Siemens 840D X-Axis Misposition on Block Search: 0.039 Fix

Problem Overview

Three Siemens 840D-controlled turning centres exhibit an intermittent but repeatable X-axis position error of exactly 0.039" (0.9912 mm, i.e. ~1 mm). The fault appears only after the part program has been interrupted and the operator initiates a block search (search-run / Suchlauf mit Berechnung, also called SERUPRO in SINUMERIK documentation) to re-enter the program. During a normal program run the lathes hold their commanded positions to within the closed-loop accuracy specification; the issue is isolated to the re-entry sequence. Because the error is always in the same direction, always in the X axis, and always the same magnitude, the root cause is a deterministic bias in how the SINUMERIK control recalculates and restores machine axes during block search and subsequent REPOS, not a random servo or encoder fault.

This article walks through the engineering analysis, the relevant machine data (MD) and setting data (SD) on the SINUMERIK 840D / 840D sl, the corrective configuration, and the recommended operator procedure so that block search no longer leaves a hidden 0.039" offset on X.

Root Cause: Inch/Metric Rounding in SERUPRO and REPOS

The 0.039" magnitude is the diagnostic fingerprint. The exact inch equivalent of 1 mm is 0.0393700787". A consistent 0.039" error, with no sub-resolution jitter, means the SINUMERIK has lost exactly one millimeter of commanded travel on X. This is a unit-system rounding artefact between the metric base of the machine and the inch display/programming mode in which the operator is running.

SINUMERIK 840D controllers have an internal representation of axis positions that is governed by the metric/inch base set in:

• MD10240 $MN_SCALING_SYSTEM_IS_METRIC — defines the fundamental measuring system of the machine. Value 1 = metric base, 0 = inch base.
• MD20150 $MC_GCODE_RESET_MODE[n] — defines which active G code (G70/G71, G700/G710) is restored after reset / M30, per channel.
• MD20151 $MC_GCODE_RESET_SAVE[n] — defines which G code persists across RESET for the listed G groups.

If the machine is metric-base (MD10240 = 1) but the operator runs the program in inch mode (G70 or G700), the control converts inch values to metric for the interpolator. During a normal run, the round-trip inch → metric → inch is continuous and self-correcting. During a block search, however, the SINUMERIK executes a SERUPRO that:

1. Recomputes the search target position by traversing the program in fast-block mode and capturing the SERUPRO-ASUP stop event at the search pointer.
2. Stores the REPOS offset for each axis as an integer value referenced to the last programmed position before the interrupt (the block-end point).
3. On REPOS, applies the saved offset using the metric-base integer representation, then re-displays the result through the inch conversion path.

The saved REPOS offset is integer-quantised in the metric domain. When that integer is re-displayed as an inch value, the conversion rounds the fractional inch part to the resolution of the inch display. The smallest meaningful integer in metric units (1 increment of MD10200 $MN_INT_INCR_PER_MM) converts to 0.0393700787", which the inch display presents as 0.039". The error is therefore not a servo bug — it is the inch display truncating to the metric-base least-increment.

Additional contributing factors include:

• MD20150 reset mode mismatch. If G70/G71 is not configured to persist across reset, the operator's inch mode silently drops on M30 or RESET and re-engages on block search — producing the same magnitude shift.
• $AC_INCHMODE not synchronised between NC and HMI: the HMI inch display and the NC channel's inch mode can briefly diverge during block search on older firmware.
• SD42600 / $SC_MM_* scaling tables that have been authored assuming a different inch/metric base than the running channel.

SINUMERIK Versions and HMI Affected

The behaviour described has been observed on:

Firmware-specific notes:

• SW 4.8 SP3 introduced $NC_SERUPRO_INC_PROG to suppress incremental block search artefacts — useful but does not address G70/G71 reset.
• SW 5.1 added Search Run with SERUPRO + REPOS via AP/RP as the recommended operator flow on Operate v5.x.
• The SERUPRO routine _N_SERUPRO_COM in the CF card's CMDS / NC.STD was extended in SW 4.7 to include a metric/inch reconciliation call.

Diagnostic Procedure

Reproduce the fault deterministically, then capture the relevant state on the NCK so the MD adjustments can be evaluated.

