802D Tool Wear Offset: Fixing Display MD 374 Input Limit

Problem Summary

On a SINUMERIK 802D-controlled lathe, operators report that the controller refuses tool wear offset (WZ-Verschleiß) values of 10 or higher in the wear offset screen. Inputs of 9 or less are accepted without fault. On a second, mechanically similar 802D lathe in the same shop floor, the same screen accepts a value of 30 without complaint. The discrepancy is identical in machine type, controller type, and software generation, and points to a per-machine data configuration rather than a hardware fault.

The governing parameter is Display Machine Data (Display-MD) 374. Field experience documented in cross-shop comparisons confirms that the value configured under MD 374 caps the permissible numeric range of the wear offset input. The default restriction 9.9999 corresponds to a one-digit integer field plus four decimal places. Values like 15 or 30, when observed on the other machine, indicate the parameter has been edited, the wear field has been re-formatted, or the input is being routed through a different (summed or extended) parameter set. This article reconstructs the diagnosis from the documented symptom and the MD 374 response, and walks through a field-proven resolution path that is safe to apply on a running production lathe.

Symptom Definition

Specifically, the fault presents as follows on the 802D operator panel (OP 010 / OP 010C / OP 010S, depending on the lathe build):

1. Operator navigates to the wear offset screen (Menu Select → Tool offset → Wear tab, or hardkey path Offset → Wear).
2. Operator highlights a wear field (length 1, length 2, length 3, or radius, depending on tool type D1 / D2).
3. Operator keys in 10, 15, 30, or any integer ≥ 10 and presses Input or Enter.
4. The controller either (a) reverts the field to its prior value with no audible alert, (b) accepts the value but truncates it to 9 (or 9.9999), or (c) raises alarm Invalid value in tool wear in the alarm bar.
5. Inputs of 9, 8.5, 9.9999, and similar sub-10 values are accepted without anomaly.

Because the second 802D lathe accepts 30, the symptom is not a controller-model limitation. It is a machine-data configuration difference between the two control blocks. The two machines are reporting two different realities because two different MD 374 values are in effect.

SINUMERIK 802D Tool Wear Offset Architecture

The 802D is a compact, three-axis (plus spindle) SINUMERIK platform intended for entry-level turning and milling machines. Tool offset storage on the 802D uses a flat (non-magazine-aware) compensation table, where each tool number (T-number) holds geometric (length L1, L2, L3; radius R) and wear (length 1, length 2, length 3; radius wear) components. Wear is added to the geometry in real time during interpolation; the wear field is what the operator adjusts on a per-part basis after measuring a finished dimension. The compensation is applied on the next block that references the affected tool.

The 802D differentiates between geometry (set once at the tool presetter, rarely changed) and wear (updated frequently). Both share the same per-tool record, but they have independent sign and magnitude limits because the wear field is the one that drifts as the insert wears. The numeric format of the wear field, including the number of integer digits and decimal places, is governed by display machine data — and that is where MD 374 enters.

The 9.9999 default that surfaced in MD 374 is the wear field's numeric format, not the geometry field's. Geometry allows a five-digit integer and four decimal places. Wear, by default, allows a one-digit integer and four decimal places — i.e., a maximum positive value of 9.9999 mm. This is by design: wear is expected to be a fraction of a millimeter on a fresh tool and a few millimeters on a worn insert, not a substitute for an uncalibrated geometry value.

Display MD 374 — Numeric Format Constraint

MD 374 (full designation: MD 374 $MM_DISPLAY_RESOLUTION_WEAR or, on some firmware branches, $MM_DISPLAY_RESOLUTION scoped to the wear field) controls the display and input resolution of the tool wear table. It is a Display Machine Data, meaning the value affects how the human-machine interface (HMI) renders and accepts the wear numeric, not the internal NCK arithmetic precision (which is IEEE-754 double-precision in all SINUMERIK 802D builds).

The encoded value is the display format string for the wear field. The internal upper limit for the wear input field, under the default format, is therefore:

When the operator types "10" or higher, the HMI performs a pre-check against this format: 10 requires a two-digit integer field, but the format string only allows one. The HMI rejects the keystroke, or it truncates the input to "9" and emits a soft warning, depending on the firmware branch and the input sequence used (TYPE then value, or direct entry). The NCK is never asked to evaluate the value, so the alarm is generated by the HMI layer, not by the interpolator. This is the same diagnostic pattern observed on the failing lathe: typing 9 is allowed because 9 is the largest single-digit integer the format permits.

