G75 Fixed Point Approach Guide for Siemens SINUMERIK CNC

Introduction

Executing a G75 fixed point approach on a Siemens control while a kinematic transformation remains active in the preprocessor registers drives the tool nose or gantry along a dangerously shifted trajectory. Because the control cannot correctly calculate the path under active tool axis transformations or coordinate rotations, the carrier bypasses soft software limits. This error leads to a catastrophic hard collision where the spindle or indexing turret crashes directly into physical workpiece workholding, such as a workpiece vise jaw, table-mounted fixture clamps, or the rotating jaws of a spindle chuck. The high-speed impact shatters the cutting tool, damages the axis ball screws, triggers a high-current axis overload alarm (such as Alarm 14800), and converts the raw stock into a completely ruined scrap part. Programmers must ensure all active transformations are deselected and zero offsets synchronized before commanding G75.

Technical Summary

Attribute	Specification
Command Code	G75
Modal Group	Group 2 (Non-modal)
Brands Supported	Siemens
Critical Parameters	FP (Fixed point selector), MD 34100 $MA_REFP_SET_POS (Reference coordinate settings)
Main Constraint	Must be programmed in a separate, independent block; target axis value must be strictly 0; active kinematic transformations must be completely deselected; preprocessor stop (STOPPRE) is required to apply active offsets.

Quick Read

• Deactivate Transformations First: Program the TRAFOOF command to deselect all active kinematic transformations prior to G75 execution to prevent trajectory errors.
• Insert a Preprocessor Stop: Program STOPPRE immediately before G75 to force the control to apply DRF and external zero offsets during preprocessing.
• Isolate the Homing Block: Write G75 in a separate, independent NC block to ensure the control interprets the non-modal command correctly.
• Set Target Coordinates to Zero: Assign a value of strictly 0 to each specified axis (e.g., X1=0 Z1=0) to trigger the simultaneous fixed-point approach.
• Verify Selected Fixed Point: Use the FP= address to select fixed point 1 or 2; omitting this address causes the control to default to fixed point 1.
• Check Machine Parameters: Confirm that coordinates are stored in the axis-specific machine data parameter 34100 $MA_REFP_SET_POS.

Basic Concepts

The G75 fixed point approach is programmed to retract machine axes to defined, high-precision coordinates within the machine coordinate system (MCS). These locations typically represent tool change stations, part loading points, or pallet exchange zones. Because these coordinates are defined globally in machine parameters, any part program can reference them regardless of active workpiece coordinate systems, zero offsets, or tool length compensations. This global definition allows setup operators to standardize safe tool changes and part loading sequences across different programs without recalculating coordinates for each job.

When the G75 block is executed, the selected axes traverse simultaneously at maximum axial velocity directly to the fixed point. The control bypasses calculations relative to the workpiece coordinate system (WCS) and targets the absolute physical machine position. This direct path trajectory minimizes transit time and eliminates incremental positioning errors. However, because changes to zero offsets or Differential Resolver Function (DRF) registers made during preprocessing are ignored, operators must execute a preprocessor stop (STOPPRE) before G75. Without this stop, the control fails to apply the updated offsets (such as those programmed via G59 Programmable Zero Offset), causing the axes to traverse along a shifted trajectory. The indexing turret then drives the cutting tool directly into workpiece workholding, causing a hard collision that shatters the tool, damages the axis ball screws, and ruins the raw stock.

Command Structure

In native Siemens mode (G290), the G75 Fixed Point Approach command is programmed as a non-modal function. Because it is non-modal, the command is valid only for the block in which it is written. The control requires G75 to be programmed in a separate, independent NC block containing no other motion commands to prevent interpreter conflicts during block look-ahead. Active kinematic transformations must be deactivated before the G75 block, ensuring that the control operates directly in the machine coordinate system.

The command syntax specifies the target axes along with the fixed point identifier. The target axes (such as X1, Y1, Z1) are targeted by writing the axis address followed strictly by a value of 0. Writing any value other than zero will cause a syntax error. The FP token selects which pre-configured fixed-point position the machine should approach. The value range is typically 1 or 2. If the FP parameter is omitted, the control defaults to the first fixed point.

