Troubleshooting SINUMERIK 840D SP1 Spindle Rotating Uncommanded

Problem Overview

On SINUMERIK 840D sl (and the older 840D/840D pl) CNC platforms, a recurring field complaint is that spindle SP1 rotates by itself when the controller is powered up or shortly after a part program terminates. The motion is initiated without any NC block (G-codes M3/M4/M5, S-word, SPOS/SPOSA, etc.), no MDI input, and—critically—no alarm or message appears on the HMI. The operator observes that the spindle either:

• Spins immediately at power-on / NCK ramp-up, or
• Spins some seconds after the part program reaches M30 / reset, or
• Spins only after the machine has been idle for a period.

Because the symptom is silent, the root cause is normally one of three families:

1. Wrong axis / spindle assignment in the NCK machine data (an axis channeled to SP1 is also being jogged, or a wrong axis number is linked to a spindle drive).
2. Encoder configuration error on the spindle (encoder type, zero-mark, or fine resolution wrong, producing a fake position that drives a closed-loop regulator to "run away").
3. PLC program issue—either a faulty user PLC block driving DB31..DB61 spindle control bits, or a stale axis enable that triggers a residual axis motion.

This reference walks through the diagnostic procedure, the relevant machine data and PLC interface signals, and the corrective action for each root cause.

SINUMERIK 840D Spindle Architecture

The spindle path on a SINUMERIK 840D is built from three cooperating subsystems. Understanding which subsystem is the source of the uncommanded motion is the first diagnostic step.

Spindle SP1 is, by default, channel-spindle 1 and is mapped through drive telegram 136 (speed-controlled spindle) or 139 (position-controlled spindle with spindle positioning). The 840D sl / SINAMICS S120 combination almost exclusively uses telegram 136 with optional 391 extensions; legacy 840D + SIMODRIVE 611 uses telegram 3/4 mapped to SSBN/SSBS drive addresses.

Root Cause 1 — Wrong Axis/Spindle Assignment (NCK Machine Data)

If SP1 rotates only when the operator selects JOG on a manual axis (and never in AUTOMATIC), the cause is almost always a misconfigured axis assignment.

Procedure to verify:

1. Open HMI: Commissioning → Machine Data → Axis MD → SP1.
2. Compare every encoder and setpoint MD against the spindle drive nameplate and the project's commissioning protocol (Lieferfreigabe).
3. Cross-check MD20070 $MC_AXCONF_MACHAX_USED (channel axis assignment) and MD35000 $MA_SPIND_ASSIGN_TO_MACHAX—the latter must reference the SP1 drive address, not an unused or wrongly-numbered axis.
4. If the system uses multiple channels, confirm MD20090 $MC_SPIND_DEF_MASTER_SPIND and per-channel spindle numbers (MD35010) are not pointing two channels at SP1.

Root Cause 2 — Encoder / Measuring-System Configuration

A second common fault is the encoder (motor encoder BISS/SSI/TTL or external spindle encoder) feeding back a corrupted or noisy signal. Because the spindle loop is normally closed in speed, a noisy encoder makes the speed controller "hunt" until the motor overspeeds. On SINAMICS S120 the drive may interpret this as a commanded speed and ramp up.

Field-proven checks:

• MD31000 $MA_ENC_IS_LINEAR = 0 for a rotary spindle (rotary encoder). Setting this to 1 by mistake is a frequent cause of "spindle moves at constant speed without command".
• MD31040 $MA_ENC_IS_DIRECT — on the spindle, encoder feedback is usually through the motor (indirect), not direct (e.g., on the spindle nose). Setting this to 1 incorrectly causes oscillation.
• MD32110 $MA_ENC_FEEDBACK_POL — if the encoder cable was rewired during service (swap A/A- and B/B-), only this MD can fix it. Wrong polarity without an alarm can drive a slow runaway.
• For SINAMICS S120 spindles, also check p0405 (encoder fault signal), p0437 (encoder simulation threshold), and r0459 (encoder fine resolution). On a sin/cos encoder with a damaged cable, the drive may report F31118 / F31117 but in older firmware these are buffered as warnings only.

On the HMI diagnostics screen select Commissioning → Drives → SP1 → Trace and capture Setpoint speed, Actual speed, and Encoder 1 fine position. If the actual speed shows steps larger than 0.5 rev/min without the setpoint moving, the encoder or its wiring is suspect.

Root Cause 3 — PLC Interface Signals (DB31 / DB61)

The PLC sends enable and command signals to each spindle through dedicated axis/spindle DBs:

• DB31 for axis/spindle 1 in channel 1 (the spindle 1st default).
• DB61 when the spindle is assigned to the second channel or as a second spindle.
• DB10 for central NCK/PLC signals (NC-Stop, EMERGENCY STOP, mode group).

If DB31.DBX18.5 or DB31.DBX30.0 are showing 1 while no NC block is being processed, the PLC program is forcing rotation. To check:

1. Open the HMI: Diagnostics → PLC → Status DB31.
2. Force DB31.DBX1.1 = 0 using PLC monitor/force and confirm SP1 stops. If yes, drive and NC are healthy and the PLC is the culprit.
3. Search the PLC program (Step 7 / TIA Portal) for any unconditional S DB31.DBX18.5 or coil on DB31.DBX30.0 outside of FC19 / FC30 / FC33 (the OEM stubs).

