Contour milling with line segments in CNC Programming - Step-by-Step Execution

Concept Flow - Contour milling with line segments

Start at first point

↓

Move to next point

↓

Mill line segment

↓

More points?

Yes→Repeat move and mill

No↓

End contour milling

The CNC machine starts at the first point, moves along each line segment milling the contour, and stops when all points are processed.

Execution Sample

CNC Programming

N10 G00 X0 Y0
N20 G01 X10 Y0 F100
N30 G01 X10 Y10
N40 G01 X0 Y10
N50 G01 X0 Y0
N60 M30

This CNC program mills a rectangular contour by moving along four line segments connecting the points.

Execution Table

Step	Command	Position (X,Y)	Feed Rate	Action	Output
1	N10 G00 X0 Y0	(0,0)	Rapid	Move to start point	At start point (0,0)
2	N20 G01 X10 Y0 F100	(10,0)	100	Mill line segment	Milled from (0,0) to (10,0)
3	N30 G01 X10 Y10	(10,10)	100	Mill line segment	Milled from (10,0) to (10,10)
4	N40 G01 X0 Y10	(0,10)	100	Mill line segment	Milled from (10,10) to (0,10)
5	N50 G01 X0 Y0	(0,0)	100	Mill line segment	Milled from (0,10) to (0,0)
6	N60 M30	(0,0)	Stop	Program end	Contour milling complete

💡 Program ends after completing all line segments of the contour.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
Position	(0,0)	(10,0)	(10,10)	(0,10)	(0,0)	(0,0)
Feed Rate	Rapid	100	100	100	100	Stop
Action	Idle	Milling	Milling	Milling	Milling	Stopped

Key Moments - 3 Insights

Why does the machine use G00 for the first move and G01 for the others?

G00 is a rapid move to position the tool quickly without cutting (see Step 1). G01 is a controlled feed move used for milling along line segments (Steps 2-5).

How does the feed rate affect the milling process?

Feed rate controls the speed of cutting. In Steps 2-5, feed rate is set to 100 for controlled milling. Without it, the machine might move too fast or too slow.

What happens if the last line segment is not closed back to the start point?

The contour would be open, milling only partial shape. Here, Step 5 moves back to (0,0) to close the contour fully.

Visual Quiz - 3 Questions

Look at the execution table, what is the position after Step 3?

A (0,10)

B (10,0)

C (10,10)

D (0,0)

💡 Hint

Check the 'Position (X,Y)' column in row for Step 3.

At which step does the machine start milling the first line segment?

A Step 1

B Step 2

C Step 3

D Step 6

💡 Hint

Look for the first G01 command and milling action in the execution table.

If the feed rate was not set in Step 2, what would change in the execution?

A The machine would use the last feed rate set or default

B The machine would stop

C The machine would mill faster than intended

D The machine would move rapidly without cutting

💡 Hint

Refer to the 'Feed Rate' column and how CNC machines remember feed rates between commands.

Concept Snapshot

Contour milling moves the tool along connected line segments.
Use G00 for rapid positioning to start point.
Use G01 with feed rate to mill each line segment.
Close the contour by returning to start point.
Feed rate controls cutting speed.
Program ends with M30.

Full Transcript

Contour milling with line segments means moving the CNC tool along straight connected lines to cut a shape. The program starts by moving rapidly to the first point using G00. Then it uses G01 commands with a feed rate to mill each line segment in order. The tool moves from point to point, cutting the contour. The feed rate controls how fast the tool cuts. The program closes the shape by returning to the start point. Finally, the program ends with M30. This step-by-step process ensures the contour is milled accurately and efficiently.

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