The G72 facing cycle is a common Computer Numerical Control (CNC) programming feature used in lathes for facing operations. It automates the process of cutting a face to a specific depth, following a predefined path.

Usage Format

The G72 cycle is typically used in two-line format, with the first line specifying the depth of cut and the retract amount, and the second line detailing the start and end points of the subroutine, finishing allowances, and feed rate.

Explanation

The G72 cycle allows for efficient and precise material removal on the X-axis, moving towards the centerline of the part. It uses a subroutine, defined between ‘N’ numbers, to follow the contour of the profile.

Example

Here’s a text diagram and example of a G72 facing cycle program:

% 
O0001 (G72 FACING CYCLE EXAMPLE)
N005 G50 S1200
N010 T0101
N015 G96 S150 M03
N020 G00 X100 Z5.0 M08
N025 G72 W1.5 R0.2
N030 G72 P035 Q045 U0.5 W0.3 F0.2
N035 G00 X90.0 Z2.0
N040 G01 Z-1.0 F0.18
N045 G00 X150.0 Z100.0
N050 M30
%

Text Diagram:

|<-- Facing Operation -->|
|                         |
|                         |
|                         |
|                         |
|-------------------------| <- Workpiece Surface
|                         |
|                         |
|                         |
|                         |
|                         |
|-------------------------| <- Final Depth

In this program:

• N025 G72 W1.5 R0.2 sets the depth of cut to 1.5mm and the retract distance to 0.2mm.
• N030 G72 P035 Q045 U0.5 W0.3 F0.2 calls the subroutine from N035 to N045, leaving a finishing allowance of 0.5mm in X and 0.3mm in Z, with a feed rate of 0.2mm/rev.
• The subroutine from N035 to N045 defines the facing profile to be machined.

This example demonstrates the simplicity and effectiveness of the G72 facing cycle in CNC lathe operations.