FANUC SV0740 / SP0740 Rigid Tap Alarm: Excess Error and Troubleshooting

FANUC SV0740, SP0740, or alarm 740 means that the spindle position deviation exceeded the permitted stop-state value during rigid tapping. Depending on the CNC generation and alarm-prefix convention, the display may use SV0740, SP0740, or simply 740 RIGID TAP ALARM: EXCESS ERROR.

FANUC alarm documentation distinguishes alarm 740 from the adjacent alarm 741. Alarm 740 is generated when the positional deviation of the stopped spindle exceeds its limit during rigid tapping. Alarm 741 is associated with excessive spindle deviation during movement or, on applicable systems, excessive rigid-tapping synchronization error.

For the surrounding alarm numbers, see the FANUC SP Alarm Codes: Rigid Tapping, Spindle Position and Deviation Alarms.

What Does FANUC SV0740 / SP0740 Mean?

During rigid tapping, the CNC controls spindle rotation and tapping-axis movement as a coordinated position relationship. The spindle position-deviation register represents the difference between commanded spindle position and detected spindle position. When the spindle is commanded to stop—for example near a reversal or at the completion of a controlled phase—the residual deviation must fall within the configured limit.

Alarm 740 indicates that this stop-state spindle deviation was too large. The alarm does not, by itself, prove that the spindle amplifier, motor, or position coder has failed. An unsuitable deviation limit, aggressive rigid-tapping adjustment, incorrect feedback or gear-ratio data, mechanical backlash, excessive load, a slipping transmission, or unstable spindle response can all contribute to excessive error.

The supplied Chinese summary states that the error occurs while the spindle is moving. In the FANUC alarm lists cited below, that description belongs to alarm 741, not alarm 740. Always diagnose the number and operating state shown by the actual CNC. This distinction determines which position-deviation parameter should be inspected.

Do Not Confuse Parameters 5310, 5312, 5311, and 5313

The four adjacent rigid-tapping limits monitor different objects and states. They are not interchangeable.

Therefore, increasing P5310/P5350 or P5312/P5352 is not the normal correction for a confirmed alarm 740 caused by spindle deviation at stop. Those parameter pairs apply to the tapping axis on the referenced controls. If the actual display is 741 or an axis-side SV0410/SV0411 alarm, the correct diagnostic branch is different.

Do not edit any of these parameters until the CNC series, software, selected spindle, parameter data type, and alarm timing have been identified.

First-Spindle and Second-Spindle Parameter Differences

FANUC parameter allocation changes across control generations. This is particularly important on machines with two or more spindles.

On classic Series 21i/210i configurations covered by parameter manual B-63610EN/01:

• Parameter 5313 sets the spindle stop-deviation limit during rigid tapping.

• When separate rigid-tapping parameter data for the second spindle is enabled by the applicable SPR setting, parameter 5353 sets the second-spindle stop-deviation limit.

• Parameter 5311 is the corresponding spindle movement limit, and 5351 is the separate second-spindle movement limit where enabled.

• Parameters 5310/5350 monitor the moving tapping axis, while 5312/5352 monitor the stopped tapping axis.

On the Series 0i-F Plus covered by B-64700EN/01, parameters 5311 and 5313 are defined as spindle-type data. The correct value is entered for the selected spindle in the spindle-data field. The same manual uses axis-type parameters 5310, 5350, 5354, and 5358 for tapping-axis movement with the first through fourth spindles, and 5312, 5352, 5356, and 5360 for tapping-axis stop limits.

This means that P5353 is not a universal second-spindle requirement. It is valid for certain older parameter structures, while newer controls may use spindle-type P5313 separately for each spindle. Use the parameter manual for the exact CNC instead of copying the parameter number or value from another machine.

