An unexpected power loss or CNC alarm can interrupt a rotary axis while it is positioning, clamping, or machining. After the machine restarts, operators may find that the indexing table no longer appears to be at the expected angle.
However, an unexpected position reading does not always mean that the table physically rotated. The deviation may be related to mechanical movement, an unconfirmed reference position, an inappropriate coordinate state, or movement of the fixture or workpiece.
At TJR Precision Technology Co., LTD., we recommend identifying the source of the deviation before resetting the rotary axis or resuming machining. A systematic inspection helps protect the workpiece, cutting tool, fixture, indexing table, and machine tool.
Quick Answer
An indexing table may lose its expected position after a power loss or CNC alarm if servo motor holding torque is removed and an external load causes the axis to move.
Whether this occurs depends on the table design, installation orientation, workpiece center of gravity, load inertia, motor brake configuration, clamping state, and CNC feedback system. Operators should verify both the physical table position and CNC coordinate state before restarting production.
1. Can an Indexing Table Lose Position After a Power Loss or CNC Alarm?
Yes. Under certain operating conditions, an indexing table may move away from its expected angular position after a power loss or CNC alarm. However, an unexpected display value does not necessarily mean that the table physically rotated.
The outcome depends on the complete rotary-axis configuration, including the table model, transmission type, mounting direction, load condition, brake configuration, encoder system, and CNC recovery procedure.
For this reason, a position discrepancy should not immediately be treated as a defective rotary table. The first step is to determine whether mechanical movement actually occurred or whether the CNC system no longer recognizes the expected position correctly.
Key Takeaways
- Position loss is possible, but it does not occur after every interruption.
- An incorrect displayed angle does not always indicate physical table movement.
- Vertical or tilted installations may be more sensitive to gravity and eccentric loading.
- The rotary table and CNC control should be evaluated as one integrated system.
- Machining should not resume until the position has been verified.
2. Did the Table Actually Move—or Is It a Coordinate Problem?
When the displayed angle differs from the expected machining position, operators should separate mechanical movement from coordinate, feedback, and setup-related conditions.
| Possible condition | What it means | What to verify |
|---|---|---|
| Physical axis movement | The table rotated after servo motor holding torque or braking support was removed. | Table marks, fixture orientation, actual angle, and load direction |
| Reference position not confirmed | The controller has not correctly re-established or confirmed the rotary-axis reference. | Homing status, absolute encoder status, and machine manual |
| Coordinate state is inappropriate | The active work offset or rotary coordinate state is unsuitable for the intended restart. | Active offset, program origin, rotary-axis command, and restart position |
| Fixture or workpiece movement | The rotary table remained in position, but the fixture or workpiece shifted. | Fasteners, workholding force, locating surfaces, and movement marks |
A work offset does not normally change simply because power is lost. Nevertheless, after an alarm recovery or interrupted program, operators should confirm that the active offset, program state, and rotary-axis coordinate remain appropriate for the intended restart point.
Reference-return requirements depend on the controller, encoder type, machine configuration, and recovery procedure. Some systems retain absolute position, while others may require reference confirmation. Follow the instructions supplied by the machine builder and rotary-table manufacturer instead of applying a universal reset procedure.
Key Takeaways
- Determine whether the deviation is mechanical or coordinate-related.
- Confirm the actual table angle instead of relying only on the CNC display.
- Check the fixture and workpiece before adjusting rotary-axis parameters.
- Follow the model-specific reference confirmation or return procedure.
3. Why Can a Rotary Axis Shift When Holding Torque Is Lost?
During normal operation, the servo motor produces torque to control and maintain the commanded axis position. If power is interrupted or the servo is disabled by an alarm, the servo motor can no longer provide active holding torque.
An external moment may then act on the rotary axis. Common sources include:
- A workpiece whose center of gravity is offset from the rotational center
- A fixture mounted eccentrically on the faceplate
- Gravity acting on a vertically installed or tilted rotary axis
- Residual cutting force at the moment of interruption
- High workpiece inertia during an emergency stop or sudden deceleration
- A motor brake or table-clamping mechanism that was not fully engaged when the interruption occurred
The result depends on the rotary table’s mechanical transmission and holding configuration. A brake motor or motor holding brake generally helps restrain the motor or axis when servo power is removed. By contrast, a pneumatic or hydraulic table-clamping mechanism is normally used to secure the table against machining torque after positioning.
These mechanisms are not interchangeable. Their engagement timing, rated holding or clamping capacity, permissible operating conditions, and control logic depend on the product and machine configuration.
A table-clamping mechanism is not a substitute for correct load calculation and balancing. The machine builder or system integrator should confirm that the workpiece load, inertia, mounting orientation, center of gravity, and control sequence remain within specified operating limits.
Key Takeaways
- Loss of servo motor holding torque may allow an external load to rotate the axis.
- Eccentric loads create torque even when the table is stationary.
- A motor holding brake and a table-clamping system perform different functions.
- Brake performance depends on its rated capacity and actual application conditions.
- Load inertia and center of gravity should be evaluated during system integration.
4. Which TJR Product Notes Should Operators Check?
TJR offers several rotary-table technologies:
- Worm Gear Rotary Tables
- Roller Gear Cam Rotary Tables
- Torque Motor Rotary Tables
- Hirth Coupling Index Tables
These products do not have identical transmission, positioning, clamping, or holding characteristics.
For selected Roller Gear Cam and direct-drive torque motor configurations, TJR product information notes that the rotary table may lose position after a power loss or alarm, potentially causing an axis shift during restart. A brake motor may therefore be required for the relevant application, and the motor-cover dimensions may change accordingly.
