G72 Facing Cycle

The G72 Facing Cycle (also known as the Transverse Roughing Cycle) is a canned cycle used on CNC lathes to remove stock by cutting across the face (X-axis) rather than along the length (Z-axis). It is essentially the "vertical" version of the G71 longitudinal cycle.

Two-Line Format (Common on Fanuc/Haas)

Most modern controls use a two-block format to define the cutting behavior and the profile range.

Line 1: Cutting Parameters

• W(d): Depth of cut for each pass in the Z-axis (given as a positive value).
• R(e): Retract amount after each cut (the distance the tool pulls back before returning for the next pass).

Line 2: Profile & Finishing Allowances

• (ns): Starting sequence number (N) of the profile subroutine.
• Q(nf): Ending sequence number (N) of the profile subroutine.
• U(u): Finishing allowance in the X-axis (diameter).
• W(w): Finishing allowance in the Z-axis (face).
• F(f): Feed rate for the roughing passes.

When to Use G72

• Facing Large Surfaces: Ideal for removing material from the front face of a part.
• Wide Shoulders: Best for parts where the radial (X) dimension is much larger than the axial (Z) dimension.
• Chip Control: Sometimes preferred over G71 to prevent long "stringy" chips from wrapping around the part or tool during OD turning.

Programming Example

This example roughs a face from an initial stock position to a specific profile.

Key Differences: G71 vs. G72

Important Tips

• Monotonicity: In a Type I cycle, the X-axis must move in one continuous direction (always increasing or always decreasing) within the P and Q blocks.
• Starting Position: Always ensure your tool is positioned at a "safe" X and Z coordinate before calling the G72 cycle to avoid crashes.
• Z-Movement: Unlike G71, the first line of your profile (N100) in a G72 cycle typically defines a movement in the Z-axis.