G75 FP=2 X1=0 Y1=0 Z1=0 ;

Parameter	Description	Value Range
G75	Non-modal fixed point approach command (Group 2).	Fixed value
FP=	Fixed point selector, targeting the stored reference coordinates.	1 or 2 (defaults to 1 if omitted)
X1=0	Selects the X1 machine axis for the fixed point approach.	Must be strictly 0
Y1=0	Selects the Y1 machine axis for the fixed point approach.	Must be strictly 0
Z1=0	Selects the Z1 machine axis for the fixed point approach.	Must be strictly 0

Brand Applications

Siemens

The G75 command represents the Fixed Point Approach in native Siemens mode (G290). This implementation allows programmers to retract specified axes simultaneously directly to a globally stored coordinate. Unlike Fanuc and Mitsubishi systems, which use G28 or G30 and require the programming of intermediate points, Siemens G75 executes coordinate-free axis mapping. This means that specifying axis variables as strictly zero (such as X1=0) initiates direct motion at maximum axial velocity to the selected machine position without intermediate moves.

This command relies on machine parameters to store physical coordinates. The SINUMERIK control stores up to two distinct reference positions per axis, configured via machine data parameter 34100 $MA_REFP_SET_POS. This parameterization permits rapid switching between tool change and part loading coordinates directly from setting data registers.

A critical difference is the bimodal toggling behavior of the compiler. When switching the system to ISO Dialect mode (G291), the G75 command is reallocated. Instead of executing a fixed-point approach, it functions as a Z-axis end-face peck grooving cycle or a plunge grinding cycle, depending on the active machine class. Thus, programmers must verify that G290 is active before calling G75 to prevent unintended cycles and workholding crashes.

Brand Comparison

SINUMERIK Series	Display and Coordinate Windows	Parameter Access and Storage	Fixed Point Features
SINUMERIK 840D sl	High-resolution widescreen multi-touch panel showing active coordinates, axis loads, and G-groups.	Full read and write capability for all axis-specific machine data parameters.	Supports up to two fixed-point coordinates per axis, configured in 34100 $MA_REFP_SET_POS.
SINUMERIK 828D	Standard HMI display displaying basic coordinates without multi-touch panel.	Standard access to machine parameters; some fields require service keys.	Supports standard dual-fixed-point referencing with standard axis travel speeds.
SINUMERIK 802D sl	Entry-level monochrome or low-resolution panels with restricted coordinate displays.	Write-protected parameter settings preventing operator modifications to 34100 $MA_REFP_SET_POS.	Basic fixed-point returns; advanced visualization and diagnostic options are locked out.

Technical Analysis

The execution and configuration of the G75 fixed-point approach differ across Siemens SINUMERIK control series. On high-end controls such as the SINUMERIK 840D sl, operators monitor real-time axis loads and active coordinate groups via high-resolution widescreen multi-touch panels. These panels provide visualization of active G-groups and axis loads during high-speed return movements. To ensure absolute path precision during high-speed returns to these fixed locations, the system can utilize coordinate checks and exact stop functions like G60 Exact Stop to ensure all axis motion is fully decelerated before initiating tool changes. In contrast, the entry-level SINUMERIK 802D sl uses a simplified HMI interface with basic coordinate displays and restricts access to critical machine data. Under the 802D sl control setup, critical parameters like the axis-specific homing coordinates in 34100 $MA_REFP_SET_POS are write-protected. This lockout prevents unauthorized or accidental user modification of reference positions on the shop floor.

The compiler architecture also determines how commands are interpreted during turning and milling. Switching between native Siemens mode (G290) and ISO Dialect mode (G291) fundamentally alters the instruction set. Under native Siemens mode, the G75 command represents the Fixed Point Approach. If the compiler is toggled to ISO Dialect mode (G291), the G75 command shifts its function to execute either peck grooving or plunge grinding. Similarly, G74 serves as the native reference point approach under G290, but translates to a left-hand tapping cycle in milling or end-face peck drilling and grooving in turning under G291. Programmers must ensure the interpreter mode matches the intended cycle to prevent collision risks.

