G93 CNC Code | Inverse Time Feed

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G93 CNC Code: Mastering Inverse Time Feed for CNC Turning and Milling

Introduction:

In CNC machining, controlling the feed rate (the speed at which the cutting tool moves relative to the workpiece) is crucial for achieving the desired surface finish, dimensional accuracy, and tool life. While traditional feed rate modes like G94 (feed per minute) and G95 (feed per revolution) are commonly used, the G93 Inverse Time Feed mode offers a unique and powerful approach, especially for complex machining operations. This article provides an in-depth explanation of G93, its syntax, its advantages and disadvantages, how it differs from G94 and G95, how it’s implemented across various control systems (Fanuc, Siemens, Haas, Mazatrol, Mitsubishi, Heidenhain, and others), best practices, and programming examples. This guide is suitable for both beginners and experienced CNC machinists and programmers. This guide will discuss G93 as it applies in both turning and milling applications.

1. What is G93 (Inverse Time Feed)?

G93 is a G-code that activates inverse time feed mode. Instead of specifying the feed rate as a distance per unit of time (like G94) or distance per revolution (like G95), G93 defines the feed rate as the reciprocal of the time it should take to complete the programmed move.

Key Concepts:

  • Inverse Time: The feed rate is expressed as 1 / time (in minutes). A larger F-value in G93 mode means a faster feed rate (shorter time), and a smaller F-value means a slower feed rate (longer time).
  • Time-Based Feed: The machine control calculates the necessary feed rate to complete the programmed movement block in the specified time. This is particularly useful when the actual path length varies due to simultaneous multi-axis movements.
  • Modal Command: G93 is a modal command, meaning it remains active until canceled by G94, G95, or a machine reset.
  • F-Word: The F-word in the same line with the axes.

Formula:

The relationship between time, distance, and feed rate in G93 mode is:

Time (minutes) = 1 / F-value
Feed Rate (units/minute) = Distance / Time

Example:

G93 G01 X100.0 Y50.0 Z25.0 F2.0
  • G93: Activates inverse time feed mode.
  • G01: Linear interpolation.
  • X100.0 Y50.0 Z25.0: The destination coordinates.
  • F2.0: The inverse time value. This means the move should take 1 / 2.0 = 0.5 minutes (30 seconds) to complete. The control will automatically adjust the feed rate along the path to achieve this.

2. Advantages and Disadvantages of G93

Advantages:

  • Consistent Surface Finish on Complex Contours: When machining complex shapes, especially in 3D milling or when turning with simultaneous X and Z movement, the actual cutting speed can vary significantly if using G94 or G95. G93 ensures a more consistent cutting speed along the entire toolpath, resulting in a better surface finish.
  • Precise Control in Multi-Axis Machining: In 5-axis machining, where the tool is moving along a complex 3D path, G93 makes it much easier to control the time it takes to complete a segment, which is often more important than the feed rate in a particular axis.
  • Simplified Programming for Complex Movements: For complex simultaneous movements, calculating the required feed rate in G94 or G95 to maintain a consistent cutting speed can be extremely difficult. G93 simplifies this by allowing you to specify the time directly.
  • Synchronization: It is very useful for synchronized motions.

Disadvantages:

  • More Complex Calculations: You need to calculate the time required for each move, rather than simply specifying a feed rate. This can be more complex, especially for simple linear moves.
  • Requires Careful Planning: Incorrect time calculations can lead to either very slow machining (reducing productivity) or excessively fast movements (risking tool breakage or poor surface finish).
  • Not Ideal for Simple Operations: For simple turning or facing operations, G94 (feed per minute) or G95 (feed per revolution) are usually much easier and more efficient to use.
  • Compatibility: Not all control units handle G93 well.
  • Block-by-Block F-word: You generally need an F-word in every block when G93 is active. This can make programs longer.
  • Debugging: It can be difficult to estimate feed.

In summary, G93 is a powerful tool for specific applications, but it’s not a replacement for G94 and G95 in all situations.

3. G93 vs. G94 (Feed Per Minute) and G95 (Feed Per Revolution)

  • G94 (Feed Per Minute):

    • Specifies the feed rate in distance units per minute (mm/min or inches/min).
    • Most commonly used for simple linear moves in milling and turning.
    • Easy to understand and program.
    • Disadvantage: The actual cutting speed can vary significantly when machining contours or using multiple axes simultaneously.

  • G95 (Feed Per Revolution):

    • Specifies the feed rate in distance units per spindle revolution (mm/rev or inches/rev).
    • Primarily used in turning operations to maintain a constant chip load, regardless of spindle speed.
    • Essential for threading operations.
    • Disadvantage: Not suitable for milling operations where the spindle speed is not directly related to the feed rate.

  • G93 (Inverse Time Feed):

    • Specifies the feed rate as the inverse of the time to complete the move.
    • Best suited for complex contours and multi-axis machining where maintaining a consistent toolpath speed is critical.
    • Disadvantage: Requires more complex calculations and is not ideal for simple operations.

