Finishing strategies (contour, scallop) in CNC Programming - Step-by-Step Execution

Concept Flow - Finishing strategies (contour, scallop)

Start: Select finishing strategy

↓

Use scallop strategy

Yes↓

Generate contour path

↓

Execute contour machining

↓

Check surface finish

↓

End

This flow shows choosing between contour and scallop finishing, generating paths, and machining to achieve smooth surfaces.

Execution Sample

CNC Programming

G01 X10 Y10 F100 ; Move to start
G01 Z-1 F50 ; Cut down
G01 X50 Y10 ; Contour path
G01 X50 Y50 ; Continue contour
G01 X10 Y50 ; Complete contour
G01 Z5 ; Retract

This CNC code executes a contour finishing pass around a rectangular shape.

Execution Table

Step	G-Code Command	Action	Position (X,Y,Z)	Feed Rate	Result
1	G01 X10 Y10 F100	Feed move to start point	(10,10,0)	100	Positioned at start edge
2	G01 Z-1 F50	Cut down into material	(10,10,-1)	50	Tool engaged at cutting depth
3	G01 X50 Y10	Cut along X axis (contour)	(50,10,-1)	50	Cutting contour edge
4	G01 X50 Y50	Cut along Y axis (contour)	(50,50,-1)	50	Cutting contour edge
5	G01 X10 Y50	Cut back along X axis (contour)	(10,50,-1)	50	Cutting contour edge
6	G01 Z5	Retract tool	(10,50,5)	50	Tool safely above workpiece
7	End	-	-	-	Contour finishing complete

💡 All contour edges cut, tool retracted, finishing pass complete

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
Position	(0,0,0)	(10,10,0)	(10,10,-1)	(50,10,-1)	(50,50,-1)	(10,50,-1)	(10,50,5)	(10,50,5)
Feed Rate	0	100	50	50	50	50	50	50
Tool State	Up	Up	Down	Down	Down	Down	Up	Up

Key Moments - 3 Insights

Why does the tool retract (Z moves up) at the end of the contour pass?

The tool retracts to move safely above the workpiece to avoid collisions when moving to the next position or finishing the program, as shown in step 6 of the execution_table.

Why is the feed rate slower when cutting compared to moving to start position?

Feed rate is slower during cutting (50) to control the tool and avoid damage, while moving to start (100) is faster since the tool is not cutting, as seen in steps 1 and 2.

What is the difference between contour and scallop finishing in terms of tool path?

Contour finishing follows the exact edge shape (like the rectangle here), while scallop finishing moves in overlapping arcs to smooth surfaces. This example shows contour path generation.

Visual Quiz - 3 Questions

Look at the execution_table, what is the tool position after step 3?

A (50,10,-1)

B (10,10,-1)

C (50,50,-1)

D (10,50,-1)

💡 Hint

Check the Position column in row for step 3 in execution_table

At which step does the tool start cutting into the material?

A Step 1

B Step 4

C Step 2

D Step 6

💡 Hint

Look for Z coordinate change from 0 to negative in execution_table

If feed rate during cutting was increased to 100, what would change in the execution_table?

A Position values would change

B Feed Rate column would show 100 for steps 2 to 5

C Tool would not retract

D No change at all

💡 Hint

Feed Rate column shows speed; changing feed rate affects this value, not position or tool state

Concept Snapshot

Finishing strategies guide the final tool path for smooth surfaces.
Contour follows exact edges; scallop moves in overlapping arcs.
Use G01 moves with controlled feed rates for cutting.
Retract tool after finishing to avoid collisions.
Feed rate is slower during cutting than rapid moves.
Choose strategy based on surface shape and finish quality.

Full Transcript

This visual execution shows how a CNC finishing strategy using contour works step-by-step. The program moves the tool to the start position quickly, then lowers it to cutting depth. It follows the rectangular contour edges with controlled feed rate to cut smoothly. After completing the contour, the tool retracts safely above the workpiece. Variables like position, feed rate, and tool state change at each step. Key moments include why the tool retracts, why feed rate changes, and the difference between contour and scallop finishing. The quiz tests understanding of tool position, cutting start, and feed rate effects. This helps beginners see how finishing strategies run in CNC programming.

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