Tool length offset (G43) in CNC Programming - Step-by-Step Execution

Concept Flow - Tool length offset (G43)

Start Program

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Call Tool with T-code

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Activate G43

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Apply Tool Length Offset H-code

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Machine moves with adjusted Z position

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Continue machining

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End Program

The CNC program calls a tool, activates G43 to apply the tool length offset stored in the specified H register, adjusting the Z-axis position before machining.

Execution Sample

CNC Programming

T1 M06
G43 H01 Z50.0
G01 X100 Y100 Z-10 F200

Select tool 1, apply tool length offset from H01, move to position with adjusted Z.

Execution Table

Step	Command	Action	Tool Length Offset (H)	Z Position Before Offset	Z Position After Offset	Notes
1	T1 M06	Tool 1 selected and changed	-	-	-	Tool ready for use
2	G43 H01 Z50.0	Activate tool length offset using H01	H01=20.0	50.0	30.0	Z adjusted by subtracting offset 20.0
3	G01 X100 Y100 Z-10 F200	Move to X100 Y100 Z-10 with offset applied	H01=20.0	-10	-30.0	Z moves relative to offset, actual position is -10 - 20 = -30
4	Program End	Stop machining	-	-	-	End of program

💡 Program ends after machining moves with tool length offset applied

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Tool	None	1	1	1	1
H01 (offset)	20.0	20.0	20.0	20.0	20.0
Z Position	Undefined	Undefined	30.0	-30.0	-30.0

Key Moments - 2 Insights

Why does the Z position change after applying G43 H01 Z50.0?

Because G43 applies the tool length offset stored in H01 by subtracting it from the programmed Z value, adjusting the tool tip position correctly as shown in execution_table step 2.

What happens if you forget to specify the H code with G43?

The machine won't know which tool length offset to apply, so the Z position won't adjust correctly, possibly causing incorrect machining depth. This is implied by the need for H01 in step 2.

Visual Quiz - 3 Questions

Look at the execution table, what is the Z position after applying G43 H01 Z50.0?

A 70.0

B 50.0

C 30.0

D -20.0

💡 Hint

Check step 2 in the execution_table for Z Position After Offset

At which step does the tool length offset get applied?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look for G43 command in the execution_table

If the offset in H01 was 25.0 instead of 20.0, what would be the Z position after applying G43 H01 Z50.0?

A 30.0

B 25.0

C 75.0

D 50.0

💡 Hint

Subtract the new offset 25.0 from Z 50.0 as in step 2

Concept Snapshot

G43 applies tool length offset stored in H register.
Syntax: G43 Hxx Znnn
Z position is adjusted by subtracting offset.
Always specify H code with G43.
Used after tool change (T-code).
Ensures correct tool tip position.

Full Transcript

This visual execution trace shows how the CNC command G43 applies a tool length offset. First, the program selects a tool with T1 M06. Then, G43 H01 Z50.0 activates the offset stored in H01, which is 20.0 units. The Z position is adjusted by subtracting this offset from the programmed Z value, changing from 50.0 to 30.0. Next, the machine moves to X100 Y100 Z-10, which after offset becomes Z-30.0. The program ends after these moves. Key points include always specifying the H code with G43 and understanding that the offset adjusts the Z position to ensure the tool tip is at the correct height for machining.