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In the NC program, verify M05 (spindle stop) is programmed immediately before G74 or reference point approach. Check spindle deceleration time via MD 3310 (Spindle acceleration time) and MD 3311 (Spindle deceleration time) in Diagnostics > Parameters > Spindle menu.

Add dwell time (G04 P...) after M05 to allow spindle mechanical deceleration. For typical systems, 0.5-2.0 seconds is sufficient depending on spindle inertia and brake configuration. Verify actual spindle coast-down time during commissioning.

Check spindle encoder feedback signal integrity at machine control cabinet (typically via encoder input module). Verify spindle feedback is wired to NCK digital input and configured in MD 3100 (Spindle type selection). Intermittent encoder signal can cause false 'not standstill' detection.

Ensure machine is in Automatic Stop state before checking electrical connections.

Review spindle brake configuration (MD 3350 - Spindle brake control) to confirm brake release/engage timing is correct. If using electric brake, verify brake delay time in MD 3351 allows spindle to fully stop before re-engagement.

Access Diagnostics > Signals > NCK Inputs menu. Locate the spindle standstill status signal (typically named 'SpindleStandstill' or 'S_Standby'). Manually stop the spindle (M05) and verify the signal toggles from 0 to 1 (or 1 to 0, depending on logic configuration). A static signal indicates PLC communication failure.

In the PLC program (typically OB 100 or technology cycle block), verify the spindle standstill bit is correctly mapped. Check the DBX address assignments against MD 3102 (Spindle feedback interface) and cross-reference with hardware configuration in the PLC symbol table. Confirm signal NOT inverted (check for negation operator in signal chain).

Test spindle encoder/tachometer feedback directly using a multimeter or scope. For analog signals, verify 0V ±0.2V at spindle stop. For digital encoder signals, verify pulse count = 0 using oscilloscope or diagnostic tool. If no signal present, replace encoder or verify drive output is energized.

De-energize spindle drive before measuring encoder connections. Use proper ESD precautions.

Run Siemens SINUMERIK Diagnostic Tool to capture live spindle speed feedback values. Access Diagnostics > Variables > Spindle Speed axis feedback. Monitor that value reaches exactly 0.0 rpm before triggering G74. If feedback shows ±5-10 rpm residual, increase dwell time (G04) after M05 or adjust spindle ramp-down in drive parameters.

Verify in the PLC startup sequence (OB 100 or equivalent) that spindle drive initialization (parameter download and status check) completes before any automatic reference point approach is triggered. Add conditional logic: IF Spindle.IsReady = 1 AND Spindle.Speed = 0 THEN execute reference point.