After the fault is corrected, the magazine completes repeated cold and hot tool changes without Error 66, and its position-confirmation signal remains stable at every stop. On a Mori MV Junior with a Fanuc 10M, an intermittent magazine index position error means the commanded motion and the controller's position confirmation failed to agree within the machine's allowed sequence. A carousel that looks correctly aligned can still fail electrically, stop marginally, or settle too slowly.
Current, heat, and stopping margin
The number that matters is the stopping margin at the end of each index. The index motor must overcome carousel drag, then the brake must remove the moving assembly's energy quickly enough for the position sensor and control sequence to agree. Excess mechanical resistance raises motor current and winding temperature. Weak braking permits coast or an unstable final position. Either condition can consume the timing margin even when the pot appears aligned afterward.
The reported pattern is temperature-sensitive: the machine ran for about three hours, began faulting after several tool changes, and worked again after cooling. This is heat, not logic. Temperature can increase coil resistance, expose a marginal contactor connection, change a rectifier fault under load, or increase resistance in a damaged bearing or tight tool-retention interface.
| Quantity or state | Observed value | Where to read it | Diagnostic meaning |
|---|---|---|---|
| Alarm | Error 66 magazine index position error |
CNC alarm display | The index sequence did not receive an acceptable position result. |
| Rectifier AC input | About 52 VAC | Rectifier input terminals | Records transformer-side voltage during the fault cycle. |
| Brake DC output | 45-48 VDC | Rectifier output or brake circuit test points | Must be compared cold versus hot and energized versus released. |
| Proposed brake rating | 90 VDC | Machine electrical drawing and brake nameplate | Treat this as a value to verify, not an adjustment target. |
| Temperature dependence | Fault after warm operation; clears after cooling | Timed tool-change test log | Prioritizes loaded voltage, contacts, coils, bearings, and mechanical drag. |
Competing fault paths
Three approaches fit the symptom. Start with the feedback and brake measurements because they do not require recalibration or alteration of tooling. Inspect mechanical drag in the same test window so an electrical symptom caused by excess load is not misclassified.
| Fault path | Expected indication | Deciding check | Corrective direction |
|---|---|---|---|
| Position feedback | Magazine stops correctly, but the CNC does not see the expected position | Monitor the encoder or pot-count/home sensor state through one complete index | Correct sensor gap, alignment, wiring, connector, or failed feedback device |
| Brake power | Carousel coasts, settles inconsistently, or fails more often hot | Measure AC input and DC output during a successful and failed index | Repair the contactor, rectifier, wiring, transformer circuit, or brake only after isolating the voltage loss |
| Mechanical drag | Index load rises, tools bind in pots, or guide motion is rough | Inspect unloaded motion, roller guide bearings, pots, and tool interfaces | Replace damaged bearings or nonconforming retention components and correct interference |
| Calibration | Feedback switches repeat, brake performance is stable, but stopping geometry is consistently wrong | Compare physical position with the documented calibration points | Perform the machine-specific calibration procedure |
Position-confirmation chain
Determine which feedback architecture is installed before adjusting anything. The magazine may use an encoder, or it may use a home flag at pot 1 plus a proximity switch that counts passing pots. These systems fail differently: an encoder can lose counts or produce an invalid position, while a home-and-count system can miss one transition and retain an incorrect logical pot number.
Watch the applicable diagnostic input while jogging or indexing under the machine's approved service procedure. A proximity signal should change cleanly at every flag, and the home input should switch only at its reference feature. An encoder position should advance without discontinuities and settle at the commanded station. Check connectors, cable flex points, sensor mounting, target damage, contamination, and supply stability if the diagnostic bit flickers or fails to repeat.
Visual alignment is not a substitute for this test. The magazine can coast into an apparently correct location after the control has already evaluated the confirmation input, or a sensor can remain electrically false while the mechanism sits on position.
Brake-supply diagnosis
The measured 52 VAC input and 45-48 VDC output are a useful baseline, but they do not by themselves identify a bad rectifier. The DC reading depends on rectifier topology, load, brake-coil condition, switching state, and meter response. Establish the required voltage from the electrical drawing and brake nameplate before comparing the circuit with the proposed 90 VDC value.
