Finishing strategies (contour, scallop) in CNC Programming - Mini Project: Build & Apply

Finishing Strategies: Contour and Scallop CNC Paths

📖 Scenario: You work in a small CNC workshop. Your job is to create toolpaths that finish parts smoothly. Two common finishing strategies are contour and scallop. Contour follows the edge of the part exactly. Scallop removes small leftover material in a wavy pattern.Understanding these helps you make parts look nice and fit well.

🎯 Goal: Build a simple CNC program script that defines a part shape, sets finishing parameters, generates contour and scallop toolpaths, and outputs the final G-code commands.

📋 What You'll Learn

Create a dictionary called part_shape with exact points defining a square: (0,0), (0,10), (10,10), (10,0)

Create a variable called finishing_params as a dictionary with keys 'stepover' set to 1.0 and 'tool_diameter' set to 2.0

Write a function called generate_contour_path that takes part_shape and returns a list of G-code strings following the contour

Write a function called generate_scallop_path that takes part_shape and finishing_params and returns a list of G-code strings for scallop finishing

Print the combined G-code commands from both finishing strategies

💡 Why This Matters

🌍 Real World

CNC machinists use finishing strategies like contour and scallop to produce smooth, precise parts. Automating G-code generation saves time and reduces errors.

💼 Career

Understanding and scripting finishing toolpaths is essential for CNC programmers and manufacturing engineers to optimize machining quality and efficiency.

1

Define the part shape points

Create a dictionary called part_shape with these exact points as keys and values: 1: (0, 0), 2: (0, 10), 3: (10, 10), 4: (10, 0)

CNC Programming

# Create the part_shape dictionary with points 1 to 4
# Your code here

Hint

Use a dictionary with keys 1 to 4 and tuple values for coordinates.

2

Set finishing parameters

Create a dictionary called finishing_params with keys 'stepover' set to 1.0 and 'tool_diameter' set to 2.0

CNC Programming

part_shape = {1: (0, 0), 2: (0, 10), 3: (10, 10), 4: (10, 0)}
# Create finishing_params dictionary
# Your code here

Hint

Use a dictionary with the exact keys and values.

3

Write functions to generate finishing paths

Write two functions: generate_contour_path(part_shape) that returns a list of G-code strings moving along the points in order, and generate_scallop_path(part_shape, finishing_params) that returns a list of G-code strings simulating scallop passes inside the shape using the stepover parameter

CNC Programming

part_shape = {1: (0, 0), 2: (0, 10), 3: (10, 10), 4: (10, 0)}
finishing_params = {'stepover': 1.0, 'tool_diameter': 2.0}

# Define generate_contour_path function
# Define generate_scallop_path function
# Your code here

Hint

Use loops to create G-code lines moving along points and scallop passes.

4

Print the combined finishing G-code

Call generate_contour_path(part_shape) and generate_scallop_path(part_shape, finishing_params), combine their results into one list called final_gcode, then print each line in final_gcode

CNC Programming

Hint

Print each G-code line from the combined list.

Practice

1. Which finishing strategy is best suited for cleaning the edges of a part by following its outline?

easy

A. Scallop finishing

B. Pocket milling

C. Contour finishing

D. Drilling

Solution

1. Step 1: Understand the purpose of contour finishing

   Contour finishing follows the outline of a part to clean and smooth its edges.

2. Step 2: Compare with other strategies

   Scallop finishing smooths curved surfaces, pocket milling removes material inside an area, and drilling creates holes.

3. Final Answer:

   Contour finishing -> Option C

4. Quick Check:

   Edge cleaning = Contour finishing [OK]

Hint: Edges cleaned by following outline means contour finishing [OK]

Common Mistakes:

• Confusing scallop finishing with contour finishing
• Choosing pocket milling for edge finishing
• Thinking drilling is a finishing strategy

2. Which of the following is the correct syntax to select scallop finishing in a CNC program snippet?

easy

A. FINISH STRATEGY = CONTOUR

B. FINISH STRATEGY = SCALLOP

C. FINISH = SCALLOP

D. STRATEGY = SCALLOP_FINISH

Solution

1. Step 1: Identify the correct keyword and value

   The standard syntax uses 'FINISH STRATEGY = SCALLOP' to select scallop finishing.

