Mitsubishi M800/M80 position mismatch: identify the counter before diagnosing a fault

In a repair handover, “the X position changed” may mean physical movement or simply a different counter on screen. Subtracting two readings of different types does not establish a positioning error. This article explains that distinction, not how to reset a reference position.

Scope and meaning of the readings

The reference is the Mitsubishi M800/M80/E80 Instruction Manual, IB-1501274-M, sections 10.2.3 and 10.2.6. Check other series, software versions and machine-builder screens separately. A counter's location on the screen does not reliably identify its type.

• Machine posn: basic machine coordinates referenced to the characteristic zero position specified for the machine.
• Work co posn: the G54–G59 work-coordinate modal number and the position in that system. Compare the modal number as well as the value.
• Program posn: the position being executed for each axis minus the tool compensation applied to that axis. Comparisons require matching compensation conditions.
• Remain command: during automatic operation or automatic halt, the remaining increment to the endpoint of the current movement block, not a position relative to machine zero.

These are controller counter definitions, not independent measurements of the tool tip or workpiece. Plausible readings alone do not establish that a part meets dimensional requirements.

Why a reading can change after returning to the screen

Section 10.2.3 explains that switching Counter #1–#5 through the display menu normally rewrites the corresponding counter-type parameters #8901–#8905. With data protection active, the displayed type can change without rewriting those parameters. Leaving and returning to the monitor can then restore the parameter-defined type. A different number may therefore reflect a restored display type, not failed memory or an encoder. This article does not call for changing those parameters or disabling protection.

The all-axis counter pop-up is not an invariant default either: section 10.2.6 describes its initial type and retained selection, with some behavior depending on available functions. Extended counters such as tool-tip work position are not available on every machine. Do not change options merely to reproduce another machine's screenshot.

A reading sequence that needs no machine movement

1. Preserve the existing full-screen image from the event and the current safe stopped state. Record the control, channel/part system, axis, unit, counter title and work-coordinate modal number, not just an X value.
2. Pair before-and-after records by channel, axis, counter type and unit. An old image without labels is a clue, not positioning-accuracy acceptance data.
3. If machine-coordinate records match but work-coordinate records differ, check the selected work coordinate system and existing offset records first. This is an investigation direction, not proof that an offset changed. Establish that both records represent the same physical state before interpreting the difference.
4. For differences involving program position, check the tool-compensation state. For remaining command, establish the same block and execution state. Do not mix distance left in a block with an absolute position.
5. Only unexplained changes remaining after these conditions match should lead to further checks of the reference position, feedback, program and mechanics by responsible personnel. This narrows the investigation; it does not identify a failed component.

Where interpretation must stop

This is an editorial read-only analysis method, not a reported machine test. If physical position is abnormal, related alarms occur, or tool-to-workpiece clearance is uncertain, do not test a screenshot by jogging axes, zeroing displays, resetting references or restoring parameters. Preserve the state and have qualified personnel follow the machine builder's procedures. Five-axis transformations, tool-center control and multiple channels require their own configuration evidence; no universal coordinate-conversion formula is supplied here.

Reference: M800/M80/E80 Series Instruction Manual, IB-1501274-M, revision M, §10.2.3, printed pages 575–579; §10.2.6, printed pages 582–583. Manual definitions are distinguished from the editorial comparison method. No machine testing was performed.