Subprogram call (M98) and return (M99) in CNC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the M98 command do in CNC programming?

M98 calls or starts a subprogram from the main program. It allows the machine to run a separate set of instructions stored in another program file or block.

beginner

What is the purpose of the M99 command in CNC programming?

M99 returns control from a subprogram back to the main program. It signals the end of the subprogram and resumes the main program where it left off.

intermediate

How do M98 and M99 work together in CNC programs?

M98 calls a subprogram to run a set of instructions, and M99 ends that subprogram and returns control to the main program. Together, they let you reuse code and organize programs better.

intermediate

Can M98 call a subprogram multiple times in a CNC program?

Yes, M98 can call the same subprogram multiple times, allowing repeated execution of the same instructions without rewriting code.

advanced

What happens if M99 is missing at the end of a subprogram?

If M99 is missing, the CNC machine may not return to the main program correctly, causing errors or unexpected behavior because the subprogram never signals it is finished.

What command do you use to start a subprogram in CNC programming?

A G00

B M99

C M03

D M98

M98 is the command used to call or start a subprogram.

Which command returns control from a subprogram to the main program?

A M98

B G01

C M99

D M05

M99 signals the end of a subprogram and returns control to the main program.

What happens if you call M98 without a matching M99 in the subprogram?

A The subprogram loops infinitely

B The machine may not return to the main program properly

C The machine stops immediately

D The subprogram runs twice automatically

Without M99, the machine does not know when to return to the main program, causing errors.

Can M98 be used to call the same subprogram multiple times?

A Yes, multiple times

B No, only once per program

C Only if the subprogram is short

D Only if M99 is omitted

M98 can call the same subprogram as many times as needed.

Which of these is true about M98 and M99?

A M98 calls a subprogram, M99 returns from it

B Both commands end subprograms

C Both commands start subprograms

D M98 ends a subprogram, M99 starts it

M98 calls a subprogram, and M99 returns control back to the main program.

Explain how M98 and M99 commands help organize CNC programs.

Think about how calling and returning from subprograms saves rewriting code.

Describe what could go wrong if M99 is missing at the end of a subprogram.

Consider the flow of program control between main and subprogram.

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