Program Examples

; Native Siemens mode fixed point approach example
N10 G290 ; Switch compiler to native Siemens mode
N20 TRAFOOF ; Deselect active kinematic transformations
N30 STOPPRE ; Force preprocessor stop to apply active zero offsets
N40 G75 FP=2 X1=0 Z1=0 ; Traverse X1 and Z1 simultaneously to Fixed Point 2
N50 STOPPRE ; Force preprocessor stop before next return
N60 G75 X1=0 ; Traverse X1 to Fixed Point 1 (FP defaults to 1) when omitted
N70 M30 ; End of program

dry run

In this program, the CNC switches to native Siemens mode in block N10. Block N20 deactivates all active coordinate transformations, ensuring that coordinate rotations or tool shifts do not alter the physical trajectory. Block N30 commands a preprocessor stop, forcing the control to apply all active zero offsets and DRF registers before initiating the movement. In block N40, the X1 and Z1 axes traverse simultaneously at maximum axial velocity to the coordinates stored for Fixed Point 2. Block N50 forces another preprocessor stop, ensuring coordinate synchronization before the next move. Block N60 commands the X1 axis to traverse to Fixed Point 1, where the FP parameter is omitted and defaults to 1. The program ends in block N70, leaving the selected axes safely retracted at their respective machine reference points.

Error Analysis

Alarm Code	Trigger Condition	Operator Symptom	Root Cause / Fix
Siemens Alarm 14011	Program CYCLE3106 not available or not released for processing when attempting fixed-point return in MDA mode.	The control interrupts automatic operation and halts the current block.	MDA mode execution of fixed-point return is prohibited. Program the return in a standard part program and verify subroutine release status.
Siemens Alarm 12080	Incorrect G75 syntax or unrecognized commands are interpreted under the active language compiler.	The control halts program execution and displays a syntax error message on the HMI screen.	Verify command syntax. Ensure G75 is programmed in an isolated block and axis values are strictly set to 0.
Siemens Alarm 14800	Feedrate is programmed as zero or omitted during travel and the fixed feedrates function is inactive.	Axis motion is blocked immediately, and the HMI displays a path velocity error.	Program a positive feedrate value (F command) or ensure the fixed feedrate function (F0–F9) is active.

Application Note

Bypassing the deactivation of kinematic transformations before executing G75 causes the tool carrier or turret to travel along a shifted trajectory. This path deviation directs the indexing turret into table-mounted fixture clamps, a workpiece vise jaw, or the rotating jaws of a spindle chuck. When an operator fails to command TRAFOOF to cancel coordinate rotations, the control computes axis coordinates using relative frame offsets. The tool nose bypasses soft safety limits and crashes into physical workpiece workholding. This high-speed impact shatters the cutting tool, damages the axis ball screws, triggers a high-current axis overload alarm (such as Alarm 14800), and converts raw stock into a ruined scrap part. Programmers must deactivate all active transformations and command a preprocessor stop (STOPPRE) in the block preceding G75 to synchronize preprocessor registers.

Related Command Network

• G74 Reference Point Approach: Performs a reference point approach in native Siemens mode (Group 2) using machine coordinate values.
• G60 Exact Stop Siemens: Controls exact stop and continuous path mode, ensuring precise deceleration before executing coordinate state changes.
• G59 Programmable Zero Offset Siemens: Configures programmable zero offsets; requires preprocessor stop synchronization to prevent offset alignment issues.
• STOPPRE: Forces a preprocessor stop, which is highly recommended before G75 to apply active offsets and resolver adjustments.
• TRAFOOF: Deactivates active kinematic transformations, which is mandatory before executing G75 to prevent axis deviation.

Conclusion

Integrating a G75 fixed-point approach into part programs requires deactivating active transformations via TRAFOOF and executing a preprocessor stop via STOPPRE. This sequence ensures the SINUMERIK control targets the stored reference coordinates in 34100 $MA_REFP_SET_POS directly. Running initial cycles in single-block mode protects the machine tools and workholding from coordinate offset errors.

FAQ

Why does G75 perform a grooving cycle instead of returning to a fixed point

The control compiler is currently operating in ISO Dialect mode (G291), which reallocates G75 to peck grooving or plunge grinding. To restore the fixed-point return function, program G290 in a separate block prior to G75 to switch the control to native Siemens mode.

What causes Alarm 14800 during a G75 fixed point approach

This alarm triggers when axis travel is commanded with a programmed feedrate of zero or when the feedrate is omitted while the fixed feedrate function is inactive. Program a positive feedrate value (F command) or verify that the fixed feedrates (F0–F9) are active before initiating the homing command.

Can G75 be executed while coordinate transformations are active

No, active coordinate rotations or tool axis transformations will cause the control to calculate an incorrect trajectory. Program TRAFOOF to deactivate all transformations and insert STOPPRE in the preceding block to synchronize the preprocessor zero offsets.