1. Establish a baseline part program. Use a part program with a known X-axis dwell at a known commanded position (e.g. X 10.0) followed by a spindle stop and tool change to give the operator a place to interrupt.
2. Interrogate the active measuring system. On the HMI, navigate to Setup → Measuring Systems and confirm whether the operator is in inch or metric mode. Check $AC_INCHMODE[<Chan>] in the NC variables view: value 1 = inch active, 0 = metric active.
3. Read the machine data. Use the HMI Commissioning menu or HMI Operate's MD Display to record the current values of MD10240, MD10200, MD20150, MD20151, and MD10886 $MN_EXTERN_INCH_SYSTEM_OF_TYPE.
4. Run a controlled SERUPRO test. Start the program, allow it to pass the dwell, press NC-STOP, then activate Block Search with Calculation from the search bar, targeting the line number immediately after the tool change. Press NC-START. The control should auto-approach the REPOS point and stop.
5. Capture actual X-axis position. Use a dial indicator (Mitutoyo 513-403-10E or equivalent, 0.0005" graduation) against a known surface, or read $AA_IM[X1] (current setpoint in the machine measuring system) and $AA_ACT_POS[X1] (encoder-derived actual position).
6. Compare values. The error magnitude must be 0.991 mm ± one increment of MD10200. If yes, the inch/metric rounding root cause is confirmed.

Machine Data Corrections

Apply the following MD/SD adjustments. Edit via Commissioning → MD on HMI Operate, then trigger an NCK reset (warm restart) for the changes to take effect.

For shops that run inch programs only, set the machine to inch base by changing MD10240 to 0 and matching all scaling data, lead screw pitch, and encoder calibrations to the inch domain. This eliminates the conversion path entirely and is the cleanest long-term solution.

Block Search Operator Procedure (SERUPRO Best Practice)

The combination of unit-system fix and disciplined operator procedure is what eliminates the recurring 0.039" fault. The following sequence is the recommended SINUMERIK workflow:

1. Stop the program. Press NC-STOP and wait for the axes to halt. Do not attempt a block search while the part program is mid-cut.
2. Move to a safe clearance position. Lift Z clear of the workpiece and any clamp before initiating search.
3. Open Block Search. Press the Search hard key on HMI Operate or use the softkey Block search.
4. Search target. Use Search for line with the cursor on the line where the program should resume. Enable Search with calculation (SERUPRO).
5. Verify measuring system. Confirm the inch/metric mode icon matches the program. If mismatched, abort and re-select the correct unit before continuing.
6. Approach the search target. Press NC-START. The control will run SERUPRO to the target block, stop, and display the REPOS offset table. Inspect the X-axis REPOS offset ($AC_REPOS_DELAY) — it must be within the resolution quantum of MD10200.
7. Approach point (AP) → Reposition point (RP). Use the AP/RP softkeys to bring the axes back to the interrupted contour. The REPOS approach mode can be RMB (return to start of block), RMI (return to mid-block), or RME (return to end of block). For turning operations prefer RMI or RME.
8. Verify position. Before pressing NC-START to resume cutting, jog X to a known datum (e.g. probe or reference edge) and confirm the residual error is < 0.001" (0.025 mm). If greater, the part has already been drifted — re-clamp and re-start.
9. Resume with reduced feed. Start the program with the dry-run switch ON or with reduced rapid traverse for the first block, then ramp to full feed.

PLC and NC Code Safeguards

For high-value components a software interlock on the PLC is recommended. The PLC/NCK interface allows a PLC to read the REPOS delay and refuse NC-START if the offset exceeds a threshold. The following is a sketch of the logic, not a drop-in program, and must be tuned to the machine's I/O assignment.

(* SCL/Structured Control Language sketch for S7-300/1500 PLC 
   coupled to SINUMERIK 840D via DB31/DB61 interface *)

FUNCTION_BLOCK FB_REPOS_GUARD
VAR_INPUT
    i_DB_NCK : WORD;              (* DB number for axis DB31.* *)
    i_AxisNo : INT;               (* axis index 1..N *)
    i_Limit_mm : REAL := 0.025;   (* 0.001" tolerance threshold *)
END_VAR
VAR_OUTPUT
    o_Fault   : BOOL;             (* TRUE = block NC-START *)
    o_RawPos  : REAL;             (* reported REPOS offset mm *)
END_VAR
VAR
    s_RawOffset : DINT;            (* $AC_REPOS_DELAY[X] from NC var selector *)
    r_MmOffset  : REAL;
END_VAR

