Finishing strategies (contour, scallop) in CNC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a contour finishing path command

What is the expected toolpath output for this contour finishing G-code snippet?

CNC Programming

G01 X10 Y10 F100
G01 X20 Y10
G01 X20 Y20
G01 X10 Y20
G01 X10 Y10

A A square path starting at (10,10) moving clockwise

B A circular path around the points

C A diagonal line from (10,10) to (20,20)

D A square path starting at (10,10) moving counterclockwise

💡 Hint

Contour finishing follows the shape edges in a closed loop.

The G01 commands move the tool in straight lines forming a square path starting at (10,10) and moving clockwise.

🧠 Conceptual

intermediate

Difference between contour and scallop finishing

Which statement best describes the difference between contour and scallop finishing strategies?

A Both contour and scallop finishing remove material in straight lines only.

B Scallop finishing follows the shape edges; contour finishing removes material in arcs.

C Contour finishing is used only for roughing; scallop finishing is for drilling.

D Contour finishing follows the exact shape edges; scallop finishing removes material in overlapping arcs.

💡 Hint

Think about how the tool moves relative to the shape edges.

Contour finishing traces the exact edges of the part, while scallop finishing uses overlapping arcs to smooth surfaces.

🔧 Debug

advanced

Identify the error in this scallop finishing G-code snippet

What error will this scallop finishing G-code cause when executed?

CNC Programming

G02 X30 Y30 I5 J0 F150
G02 X40 Y30 I5 J0
G02 X40 Y40 I0 J5
G02 X30 Y40 I0 J5

A Syntax error due to missing feed rate on second line

B Runtime error because arcs do not connect smoothly

C No error; code runs correctly producing scallop arcs

D Tool will move in straight lines instead of arcs

💡 Hint

Check if the arcs connect properly at endpoints.

The arcs defined do not connect smoothly, causing a runtime error or unexpected toolpath.

🚀 Application

advanced

Calculate scallop height for a given tool and step-over

Given a ball-end mill with radius 5mm and a scallop step-over of 4mm, what is the approximate scallop height?

A 0.2 mm

B 0.5 mm

C 0.4 mm

D 1.0 mm

💡 Hint

Use the formula h = r - sqrt(r^2 - (s/2)^2), where r is radius and s is step-over.

Plugging in r=5 and s=4 gives h ≈ 0.4 mm scallop height.

📝 Syntax

expert

Identify the correct G-code snippet for contour finishing with a clockwise path

Which G-code snippet correctly programs a contour finishing path moving clockwise around a rectangle from (0,0) to (50,30)?

A

G01 X0 Y0
G01 X50 Y0
G01 X50 Y30
G01 X0 Y30
G01 X0 Y0

B

G01 X0 Y0
G01 X0 Y30
G01 X50 Y30
G01 X50 Y0
G01 X0 Y0

C

G01 X0 Y0
G01 X50 Y30
G01 X50 Y0
G01 X0 Y30
G01 X0 Y0

D

G01 X0 Y0
G01 X50 Y0
G01 X0 Y30
G01 X50 Y30
G01 X0 Y0

💡 Hint

Clockwise movement around a rectangle starts at bottom-left, moves right, then up, then left, then down.

Option A moves from (0,0) right to (50,0), up to (50,30), left to (0,30), then down to (0,0), which is clockwise.

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