- Identify the brake coil, rectifier, contactor, transformer feed, and associated terminals from the machine drawing. Apply the machine builder's electrical safety and stored-energy procedure before touching conductors.
- Record AC input and DC output when cold during a successful tool change. Note whether the circuit applies voltage to set the brake or to release it; the schematic decides the expected state.
- Repeat the measurements after the machine reaches the condition that produces
Error 66. Capture voltage during the index and at the commanded stop, not only while the machine is idle. - Measure across each closed contact and connection under load. A substantial voltage appearing across a supposedly closed contact identifies resistance at that contact or terminal.
- If AC input stays stable while DC output collapses hot, isolate the rectifier, brake coil, and DC wiring. If both fall, work upstream through the contactor, transformer circuit, terminals, and control command.
- Inspect for heat discoloration, loose terminations, damaged insulation, contact chatter, and abnormal coil temperature. Replace the failed component with the specified type and rating.
Mechanical load and tooling
A tight tool-to-pot interface can add intermittent load and disturb the changer sequence. Some pull studs in this installation had a sharp edge at a 90-degree feature and were difficult to pull from the pot by hand; versions with a small chamfer released easily. The installation also experienced dropped tools, making this a retention-safety issue as well as a positioning issue.
Quarantine any holder that binds, shows damaged retention geometry, or has been associated with a dropped tool. Compare its pull stud and release behavior with a known-good assembly, then verify dimensions against the machine and tooling specifications. Do not hand-file or otherwise alter a pull stud used for tool retention without an approved rework specification; changing its geometry can compromise retention strength or engagement.
Inspect the carousel's roller guide bearings for roughness, play, flat spots, or damage. Bearing replacement has been required on the same class of changer after abnormal handling. Correct mechanical faults before changing circuit-board potentiometers: calibration can mask drag or weak braking temporarily while reducing margin elsewhere in the cycle.
Controlled repair and verification
- Record the current pot, commanded pot, alarm text, machine temperature, elapsed run time, installed holder, and whether the magazine visibly coasted or hesitated.
- Establish the feedback type and verify every count or encoder transition through a full magazine revolution.
- Run an empty or approved low-risk test sequence and compare mechanical motion with loaded operation. Stop if a holder binds or retention is uncertain.
- Trend the brake circuit's AC input and DC output cold and hot. Correlate each voltage trace with the index command, stop command, confirmation input, and alarm occurrence.
- Repair the isolated electrical or mechanical defect. Use calibration only when the feedback, brake, bearings, and tooling interfaces are repeatable and the documented stop position remains incorrect.
- Repeat enough tool changes to cover both the cold state and the elapsed operating period that previously produced the fault. Test all pot positions, because a damaged flag, pot, or guide segment can make the problem station-dependent.
- Pass the repair only when
Error 66remains absent, feedback changes cleanly, the magazine stops without visible coast or rebound, measured voltages stay within the documented limits, and every holder releases normally.
Frequently asked questions
How do I troubleshoot Mori MV Junior Error 66?
Monitor the magazine position input while recording brake-circuit voltage during cold and hot tool changes. Then inspect bearings, pots, and holder retention for drag before considering calibration.
How do I know whether the magazine sensor caused Error 66?
Identify whether the changer uses an encoder or a pot-1 home flag with a counting proximity switch. A missing, flickering, discontinuous, or nonrepeatable diagnostic signal during indexing points to the sensor, target, wiring, connector, or supply.
How do I evaluate the 45-48 VDC brake reading?
Compare the measured 45-48 VDC and approximately 52 VAC input cold versus hot and under the same switching state. Read the required coil voltage and circuit function from the machine schematic and brake nameplate before treating 90 VDC as the correct target.
When should I stop troubleshooting Error 66 and call support?
Stop if a tool drops, a holder binds in a pot, the carousel can move unexpectedly, or safe energized measurements are not possible. Escalate to the machine builder's official support channel when the schematic, brake rating, feedback calibration method, or circuit-board potentiometer procedure is unavailable. Provide the cold and hot voltage records, diagnostic-input trace, affected pot numbers, and tool-change history.