G72 Cycle | Stock Removal in Facing | Turning

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G72 CNC Code: Mastering the Facing Stock Removal Cycle in Turning

Introduction:

In CNC turning, efficiently removing material from the face of a workpiece is a common and critical operation. The G72 Facing Stock Removal Cycle is a powerful canned cycle designed to automate this process, making it faster and more consistent than programming individual facing cuts. This guide provides a in-depth explanation of G72, covering its function, syntax, different types (Type I and Type II), applications, variations across control systems (Fanuc, Siemens, Haas, Mazatrol, Mitsubishi, Heidenhain, and others), best practices, troubleshooting, and programming examples. This guide is for all levels of CNC users.

1. What is the G72 Facing Stock Removal Cycle?

G72 is a G-code command that initiates a canned cycle for rough facing on a CNC lathe. Like G71 (for longitudinal turning), G72 simplifies programming by automatically generating a series of facing cuts to remove material up to a defined profile, leaving a specified allowance for finishing.

Key Concepts:

• Facing: A turning operation where the tool moves perpendicular to the axis of rotation (typically along the X-axis) to machine the end face of the workpiece.
• Rough Facing: Removing the majority of the material from the face, leaving a small amount for a subsequent finishing operation.
• Canned Cycle: A pre-programmed sequence of operations that simplifies programming. The programmer provides key parameters, and the control generates the toolpath.
• Stock Removal: The process of removing material.
• P and Q: These parameters are used to define the toolpath for G72.

2. Type I and Type II G72 Cycles

Similar to G71, the G72 cycle has two main types: Type I and Type II. The type dictates the direction of the roughing cuts and the allowable shapes of the profile.

• G72 Type I:

  • The roughing cuts are made parallel to the X-axis (facing cuts).
  • The defined profile must be monotonic in the Z-axis. This means the Z-dimension must either continuously increase or continuously decrease along the profile. No “undercuts” or “pockets” are allowed in the Z-direction.
  • This is the most common type of G72 cycle.

• G72 Type II:

  • The roughing cuts are made parallel to the Z-axis (longitudinal cuts).
  • The defined profile must be monotonic in the X-axis (diameter). The X-diameter must either continuously increase or continuously decrease along the profile.
  • Type II allows for facing profiles with “undercuts” in the Z-direction.
  • Not all CNC controls support Type II G72. Check your machine’s manual.

Visualization:

Imagine facing a large disc:

• Type I: The tool would move across the face of the disc (X-axis), taking multiple passes and stepping in along the Z-axis.
• Type II: The tool would move along the Z-axis for each pass, stepping across the face (X-axis) for each new pass. This is less common for facing.

3. G72 Syntax and Parameters

The syntax for the G72 command typically involves two G72 blocks, similar to G71:

First G72 Block:

G72 W[Depth of Cut] R[Retract Amount]

• G72: The G-code for the facing stock removal cycle.
• W[Depth of Cut]: Specifies the depth of cut for each roughing pass along Z axis. This is an incremental value, representing the amount of material removed in each pass.
• R[Retract Amount]: Specifies the amount the tool retracts (in all axes) at the end of each cutting pass. This provides clearance for chip evacuation and prevents rubbing.

Second G72 Block:

G72 P[Start Block] Q[End Block] U[X Allowance] W[Z Allowance] F[Feed Rate] S[Spindle Speed] T[Tool Number]

• G72: Again, the G-code for the facing stock removal cycle.
• P[Start Block]: The starting block number (N-number) of the sequence of blocks that defines the finished profile of the face.
• Q[End Block]: The ending block number (N-number) of the profile definition sequence.
• U[X Allowance]: The amount of material to leave for finishing in the X-axis (diameter). This is a signed value. A positive U value leaves material on the outside diameter (for OD facing – less common). A negative U value leaves material on the inside diameter (for ID facing – more common, as you’re typically facing towards the center).
• W[Z Allowance]: The amount of material to leave for finishing in the Z-axis. This is a signed value. A positive W value leaves material on the face (in the +Z direction).
• F[Feed Rate]: The feed rate for the roughing cuts.
• S[Spindle Speed]: The spindle speed (RPM) (if using G97) or the surface speed (if using G96).
• T[Tool Number]: The tool number (often redundant if already selected).

Example:

G72 W2.0 R1.0 ; Depth of cut 2mm, retract 1mm
G72 P100 Q200 U-0.4 W0.2 F0.3 S1000 T0101 ; Profile from N100-N200, ID facing

4. Control System Variations

The implementation of G72, like other canned cycles, varies between CNC control manufacturers:

• Fanuc:
  • Type I and Type II: Fanuc controls generally support both Type I and Type II G72 cycles. The type is determined by the shape of the defined profile.
  • Two-Line Format: The two-line format is standard.
  • Widely Used: Fanuc is a very common control.
• Siemens (SINUMERIK):
  • CYCLE95 (Stock Removal Cycle): Siemens uses CYCLE95 for stock removal, which can handle both turning (like G71) and facing (like G72) operations. You would select the appropriate machining type (facing) and parameters within the CYCLE95 call.
  • Parameters: CYCLE95 uses a set of parameters to define the cutting strategy, depth of cut, finishing allowance, etc.
  • ShopTurn/ShopMill: Siemens’ conversational interfaces simplify the process.
• Haas:
  • Syntax: Very similar to Fanuc (two-line G72).
  • Type I and Type II: Supports both types.
• Mazatrol (Mazak):
  • Conversational Programming: Facing stock removal is defined within the conversational programming interface, as part of the overall machining process definition. You wouldn’t typically write a G72 command directly. The system automatically generates the facing passes based on the part geometry and parameters.
  • EIA/ISO (G-code): Mazak machines can run standard G-code, and G72 would be used similarly to Fanuc.
  • FACE CUT: Mazatrol uses FACE CUT in conversational programming.
• Mitsubishi:
  • Syntax: Similar to Fanuc (two-line G72).
  • Type I and Type II: Supports both types.
• Heidenhain:
  • CYCLE: Heidenhain uses different cycles.
  • Manual Programming: Using G0, G1, G2, G3.
• Other Controls (Okuma, Fagor, etc.): Consult the specific programming manual.

