Subprogram for repeated patterns in CNC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a subprogram in CNC programming?

A subprogram is a separate block of code that performs a specific task and can be called multiple times within the main program to repeat patterns without rewriting the code.

beginner

Why use subprograms for repeated patterns in CNC?

Using subprograms saves time and reduces errors by reusing code for repeated shapes or movements, making the program shorter and easier to maintain.

beginner

How do you call a subprogram in CNC code?

You call a subprogram using the M98 command followed by the subprogram number, for example, M98 P1000 calls subprogram O1000.

beginner

What is the purpose of the M99 command in a subprogram?

M99 ends the subprogram and returns control back to the main program to continue execution after the subprogram call.

intermediate

How can you repeat a subprogram multiple times?

You can repeat a subprogram multiple times by adding the L parameter to M98, for example, M98 P1000 L5 calls subprogram O1000 five times.

Which command is used to call a subprogram in CNC?

A M98

B M30

C M99

D G00

M98 is the command used to call a subprogram by specifying its program number.

What does the M99 command do in a subprogram?

A Starts the subprogram

B Ends the subprogram and returns to main program

C Calls another subprogram

D Stops the machine

M99 ends the subprogram and returns control to the main program.

How do you repeat a subprogram 3 times?

A M98 P1000 L3

B M98 P1000 R3

C M99 P1000 L3

D M30 P1000 L3

The L parameter with M98 specifies how many times to repeat the subprogram.

Why use subprograms for repeated patterns?

A To make the program longer

B To slow down the machine

C To avoid repeating code and reduce errors

D To confuse the operator

Subprograms help reuse code, making programs shorter and easier to maintain.

What is the correct format to define a subprogram named 1000?

A G00 O1000 ... M99

B M98 P1000 ... M30

C O1000 ... M30

D O1000 ... M99

Subprograms start with O followed by the number and end with M99.

Explain how subprograms help in CNC programming for repeated patterns.

Think about how you avoid writing the same instructions many times.

Describe the commands used to call and end a subprogram in CNC code.

Focus on the commands that start and finish the subprogram.

Practice

1. What is the main purpose of using a subprogram in CNC programming?

easy

A. To stop the machine immediately

B. To speed up the machine spindle

C. To change the tool automatically

D. To repeat a set of instructions multiple times without rewriting them

Solution

1. Step 1: Understand subprogram concept

   Subprograms are blocks of code that can be reused multiple times in CNC programming.

2. Step 2: Identify main purpose

   They help avoid rewriting the same instructions repeatedly, making code shorter and easier to manage.

3. Final Answer:

   To repeat a set of instructions multiple times without rewriting them -> Option D

4. Quick Check:

   Subprograms repeat code = D [OK]

Hint: Subprograms repeat code blocks to save writing [OK]

Common Mistakes:

• Confusing subprograms with tool changes
• Thinking subprograms control spindle speed
• Assuming subprograms stop the machine

2. Which of the following is the correct syntax to call a subprogram numbered 100 in CNC code?

easy

A. M99 P100

B. M98 P100

C. M100 P98

D. M98 L100

Solution

1. Step 1: Recall subprogram call command

   M98 is the standard command to call a subprogram in CNC programming.

2. Step 2: Match syntax with subprogram number

   The correct syntax is M98 followed by P and the subprogram number, e.g., M98 P100.

3. Final Answer:

   M98 P100 -> Option B

4. Quick Check:

   Call subprogram = M98 P[number] [OK]

Hint: Use M98 P[number] to call subprograms [OK]

Common Mistakes:

• Using M99 to call instead of return
• Swapping M98 and M99 commands
• Using L instead of P for subprogram number

3. Given the following CNC code snippet, what will be the output movement commands executed?

O1000;
G01 X10 Y10 F100;
M99;

N10 M98 P1000 L3;
M30;

medium

A. Moves to X10 Y10 three times in a row

B. Moves to X10 Y10 once only

C. Moves to X10 Y10 infinitely

D. No movement commands executed

Solution

1. Step 1: Understand subprogram call with repetition

   M98 P1000 L3 calls subprogram 1000 three times in a row.

2. Step 2: Analyze subprogram content

   Subprogram 1000 moves the tool to X10 Y10 with feedrate 100, then returns with M99.

3. Final Answer:

   Moves to X10 Y10 three times in a row -> Option A

4. Quick Check:

   M98 P1000 L3 repeats subprogram 3 times = C [OK]

Hint: M98 with L repeats subprogram calls multiple times [OK]

Common Mistakes:

• Ignoring the L3 repeat count
• Confusing M99 as a stop command
• Assuming infinite loop without L parameter

4. Identify the error in this CNC subprogram call:

O2000;
G01 X20 Y20 F150;
M99;

N20 M98 P2000 L0;
M30;

medium

A. M99 should be M98 inside subprogram

B. P2000 is invalid subprogram number

C. L0 causes no repetition, so subprogram never runs

D. Missing G00 command before M98

Solution

1. Step 1: Check repetition count in subprogram call

   L0 means repeat zero times, so the subprogram is not executed at all.

2. Step 2: Confirm other commands are correct

   M99 is correct to return from subprogram, P2000 is valid, and G00 is not mandatory before M98.

3. Final Answer:

   L0 causes no repetition, so subprogram never runs -> Option C

4. Quick Check:

   L0 means zero repeats = B [OK]

Hint: L0 means no repeats, so subprogram won't run [OK]

Common Mistakes:

• Thinking M99 is wrong inside subprogram
• Assuming P2000 is invalid
• Believing G00 is required before M98

5. You want to drill 5 holes spaced 10mm apart along the X-axis starting at X0 Y0 using a subprogram. Which code snippet correctly uses a subprogram with a loop to achieve this?

hard

A. O3000;
G81 X#1 Y0 Z-5 R1 F100;
M99;

N30 G91;
M98 P3000 L5;
M30;

B. O3000;
G81 X0 Y0 Z-5 R1 F100;
M99;

N30 G90;
M98 P3000 L5;
M30;

C. O3000;
G81 X10 Y0 Z-5 R1 F100;
M99;

N30 G91;
M98 P3000 L5;
M30;

D. O3000;
G81 X#1 Y0 Z-5 R1 F100;
M30;

N30 G91;
M98 P3000 L5;
M30;

Solution

1. Step 1: Understand incremental positioning for spacing

   G91 sets incremental mode, so each call moves relative to last position, spacing holes 10mm apart.

2. Step 2: Check subprogram and loop usage

   Subprogram O3000 uses G81 drilling cycle with X#1 (variable), M99 returns correctly, and M98 P3000 L5 calls it 5 times.

3. Final Answer:

   Option A correctly uses incremental mode, subprogram, and loop for 5 holes spaced 10mm apart -> Option A

4. Quick Check:

   Incremental + M98 with L5 = 5 spaced holes [OK]

Hint: Use G91 for incremental moves and M98 L5 to repeat 5 times [OK]

Common Mistakes:

• Using G90 (absolute) instead of G91 (incremental)
• Forgetting M99 to return from subprogram
• Using M30 inside subprogram instead of M99