Contour milling with line segments in CNC Programming - Mini Project: Build & Apply

Contour milling with line segments

📖 Scenario: You are programming a CNC machine to mill a simple contour made of straight line segments. The contour is defined by points on a 2D plane. You want to create a program that generates the G-code commands to move the milling tool along these line segments.

🎯 Goal: Build a CNC program script that defines the contour points, sets a feed rate, generates the G-code commands to move along each line segment in order, and outputs the final G-code commands.

📋 What You'll Learn

Create a list of contour points with exact coordinates

Define a feed rate variable

Generate G-code commands to move the tool along each line segment between points

Print the final G-code commands

💡 Why This Matters

🌍 Real World

CNC programmers often need to convert geometric contours into G-code commands that machines understand to mill parts accurately.

💼 Career

Understanding how to automate G-code generation for contours is useful for CNC operators, manufacturing engineers, and automation specialists.

1

Define the contour points

Create a list called contour_points with these exact points as tuples: (0, 0), (10, 0), (10, 5), (0, 5).

CNC Programming

# Create the list contour_points with the given points
# Your code here

Hint

Use a Python list with tuples for each point.

2

Set the feed rate

Create a variable called feed_rate and set it to 1500.

CNC Programming

contour_points = [(0, 0), (10, 0), (10, 5), (0, 5)]
# Set the feed_rate variable to 1500
# Your code here

Hint

Just assign the number 1500 to feed_rate.

3

Generate G-code commands for contour milling

Create a list called gcode_commands. Use a for loop with variables start and end to iterate over pairs of consecutive points in contour_points. For each pair, append a string to gcode_commands in this format: f"G1 X{end[0]} Y{end[1]} F{feed_rate}".

CNC Programming

contour_points = [(0, 0), (10, 0), (10, 5), (0, 5)]
feed_rate = 1500
# Create gcode_commands list and fill it with G1 commands for each line segment
# Your code here

Hint

Use a for loop with index to access pairs of points and format the G-code string.

4

Print the G-code commands

Use a for loop to print each command in gcode_commands on its own line.

CNC Programming

contour_points = [(0, 0), (10, 0), (10, 5), (0, 5)]
feed_rate = 1500
gcode_commands = []
for i in range(len(contour_points) - 1):
    start = contour_points[i]
    end = contour_points[i + 1]
    gcode_commands.append(f"G1 X{end[0]} Y{end[1]} F{feed_rate}")
# Print each G-code command on its own line
# Your code here

Hint

Use a for loop to print each string in gcode_commands.

Practice

1. What does the G01 command do in contour milling with line segments?

easy

A. Sets the tool to a circular interpolation mode

B. Moves the tool in a straight line at a set feed rate

C. Moves the tool rapidly without cutting

D. Stops the machine immediately

Solution

1. Step 1: Understand G-code commands for movement

   The G01 command is used for linear interpolation, meaning the tool moves in a straight line at a controlled feed rate.

2. Step 2: Differentiate from other movement commands

   G00 is rapid move without cutting, G02/G03 are circular moves, and M00 stops the machine.

3. Final Answer:

   Moves the tool in a straight line at a set feed rate -> Option B

4. Quick Check:

   G01 = straight cutting move [OK]

Hint: G01 always means straight cutting move [OK]

Common Mistakes:

• Confusing G01 with rapid move G00
• Thinking G01 is for circular moves
• Assuming G01 stops the machine

2. Which of the following is the correct syntax to move the tool to X=10, Y=20 using linear interpolation in CNC code?

easy

A. G01 X10 Y20

B. G00 X10 Y20

C. G02 X10 Y20

D. G03 X10 Y20

Solution

1. Step 1: Identify the command for linear interpolation

   G01 is used for linear moves with cutting at a set feed rate.

2. Step 2: Check the coordinate format

   Coordinates are given as X10 Y20 to specify the target position.

