G42 right compensation in CNC Programming - Step-by-Step Execution

Concept Flow - G42 right compensation

Start Program

↓

Move to Start Point

↓

Activate G42 (Right Compensation)

↓

Tool Path Offset Right of Programmed Path

↓

Execute Cutting Along Offset Path

↓

Deactivate Compensation

↓

End Program

The flow shows starting the program, activating G42 to offset the tool path to the right of the programmed line, cutting along this offset path, then turning off compensation.

Execution Sample

CNC Programming

N10 G00 X10 Y10
N20 G42 D01
N30 G01 X50 Y10 F100
N40 G40
N50 M30

This CNC code moves to start, activates G42 with tool offset 1, cuts a straight line with right compensation, then cancels compensation and ends.

Execution Table

Step	Code Line	Action	Tool Position	Compensation Status	Output/Note
1	N10 G00 X10 Y10	Rapid move to start point	(10,10)	Off	Tool moves to programmed start point
2	N20 G42 D01	Activate right compensation with tool offset 1	(10,10)	On (Right)	Tool path will offset right by tool radius
3	N30 G01 X50 Y10 F100	Linear cut with feedrate 100	(50,9) approx	On (Right)	Tool cuts offset path right of programmed line
4	N40 G40	Cancel compensation	(50,9)	Off	Tool returns to programmed path, compensation off
5	N50 M30	Program end	(50,9)	Off	Program stops
6	-	-	-	-	End of program execution

💡 Program ends after M30; compensation cancelled at step 4.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
Tool Position	(0,0)	(10,10)	(10,10)	(50,9)	(50,9)	(50,9)
Compensation Status	Off	Off	On (Right)	On (Right)	Off	Off

Key Moments - 3 Insights

Why does the tool position at step 3 show Y=9 instead of Y=10?

Because G42 applies right compensation, the tool path is offset to the right of the programmed line, shifting the tool position slightly off the programmed Y=10 line as shown in execution_table row 3.

What happens if G40 is not used after G42?

Without G40 to cancel compensation (step 4), the tool would continue offsetting the path, which might cause incorrect machining or crashes, as shown by compensation status remaining 'On (Right)' in execution_table.

Does the tool move instantly to the offset path when G42 is activated?

No, the tool stays at the programmed position when G42 activates (step 2), but the cutting moves offset to the right start from the next move (step 3), as seen in tool position changes.

Visual Quiz - 3 Questions

Look at the execution table, what is the compensation status after executing line N20?

A On (Right)

B Off

C On (Left)

D Cancelled

💡 Hint

Check the 'Compensation Status' column at step 2 in the execution_table.

At which step does the tool start cutting along the offset path?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' and 'Tool Position' columns in execution_table to see when cutting begins with offset.

If G40 was omitted, how would the compensation status change after step 4?

A It would switch to On (Left)

B It would remain On (Right)

C It would turn Off automatically

D It would cause an error

💡 Hint

Refer to key_moments explanation about cancelling compensation and execution_table step 4.

Concept Snapshot

G42 activates right tool radius compensation.
Tool path shifts right of programmed line.
Use G42 before cutting moves.
Cancel with G40 after compensation.
Prevents tool from cutting inside the programmed path.
Essential for precise machining with tool diameter.

Full Transcript

This visual execution trace shows how G42 right compensation works in CNC programming. The program starts by moving the tool to the start point without compensation. Then G42 is activated with a tool offset number, turning on right compensation. The cutting move follows an offset path to the right of the programmed line, shifting the tool position slightly. After cutting, G40 cancels the compensation, returning the tool path to programmed coordinates. The program ends with M30. Key points include understanding that compensation shifts the tool path, that G40 must be used to cancel compensation, and that the tool does not instantly move when G42 activates but offsets during cutting moves. The execution table and variable tracker clearly show tool positions and compensation status at each step, helping beginners visualize how G42 affects machining.

Practice

1. What does the G42 command do in CNC programming?

easy

A. Cancels all tool radius compensation

B. Activates right side tool radius compensation

C. Activates left side tool radius compensation

D. Changes the tool without compensation

Solution

1. Step 1: Understand G42 function

   G42 is used to activate tool radius compensation on the right side of the programmed path.

2. Step 2: Compare with other compensation codes

   G41 activates left compensation, G40 cancels compensation, so G42 is specifically right compensation.

