Post-processor and G-code output in CNC Programming - Mini Project: Build & Apply

Post-processor and G-code output

📖 Scenario: You work in a small workshop that uses CNC machines. You need to create a simple program that converts a list of machining steps into G-code commands. This helps the CNC machine understand what to do.

🎯 Goal: Build a Python script that takes a list of machining steps and outputs the correct G-code commands as text.

📋 What You'll Learn

Create a list of machining steps with exact values

Add a configuration variable for feed rate

Use a for loop to convert steps into G-code lines

Print the final G-code output

💡 Why This Matters

🌍 Real World

CNC machines need G-code instructions to move tools and cut materials precisely. Post-processors convert design steps into these instructions.

💼 Career

Understanding how to automate G-code generation helps CNC programmers and manufacturing engineers save time and reduce errors.

1

Create the machining steps list

Create a list called steps with these exact tuples: ("move", 10, 20), ("cut", 15, 25), ("move", 20, 30)

CNC Programming

# Create the list called steps with the given tuples
# Your code here

Hint

Use square brackets to create a list. Each step is a tuple with an action and two numbers.

2

Add a feed rate configuration

Create a variable called feed_rate and set it to 100 to use as the speed for cutting.

CNC Programming

steps = [("move", 10, 20), ("cut", 15, 25), ("move", 20, 30)]
# Create feed_rate variable and set it to 100
# Your code here

Hint

Just write feed_rate = 100 on a new line.

3

Convert steps to G-code lines

Create a list called gcode_lines. Use a for loop with variables action, x, and y to go through steps. For each step, add a string to gcode_lines: if action is "move", add "G0 X{x} Y{y}"; if action is "cut", add "G1 X{x} Y{y} F{feed_rate}".

CNC Programming

steps = [("move", 10, 20), ("cut", 15, 25), ("move", 20, 30)]
feed_rate = 100
# Create gcode_lines list and fill it using a for loop over steps
# Your code here

Hint

Start with an empty list. Use a for loop to check each action and add the right formatted string.

4

Print the G-code output

Use a for loop to print each line in gcode_lines.

CNC Programming

steps = [("move", 10, 20), ("cut", 15, 25), ("move", 20, 30)]
feed_rate = 100
gcode_lines = []
for action, x, y in steps:
    if action == "move":
        gcode_lines.append(f"G0 X{x} Y{y}")
    elif action == "cut":
        gcode_lines.append(f"G1 X{x} Y{y} F{feed_rate}")
# Print each line in gcode_lines
# Your code here

Hint

Use a for loop to print each string in the list on its own line.

Practice

1. What is the main purpose of a post-processor in CNC programming?

easy

A. To measure the dimensions of the finished part

B. To design 3D models for CNC machining

C. To convert toolpath data into machine-specific G-code instructions

D. To operate the CNC machine manually

Solution

1. Step 1: Understand the role of post-processors

   Post-processors take the generic toolpath data and convert it into G-code that a specific CNC machine can understand.

2. Step 2: Differentiate from other CNC tasks

   Designing models, manual operation, and measuring parts are separate tasks not handled by post-processors.

3. Final Answer:

   To convert toolpath data into machine-specific G-code instructions -> Option C

4. Quick Check:

   Post-processor = G-code conversion [OK]

Hint: Post-processor = toolpath to machine code converter [OK]

Common Mistakes:

• Confusing post-processor with CAD design software
• Thinking post-processor operates the machine
• Mixing up measuring tools with post-processing

2. Which of the following is the correct syntax to output a G-code line for moving to X=10, Y=20 in a simple post-processor script?

easy

A. print('G01 X10 Y20')

B. writeLine(`G01 X10 Y20`);

C. echo G01 X10 Y20;

D. output G01 X10 Y20

Solution

1. Step 1: Identify common post-processor output syntax

   Many post-processors use a function like writeLine() to output G-code lines as strings.

2. Step 2: Check syntax correctness

   writeLine(`G01 X10 Y20`); uses backticks for string and a function call, which is typical in scripting post-processors. Other options lack proper function or string syntax.