Choosing the Right Feed Mode:

Operation Recommended Feed Mode
Simple turning (facing, straight turning) G95
Simple milling (linear moves) G94
Complex contouring (turning or milling) G93
5-axis machining G93
Threading (turning) G95

4. Control System Variations

  • Fanuc and Similar (Haas, Mitsubishi): The syntax and behavior are generally consistent: G93 G01 X... Y... Z... F.... G93 is modal and requires an F-word in each subsequent G01, G02, or G03 block. G94 or G95 cancels G93.

  • Siemens (SINUMERIK):

    • G93: Siemens supports G93. The F value represents the inverse of the time in minutes.
    • FGROUP: Siemens also has the FGROUP command, which can be used to specify which axes are considered for the inverse time feed calculation. This allows you to use G93 for a subset of the machine’s axes.
    • FFWON / FFWOF: Feed Forward Control. It can affect G93.

  • Mazatrol (Mazak):

    • Conversational Programming: Inverse time feed is typically handled implicitly within the conversational programming interface. You might specify a desired machining time for a particular operation, and the control will calculate the necessary feed rate.
    • EIA/ISO (G-code): Mazak machines can run standard G-code, and G93 would function as expected.

  • Heidenhain: Heidenhain controls fully support G93. The syntax is standard.

  • Other Controls: Always consult the programming manual for your specific control. Okuma, Fagor, and others will have their own implementations and potential variations.

Key Takeaways:

  • G93 syntax is generally consistent: G93 G01/G02/G03 X... Y... Z... F...
  • Siemens has FGROUP: For more granular control over which axes are affected by G93.
  • Mazatrol handles it implicitly: Within the conversational programming environment.
  • Always check your machine’s manual!

5. Programming Examples

Example 1: Simple 3D Milling (Fanuc)

O0001 (G93 Example - 3D Milling)
G90 G21 G17 G40 G80 ; Safety line
T01 M06 ; Select Tool 1
G54 ; Select Work Coordinate System
G00 X0 Y0 Z10.0 ; Rapid to starting position

; --- G93 Inverse Time Feed ---
G93 ; Activate inverse time feed
G01 X50.0 Y25.0 Z-5.0 F2.0 ; Move in 30 seconds (1/2.0 = 0.5 minutes)
G01 X75.0 Y50.0 Z-10.0 F4.0; Move in 15 seconds (1/4.0=0.25 minutes)
G01 X100.0 Y75.0 Z-15.0 F1.0 ; Move in 60 seconds

; --- Cancel G93 ---
G94 ; Switch back to feed per minute
G00 Z100.0 ; Rapid retract
M30

Explanation:

Each of moves should complete according to F value.

Example 2: Contour Turning (Fanuc)

O0002 (G93 Example - Contour Turning)
G90 G21 G40 G80
T0202
G97 S1000 M03 ; Constant RPM
G00 X50.0 Z5.0 ; Rapid to starting position

; --- Contour with varying diameters ---
G93 ; Activate inverse time feed
G01 Z-20.0 F2.0      ; Feed to Z-20 in 30 seconds
G01 X40.0 Z-30.0 F3.0  ; Tapered move in 20 seconds
G01 Z-50.0 F2.5      ; Straight cut in 24 seconds
G01 X30.0 Z-60.0 F4.0 ; Tapered move in 15 seconds.

; --- Cancel G93 ---
G94 ; Switch back to feed per minute
G00 X100.0 Z100.0
M30

Explanation: Inverse time feed can be used in turning.

6. Best Practices

  • Use G93 Strategically: Reserve G93 for situations where precise control over the time of a move is critical, such as complex contouring or 5-axis machining.
  • Calculate Time Accurately: Carefully calculate the time required for each move based on the desired cutting speed and the path length.
  • Use CAM Software: For complex 3D paths, use CAM software that supports G93 output. This will automate the time calculations and ensure accuracy.
  • Simulate Your Program: Always simulate your program thoroughly before running it on the machine, paying close attention to the feed rate and toolpath.
  • Start with Conservative Values: When first using G93, start with longer times (smaller F-values) to avoid excessively fast movements.
  • Monitor the Machining Process: Carefully observe the machining process, paying attention to the surface finish, tool load, and any signs of vibration or chatter.
  • Check Machine’s Manual: Consult it.

7. Troubleshooting

  • Unexpected Feed Rate:
    • Cause: Incorrect time calculation, incorrect F-value, or a misunderstanding of how G93 works.
    • Solution: Double-check your calculations, verify the F-value, and ensure you understand that the F-value represents inverse time.
  • Machine Alarms:
    • Cause: Syntax errors, unsupported G-code, or exceeding the machine’s feed rate limits.
    • Solution: Check your machine’s manual for specific alarm codes and troubleshooting information. Ensure that your control system supports G93.
  • Poor Surface Finish:
    • Cause: Incorrect G93.
    • Solution: Optimize parameters.

8. Conclusion

The G93 Inverse Time Feed mode is a powerful tool for CNC programmers, providing a way to control feed rates based on the time it takes to complete a move, rather than a fixed distance per unit of time or revolution. While it requires more careful planning and calculation than traditional feed rate modes, G93 offers significant advantages for complex contouring, multi-axis machining, and situations where maintaining a consistent toolpath speed is critical for achieving the desired surface finish and accuracy. Understanding the nuances of G93 and how it’s implemented across different control systems is essential for any CNC programmer or machinist working with advanced machining operations.