3. Final Answer:

   G01 X10 Y20 -> Option A

4. Quick Check:

   Linear move syntax = G01 X10 Y20 [OK]

Hint: Use G01 for cutting moves with coordinates [OK]

Common Mistakes:

• Using G00 which is rapid move without cutting
• Using circular move commands G02 or G03 incorrectly
• Omitting the G-code command before coordinates

3. Given the CNC code snippet:

G21
G90
G01 X10 Y0 F100
G01 X10 Y10
G01 X0 Y10
G01 X0 Y0

What shape will the tool cut?

medium

A. A square with 10 units sides

B. A circle with radius 10

C. A diagonal line from (0,0) to (10,10)

D. A triangle with vertices at (0,0), (10,0), (10,10)

Solution

1. Step 1: Analyze the coordinate moves

   The tool moves from (0,0) to (10,0), then to (10,10), then to (0,10), and back to (0,0).

2. Step 2: Visualize the path

   These points form the four corners of a square with sides of length 10 units.

3. Final Answer:

   A square with 10 units sides -> Option A

4. Quick Check:

   Coordinates form square corners [OK]

Hint: Trace points step-by-step to visualize shape [OK]

Common Mistakes:

• Assuming circular shape from linear moves
• Ignoring the return to start point
• Confusing coordinate order

4. Identify the error in this CNC code snippet for contour milling:

G21
G91
G01 X10 Y0 F150
G01 X0 Y10
G01 X-10 Y0
G01 X0 Y-10

medium

A. Feed rate (F150) is missing

B. Coordinates must be absolute with G90, not incremental

C. Using incremental positioning (G91) causes incorrect final position

D. Negative coordinates are not allowed

Solution

1. Step 1: Understand positioning modes

   G91 sets incremental positioning, so each move is relative to the current position.

2. Step 2: Check if moves form a closed contour

   Contour milling typically uses absolute positioning (G90); G91 leads to incorrect final position relative to absolute expectations.

3. Step 3: Identify the error

   Using incremental mode without adjusting coordinates causes unexpected tool path.

4. Final Answer:

   Using incremental positioning (G91) causes incorrect final position -> Option C

5. Quick Check:

   Incremental mode changes move reference [OK]

Hint: Check G90 vs G91 for coordinate reference [OK]

Common Mistakes:

• Confusing incremental and absolute positioning
• Ignoring feed rate presence
• Believing negative coordinates are invalid

5. You want to mill a rectangular contour 50 units wide and 30 units high starting at the origin (0,0). Which CNC code sequence correctly uses line segments (G01) to cut this shape with absolute positioning and feed rate 200?

hard

A. G21\nG90\nG01 X0 Y50 F200\nG01 X30 Y50\nG01 X30 Y0\nG01 X0 Y0

B. G21\nG91\nG01 X50 Y0 F200\nG01 X0 Y30\nG01 X-50 Y0\nG01 X0 Y-30

C. G20\nG90\nG01 X50 Y0 F200\nG01 X50 Y30\nG01 X0 Y30\nG01 X0 Y0

D. G21\nG90\nG01 X50 Y0 F200\nG01 X50 Y30\nG01 X0 Y30\nG01 X0 Y0

Solution

1. Step 1: Confirm units and positioning mode

   G21 sets millimeters, G90 sets absolute positioning as required.

2. Step 2: Check the coordinate moves for rectangle

   Moves go from (0,0) to (50,0), then (50,30), then (0,30), and back to (0,0), forming a 50x30 rectangle.

3. Step 3: Verify feed rate and command correctness

   Feed rate F200 is set on first move, and all moves use G01 for linear cutting.

4. Final Answer:

   G21 G90 G01 X50 Y0 F200 G01 X50 Y30 G01 X0 Y30 G01 X0 Y0 -> Option D

5. Quick Check:

   Absolute moves with G90 and correct coords [OK]

Hint: Use G21 + G90 + G01 with correct coords for rectangle [OK]

Common Mistakes:

• Using incremental mode instead of absolute
• Setting wrong units (G20 instead of G21)
• Incorrect coordinate order or values