Troubleshooting SINUMERIK 840D sl Alarms 201711 25001 27001

1. Problem Summary and Affected Hardware

A five-axis milling machine controlled by a SINUMERIK 840D sl with NCU 730.3B PN and PROFINET, operating on CNC software version 4.4 with paired SINAMICS S120 drive modules, reports a cluster of drive- and encoder-related alarms during real cutting. Simulation (dry run without tool contact) is clean. The alarm cluster appears only when material is being removed and intensifies over time, typically one to several minutes after the tool first contacts the workpiece. The same alarm cluster re-appears on the X axis when it is run alone for several minutes, with no other axes in motion. The programmed tool path is correct: the same program run on a different physical workpiece on a different table position completes without alarms.

The reported alarms are:

• 201711 - position/encoder error on the X axis (motor encoder path)
• 201750 - PROFIsafe / Safety Integrated motion error, follow-on from 201711
• 231116 - encoder comparison or plausibility error on the C axis
• 25001 - drive-side encoder or motor encoder fault
• 27001 - axis stop / monitoring error
• 27013 - following error actual value
• 27023 - following error model
• 27024 - following error actual value extended
• 232 - position controller monitoring error (raised with 201711)

The complete set of definitions, clear conditions, and acknowledgement rules for these alarms is published in the Siemens Diagnostics Manual, Alarms for the 840D sl / 840DE sl (software version 4.4 / SINAMICS S120), available from Siemens Industry Online Support at DAsl_0911_en_en-US.pdf. Always treat the official alarm help text shown on the HMI as the binding interpretation; the categorization in this article is for triage only.

2. System Configuration Snapshot

Before diagnosing, document the as-built configuration. The information in the source incident is summarized below; the same checklist is used in service reports.

3. Alarm Code Reference Table

The codes below are grouped by their role in the incident. Exact wording, clearing conditions, and remedies must be confirmed against the live alarm help in the HMI and against the official diagnostics manual.

4. Failure Pattern and Trigger Conditions

The incident exhibits a very specific signature that is itself diagnostic. Map observations to causes before touching hardware:

1. Simulation is clean. The NCK, drives, encoders in standby, and the part program are not the root cause. The fault only appears when real mechanical load is applied through the tool.
2. Time-dependent onset. Alarms appear "several minutes" after tool contact. This rules out a hard electrical open (which would trip at first motion) and points to a slowly-degrading signal: thermal drift, contamination creep, intermittent shield loss, or a developing connector problem.
3. 5-axis simultaneous path is the most sensitive trigger. All five axes share the same PROFINET / DRIVE-CLiQ chassis, so a marginal encoder problem anywhere is most likely to surface when the controller is sampling all channels at the high axis dynamics of a 5-axis move.
4. X axis alone reproduces the fault. Without C, B, Y, or Z in motion, the X axis is still able to raise 201711 + 27001/27013/27023/27024. The X axis is therefore the source of those codes, not a cascade from another axis.
5. Different workpiece, different physical location on the table, same machine, same program - runs clean. This is the most important observation. The root cause is not a global program, mechanical, or electrical fault; it is local to the work area of the original setup. This rules out spindle power stage, NCK hardware, and SI configuration as primary causes.
6. Surface finish is degraded and acoustic noise is elevated before the alarm appears. This is a pre-failure symptom. The cutter is doing more work than it should, or the workpiece / fixture is moving, before the drive system detects a hard error. Vibration and cut noise are mechanical precursors to the encoder fault trip.
7. Coolant ingress possible. QUAKERCOOL 3618 HBFF C is oil-miscible, high-pressure through-tool. Past field experience with HiPer-finish milling on 840D sl machines (the historical 2015 INDEX multi-spindle case) is that high-pressure oil returns through axis motor bearing seals into the motor encoder housing and slowly degrades the encoder or the cable. This must be ruled in or out early.
8. EMC context. The PROFINET 840D sl chassis sits in the same cabinet as the S120 line modules. The increased spindle current under cut load elevates radiated and conducted noise, which couples into marginal encoder wiring or PROFINET cables more easily when the cable shield or connector is already slightly compromised.

