G25 CNC Code | Spindle Speed Limits and Control

[Hakan]

G25 on CNC: Understanding Spindle Speed Limits and Control

In CNC machining, controlling spindle speed is crucial for tool life, surface finish, and overall machining efficiency. You might encounter information suggesting that the G25 G-code is used to set spindle speed limits or detect spindle speed fluctuations. However, G25 is not a universally standardized G-code for these functions on modern CNC controls. Its purpose, if any, is highly control-system specific.

This article clarifies the often-misunderstood role of G25, explains the general concept of spindle speed control, and, most importantly, presents the correct and recommended ways to manage spindle speed limits on your CNC lathe or machining center.

Crucial Note: Before using any G-code, especially less common ones like G25, always consult the programming manuals for your specific CNC control system and machine tool. Never assume a G-code’s function without verification.

1. G25: A Non-Standard G-Code

The most important point to understand is that you should not expect G25 to have a consistent, predictable function across different CNC machines. It is not a reliable command for:

• Setting Both Minimum and Maximum Spindle Speed Limits: The syntax G25 S[min] S[max] presented in one of the articles is incorrect for most modern controls.
• General Spindle Speed Fluctuation Detection: While some older controls might have used G25 for this, it’s not a standard practice.
• Reference Point Return, or coordinate modification.

2. The Concept of Spindle Speed Clamping

While G25 itself might not be the standard command, the concept of setting limits on spindle speed is important. This is often called spindle speed clamping. There are two main types of limits:

• Maximum Spindle Speed: This prevents the spindle from exceeding a certain RPM, which is crucial for:
  • Safety: Preventing dangerously high speeds that could cause tool breakage, workpiece ejection, or machine damage.
  • Tool Life: Protecting cutting tools from excessive speeds.
  • Machine Limits: Staying within the machine’s maximum spindle speed rating.
• Minimum Spindle Speed: This prevents the spindle from running too slowly, which can be important for:
  • Certain Cutting Tools: Some tools (e.g., certain types of carbide inserts) require a minimum cutting speed to function correctly.
  • Maintaining Chip Load: Ensuring adequate chip evacuation.
  • Preventing Stalling: Avoiding situations where the spindle might stall due to insufficient speed.

3. G25 on Specific Control Systems (and What to Use Instead)

Here’s a breakdown of G25 (and more importantly, the correct methods for spindle speed control) on various CNC control systems:

• Fanuc:
  • On most modern Fanuc lathe and mill controls, G25 has no standard, documented function related to setting general spindle speed limits.
  • Some older Fanuc controls (or custom macros) might have used G25 for spindle speed fluctuation detection disable. This is not the same as setting speed limits. It would prevent the control from generating an alarm if the spindle speed deviates from the programmed speed. This function is likely not what most users are looking for.
  • The Correct Ways to Control Spindle Speed on Fanuc:
    • Parameters: The primary and most reliable method is to set the maximum and minimum spindle speed limits through machine parameters. Consult your Fanuc parameter manual for the specific parameter numbers (they will vary depending on the control series and machine configuration). Common parameters are in the range of 3700, 4000, and 4600, but always double check.
    • G92 (Lathes): On lathes, the G92 command (which is traditionally used for setting the coordinate system – use with caution!) can also be used to set the maximum spindle speed. This is often used in conjunction with G96 (Constant Surface Speed). Example: G92 S3000 (sets the maximum spindle speed to 3000 RPM).
    • G50 (Lathes): Some older Fanuc controls use G50 to set the maximum spindle speed, often in conjunction with G96.
    • S Word with G96 (Lathes): When using Constant Surface Speed (G96), you typically specify a maximum spindle speed using the S word. Example: G96 S200 G92 S3000 (CSS with a maximum of 3000 RPM).
• Siemens: Siemens controls do not typically use G25 for spindle speed clamping. Siemens uses:
  • Parameters: Maximum and minimum spindle speeds are set via machine data (parameters).
  • G92 (Similar to Fanuc): For maximum spindle speed in turning, often with G96.
  • LIMS: On newer Siemens controls, the LIMS command can be used to set a maximum spindle speed that applies even in G96 mode.
  • Cycle Settings: Specific cycles might have their own spindle speed limit settings.
• Haas: Haas controls do not typically use G25 for spindle speed clamping. Haas uses:
  • Settings: Maximum and minimum spindle speeds are set through the control’s settings menus.
  • G92 (Similar to Fanuc): For maximum spindle speed in turning (often with G96).
• Mazak:
  • Mazatrol: Spindle speed limits are typically set within the conversational programming interface, not with a G25 G-code.
  • EIA/ISO: Consult the specific Mazak control documentation.
• Mitsubishi:
  • MELDAS: Refer to specific machine documentation.
• Other Controls: Always consult your documentation.