Root Cause

Per-machine MD 374 has been set to a value that constrains the wear input to 9.9999. The second machine in the same shop, although mechanically identical, has either (a) MD 374 set to a different format (e.g., a wider format allowing two integer digits), (b) its wear record distributed across multiple fields that sum on read-out, or (c) the operator's "30" being read from a different field (a tool-life count, a usage counter, or an OEM-specific extended wear slot) that resembles a wear value visually but is governed by a different MD.

The most likely cause, given the documented observation, is (a) or (c): the second machine was commissioned with a different Display MD 374 value — either the original 802D default (which can differ from the 802Dsl default) or a custom value chosen by the lathe builder. Siemens documentation for 802D does not guarantee a uniform default for Display MD 374 across all firmware branches; the parameter is a builder-configurable value, not a factory-fixed one. A secondary possibility is that Machine B is reporting the value 30 in a different MD (e.g., MD 375 geometry resolution or MD 376 total read-out) and the operator is reading it as the same screen as Machine A.

Diagnostic Procedure

1. Power on the 802D and log in at access level Manufacturer (password: SUNRISE on default 802D builds, or the password set by the integrator). Access level Service is insufficient to view display machine data on most 802D firmware branches.
2. Press the Menu Select hardkey, then navigate to Setup → Machine Data → Display MD. On 802Dsl, the path is Menu Select → Commissioning → Machine Data → Display Data.
3. Locate MD 374. Use the Search softkey; type 374 and press Input.
4. Read the current value of MD 374. Record it on the commissioning sheet, dated and initialed.
5. Compare the recorded value against the value on the second 802D (the one that accepts 30). If the two values differ, the diagnosis is complete: the input range is governed by the format string in MD 374.
6. Cross-check the input field's actual behavior. On the screen of the failing machine, attempt to type 9.9999 and confirm it is accepted. Attempt 10 and confirm rejection. The asymmetry confirms a one-digit integer cap.
7. Optional: navigate to MD 20150 (default G-code profile) and MD 20360 (tool compensation parameter) to confirm no program-level wear redirection is in effect. If a custom macro is reading wear from a non-standard location, it will not show under MD 374.

Step-by-Step Resolution

The fix is to change MD 374 to a value that allows the needed input range, or to move the bulk of the value into the geometry field that has a wider native range. There are two field-proven approaches; choose based on whether the bulk of the value is a one-time correction (use Approach B) or a recurring wear trend (use Approach A).

Approach A — Widen the input format

1. From Manufacturer access, open MD 374.
2. Replace the current value (e.g., 9.9999) with a wider format that allows two integer digits, e.g., 99.9999 or 99.999.
3. Press Input (or OK depending on HMI revision) to commit the value.
4. Cycle NCK power (or execute the equivalent soft reset) for the change to take effect on 802D; 802Dsl applies display MD changes immediately.
5. Re-attempt the wear input. Typing 15 or 30 should now be accepted in the wear field.

Approach B — Distribute the wear across the existing format

If the integrator wants to keep the input format narrow (9.9999) for operator-friendliness, but the actual compensation is, e.g., 30 mm of length compensation on a worn slide, the correct procedure is to move most of the value into the geometry field and leave only the per-pass wear in the wear field. This is the standard Siemens-recommended practice for 802D lathes with limited wear range.

1. Identify the worn dimension. Suppose a turning tool's effective length compensation is 28.5 mm because the tool post has shifted, not because the insert is wearing.
2. Navigate to the tool's geometry record. Set the geometry L1 to the new "base" length, e.g., re-measured on the tool presetter.
3. Reset the wear field to a small value (≤ 9.9999) reflecting only the per-insert wear.
4. Confirm the part dimension is correct after the change by cutting a single test piece and measuring.

Approach B is the most common in 802D turn-mill applications and is the path Siemens documentation recommends. Approach A is appropriate when the wear truly must be in the wear field — for example, when a custom post-processor is summing wear entries across a tool family and the result can exceed 9.9999 mm, or when a process routinely requires multi-millimeter per-part correction.