5. Root Cause Analysis Matrix

Use the table below to triage. "Probability" is the likelihood in this specific incident signature, not a general ranking. "Evidence to gather" is the test that confirms or eliminates the cause.

6. Encoder Cable and DRIVE-CLiQ Diagnostics

The DRIVE-CLiQ cable between the SINAMICS S120 motor module and the motor encoder is the highest-risk single component in this fault pattern. On the X axis it is also routed through a drag chain that flexes millions of cycles. Use the following sequence.

1. Read the drive-side fault buffer. On the HMI go to Diagnosis > Drive System > Drive Parameters > Fault buffer and capture the last 16 faults on the X axis and on the spindle. Note the detailed fault code (e.g. F3xxx for SINAMICS encoder faults) and the error value.
2. Visual inspection of connectors. With the machine safely de-energized and locked-out, unplug the motor-side DRIVE-CLiQ connector on the X motor and on the C motor. Inspect for: oil film, coolant residue, bent pins, recessed pins, signs of arcing (blackening of the shield), and correct seating of the inner connector. Any oil film is grounds for a full replacement, not a clean-and-reinstall.
3. Continuity and shield check. With the cable off the motor, ring out each conductor end-to-end and check that the overall shield is continuous from connector shell to connector shell. A shield that is open at one end is a near-certain EMC problem.
4. Resistance to PE. With a megohmmeter set to 500 V, measure the insulation resistance between each encoder conductor and PE. A reading that drops below ~10 MOhm under cable flex is a hard fault; the cable must be replaced.
5. Swap test. If the inspection is not conclusive, swap a known-good DRIVE-CLiQ cable onto the X axis (same length, same part number) and run the X axis in isolation for at least 15 minutes under the original part program, with the spindle running and coolant on. If the alarm is gone, the cable is the cause.
6. Check the cable specification. Siemens mandates specific DRIVE-CLiQ cable types (6FX.002 series, with MC-certified pre-assembled variants). Aftermarket cables must meet the same 24 V / data and shield specification. Wrong cable = intermittent, EMC-sensitive, non-recoverable under load.

7. EMC Interference Verification

The 840D sl / S120 system is designed to be EMC-robust in the as-shipped configuration, but field conditions degrade that margin quickly. Use the EMC checklist below before re-routing any cable.

1. Confirm 360 deg shield bonding at both ends of every encoder / PROFINET cable. The shield must clamp onto a metallized backshell, not onto a plastic strain relief. A pigtail shield termination is the most common EMC defect in field service.
2. Verify that PROFINET cables do not run parallel to S120 motor power cables in the same tray for more than ~200 mm. The 840D sl cabinet is laid out for separation; field modifications often break this. A 100 mm minimum gap is a hard rule; in a retrofit, fit a grounded metal divider.
3. Check the equipotential bonding at the spindle motor. The spindle motor frame must bond to the cabinet PE with a short, fat strap. Paint, anodizing, and oil film between the motor foot and the machine bed break the bond.
4. Verify the filter / line reactor on the S120 line module is correctly sized and torqued. A loose filter leg is a near-field noise radiator.
5. Add a temporary ferrule test. Clip-on ferrules on the motor power leads and on the encoder cable near the drive end will often silence a marginal EMC problem. If they do, the cable plant is the weak link and must be repaired, not just filtered. Filtering hides the problem; it does not fix it.
6. Inspect recent electrical changes near the machine. New welding equipment, new VFDs on adjacent machines, a new battery charger, a new HVAC inverter - all of these are common EMC noise sources that surface as drive faults on the affected machine while the drive sits inside its EMC envelope.

