Finishing strategies (contour, scallop) in CNC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a contour finishing strategy in CNC programming?

Contour finishing is a method where the tool follows the exact outline or edge of the part to create a smooth, precise surface finish.

beginner

Describe the scallop finishing strategy.

Scallop finishing moves the tool in a pattern that keeps a constant step-over distance, creating small overlapping arcs or 'scallops' to smooth the surface.

intermediate

Why is scallop finishing preferred for complex 3D surfaces?

Because scallop finishing maintains a consistent step-over, it can evenly smooth curved or irregular surfaces, avoiding marks or unevenness.

intermediate

What is the main difference between contour and scallop finishing?

Contour finishing follows the exact edge or profile, while scallop finishing moves in overlapping arcs to maintain a constant surface finish across the whole area.

intermediate

How does step-over affect scallop finishing?

Step-over controls the distance between tool passes; smaller step-over means finer finish but longer machining time, larger step-over means faster but rougher finish.

Which finishing strategy follows the exact outline of the part?

A Contour finishing

B Scallop finishing

C Pocket milling

D Drilling

Contour finishing moves the tool along the exact edge or profile of the part.

What pattern does scallop finishing create on the surface?

A Random paths

B Overlapping arcs or scallops

C Straight lines

D Circular holes

Scallop finishing creates overlapping arcs to maintain a smooth surface.

Why might scallop finishing take longer than contour finishing?

A Because it maintains a constant step-over for a smooth finish

B Because it only follows edges

C Because it drills holes

D Because it uses larger tools

Maintaining a constant step-over means more passes and longer machining time.

Which finishing strategy is better for smoothing complex 3D shapes?

A Contour finishing

B Face milling

C Scallop finishing

D Slotting

Scallop finishing adapts well to curved surfaces by keeping a consistent finish.

What happens if you increase the step-over in scallop finishing?

A Tool path follows edges only

B Machining time increases

C Finish becomes smoother

D Finish becomes rougher

Increasing step-over means fewer passes and a rougher surface finish.

Explain the difference between contour and scallop finishing strategies in CNC programming.

Think about how the tool moves and the surface it creates.

Describe when and why you would choose scallop finishing over contour finishing.

Consider the shape of the part and the finish quality needed.

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