A frequent defect on older 840D machines is that DB31.DBX6.3 (Feed/rapid stop) or DB31.DBX6.7 (Spindle stop) is latched incorrectly, releasing on NC-Reset and re-asserting as a transient motion. Insert a cross-reference to OB1, OB35, OB100 (restart) and OB102 (cold restart).

Root Cause 4 — Drive-Level Speed Setpoint Leakage (SINAMICS / SIMODRIVE)

If the NCK and PLC are correct, the drive itself may still hold a small residual speed setpoint that gets amplified by the closed loop. Check the drive side:

On SIMODRIVE 611 digital (legacy), the equivalent parameters live under MD1100..MD1400—check MD1104 (setpoint default), MD1112 (jog speed 1), and confirm that SSBN (setpoint normalized) reads 0 after NC Reset.

Step-by-Step Diagnostic Procedure

1. Verify the symptom class: confirm whether rotation appears in JOG, MDI, AUTOMATIC, or all three. If only JOG → axis assignment / JOG MD issue. If AUTOMATIC after M30 → PLC signals or drive leakage.
2. Read alarms from the alarm bar (top of HMI). Even if no alarm appears, run Diagnostics → Alarm Log → History to capture the previous 200 events, including 8022/8023 axis-stop codes and 2xxxx drive alarms.
3. Open the axis service screen for SP1 (Commissioning → Machine Data → Axis MD → SP1 → Service Display) and record: Setpoint, Actual, Following error, Position, Gear stage, Controller mode.
4. Disable the drive via PLC DB31.DBX1.1 = 0 and observe whether the setpoint at the drive (SINAMICS r0062 or 611 MD1130) still holds a value. If yes, drive is being commanded by PLC/NCK. If no, drive is self-exciting.
5. Force a Reset (DB10.DBX56.2) and watch SP1 for 60 s. If motion returns >5 s after Reset → suspect PLC timeouts, OB100/OB102, or DC link residual charge.
6. Compare the active and inactive drives: if the machine has a second spindle SP2, swap drive addresses (only with OEM authorisation) and observe whether the uncommanded motion follows the drive or stays with SP1. This isolates hardware vs. configuration.
7. Check the part program: even with no NC block active, an asynchronous subprogram, an ASUB (SETINT), or a cyclic FC9 trigger from the PLC can issue an M3. Use the HMI Program overview and $AC_TIMER diagnostics.
8. Compare against a known-good commissioning protocol (Lieferfreigabe backup). Differences in MD30130/MD30200/MD31000/MD31020/MD31040/MD32110 typically explain a "spindle moves by itself" fault after a service intervention.

Corrective Action by Root Cause

Verification After Repair

1. NCK restart: HMI → Commissioning → NCK Reset (po clear) (warm restart).
2. Drive restart: SINAMICS p0972 = 1 (factory reset of drive object); for 611 digital, power-cycle the line infeed.
3. PLC restart: stop-run on the S7 station; confirm no force table is active.
4. Power on the machine. Monitor SP1 for 5 minutes with no part program loaded: SPINDLE.SPEED.ACT must remain 0 rev/min, r0062 on the drive must remain 0.
5. Run a dry-cycle test: S500 M3, dwell 30 s, M5, dwell 60 s, repeat 10 cycles. Confirm no overspeed (F07901), no encoder alarms (F31117/F31118), and that SP1 stops cleanly at M5.
6. Check the following-error on the service display stays below the MD36000 $MA_SPIND_POSCTRL_SAMPLE_TIME × 0.5 limit and below the MD36400 $MA_CONTOUR_TOL value.

Diagnostic Quick Reference (Troubleshooting Matrix)

Field-Proven Caveats

• If the machine uses an SMC/SME module to add an external spindle encoder, a loose DRIVE-CLiQ connector produces no alarm on older firmware but a position drift that the regulator compensates by accelerating. Re-seat and re-code the SMC module.
• On 840D pl with NCU 561/571/573 (older generations), the spindle encoder may be routed through the SIMODRIVE 611 FM-E measuring-system card. Confirm the correct module (2- or 3-box) is installed; mixing modules is a documented source of uncommanded motion.
• For 840D sl with SINAMICS S120, if F31118 appears only in the history log (not active), enable p0437.7 = 1 to convert this into a fault instead of a warning. Always cross-check after upgrading firmware (e.g. 4.x → 5.x or 6.x) where encoder monitoring behavior changed.
• The HMI Service Display shows gear stage (M40..M45). If the machine enters a wrong gear stage because of a faulty PLC cam, the spindle may rotate at an apparently commanded speed even when M40 is not active.
• Always verify the spindle parameter set selection (parameter set 1..8) via PLC signals DB31.DBX16.0..16.2. A stuck parameter set selection can load a dataset with a non-zero fixed setpoint.

Related Standards and Siemens Documentation

For deep commissioning context and parameter listings beyond this article, the following official Siemens manuals apply:

• SINUMERIK 840D sl Commissioning Manual (06/2023) – chapters 4 (axis commissioning) and 6 (spindle commissioning).
• SINUMERIK 840D sl Basic Programming and NC Variables – axis/spindle PLC interface (DB31/DB61).
• SINUMERIK 840D sl List Manual (Machine Data) – Volume 1 (Axes / Spindles) – MD30xxx, MD31xxx, MD35xxx.
• SINAMICS S120/S150 List Manual (Parameters) – p1001..p1020, p1460, p1470, r0062.
• SINUMERIK 840D sl Diagnostics Manual – alarm 2xxxx, 3xxxx, 8xxxx.