8. Coolant Contamination and Motor Seal Inspection

With QUAKERCOOL 3618 HBFF C fed through the spindle at high pressure, and with a finish-mill application that uses high-pressure coolant at the cutting edge, the most likely non-cable cause is oil migration into the motor encoder housing or the encoder cable jacket. The historical case (a 2015 INDEX multi-spindle machine on 840D sl, oil coolant through high-pressure rotary union, oil returned past the axis motor bearing seal into the encoder housing) is the closest analogue to this incident.

1. Remove the motor encoder cover on the X axis. Inspect for any oil film, darkening of the encoder disc, or varnish smell. Any oil film means the bearing seal is leaking and the encoder is at the end of its life.
2. Inspect the encoder cable gland on the motor. If the gland is loose, the cable jacket is cut, or the heat-shrink boot is cracked, coolant wicks along the cable strands and reaches the connector.
3. Inspect the bearing seal on the motor shaft end. The seal lip must be intact and free of scoring. A 0.5 mm groove on the seal lip is enough to allow oil to enter over a 30 minute cut.
4. Check the rotary union on the spindle. The 2015 INDEX case was the rotary union feeding oil back along the motor shaft. A 5-axis head typically has its own rotary union; check the seal ring set.
5. Replace the encoder if contamination is present. Cleaning the encoder disc and re-calibrating is a stop-gap; the oil has already attacked the bearings and the disc is permanently degraded.
6. Replace the cable if there is any oil in the jacket. The oil will re-migrate to the new encoder if the same cable is reused.

9. Spindle Encoder and Electrical Spindle Checks

Once the X axis was tested in isolation, the spindle also began to refuse reset. The spindle fault is therefore a separate, parallel failure that needs its own work stream; do not assume the spindle fault will clear once the X axis is fixed.

1. Capture the spindle drive fault buffer. The F-code (e.g. F31110, F31111, F31112, F31115 family) tells you whether the fault is on the encoder itself, the encoder cable, or the speed calculation. F31110 / F31111 family most often points to the encoder or the cable; F7901 / motor-side faults point to the power stage.
2. Pull and inspect the spindle encoder connector. Spindle encoder connectors sit near the spindle motor, where vibration and oil are the highest. Inspect for fretting at the pins.
3. Megger the spindle encoder cable to PE at 500 V. Replace if below ~10 MOhm.
4. Check the spindle motor insulation to PE at 1 kV (line-to-line to PE). Replace the spindle motor if below typical acceptance values for the frame size.
5. Verify the spindle parameters in the drive (p2900, p2901, p2902 family) match the spindle nameplate. A parameter drift will not normally produce an encoder alarm, but a misloaded drive will produce following errors that look identical to the X axis faults.
6. Confirm PROFINET slot mapping for the spindle. A misassigned PROFINET slot will produce intermittent telegram timeouts that surface as 25000x-series alarms under load.

10. Safety Integrated Monitoring Verification

Because 201750 is a Safety Integrated motion error and the machine uses the SI option, confirm that the SI limit values are not the underlying trigger.

1. Compare the SGES / SBR / SCA limit values (machine data 36901 ... 36990 family) against the as-shipped commissioning sheet. A drift in safe actual position tolerance is the most common SI false trip on machines that have been re-homed after a long stop.
2. Confirm the PROFIsafe telegram selection on every axis matches the F-CPU configuration. A mixed telegram type is the most common PROFINET SI error and surfaces under load only.
3. Verify the SI clock and the PROFINET update time. With 5 axes on the line, the PROFINET update time must be sized for the worst case. A 1 ms update time on a 5-axis 840D sl with SI is normal; 4 ms can fail intermittently under load.
4. After any encoder replacement, re-validate the SI functions in the standard Siemens sequence: Commissioning > Safety Integrated > Acceptance test. Document the test results; SI acceptance is a regulatory requirement in every major market.

11. Mechanical, Tool-Holder, and Cutting-Parameter Cross-Check

The clue that "a different physical workpiece, in a different table position, on the same machine, on the same program, runs clean" is the most important single observation in the incident. The mechanical chain must be examined even after a cable is found, because the cable is the symptom, and the cutting condition is the trigger that pushes a marginal cable over the edge.