BEGIN
    (* Read REPOS delay for axis i_AxisNo via PUT/GET to NCK *)
    s_RawOffset := "ncVarSel".repos_delay[i_AxisNo];
    r_MmOffset  := DINT_TO_REAL(s_RawOffset) / 10000.0; (* MD10200=10000 *)

    o_RawPos := r_MmOffset;

    (* If absolute offset exceeds limit, latch a fault and disable NC-START *)
    IF ABS(r_MmOffset) > i_Limit_mm THEN
        o_Fault := TRUE;
        "axis_db".nc_start_enable[i_AxisNo] := FALSE;
    ELSE
        o_Fault := FALSE;
        "axis_db".nc_start_enable[i_AxisNo] := TRUE;
    END_IF;
END_FUNCTION_BLOCK

For an NC-side safeguard, add a synchronous action at the start of the program that logs the active measuring system and verifies the channel state after SERUPRO:

; NC subroutine _N_SAFECHECK_SPF
def int _inch, _meas
_inch = $AC_INCHMODE          ; 1 = inch, 0 = metric
_meas = $AC_MEAS_TYPE         ; 0 = workpiece, 1 = tool

; force a metric round-trip at SERUPRO entry to reconcile
IF _inch == 1
  G710                       ; ensure metric interpolation
  G70                        ; restore inch display
ENDIF

; alarm 61100 if REPOS residual exceeds 25 µm
IF ABS($AC_REPOS_DELAY[X]) > 250
  SETAL(61100, $AC_REPOS_DELAY[X])
ENDIF

The SETAL alarm 61100 is a user alarm and can be wired to the PLC to force an NC-START lockout until acknowledged. Reference the SINUMERIK 840D sl Alarms manual for alarm class configuration.

Verification and Acceptance Test

After applying the MD corrections and operator procedure, run the following acceptance test before returning the machine to production:

1. Static test. Run the baseline part program (X dwell, tool change) ten consecutive times without interruption. Confirm the X-axis actual position matches the commanded position within MD10200 quantum.
2. SERUPRO test. Interrupt the program at the dwell point ten times and perform block search with calculation to the same target. Confirm that no NC stop with REPOS residual > 0.025 mm occurs.
3. Unit-flip test. Run the program in inch (G70) ten times, then in metric (G71) ten times, with interruption in each cycle. Confirm the REPOS residual is consistent regardless of the active unit.
4. Reset persistence test. Reset the NCK between runs and confirm that $AC_INCHMODE returns to the expected state after restart (this verifies MD20150/MD20151).
5. Encoder trace. Capture $AA_IM[X1], $AA_ACT_POS[X1], and $VA_ENC1_COMP_VAL[X1] over a 60 s window. The setpoint and actual must converge to within the position-controller tolerance.

Troubleshooting Matrix

Operator Procedure: Block Search Quick Reference Card

Print and post at the operator station:

1. NC-STOP at the safe Z position.
2. Search hard-key → "with calculation" (SERUPRO).
3. Pick the resume line — never use "without calculation" on a cutting block.
4. Confirm the inch/metric icon in the status bar matches the program header.
5. NC-START — control auto-repositions via AP/RP.
6. Jog X to a reference edge, check residual < 0.001" before resuming cut.
7. If residual ≥ 0.001": re-clamp the part and restart from a clean datum.

Related SINUMERIK Reference Material

For deeper study, the following official Siemens manuals are recommended. Always cross-check the firmware version of your NCU against the manual edition before applying any parameter.

• SINUMERIK 840D sl Programming Manual (PG) — chapter on SERUPRO, REPOS, and G70/G71 grouping.
• SINUMERIK 840D sl Basic Functions (BAsics) — machine data MD10200, MD10240, MD20150, MD20151.
• SINUMERIK 840D sl Axes and Spindles — REPOS offset mechanics and block-end-point storage.
• SINUMERIK 840D sl Measuring Cycles / HMI Operate — inch/metric display resolution under different HMI versions.
• SINUMERIK 840D sl ISO Dialects — MD10886 $MN_EXTERN_INCH_SYSTEM_OF_TYPE selection.
• SINUMERIK 840D sl Alarms — alarm 61100 user area and 21612 REPOS error class.