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Open the part program or GUD initialization block. Locate the CYCLE971 call and trace back to identify where _MFS[2] should be assigned.

Add explicit assignment before CYCLE971 invocation: _MFS[2]=550,3500 (example: 550 F/min feed, 3500 RPM spindle). Verify both feed and speed values are present in the array structure.

Feed rate must be in active unit system (mm/min or inch/min per MD1030); spindle speed in RPM must comply with spindle capability limits.

Verify _MFS array declaration in the GUD table. Check Diagnostics > Program > Variables to confirm _MFS is defined as REAL array with sufficient size (minimum [2] indices).

Navigate to NCK > GUD > Variable Editor in Siemens HMI and search for variable _MFS. Confirm data type is REAL and array dimension is at least [1..2].

Do not modify standard GUD variables without Siemens documentation reference; backup current GUD before changes.

Cross-reference CYCLE971 documentation (typically in Siemens SINUMERIK Cycles Manual) to verify exact _MFS structure requirement. Standard definition: _MFS[1]=feed_rate, _MFS[2]=spindle_speed.

If _MFS is corrupted or missing, reinitialize it using standard GUD template provided in your system's cycle library. Reload all cycles via Siemens system software update or manual GUD restoration from backup.

Review NC program sequence before CYCLE971 call. Verify M03 or M04 (spindle start) is executed and spindle is in READY state. Check machine status via HMI Spindle panel.

Ensure spindle acceleration is complete before CYCLE971 invocation; add dwell (G04 Px.x) if needed based on spindle ramp-up time (MD1500 spindle time constant).

Confirm _MFS[2] value is non-zero and within spindle rated speed range (check machine data MD1305 max spindle speed). If speed is below 50 RPM or exceeds limit, CYCLE971 may reject it as invalid.

Execute manual spindle test via HMI: set spindle to test speed, verify rotation, then assign that verified speed to _MFS[2] in part program. Rerun CYCLE971 with confirmed spindle parameter.