1. Inspect the tool holder. Pull-stud condition, HSK or SK taper cleanliness, balance class, and runout. A damaged pull-stud allows micro-movement of the cutter in cut, which is the same mechanical signature as a faulting drive.
2. Inspect the cutter. Edge condition, flutes not chipped, correct insert seating. A chipped insert produces an intermittent cutting force that mimics a drive following error.
3. Inspect the workpiece fixture and the table contact. Loose clamps, dirt under the workpiece, an uneven table surface all create a localized resonance that couples back into the head structure and onto the spindle encoder.
4. Re-examine the cutting parameters. A radial engagement or a chip load that was at the upper limit of what the spindle can deliver will not cause an alarm at sim, but will produce a torque pulse and a vibration peak on every tooth pass. With a marginal encoder, that peak is the trip.
5. Run a vibration survey on the X axis and the spindle bearing housings with a portable accelerometer if available. Peaks at the cutter tooth-pass frequency or its harmonics, present in cut and absent in air-cut, point to a cutting parameter problem rather than a drive problem.

12. Step-by-Step Remediation Procedure

Run the following in the order given. Each step is sized to be one service shift or less. Do not skip to a deeper step before the shallower one is ruled in or out.

1. Capture and back up. Archive the NCK and drive parameter sets, the alarm log, and the HMI screenshots. Required before any hardware swap.
2. Read the drive fault buffers on every axis and on the spindle. Note the F-code and the error value. This is your baseline.
3. Inspect the X axis DRIVE-CLiQ cable as a complete assembly. If there is any sign of oil, mechanical damage, or shield discontinuity, replace the cable. Re-test.
4. Inspect the X motor encoder for oil ingress. If oil is present, replace the encoder and the bearing seal. Re-test.
5. Inspect the C axis DRIVE-CLiQ cable and motor encoder in the same intervention.
6. Inspect the spindle encoder cable, connector, and motor insulation. Replace as needed. Re-test.
7. EMC audit: verify shield bonding, equipotential bonding, and cable separation per Siemens Diagnostics Manual 4.4 / SINAMICS S120 and the cabinet layout drawing. Repair any defects.
8. Tool and fixture inspection for the original setup. Clean, re-clamp, re-measure. Re-test.
9. Cutting parameter review with the part program. Back off the radial engagement or the chip load by 10-15% as a verification run.
10. SI acceptance test after every encoder or cable change, per the standard Siemens procedure.
11. Production acceptance run on the original workpiece, in the original position, with the original part program, for at least 30 minutes continuous cut. Compare cycle time and surface finish against the pre-incident reference.

13. Verification and Acceptance

Verification is the difference between a fix and a re-visit. The acceptance criteria below are the minimum required to close the incident.

14. Field-Proven Lessons

The following are condensed from the kind of incident that produces the alarm cluster in this article. They are general field guidance, not Siemens publication.

• On a 5-axis head on a 840D sl with PROFINET, the high-pressure oil path through the spindle is the most common encoder-degradation source. Make a habit of inspecting bearing seals on a 12-month cycle on any machine with oil-miscible coolant and through-tool supply.
• DRIVE-CLiQ cables are wear parts in any machine that uses a drag chain. Replace them at the scheduled interval; do not wait for a fault. The cost of the cable is one hour of machine time.
• Simulation and air-cut are good filters for NCK and program faults; they are useless for EMC, encoder, and coolant contamination faults. A clean sim never rules out a drive-side fault.
• When the same alarm cluster appears on one axis alone, that axis is the source. The other axes are reporting cascaded alarms only.
• When a fault appears on one physical setup and not on another on the same machine, the cause is local. The most common local causes on 5-axis heads are fixture / clamping, cutting parameters, and tool condition.
• Always post the alarm text (or a screenshot) and not just the alarm number. Different 840D sl software versions use the same number for different conditions. The diagnostics manual is the source of truth, not the service person's memory.