1. Record the Alarm Timing and Active Spindle

Before resetting the CNC or enabling parameter write, record:

• The complete alarm text and prefix: SV0740, SP0740, or 740

• The CNC series, model, and software version

• The spindle amplifier and spindle motor models, including all suffixes

• The active path and selected spindle number

• The rigid-tapping M code and G84/G74 sequence used by the machine

• Spindle speed, thread pitch, feed command, tapping depth, gear range, and return setting

• Whether the alarm occurs on entry, at the hole bottom, during reversal, at the R point, or after cycle completion

• Any simultaneous spindle, servo, position-coder, overload, mode-switching, or machine alarms

• Any recent parameter restore, spindle-drive work, encoder replacement, belt adjustment, or mechanical repair

The cycle point is highly diagnostic. A true 740 alarm commonly appears when the spindle is expected to settle. An alarm generated while the spindle is rotating continuously should prompt confirmation that the actual number is 741 or another spindle/servo alarm.

2. Observe Spindle Deviation Before Widening the Limit

Use the rigid-tapping diagnosis data and waveform functions supported by the exact control to observe spindle position deviation, tapping-axis position deviation, synchronization error, speed command, and actual speed. Record the maximum values during entry, cutting, reversal, and extraction.

Compare the measured spindle stop deviation with the current effective limit. Determine whether the error:

• Remains high only briefly at reversal

• Oscillates instead of settling

• Increases sharply with spindle speed

• Appears only in one gear range

• Appears only with the second spindle

• Becomes excessive under cutting load but not during a safe no-load test

• Changed after parameter, drive, encoder, belt, or gearbox work

Parameter 5313 or its exact system-specific equivalent is an alarm threshold, not a tuning control. Increasing it permits a larger residual spindle error before the CNC stops. A small, justified increase may be appropriate when the original value is demonstrably inconsistent with the machine builder's verified data and measured normal deviation, but repeatedly enlarging the value can mask unstable control, feedback error, transmission slip, or mechanical resistance.

Back up all CNC and spindle data before any authorized change. Record the original value, new value, reason, and test result.

3. Confirm the Program, Pitch, Speed, and Gear Data

Rigid tapping depends on a correct relationship between spindle rotation and linear-axis feed. Verify:

• The programmed thread pitch or feed value

• Inch/metric mode and feed-per-revolution interpretation

• The commanded spindle speed and maximum rigid-tapping speed for the active gear

• The selected gear and the gear-selection signals

• Spindle-side and position-coder-side gear-tooth parameters

• Position-coder resolution and spindle-to-coder mechanical ratio

• Whether the postprocessor generated the correct G84/G74 cycle and return settings

Incorrect gear-ratio or detector data changes the calculated position relationship. Excessive speed or an unsuitable acceleration schedule can also leave the spindle unable to settle within the permitted stop error, especially at reversal.

If the problem occurs only above a certain S command, reduce speed temporarily for diagnosis. Do not treat reduced speed as the final repair until the source of the excessive deviation has been identified.

4. Inspect Spindle Feedback and Mechanical Transmission

The CNC cannot control spindle position correctly if feedback does not represent the actual spindle motion. With power safely isolated where required, inspect:

• Position coder, spindle sensor, or built-in motor sensor mounting

• Feedback connectors, cable condition, shielding, grounding, and contamination

• Coupling, belt, pulley, gear, and key condition

• Belt tension and evidence of slipping at reversal

• Backlash, looseness, or lost motion between motor, spindle, and position coder

• Unusual bearing drag, spindle resistance, or abnormal noise

• Changes made during recent spindle, encoder, belt, or gearbox repair

A loose encoder coupling or slipping belt may allow the spindle and feedback device to disagree most strongly during rapid deceleration and reversal. Excessive mechanical backlash can also create a transient position error that does not settle before the 740 check is made.

Do not replace the position coder solely because alarm 740 appears. Confirm abnormal feedback, physical damage, connector trouble, waveform irregularity, or a related detector alarm before condemning it.

5. Check Rigid-Tapping Position Loop Gain

On applicable 0i-F and 0i-F Plus controls, parameter 5280 sets a position control loop gain common to all gears. When 5280 is zero, parameters 5281 through 5284 provide gear-specific gains. FANUC states that these settings apply to both the spindle and tapping axis and significantly affect threading precision.

