answer:

   M98 P100 L3 -> Option B

4. Quick Check:

   Call syntax = M98 P100 L3 [OK]

Hint: M98 P[number] L[repeat] calls subprogram [OK]

Common Mistakes:

• Using M99 instead of M98 to call
• Swapping P and L parameters
• Using invalid command like M100

3. Given this CNC program snippet:

N10 M98 P200 L2
N20 M30

O200
N10 G01 X10 Y10
N20 M99

What will happen when the program runs?

medium

A. The subprogram 200 runs twice, then the main program ends

B. The subprogram 200 runs once, then the main program loops infinitely

C. The program will cause a syntax error at M99

D. The main program ends immediately without running subprogram

Solution

1. Step 1: Analyze M98 call with L2

   M98 P200 L2 calls subprogram 200 two times.

2. Step 2: Understand M99 in subprogram

   M99 ends the subprogram and returns to main program after each call.

3. Step 3: Main program ends with M30

   After subprogram calls, M30 ends the main program.

4. Final Answer:

   The subprogram 200 runs twice, then the main program ends -> Option A

5. Quick Check:

   M98 L2 calls twice, M99 returns, M30 ends [OK]

Hint: M98 with L repeats subprogram; M99 returns to main [OK]

Common Mistakes:

• Thinking M99 causes error
• Assuming infinite loop without M99
• Ignoring L parameter repeat count

4. Identify the error in this CNC code snippet:

N10 M98 P300
N20 M30

O300
N10 G01 X20 Y20
N20 M30

medium

A. Subprogram 300 does not have M99 to return

B. M98 command is missing the repeat count L

C. Main program should not have M30

D. Subprogram number should be 200, not 300

Solution

1. Step 1: Check subprogram termination

   Subprogram 300 ends with M30, which ends the entire program, not returns to main.

2. Step 2: Identify missing M99

   Subprograms must end with M99 to return control to main program after call.

3. Final Answer:

   Subprogram 300 does not have M99 to return -> Option A

4. Quick Check:

   Missing M99 causes no return from subprogram [OK]

Hint: Subprograms must end with M99 to return [OK]

Common Mistakes:

• Using M30 inside subprogram instead of M99
• Thinking L parameter is mandatory
• Confusing subprogram number

5. You want to machine 5 identical pockets using subprogram 150. Which code snippet correctly calls the subprogram 5 times and returns properly?

hard

A. N10 M98 P150 L5
N20 M99

O150
N10 G01 X0 Y0
N20 M99

B. N10 M98 P150
N20 M99

O150
N10 G01 X0 Y0
N20 M30

C. N10 M98 P150 L5
N20 M30

O150
N10 G01 X0 Y0
N20 M99

D. N10 M98 P150 L5
N20 M30

O150
N10 G01 X0 Y0
N20 M30

Solution

1. Step 1: Confirm main program calls subprogram 5 times

   M98 P150 L5 calls subprogram 150 five times, correct for repetition.

2. Step 2: Check subprogram ends with M99

   Subprogram ends with M99 to return control after each call.

3. Step 3: Main program ends with M30

   M30 properly ends the main program after calls.

4. Final Answer:

   N10 M98 P150 L5
   N20 M30
   
   O150
   N10 G01 X0 Y0
   N20 M99 -> Option C

5. Quick Check:

   M98 with L5 and M99 in subprogram [OK]

Hint: Use M98 P[number] L5 and M99 to return [OK]

Common Mistakes:

• Ending subprogram with M30 instead of M99
• Using M99 in main program instead of M30
• Omitting repeat count L5