2. Step 2: Check other options for syntax errors

   Options B, C, and D use incorrect keywords or incomplete values.

3. Final Answer:

   FINISH STRATEGY = SCALLOP -> Option B

4. Quick Check:

   Correct syntax = FINISH STRATEGY = SCALLOP [OK]

Hint: Look for exact keyword 'FINISH STRATEGY = SCALLOP' [OK]

Common Mistakes:

• Using incomplete or wrong keywords
• Mixing contour and scallop keywords
• Missing equal sign or wrong casing

3. Given this CNC code snippet for scallop finishing:

TOOLPATH FINISH SCALLOP
  STEP_OVER = 0.5
  CUT_DEPTH = 0.2
END_TOOLPATH

What is the main effect of reducing the STEP_OVER value?

medium

A. Increase surface smoothness by making tool passes closer

B. Decrease machining time by skipping passes

C. Increase cut depth per pass

D. Change tool diameter automatically

Solution

1. Step 1: Understand STEP_OVER in scallop finishing

   STEP_OVER controls the distance between tool passes; smaller values mean closer passes.

2. Step 2: Effect of reducing STEP_OVER

   Closer passes improve surface smoothness but increase machining time.

3. Final Answer:

   Increase surface smoothness by making tool passes closer -> Option A

4. Quick Check:

   Smaller STEP_OVER = smoother surface [OK]

Hint: Smaller step over means closer passes and smoother finish [OK]

Common Mistakes:

• Thinking smaller step over reduces machining time
• Confusing step over with cut depth
• Assuming tool diameter changes automatically

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

TOOLPATH FINISH CONTOUR
  STEP_OVER = 1.0
  CUT_DEPTH = -0.1
END_TOOLPATH

medium

A. Missing tool diameter specification

B. STEP_OVER value is too large for contour finishing

C. FINISH keyword should be SCALLOP, not CONTOUR

D. CUT_DEPTH should be positive, not negative

Solution

1. Step 1: Check CUT_DEPTH value

   CUT_DEPTH represents how deep the tool cuts; it should be positive to indicate depth.

2. Step 2: Analyze other parameters

   STEP_OVER can be 1.0 if suitable, tool diameter is optional here, and CONTOUR is correct for contour finishing.

3. Final Answer:

   CUT_DEPTH should be positive, not negative -> Option D

4. Quick Check:

   Negative CUT_DEPTH is invalid [OK]

Hint: Cut depth must be positive number in finishing [OK]

Common Mistakes:

• Ignoring negative cut depth as error
• Assuming STEP_OVER is always too large
• Confusing contour and scallop keywords

5. You want to finish a complex curved surface with minimal scallop marks and efficient machining time. Which combination of finishing strategy and parameter adjustment is best?

hard

A. Use scallop finishing with a small STEP_OVER value

B. Use contour finishing with a large STEP_OVER value

C. Use scallop finishing with a large CUT_DEPTH value

D. Use contour finishing with a small CUT_DEPTH value

Solution

1. Step 1: Choose finishing strategy for curved surfaces

   Scallop finishing is designed to smooth curved surfaces effectively.

2. Step 2: Adjust parameters for minimal scallop marks and efficiency

   A small STEP_OVER reduces scallop marks by making passes closer, balancing smoothness and machining time.

3. Final Answer:

   Use scallop finishing with a small STEP_OVER value -> Option A

4. Quick Check:

   Curved surface + small STEP_OVER = smooth finish [OK]

Hint: Small step over + scallop finishing smooths curves best [OK]

Common Mistakes:

• Choosing contour finishing for curved surfaces
• Using large STEP_OVER causing rough finish
• Increasing CUT_DEPTH unnecessarily