Cross-Machine Configuration Comparison

The "30" reading on Machine B is itself unusual. A value of 30 in MD 374 would not be a valid format string in the strict sense (a format string is 9.9999-style). It is more likely that Machine B is reporting a maximum numeric value in a different units slot, or that MD 374 has been re-purposed by the builder, or that the "30" is being read from a related MD such as 375 or 376. Field verification on Machine B is required to disambiguate. The exact MD being read should be confirmed before treating Machine B as the configuration reference.

Related Machine Data Parameters

When a 30 mm value appears on the working machine, the natural follow-up reads are MD 375 (geometry resolution) and MD 376 (total read-out). If MD 375 on Machine B has been set to 30.0000, the operator is likely looking at a geometry field, not a wear field, and the symptom on Machine A is not a defect — it is the correct behavior of the wear field under default MD 374. The naming convention used in your build (HMI version, firmware branch) can shift which MD controls the read-out; confirm against the on-controller help text for MD 374 before assuming.

Verification and Validation

1. After editing MD 374, restart the NCK (on 802D) or wait for the soft reload (on 802Dsl).
2. Navigate to the wear offset screen and re-test the input sequence: enter 10, 15, 30, and a fractional value (e.g., 0.0123). All should be accepted.
3. Run a no-cut dry program (insert an M00 after the first tool change) and observe whether the wear value is being applied. The active wear value can be read from the HMI's Active tool data screen under Menu Select → Diagnostics → Tool.
4. Cut a single part, measure, and adjust wear by no more than the offset needed. Confirm the part is within tolerance.
5. Run a multi-tool program with several tool changes. Confirm that wear is applied independently per tool and that the per-tool values are not summed unexpectedly.
6. Sign the commissioning sheet and note the new MD 374 value, the date, the firmware version (read from Menu Select → Diagnostics → Version), and the name of the technician who performed the change.

Field-Commissioning Notes and Caveats

802D vs 802Dsl

The original 802D and the 802Dsl (the "solution line" successor) share most MD numbers but not all defaults. Display MD 374 default is one of the parameters that may differ between the two branches. When cross-shop comparison shows asymmetric behavior between an 802D and an 802Dsl lathe, the first diagnostic step is to confirm the firmware version on each. The 802Dsl additionally supports a soft reset that re-applies Display MD without an NCK power cycle; the 802D requires a hard power cycle.

Alarm text on invalid wear input

On 802D firmware < SW 2.x, the HMI silently reverts the wear input. On firmware ≥ SW 2.6 and on 802Dsl, the HMI raises alarm 6481 Invalid value in tool wear, channel %1, T %2, D %3 with a Clear/Reset key sequence to dismiss. If a different alarm number is observed, capture it from the alarm bar and cross-check against the Siemens 802D Diagnostics Manual alarm list; do not generalize from one alarm number to the next. The complete alarm list is in the SINUMERIK 802D diagnostics manual available on the Siemens Industry Online Support portal.

Wear vs geometry semantic discipline

Operators under time pressure will sometimes enter a large wear value as a substitute for "I don't have time to re-measure geometry." This is the root cause of most "MD 374 won't accept my value" tickets in the field. The fix is to re-establish the geometry-wear discipline on the shop floor: geometry is set at the tool presetter once per tool life, wear is adjusted per part. A wide MD 374 is a workaround, not a process improvement.

OEM customizations

Many lathe builders (e.g., from the China and India machine-tool ecosystem that commonly uses 802D controls) customize the HMI software and may relabel or hide the wear field. The 30 mm value observed on Machine B is most likely an OEM-customized readout, not a stock Siemens value. Before changing MD 374, confirm with the lathe OEM that the value you are reading is the standard Display MD and not an OEM label. The 802D HMI manual is available on the Siemens Industry Online Support portal as a downloadable PDF for cross-reference.

Backup before change

Use the 802D's series commissioning file (file extension .arc for 802D, .dsf for 802Dsl) to back up the current NCK and HMI state before editing any machine data. The file is exported via Menu Select → Commissioning → Series commissioning → Export. Store the file on a CF card or USB stick with a timestamp. If the MD change destabilizes the controller, the file can be re-imported to roll back within minutes.

Frequently Asked Questions