Why Does G01 Feed Motion Stop on a Fanuc 18iTB Lathe?

A Spinner SB lathe with a Fanuc 18iTB control completed G0 rapid moves but stopped whenever it reached a G1 feed move. Changing the programmed feed, entering the move through MDI, and restarting the machine did not restore motion. The fault was a broken cable. The cable function was not identified, so prove which signal is missing before replacing or bypassing anything.

Stop repeating the quick fixes

Because the same symptom occurred in both the program and MDI, stop editing production code. Capture the active modal state and machine diagnostics before disturbing connectors.

Read the symptom as a missing feed dependency

G0 and G1 do not use the same command path. A rapid move uses the machine's rapid-motion limits and does not depend on the programmed F value. A linear feed move uses the active feed mode, feed command, override state, axis permissions, and any feedback required to calculate that feed.

This split matters. Successful rapid motion shows that the control can command the axis and that the axis can move under at least one operating condition. It does not prove that every feed-related input, feedback channel, interlock, or cable is healthy.

On a lathe, the spindle-feedback branch deserves early attention when feed per revolution is active. A spindle encoder may be mechanically driven directly or through a belt. A failed encoder drive or an open signal path can leave the control without the spindle information needed for synchronized feed calculation. The confirmed repair in this case was a cable fault, but identify the failed circuit from diagnostics and electrical tests rather than labeling it as an encoder cable without measurement.

Check the active feed mode before opening cabinets

G94 selects feed per minute, while G95 selects feed per spindle revolution. The numerical meaning of F therefore changes with the mode. Never insert either code into a production cycle until the intended feed units and resulting motion have been checked.

1. Record the active modal codes, commanded F value, spindle command, actual spindle indication, feed override, axis position, and every active alarm or machine message.
2. Confirm that feed hold is released and that the feed override is not at zero. Check builder-specific inhibit and interlock indications on the machine diagnostic screens.
3. Determine whether the failing block expects G94 or G95. Read the setup sheet and surrounding program blocks; do not infer the intended mode from the F number alone.
4. If G95 is active, compare commanded spindle operation with the control's actual spindle-speed or position indication. A running spindle with missing or implausible feedback points toward the encoder, its mechanical drive, connectors, cable, or input electronics.
5. If an approved test using G94 moves correctly while the equivalent controlled test using G95 waits, concentrate on spindle feedback. If neither mode moves, expand the check to feed override, interlocks, motion permission, command generation, and axis diagnostics.

Trace the cable fault to the failed circuit

Use the machine electrical drawings to follow the affected feedback or command path from device to control input. Do not assume the first damaged-looking cable is the failed one.

1. Place the machine in a safe state and isolate power as required before disconnecting plugs or testing conductors.
2. Inspect encoder couplings or drive belts, cable carriers, flex points, cabinet entries, connector backshells, strain reliefs, and areas exposed to coolant or chips.
3. Check that connectors are fully seated and that pins are not backed out, bent, contaminated, or loose. Inspect shielding and grounding terminations without changing their arrangement.
4. Disconnect the circuit only according to the electrical documentation. Test conductor continuity end to end and test for shorts between conductors and to the shield where the circuit design permits that measurement.
5. Flex the cable gently at normal bend points while observing the meter. A continuity value that changes with movement identifies an intermittent conductor even when a stationary test passes.
6. Correlate the electrical result with the control indication. A repaired conductor should restore a stable feedback or input value before any axis test begins.

Stop before applying an external voltage, insulation tester, or improvised jumper to an encoder or control input. Those actions can damage interface electronics and can defeat machine interlocks.

Repair the failed cable without hiding the defect

Replace the damaged cable or repair it using the machine builder's wiring specification, connector assignment, conductor type, shielding arrangement, and routing requirements. A temporary splice in a continuously flexing carrier is likely to fail again, particularly if the original bend radius, strain relief, or abrasion problem remains.

Before reconnecting the circuit, compare every conductor with the electrical drawing and check for unintended cross-connections. Secure the cable so axis, turret, spindle, door, and carrier movement cannot pull on the connector. Correct the mechanical cause—sharp edge, tight bend, failed carrier support, coolant entry, or missing clamp—rather than replacing only the conductor.

Get production moving only after the repair passes static and moving checks. Schedule permanent cable replacement if an approved temporary repair was required for recovery.

Prove the repair under controlled motion

1. Restore power and confirm that the repaired signal is present and stable in the control diagnostics. Check the spindle's actual indication if the failed circuit serves spindle feedback.
2. Verify that no new alarm, interlock, or implausible value appeared after reconnection.
3. Use an approved test program or MDI in single-block mode, with a conservative override and clear machine envelope. Test G0 separately from G1.
4. Test the feed mode required by the production program. If both G94 and G95 are used, validate each with feed values selected for that mode rather than reusing one value blindly.
5. Observe the cable through the relevant machine travel. Feedback and motion must remain stable at the flex points where the break was found.
6. Run the production cycle first without a workpiece if the process and machine procedures allow it. Confirm actual feed behavior before returning overrides to normal.

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