G94 Cycle | End Face Turning | Turning

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G94 CNC Code: The End Face Turning Cycle for CNC Lathes

Introduction:

In CNC turning, canned cycles are pre-programmed routines that simplify the programming of common machining operations. The G94 End Face Turning Cycle is a canned cycle specifically designed for facing operations on a CNC lathe – machining the end face of a workpiece to create a flat, smooth surface perpendicular to the axis of rotation. This article provides a detailed explanation of the G94 cycle (as a canned cycle), its syntax, parameters, how it differs from other turning cycles (like G90 and G71/G72), how it’s implemented across various control systems (Fanuc, Siemens, Haas, Mazatrol, Mitsubishi, Heidenhain, and others), best practices, and programming examples. Note that this G94 is distinct from the modal G94 command for Feed Per Minute used primarily in milling. This article is specifically about the canned cycle G94 used in turning.

1. What is the G94 End Face Turning Cycle?

The G94 End Face Turning Cycle automates the process of facing a workpiece on a CNC lathe. It allows the programmer to define the final dimensions and the number of passes, and the control automatically generates the necessary tool movements. The cycle typically involves these steps:

  1. Rapid Positioning: The tool rapids to a starting position (clearance position) near the workpiece.
  2. Infeed: The tool moves to the starting X-diameter for the first pass.
  3. Facing Cut: The tool feeds across the face of the workpiece (typically towards the center – X0).
  4. Retraction: The tool retracts slightly in the Z-axis.
  5. Repositioning: The tool returns to the starting Z-position and moves to the next X-diameter for the subsequent pass.
  6. Repeat: Steps 2-5 are repeated until the final X-diameter is reached.
  7. Final Retract: The tool rapids back to the initial clearance position.

Key Concepts:

  • Canned Cycle: A pre-programmed routine that simplifies programming.
  • Facing: Machining the end face of a workpiece.
  • Depth of Cut: The amount of material removed in each pass.
  • Number of Passes: The total number of passes required to reach the final dimensions.
  • Feed Rate: The speed at which the tool moves during the cutting pass (typically specified in G95 – feed per revolution).

2. G94 vs. Other Turning Cycles (G90, G71/G72)

It’s crucial to distinguish G94 from other common turning cycles:

  • G90 (Canned Cycle for Straight/OD Turning):

    • Used for straight turning (also called OD turning or cylindrical turning) – machining along the length of the workpiece (Z-axis) to reduce its diameter.
    • The tool moves parallel to the axis of rotation.
    • Not designed for facing.
    • Can be used for facing with a series of X moves.

  • G71/G72 (Roughing Cycles):

    • G71 is a Type I and Type II OD/ID roughing cycle. It removes material in a series of passes along the Z-axis, following a defined profile.
    • G72 is a facing roughing cycle. It removes material in a series of passes along the X-axis, following a defined profile.
    • These cycles are more complex than G94 and are used for roughing out complex shapes.

  • G70 (Finishing Cycle): This cycle finishes the part, and generally follows after G71 or G72

  • G94 (End Face Turning Cycle):

    • Specifically designed for facing operations.
    • The tool moves perpendicular to the axis of rotation (primarily in the X-axis, with small Z retractions).
    • Simpler than G71/G72 and ideal for flat facing.

In summary:

  • Use G90 for straight turning along the Z-axis.
  • Use G71/G72 for roughing complex profiles.
  • Use G94 for facing operations.

3. Control System Variations

The specific implementation of the G94 End Face Turning Cycle can vary significantly between different CNC control systems.

  • Fanuc and Similar (Haas, Mitsubishi):

    • Syntax (Common Format): G94 X[Final X-diameter] Z[Final Z-position] F[Feed rate] (K[Number of passes]);
      • X: The final X-diameter after facing (absolute or incremental, depending on G90/G91). Usually, X0 for a full face.
      • Z: The final Z-position (absolute or incremental). This determines how much material is removed from the face.
      • F: The feed rate (usually in mm/rev or inches/rev, as G95 is typically active in turning).
      • K: (Optional) The number of passes. If omitted, a single pass is performed. If included, the control automatically calculates the depth of cut per pass.
    • Example:
    G90 G95 ; Absolute, Feed per rev
G00 X80.0 Z5.0 ; Rapid to starting point
G94 X0 Z-2.0 F0.1 K3 ; Face to X0, Z-2, feed 0.1 mm/rev, 3 passes
G00 X80.0 Z5.0 ; Rapid retract

  • Siemens (SINUMERIK):

    • CYCLE93 (Stock Removal Cycle): Siemens typically uses CYCLE93 for both stock removal in turning and facing. The specific parameters within CYCLE93 determine the type of operation. This is a much more powerful cycle than the simple G94 found on Fanuc-style controls.

    • Parameters (Simplified): CYCLE93(SPD, SPL, MID, FALZ, FALX, VARI, ...)

      • SPD: Starting Point Diameter
      • SPL: Starting Point Z
      • MID: Maximum Infeed Depth
      • FALZ: Finishing Allowance Z
      • FALX: Finishing Allowance X
      • VARI: Machining Type (determines if it’s OD turning, facing, or grooving)

    • Direct G94: Siemens controls might offer G94 for compatibility, but Cycle93 is preferred.

    • Example (CYCLE93 for Facing):

      G90 G95
G00 X80.0 Z5.0
CYCLE93(80, 2, 1, 0.2, 0.2, 7) ; Simplified example - facing operation.
G00 X80.0 Z5.0
      • In this example, VARI = 7 would (typically) indicate a facing operation. Always refer to the Siemens programming manual for the exact meaning of VARI and other parameters.