Gain that is too low can allow excessive lag and slow settling. Gain that is too high can cause hunting, vibration, overshoot, or unstable reversal. The correct value must suit the spindle system, tapping axis, detector resolution, mechanical transmission, and active gear.

Do not raise gain simply to force the error register toward zero. Observe the position-error, speed, and torque response, compare the current parameters with the machine builder's verified data, and conduct controlled tests. Correct mechanical looseness or feedback instability before attempting to tune around it.

Some older analog-spindle configurations also use rigid-tapping spindle loop-gain multipliers such as parameters 5291 through 5294, while separately parameterized second-spindle systems may use another gain group. These are configuration-specific and should not be copied into a serial-spindle procedure without the applicable manual.

6. Verify Rigid-Tapping Feed Forward

On the 0i-F Plus, bit RFF, parameter 5203#2, enables rigid-tapping feed forward. FANUC recommends the enabled state as the standard setting for an applicable serial spindle. The same manual requires the advanced-preview feed-forward coefficient for the tapping axis to match the corresponding spindle coefficient:

• Tapping-axis coefficient: parameter 2092, or 2144 when the cutting/rapid-traverse feed-forward function selects that parameter

• Spindle coefficient: parameter 4344

Feed forward can reduce command-following delay, but mismatched coefficients can undermine spindle/axis synchronization. Confirm the feature, parameter-selection bit, and data type before editing. These numbers are not universal for every FANUC generation, analog spindle, smart rigid-tapping option, or machine-builder implementation.

Do not use feed forward to conceal backlash, slipping transmission components, intermittent feedback, or an underperforming spindle drive.

7. Adjust the Rigid-Tapping Time Constant Carefully

Parameters 5261 through 5264 set the rigid-tapping acceleration/deceleration time constant for the active gear on applicable systems. The value applies to the spindle and tapping axis. A longer time constant makes acceleration and deceleration more gradual; a shorter value demands a faster response.

If the spindle cannot settle at reversal, an overly aggressive time constant may be contributing to the residual stop error. However, changing it also changes tapping-cycle motion and may affect cycle time, synchronization, torque demand, and thread quality.

When bit TDR in parameter 5201 enables separate extraction settings, parameters 5271 through 5274 set the extraction-side time constants. Verify whether the alarm occurs during cutting reversal or extraction before deciding which group is active.

Change one controlled item at a time, preserve the original values, and verify the result across every used speed and gear. Do not randomly change gain, feed forward, and time constants together; doing so removes the evidence needed to identify the real cause.

8. Check Load, Tooling, and Reversal Conditions

Excessive tapping torque can delay spindle response and increase deviation. Inspect:

• Tap condition, size, coating, and suitability for the material

• Correct drilled-hole diameter and depth

• Chip packing, bottoming, and insufficient chip clearance

• Coolant or lubrication delivery

• Collet, holder, and spindle-taper condition

• Workpiece material variation

• Spindle load and tapping-axis load near reversal

Compare a safe air-cut or non-cutting diagnostic cycle with the loaded cut where the machine builder permits it. If the alarm occurs only in material, correct the cutting condition or mechanical load before widening the stop-error limit.

Never repeat a tapping cycle blindly after the spindle has failed to reverse or settle. A damaged tap, workpiece, holder, or spindle can create a more serious collision on the next attempt.

9. Evaluate the Spindle Drive Only After External Causes

A spindle amplifier or motor problem can prevent accurate position response, but alarm 740 alone does not identify a failed module. Review the spindle amplifier display and all simultaneous alarms. Check whether the spindle can follow commanded speed and position in other position-control functions, and compare current, torque, speed, and feedback behavior with known-good data.

Inspect the motor power circuit, amplifier connections, grounding, cooling, and motor condition using the applicable FANUC and machine-builder procedures. If a substitution or bench test is justified, use an exact compatible spindle amplifier identified by complete model and suffix, and preserve all spindle parameters.