3. Final Answer:

   writeLine(`G01 X10 Y20`); -> Option B

4. Quick Check:

   Output G-code line with writeLine() [OK]

Hint: Use writeLine() with backticks for G-code output [OK]

Common Mistakes:

• Using print() instead of writeLine() in post-processor
• Missing quotes or backticks around G-code string
• Using shell commands like echo incorrectly

3. Given this snippet from a post-processor script:

writeLine(`G00 X${posX} Y${posY}`);
posX = 50;
posY = 100;
writeLine(`G01 X${posX} Y${posY} F1500`);

What will be the output G-code lines?

medium

A. G00 Xundefined Yundefined
G01 X50 Y100 F1500

B. G00 X50 Y100
G01 X50 Y100 F1500

C. G00 X0 Y0
G01 X50 Y100 F1500

D. G00 Xundefined Yundefined
G01 Xundefined Yundefined F1500

Solution

1. Step 1: Analyze variable values at first writeLine()

   posX and posY are used before assignment, so they are undefined at first output.

2. Step 2: Analyze variable values at second writeLine()

   After assigning posX=50 and posY=100, the second line outputs correct values with feedrate F1500.

3. Final Answer:

   G00 Xundefined Yundefined G01 X50 Y100 F1500 -> Option A

4. Quick Check:

   Variables undefined before assignment [OK]

Hint: Check variable assignment order before output [OK]

Common Mistakes:

• Assuming variables have default zero values
• Ignoring variable initialization order
• Confusing G00 and G01 commands

4. A post-processor script contains this code snippet:

writeLine(`G01 X${x} Y${y} F${feedrate}`);
let x = 10;
let y = 20;
let feedrate = 1000;

What is the main error and how to fix it?

medium

A. Incorrect G-code command; change G01 to G00

B. Missing semicolons; add semicolons after each line

C. Wrong string quotes; use single quotes instead of backticks

D. Variables used before declaration; declare variables before writeLine call

Solution

1. Step 1: Identify variable usage order

   The writeLine uses variables x, y, feedrate before they are declared and assigned, causing undefined values.

2. Step 2: Fix variable declaration order

   Move the let declarations and assignments before the writeLine call to ensure variables have values.

3. Final Answer:

   Variables used before declaration; declare variables before writeLine call -> Option D

4. Quick Check:

   Declare variables before use [OK]

Hint: Declare variables before using them in output [OK]

Common Mistakes:

• Assuming variables can be used before declaration
• Changing G-code commands unnecessarily
• Confusing string quote types

5. You want to write a post-processor script that outputs G-code to drill holes at multiple XY positions stored in an array. Which approach correctly generates the G-code lines for each hole with feedrate 800?

hard

A. for (const pos of positions) { writeLine(`G81 X${pos.x} Y${pos.y} Z-5 F800`); }

B. positions.forEach(pos => writeLine(`G00 X${pos.x} Y${pos.y}`)); writeLine(`G81 Z-5 F800`);

C. writeLine(`G81`); for (let i=0; i

D. for (let pos in positions) { writeLine(`G81 Xpos.x Ypos.y Z-5 F800`); }

Solution

1. Step 1: Understand G81 drilling cycle usage

   G81 command includes X, Y, Z, and feedrate parameters per hole position.

2. Step 2: Check loop and string interpolation correctness

   for (const pos of positions) { writeLine(`G81 X${pos.x} Y${pos.y} Z-5 F800`); } uses a for-of loop with correct template literals to output each hole's G81 line properly.

3. Step 3: Identify errors in other options

   positions.forEach(pos => writeLine(`G00 X${pos.x} Y${pos.y}`)); writeLine(`G81 Z-5 F800`); separates move and drill incorrectly; writeLine(`G81`); for (let i=0; i

4. Final Answer:

   for (const pos of positions) { writeLine(`G81 X${pos.x} Y${pos.y} Z-5 F800`); } -> Option A

5. Quick Check:

   Use for-of loop with template literals for each hole [OK]

Hint: Use for-of loop and template literals for each position [OK]

Common Mistakes:

• Using for-in loop incorrectly for arrays
• Splitting G81 command across lines improperly
• Not including feedrate in each drilling command