  • Mazatrol (Mazak):

    • Conversational Programming: Facing is typically defined within a “FACE” unit in the conversational programming interface. You would specify the starting and ending diameters, the Z-depth, the number of passes, the feed rate, and other relevant parameters. The control automatically generates the necessary toolpath.
    • EIA/ISO (G-code): Mazak machines can run standard G-code, and a G94 command (in the Fanuc-style format) might be supported, but it’s not the typical Mazatrol way.

  • Heidenhain:

    • CYCLE 811 (SHOULDER, LONGITD.): Similar to Siemens, Heidenhain uses named cycles, and cycle 811 is appropriate.
    • Direct G94: Might be supported for compatibility.

  • Other Controls: Refer to programming manual.

4. Syntax and Parameters (Fanuc-Style)

As the Fanuc-style syntax is common, let’s break it down further:

G94 X[Final X-diameter] Z[Final Z-position] F[Feed rate] (K[Number of passes]);
  • G94: The G-code to initiate the End Face Turning Cycle.
  • X[Final X-diameter]:
    • In absolute mode (G90), this is the final X-diameter after facing. For a full face cut to the center, this will usually be X0.
    • In incremental mode (G91), this is the change in X-diameter. For a full face cut, this would be a negative value equal to the starting radius.
  • Z[Final Z-position]:
    • In absolute mode (G90), this is the final Z-position of the faced surface.
    • In incremental mode (G91), this is the change in Z-position (the total depth of material to be removed). This will be a negative value.
  • F[Feed rate]: The feed rate, typically in mm/rev or inches/rev (because G95 is the standard feed mode for turning).
  • K[Number of passes] (Optional):
    • The number of passes to take. If omitted, a single pass is performed.
    • If included, the control automatically calculates the depth of cut per pass: (Total Z-depth) / K.

Example (Incremental, Fanuc):

G91 G95 ; Incremental, Feed per rev
G00 X80.0 Z5.0 ; Rapid to starting position (assume diameter is 80mm)
G94 X-40.0 Z-2.0 F0.15 K2 ; Face to center (X-40), Z-2, feed 0.15 mm/rev, 2 passes
G00 X40.0 Z2.0 ; Rapid retract (back to original position)
G90 ; Switch back to Absolute

5. Programming Examples (Fanuc-Style)

Example 1: Single-Pass Facing

O0001 (Single-Pass Facing)
G90 G95 G21 ; Absolute, Feed per rev, Metric
T0101 ; Select Tool 1, Offset 1
G97 S1000 M03 ; Constant RPM, Spindle on
G00 X80.0 Z2.0 ; Rapid to starting position (assuming 80mm diameter)

G94 X0 Z-1.0 F0.1 ; Face to center (X0), Z-1, feed 0.1 mm/rev
G00 X80.0 Z2.0 ; Rapid retract

M30 ; Program end

Example 2: Multi-Pass Facing

O0002 (Multi-Pass Facing)
G90 G95 G21
T0202
G97 S800 M03
G00 X60.0 Z3.0 ; Rapid to starting position (60mm diameter)

G94 X0 Z-3.0 F0.2 K5 ; Face to center, Z-3, feed 0.2 mm/rev, 5 passes
G00 X60.0 Z3.0 ; Rapid retract

M30 ;

Example 3: Facing with Incremental Positioning

O0003 (Incremental Facing)
G91 G95 G21
T0303
G97 S1200 M03
G00 X80.0 Z2.0 ; Starting position (80mm diameter)

G94 X-40.0 Z-2.5 F0.12 K3 ; Face to center (incremental X-40), Z-2.5, 3 passes
G00 X40.0 Z2.5 ; Rapid retract (incremental back to start)

G90 ; Switch back to absolute
M30

6. Best Practices

  • Use G95 (Feed Per Revolution): Always use G95 (feed per revolution) with G94. This ensures a constant chip load, which is critical for good surface finish and tool life in turning.
  • Calculate Depth of Cut Appropriately: Choose a depth of cut that is suitable for the material, tool, and machine rigidity. Too large a depth of cut can lead to chatter, tool breakage, or poor surface finish.
  • Use the Correct Tool: Use a tool designed for facing. Consider the tool nose radius and its effect on the final surface.
  • Use Coolant: Use appropriate cutting fluid to cool the tool and workpiece, and to help flush away chips.
  • Check Clearance: Ensure sufficient clearance between the tool and the workpiece before and after the cycle.
  • Verify the Program: Always simulate your program before running it on the machine to check for errors and potential collisions.
  • Check Machine’s Manual: Consult it.
  • Optimize for Surface Finish: Reduce feed and increase number of passes.

7. Troubleshooting

  • Incorrect Dimensions:
    • Cause: Incorrect X or Z values in the G94 block, incorrect tool offset, or using absolute/incremental mode incorrectly.
    • Solution: Double-check the X and Z values, verify tool offsets, and ensure you’re using the correct positioning mode (G90 or G91).
  • Uneven Facing:
    • Cause: Tool deflection, insufficient rigidity, or incorrect depth of cut per pass.
    • Solution: Use a more rigid toolholder, reduce the depth of cut, or increase the number of passes.
  • Poor Surface Finish:
    • Cause: Feed rate too high, tool wear, incorrect spindle speed, or insufficient coolant.
    • Solution: Reduce the feed rate, replace the tool, adjust the spindle speed, ensure proper coolant flow.
  • Chatter:
    • Cause: Excessive depth of cut, tool overhang, insufficient rigidity, or worn spindle bearings.
    • Solution: Reduce the depth of cut, reduce tool overhang, improve workpiece clamping, or check the machine’s condition.

8. Conclusion

The G94 End Face Turning Cycle is a valuable tool for CNC lathe programming, providing a simple and efficient way to perform facing operations. By understanding its syntax, parameters, how it differs from other turning cycles, and its implementation across various control systems, you can create accurate and efficient CNC programs.