REACO CNC can help test, repair, or select compatible FANUC servo, spindle, and power drives.

Difference Between Alarm 740 and Alarm 741

If the machine reports 741, do not troubleshoot it as a stop-state 740 alarm. If it reports an axis-side SV0410 or SV0411 during rigid tapping, identify the tapping-axis limit instead of changing the spindle limit.

FANUC SV0740 / SP0740 Diagnostic Table

Recommended Troubleshooting Order

1. Record the complete alarm text, prefix, selected spindle, gear, S command, pitch, cycle point, and simultaneous alarms.

2. Confirm whether the spindle was stopped or moving when the alarm was detected; distinguish 740 from 741 and from axis-side SV0410/SV0411.

3. Back up the CNC and spindle data and preserve rigid-tapping diagnostic or waveform evidence.

4. Identify the effective spindle stop-deviation parameter for the exact control: commonly 5313, or 5353 for an independently parameterized second spindle on applicable older systems.

5. Do not substitute tapping-axis parameters 5310/5350 or 5312/5352 for the spindle-side 740 limit.

6. Compare actual spindle stop deviation with the verified limit; widen the threshold only when the machine specification and measured normal behavior justify it.

7. Verify thread pitch, feed mode, spindle speed, active gear, maximum rigid-tapping speed, detector resolution, and spindle/position-coder gear-ratio data.

8. Inspect the position coder, feedback cable, coupling, belt, gears, backlash, spindle resistance, and any recent mechanical work.

9. Check tapping load, hole diameter, tap condition, chips, lubrication, holder, and reversal behavior.

10. Review rigid-tapping position gain: P5280 or gear-specific P5281–P5284 on applicable systems.

11. Verify rigid-tapping feed forward and matching spindle/tapping-axis coefficients only if supported by the installed control.

12. Review cutting and extraction time constants, including P5261–P5264 and, where enabled, P5271–P5274.

13. Change one parameter group at a time, document every change, and test first at a safe speed under controlled conditions.

14. Evaluate the spindle motor, feedback device, and amplifier only after the program, settings, load, and mechanical transmission have been checked.

15. Complete a machine-builder-approved rigid-tapping test across the required speeds, gears, pitches, and spindles before returning the machine to production.

Technical References

• FANUC Series 16i/18i/21i Operation and Maintenance Handbook, rigid-tapping alarms 740–742

• FANUC Series 21i/210i Parameter Manual, B-63610EN/01, parameters 5204, 5261–5294, 5310–5314, 5341–5353

• FANUC Series 0i-MODEL F Plus Parameter Manual, B-64700EN/01, parameters 5201, 5203, 5214, 5261–5284, 5300–5313, 5350–5360

• FANUC Series 30i/31i/32i-MODEL B Maintenance Manual, B-64485EN, SP0740 and SP0741 alarm descriptions

• Applicable FANUC spindle motor, spindle amplifier, parameter, connection, maintenance, and machine-builder manuals for the installed equipment

Always use the manual edition that applies to the exact CNC, software, spindle motor, amplifier, position detector, number of spindles, rigid-tapping option, and machine. Parameter changes, powered waveform tests, mechanical work, and drive replacement must be performed by qualified personnel under the machine builder's safety procedure.

Need Help With FANUC SV0740 / SP0740 Alarm?

REACO CNC provides independent FANUC alarm analysis, spindle amplifier testing and repair, position-feedback checks, parameter compatibility support, and replacement-drive supply. Send us the complete alarm screen, CNC model, spindle amplifier model and suffix, spindle motor model, selected spindle, current rigid-tapping parameter values, tapping program, and a description of exactly when the alarm occurs.

Visit the FANUC Technical Support Center for more FANUC alarm guides, or contact REACO CNC for assistance.

Reference Source: Beijing FANUC. This article is based on FANUC technical documentation. The applicable parameter, tuning, feedback, and verification procedure depends on the exact CNC, spindle system, software, rigid-tapping configuration, and machine.