The panel shows a rotary axis that starts the home cycle but keeps turning instead of completing reference. Start with the reference-feedback circuit: the table can report changing position and run correctly after startup while the reference-mark signal, its conductor, connector, or control input remains open. Test the cable end to end before replacing the rotary transducer; a replacement was quoted at about $6,200 for this machine.
Read the symptoms before replacing parts
The DMU 80T runs full five-axis motion correctly once it is operating. That matters. Normal commanded motion indicates that the drive, motor, and at least the main position-feedback function can work, while continuous rotation during power-up homing points toward the separate reference channel.
Watch the displayed rotary position during the failed home attempt. Position feedback and reference detection are related, but they are not the same signal.
| Observed symptom | Most likely area to test |
|---|---|
| Position changes normally, but the table never stops searching | Reference-mark channel, cable conductor, connector pin, or reference input |
| Position is frozen, jumps, or moves in the wrong direction | Main feedback channels, transducer supply, common, cable, or connector |
| Homing succeeds when the cable or connector position changes | Intermittent conductor, loose contact, strain damage, or connector contamination |
| The control detects reference but completes home at the wrong position | Reference sequence, offset, axis configuration, coupling, or control processing |
| The axis cannot run normally after homing | Broader servo, drive, mechanical, or feedback fault rather than an isolated reference circuit |
If the position count remains smooth while the table rotates continuously, that is not proof that the complete transducer is healthy. It proves only that the channels used for position counting are reaching the control.
Follow the reference signal
An incremental rotary feedback device normally sends position pulses while the axis moves. A separate reference mark establishes an absolute machine datum after power-up. The control searches until it sees that mark, then executes the configured reference sequence and assigns the machine position.
The installation was suspected to use a once-per-revolution reference pulse. Confirm the actual mark arrangement from the transducer documentation or machine electrical drawings because a feedback device may provide one or more marks. The decisive question is whether the expected reference transition reaches the control during rotation.
Older comparable rotary-axis systems used Heidenhain feedback and passed the scale reference signal to an R/M board. Treat that as a diagnostic pattern, not identification of the installed hardware. Read the transducer nameplate and trace the machine drawings before selecting pins or ordering a device.
A single broken reference conductor explains the apparent contradiction: normal position counting during operation and endless rotation during the reference search. The control never receives the event that tells it to leave search mode.
Test the feedback path first
Record the displayed rotary position while commanding the approved home sequence. Stop if the value is frozen, erratic, or inconsistent with physical direction; diagnose the complete feedback path instead of only the reference conductor.
Open the electrical drawings and identify the rotary transducer, intermediate connectors, cable junctions, and destination input. Use the documented pinout. Do not infer pin assignments from another table or encoder family.
Remove power using the machine service procedure. Disconnect the feedback cable at both ends so connected electronics and parallel circuits cannot create a false continuity reading.
Measure every documented conductor end to end. Compare the reference conductor with the position channels, supply, and common rather than accepting a continuity buzzer alone.
Check each conductor for shorts to adjacent conductors, shield, and protective ground where the machine documentation permits that test. Do not apply an insulation tester to a connected transducer or control board.
Flex the cable gently at the table entry, strain reliefs, drag-chain transitions, connector backshells, and other moving points while watching the meter. A conductor can pass a stationary check and open only when the rotary axis bends or twists the cable.
Inspect both connector ends for recessed pins, loose sockets, contamination, damaged threads, broken backshell bonding, and strain transferred directly into the contacts.
If cable tests pass, reconnect the circuit and observe the documented reference diagnostic at the control input or
R/Mboard, if fitted. Rotate the table through the reference region using the approved service method and confirm that the input changes state.
Divide the circuit at an accessible connector when the signal is absent. A reference event present near the transducer but absent at the control isolates the harness or intermediate connection. No event at the transducer output moves the diagnosis toward the transducer, its supply, its coupling, or the sensing target.
Repair the isolated fault
Repair a failed conductor with machine-tool cable and termination methods suitable for the axis movement and installed environment. Preserve the documented shield termination, conductor pairing, connector pinout, bend radius, and strain relief. A stationary patch that pulls on the connector will return as an intermittent homing fault.
Replace or reterminate damaged contacts when a pin cannot maintain stable electrical contact. Clean only with materials approved for the connector and feedback device. After repair, secure the cable so table motion cannot crush, stretch, or twist it outside its intended path.
Replace the rotary transducer only after the cable, connectors, supply, mechanical coupling, and control input pass their checks and the reference signal is still absent at the device output. Do not change home offsets or axis parameters to compensate for a missing electrical reference event.
Verify the complete home sequence
Reconnect all feedback components and restore the original cable routing, shields, backshells, and strain reliefs.
Start the machine with the table at a permitted position and command the normal reference cycle. Confirm that the displayed position changes smoothly and that continuous search rotation ends when the control processes the reference event.
Repeat the power-up and home test from different permitted rotary starting positions. The table must find the same machine datum rather than succeed only when the cable rests in one position.
Observe the reference diagnostic during the test. It must produce the transition pattern specified for the installed transducer and control.
Run the rotary axis through its permitted operating range and watch for position jumps, feedback alarms, or reference-input flicker. Reinspect any moving cable section that reacts to table position.
Confirm that five-axis operation remains correct and that the rotary machine position agrees with the machine's established datum-check procedure.
Avoid the recurring diagnostic traps
Do not replace the transducer because the table rotates continuously. First prove where the reference signal disappears.
Do not treat a changing position display as proof that the reference channel works. Incremental count channels can remain healthy after the reference conductor opens.
Do not test only at rest. Moving-axis cables commonly fail at flex points and pass a static continuity check.
Do not measure continuity through connected boards or feedback electronics. Parallel paths can hide an open conductor and test voltage can damage electronics.
Do not identify the installed transducer as Heidenhain solely because older comparable machines used that hardware. Match the nameplate, drawings, connector, and pinout.
Do not alter offsets, reference direction, or control parameters until the reference input has been electrically verified. Parameter changes cannot restore a missing pulse.
FAQ
Why does the DMU 80T rotary table keep turning during home?
The control remains in reference-search mode when it does not receive the rotary reference event. Start with the dedicated reference conductor, connectors, and control input, especially when the displayed position changes normally.
Why can the table position count correctly with a broken feedback wire?
The position-count channels and reference-mark channel perform different jobs. A break limited to the reference channel can leave normal incremental position updates intact while preventing the power-up home sequence from completing.
When should I stop troubleshooting and contact official support?
Stop when the pinout or service method cannot be identified, the test requires operating near hazardous motion, or the reference signal reaches the documented control input but the sequence still does not advance. Contact the machine manufacturer's official support channel with the electrical measurements, connector locations, transducer identification, diagnostic-input state, and observed home sequence; request control-board or transducer evaluation before authorizing replacement.