Subprogram call (M98) and return (M99) in CNC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What does this CNC program output when calling subprogram with M98?

Consider the following CNC program snippet. What will be the final position of the tool after execution?

CNC Programming

N10 G00 X0 Y0
N20 M98 P100 L2
N30 G01 X50 Y50 F100
N40 M30

O100
N10 G01 X10 Y10 F50
N20 M99

A The tool ends at X50 Y50 after running the main program and two subprogram calls.

B The tool ends at X10 Y10 because the subprogram moves it there and returns immediately.

C The tool stays at X0 Y0 because M98 does not move the tool.

D The program causes an error because M99 is used outside a subprogram.

💡 Hint

Remember M98 calls the subprogram multiple times, and M99 returns control to the main program.

M98 calls subprogram O100 twice (L2). Each call moves the tool to X10 Y10. After returning, the main program moves the tool to X50 Y50. So final position is X50 Y50.

🧠 Conceptual

intermediate

What is the role of M99 in CNC subprograms?

In CNC programming, what does the M99 command do inside a subprogram?

A It returns control to the main program after subprogram execution.

B It ends the entire CNC program immediately.

C It calls another subprogram recursively.

D It pauses the program until operator intervention.

💡 Hint

Think about how the program flow returns after a subprogram finishes.

M99 signals the end of a subprogram and returns control to the main program at the point after the M98 call.

🔧 Debug

advanced

Identify the error in this CNC subprogram usage

What error will occur when running this CNC code snippet?

CNC Programming

N10 G00 X0 Y0
N20 M99
N30 G01 X100 Y100 F200
N40 M30

A The program will loop infinitely due to M99.

B No error; M99 will return control to the main program.

C Runtime error because G00 and G01 commands conflict.

D Syntax error because M99 is used outside a subprogram.

💡 Hint

M99 should only be used inside subprograms.

M99 is a return command and must be inside a subprogram. Using it in the main program causes a syntax error.

🚀 Application

advanced

How many times will the subprogram run?

Given this CNC program, how many times does the subprogram O200 execute?

CNC Programming

N10 G00 X0 Y0
N20 M98 P200 L3
N30 G01 X100 Y100 F150
N40 M30

O200
N10 G01 X20 Y20 F100
N20 M99

A The subprogram does not run because P200 is invalid.

B The subprogram runs once because M98 ignores L parameter.

C The subprogram runs 3 times because of L3 in the M98 call.

D The subprogram runs 2 times because L counts start at zero.

💡 Hint

L parameter in M98 specifies the number of repetitions.

M98 P200 L3 calls subprogram O200 three times before continuing the main program.

🧠 Conceptual

expert

What happens if M99 is missing in a subprogram called by M98?

If a subprogram called with M98 does not contain an M99 command, what is the expected behavior?

A The program will cause a hang or error because the return command is missing.

B The CNC machine will continue executing the subprogram until it reaches M30 or program end.

C The machine will return immediately to the main program after the first line of the subprogram.

D The subprogram will run infinitely in a loop.

💡 Hint

Think about how the program flow ends if no explicit return is given.

Without M99, the subprogram runs until it reaches M30 or the program end, effectively ending the entire program or stopping execution.

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