3. Final Answer:

   Activates right side tool radius compensation -> Option B

4. Quick Check:

   G42 = Right compensation [OK]

Hint: G42 always means right side compensation [OK]

Common Mistakes:

• Confusing G42 with G41 (left compensation)
• Thinking G42 cancels compensation
• Assuming G42 changes tools

2. Which of the following is the correct syntax to activate right compensation with tool offset 3?

easy

A. G42 D3

B. G41 D3

C. G40 D3

D. G43 D3

Solution

1. Step 1: Identify G42 usage with tool offset

   G42 activates right compensation and requires a tool offset number with D, e.g., D3.

2. Step 2: Check other codes

   G41 is left compensation, G40 cancels compensation, G43 is tool length offset, so only G42 D3 is correct.

3. Final Answer:

   G42 D3 -> Option A

4. Quick Check:

   Right compensation syntax = G42 D3 [OK]

Hint: G42 + D number activates right compensation [OK]

Common Mistakes:

• Using G41 instead of G42
• Adding D number with G40 or G43 incorrectly
• Omitting the D number

3. Given the CNC code snippet:
G00 X10 Y10
G42 D1
G01 X20 Y20 F100
What is the effect of the G42 command here?

medium

A. Tool radius compensation is canceled before moving

B. Tool radius compensation starts on the left side from X10 Y10 to X20 Y20

C. Tool radius compensation starts on the right side from X10 Y10 to X20 Y20

D. Tool length offset is applied instead of radius compensation

Solution

1. Step 1: Analyze G42 command with D1

   G42 activates right side tool radius compensation using tool offset 1.

2. Step 2: Understand movement commands

   G00 moves rapidly to X10 Y10, then G42 activates compensation, and G01 moves linearly to X20 Y20 with compensation active.

3. Final Answer:

   Tool radius compensation starts on the right side from X10 Y10 to X20 Y20 -> Option C

4. Quick Check:

   G42 D1 activates right compensation on next move [OK]

Hint: G42 activates right compensation starting at next move [OK]

Common Mistakes:

• Confusing right and left compensation
• Thinking compensation cancels movement
• Mixing tool length offset with radius compensation

4. Identify the error in this CNC code snippet:
G42 D2
G01 X50 Y50
G40
G01 X60 Y60

medium

A. G40 should be before G42

B. G42 must be followed by a move command before G40

C. G40 cancels compensation but is placed after a move command

D. No error, code is correct

Solution

1. Step 1: Check G42 and move sequence

   G42 activates right compensation with D2, followed by a move command G01 X50 Y50 which uses compensation.

2. Step 2: Check G40 placement

   G40 cancels compensation after the compensated move, then next move G01 X60 Y60 is without compensation, which is correct practice.

3. Final Answer:

   No error, code is correct -> Option D

4. Quick Check:

   G42 then move then G40 cancel is correct order [OK]

Hint: G40 cancels compensation after moves, not before [OK]

Common Mistakes:

• Placing G40 before any compensated move
• Missing move command after G42
• Assuming G40 cancels before compensation starts

5. You want to mill a square path with right side tool radius compensation using tool offset 4. Which sequence correctly applies G42 and cancels it after the path?

hard

A. G42 D4G01 X100 Y0G01 X100 Y100G01 X0 Y100G01 X0 Y0G40

B. G40G42 D4G01 X100 Y0G01 X100 Y100G01 X0 Y100G01 X0 Y0

C. G42 D4G40G01 X100 Y0G01 X100 Y100G01 X0 Y100G01 X0 Y0

D. G01 X100 Y0G42 D4G01 X100 Y100G01 X0 Y100G01 X0 Y0G40

Solution

1. Step 1: Start compensation before path moves

   G42 D4 activates right compensation with tool offset 4 before starting the square path moves.

2. Step 2: Cancel compensation after completing path

   G40 cancels the compensation after all moves are done, ensuring no compensation on subsequent moves.

3. Final Answer:

   G42 D4 followed by moves then G40 to cancel -> Option A

4. Quick Check:

   Activate G42 before moves, cancel with G40 after [OK]

Hint: Activate G42 before moves, cancel with G40 after [OK]

Common Mistakes:

• Placing G40 before moves cancels compensation too early
• Inserting G40 in middle of path
• Starting moves before activating G42