Key Takeaways:

• Fanuc is the most common reference: The two-line G72 format is widely used.
• Siemens uses CYCLE95: A more comprehensive cycle that handles various stock removal operations.
• Mazatrol is conversational: Facing is defined within the conversational programming environment.
• Always check your machine’s manual! This is critical for correct implementation.

5. The Profile Definition (P and Q Blocks)

The P and Q parameters in the second G72 line define the start and end block numbers of the sequence that describes the finished face profile.

Example:

N10 G72 W2.0 R1.0
N20 G72 P100 Q200 U-0.4 W0.2 F0.3 S1000 T0101
; ...
N100 G00 X50.0 Z2.0  ; Start of profile (P100) - Rapid to X only
N110 G01 Z0.0 F0.15  ; Feed to Z0 (face of part)
N120 G01 X30.0       ; Face to X30 (towards center)
N130 G03 X20.0 Z-5.0 R5.0 ; Circular interpolation (CW)
N140 G01 Z-15.0      ; Linear move in Z
; ... (More profile blocks, if needed) ...
N200 G01 X10.0       ; End of profile (Q200)
; ...

Important Points:

• Allowed Commands: Only G00, G01, G02, and G03 are allowed within the P-Q block sequence. No tool changes, spindle speed changes, etc.
• First Move: The first move in the P-Q sequence should be a rapid move (G00) or feed move (G01) only in X axis. The control uses this to determine the starting point.
• Monotonicity: For G72 Type I, the profile must be monotonic in Z (continuously increasing or decreasing Z). For Type II, it must be monotonic in X.
• The Shape: Machine will cut the material according to P and Q blocks.
• No other G-codes: You can not use different G-codes between P and Q blocks.

6. Using G70 for Finishing

After the G72 rough facing cycle, a G70 Finishing Cycle is typically used to machine the face to its final dimensions and achieve the desired surface finish.

• G70 Syntax: G70 P[Start Block] Q[End Block]
• Relationship to G72: The G70 command uses the same P and Q block numbers as the preceding G72 command.

Example (Continuing from previous example):

; ... (G72 roughing cycle and profile definition as above) ...
N210 T0202 ; Select finishing tool
N220 G96 S250 M03 ; Constant Surface Speed for finishing
N230 G70 P100 Q200 ; Finishing cycle (using the same P and Q as G72)
N240 G00 X100.0 Z100.0 ; Rapid retract
N250 M30 ; Program end

7. Best Practices for Using G72

• Correct Type (I or II): Choose Type I or Type II based on the part geometry.
• Appropriate Depth of Cut (W): Choose a depth of cut that is appropriate for the material, tool, and machine.
• Appropriate Retract Amount (R): Ensure the retract amount is sufficient to clear chips.
• Correct Finishing Allowances (U and W): Leave enough material for G70, but not too much. Remember the sign conventions for U (positive for OD, negative for ID).
• Optimize Feed and Speed: Use appropriate feed rates and spindle speeds for rough facing.
• Use Coolant: Use appropriate coolant.
• Verify the Toolpath: Always simulate your program (including G72 and G70) to visually verify.
• Consider Tool Nose Radius Compensation: You can use G41/G42 in P and Q blocks.

8. Troubleshooting Common G72 Problems

• “Sequence Number Not Found” Alarm:
  • Cause: Incorrect P or Q block numbers.
  • Solution: Double-check the P and Q values.
• “Illegal Argument” or “Invalid G-Code” Alarm:
  • Cause: Syntax errors or unsupported parameters.
  • Solution: Consult your machine’s manual and review the G72 syntax.
• “Non-Monotonic Motion” Alarm (Type I):
  • Cause: The profile is not monotonic in Z.
  • Solution: Redesign the profile or use Type II (if supported).
• Incorrect Dimensions:
  • Cause: Incorrect U and W values, incorrect tool offsets, or errors in the profile.
  • Solution: Double-check all parameters and offsets.
• Excessive Tool Wear/Breakage:
  • Cause: Too large a depth of cut (W), excessive feed rate, or inappropriate tool.
  • Solution: Reduce the depth of cut, reduce the feed rate, use a more robust tool.
• Poor Surface Finish:
  • Cause: Excessive depth of cut, incorrect feed, tool vibration, or using a roughing tool for finishing.
  • Solution: Reduce the depth, adjust feed and speed, ensure machine stability, select a finishing tool.

9. Conclusion: Efficient Facing with G72

The G72 Facing Stock Removal Cycle is a fundamental and highly efficient canned cycle in CNC turning. It automates the process of rough facing, saving programming time and ensuring consistent material removal. By understanding the two types of G72, the key parameters, the variations between control systems, best practices, and how to troubleshoot common problems, CNC machinists and programmers can significantly improve the productivity and quality of their facing operations. G72, used in conjunction with G70, is a cornerstone of efficient and accurate CNC turning.