Why Subprogram call (M98) and return (M99) in CNC Programming? - Purpose & Use Cases

The Big Idea

What if you could write a complex machining step once and use it everywhere without copying it?

The Scenario

Imagine you have a CNC machine that needs to perform the same drilling pattern multiple times in different places on a workpiece. Without subprograms, you would have to write the same drilling code over and over again in your main program.

The Problem

Writing repeated code manually is slow and boring. It increases the chance of mistakes, wastes time, and makes your program longer and harder to read. If you want to change the drilling pattern, you must update every copy, risking errors.

The Solution

Using subprogram calls with M98 and returns with M99 lets you write the drilling pattern once as a separate subprogram. You can then call it whenever needed, and the machine will return to the main program automatically. This saves time, reduces errors, and keeps your code clean.

Before vs After

✗ Before

G81 X10 Y10 Z-5
G81 X20 Y20 Z-5
G81 X30 Y30 Z-5

✓ After

M98 P1000 X10 Y10
M98 P1000 X20 Y20
M98 P1000 X30 Y30
...
O1000
G81 X#1 Y#2 Z-5
M99

What It Enables

You can reuse complex machining steps easily, making your CNC programs shorter, clearer, and faster to update.

Real Life Example

A machinist programs a hole pattern once as a subprogram and calls it multiple times for different parts of a metal plate, saving hours of repetitive coding.

Key Takeaways

Subprogram calls avoid repeating code.

M98 calls a subprogram; M99 returns to the main program.

This makes CNC programs easier to write, read, and maintain.

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