4. Programming Examples (Using Correct Methods)

Example 1: Setting Maximum Spindle Speed with G92 (Lathe - Fanuc, Haas, others):

G21 G18 G40 G99; Metric, XZ Plane
T0101 ; Select tool
G96 S200 ; Constant Surface Speed (200 m/min)
G92 S3000 ; Set maximum spindle speed to 3000 RPM  <-- IMPORTANT
; ... rest of program ...

This is the most common way to limit spindle speed on a lathe, especially when using G96.

Example 2: Setting Maximum and Minimum Spindle Speed via Parameters (Conceptual):

• You would not typically do this within a part program. You would access the machine’s parameter settings (usually through a dedicated screen on the control) and change the appropriate parameters.
• Fanuc (Conceptual Example - Do Not Use Without Verification):
  • Find the parameters for maximum spindle speed (e.g., might be in the 4000 range, or 3700, or others).
  • Find the parameters for minimum spindle speed (e.g., might be in the 4600 range, or others).
  • Set these parameters to the desired values.

Example 3: Using LIMS on Siemens (Conceptual):

; ...
LIMS=3000 ; Set maximum spindle speed to 3000 RPM (even in G96)
G96 S200 ; Constant Surface Speed
; ...

5. Troubleshooting

• Spindle Speed Higher/Lower Than Expected:
  • Check the G96 and G92 (or LIMS) commands in your program (lathes).
  • Check the machine parameters for maximum/minimum spindle speed.
  • Make sure you’re using the correct units (RPM or surface speed).
• Machine Alarms:
  • “Spindle Speed Limit Exceeded”: You’ve programmed a speed higher than the maximum allowed (either by parameter or by G92/LIMS).

6. CAM Software

CAM software typically handles spindle speed limits automatically. You specify the maximum and minimum spindle speeds for your machine and/or your tools, and the CAM system will generate code that respects those limits (often using G92, LIMS, or parameters).

7. Frequently Asked Questions (FAQ)

• Q: What is G25 used for on CNC lathes and mills?
  • A: On most modern controls, G25 has no standard, documented function for setting spindle speed limits. It might be unused or assigned to a machine-specific function. Some older Fanuc controls might have used it for spindle speed fluctuation detection disable, but this is not common.
• Q: How do I set spindle speed limits on my CNC machine?
  • A: The most reliable method is to set the limits in the machine’s parameters. On lathes, you can also often use G92 (or LIMS on Siemens) to set a maximum spindle speed, especially when using G96 (Constant Surface Speed).
• Q: How to I limit the RPM when using G96?
  • A: Use G92 (or LIMS on Siemens) to set a maximum RPM.

8. Conclusion

The G25 command is not a standard, universally defined G-code for setting spindle speed limits on modern CNC lathes and machining centers. The correct way to manage spindle speed limits is typically through machine parameters and, on lathes, often in conjunction with G96 (Constant Surface Speed) and G92 (or LIMS on Siemens). Always consult your machine’s and control system’s documentation to determine the correct procedures and parameters for your specific equipment. Do not rely on potentially outdated or inaccurate information about non-standard G-codes like G25. Use the proper, documented methods for your control system to ensure safe and efficient machining.