This warning should not be generalized to every TJR indexing table. The appropriate product and brake configuration must be determined using the exact table model, mounting orientation, motor specification, workpiece load, center of gravity, and CNC integration method.
Before selecting a product or modifying an installation, confirm:
- Complete rotary-table model and serial number
- Horizontal, vertical, tilted, or trunnion installation
- Workpiece and fixture weight
- Distance between the load’s center of gravity and rotational center
- Required indexing speed and acceleration
- Motor brake or holding-brake configuration
- CNC controller, servo interface, and table-clamping method
Key Takeaways
- The power-loss warning applies to specific configurations rather than every indexing table.
- Similar-looking tables may use different transmission and holding mechanisms.
- Confirm the exact model and serial number before making changes.
- Evaluate brake requirements using actual load and installation data.
5. What Should Be Checked Before Machining Restarts?
Restarting immediately after clearing an alarm can damage the workpiece, cutting tool, fixture, rotary indexing table, or machine. Operators should complete a controlled inspection before automatic operation resumes.
Recommended Restart Checklist
- Keep the machine in a safe condition. Stop automatic operation and ensure that no person is inside the hazardous movement area.
- Record the alarm information. Save the alarm number, complete message, displayed axis position, program block, and operating condition before clearing the alarm.
- Inspect the actual table position. Compare the faceplate, fixture, and workpiece orientation with known reference marks or a verified setup position.
- Check the workholding system. Inspect fixture bolts, locating components, workpiece clamps, hydraulic or pneumatic pressure, and visible evidence of movement.
- Confirm feedback and reference status. Verify the encoder state, reference status, rotary-axis display, active work offset, and controller recovery condition according to the applicable manuals.
- Verify the axis at reduced speed. Following the machine builder’s procedure, use an appropriate manual, single-block, or dry-run mode without cutting.
- Reconfirm the machining reference. Verify the rotary datum, critical coordinates, tool clearance, and intended restart block before returning to automatic machining.
Do not alter servo parameters, encoder settings, zero-return data, or mechanical alignment simply to make the displayed position match the expected value. An incorrect reset can hide the original problem and create a more serious collision risk.
If the source of the deviation cannot be confirmed, contact TJR, the machine builder, or the system integrator before restarting production.
Key Takeaways
- Save alarm and position data before clearing the system.
- Verify physical position, feedback position, fixture condition, and work offset separately.
- Begin verification at reduced speed and without cutting.
- Do not change parameters without model-specific instructions.
- Stop troubleshooting if safe axis movement cannot be guaranteed.
6. What Information Should You Send to TJR?
Accurate application information helps us review whether the issue may be associated with the rotary-table configuration, installation, load condition, motor brake, clamping sequence, or CNC integration.
When contacting TJR, please prepare:
- Rotary-table model and serial number
- Machine-tool manufacturer and model
- CNC controller and servo-system information
- Alarm number and complete alarm message
- Displayed rotary-axis position before and after the event
- Installation orientation, fixture weight, and workpiece weight
- Approximate center-of-gravity position
- Photos of the table, motor, fixture, and installation
- A video of the abnormal movement, if it can be recorded safely
- Recent parameter, wiring, fixture, or program changes
- Whether the same condition can be reproduced
TJR can help review product specifications, rotary-table configuration, application conditions, and available product documentation. CNC alarms, machine interlocks, ladder logic, servo parameters, and machine-specific recovery procedures may also require assistance from the machine builder or system integrator.
A correct diagnosis may require verification of both the rotary-table configuration and the CNC machine’s control and feedback conditions.
Key Takeaways
- Always provide the exact model and serial number.
- Alarm records and before-and-after position data are especially useful.
- Photos and videos can help distinguish mechanical movement from a control issue.
- Machine-specific controls may need to be checked by the machine builder or integrator.
7. FAQ: Indexing Table Position Loss
Can every indexing table lose position after a power failure?
No. The result depends on the table design, installation direction, external load, center of gravity, motor brake, clamping state, encoder system, and CNC configuration. Some axes remain stationary, while others may move if the external torque exceeds the available holding capacity.
Does an incorrect displayed angle prove that the table physically moved?
No. The difference may result from physical movement, an unconfirmed reference position, an encoder or feedback condition, an inappropriate coordinate state, or movement of the fixture or workpiece. Check both the actual table angle and CNC position.
Must the rotary axis always return to reference after power is restored?
Not necessarily. The correct procedure depends on the controller, encoder type, machine configuration, and recovery status. Follow the rotary-table manual and machine builder’s instructions. Do not initiate axis movement if safe clearance and position cannot be confirmed.
Is a table clamp the same as a motor holding brake?
No. A motor holding brake helps restrain the motor or axis when servo power is removed. A pneumatic or hydraulic table clamp is generally used to resist machining torque after positioning. Their functions and control sequences depend on the specific system configuration.
Can machining restart after the CNC alarm is cleared?
No. Clearing the alarm does not confirm that the rotary axis, fixture, and coordinate state are correct. Complete the required position, workholding, reference, and safety checks before resuming automatic machining.
Restore Position with Facts, Not Assumptions
A power loss or CNC alarm does not automatically mean that an indexing table is damaged. It does mean that the rotary-axis position, coordinate state, fixture condition, and holding configuration should be verified before machining continues.
If your rotary indexing table shows an unexpected angular position, record the alarm and axis information before resetting the system. Send the model, serial number, installation details, load data, and available photos to our team for review.
Contact TJR for Application Support