Siemens SINUMERIK Axis Faults: What the CNC Operator Can Check

When an axis or spindle alarm occurs on a CNC machine with a Siemens SINUMERIK control, the operator’s first job is not to change parameters.

The operator should first check:

• the alarm number,
• which axis triggered the alarm,
• the program block where execution stopped,
• whether the axis is moving at all,
• the reference (homing) status,
• the condition of the tool and the workpiece.

The diagnostic screens available on SINUMERIK 840D, 840Di and 810D controls can display the commanded and actual positions of each axis, the following error, the actual speed and the reference status.

1. Record the Alarm Number and the Axis Name

When an alarm appears, read the information on the screen before pressing RESET.

Note down the following:

• Alarm number
• Alarm text
• The axis involved: X, Y, Z, C or spindle
• Program block number
• Operating mode
• Which movement the program stopped in

For example, if the alarm only occurs on the Z axis, the problem may be related to the Z axis rather than the whole machine.

If an alarm history or alarm log screen is available, the time the alarm occurred can also be checked. Siemens documentation states that active alarms can be viewed in the Diagnosis area and past alarms in the alarm log section.

2. Check the Tool and the Workpiece

When an axis or contour alarm occurs, the first physical checks should be:

• Is the tool broken?
• Is the tool worn or dull?
• Is there chip jamming?
• Has the tool collided with the workpiece or the fixture?
• Has the workpiece shifted in the clamping?
• Is there an obstacle in front of the table or a moving element?

Sudden load increases during cutting can make axis movement more difficult.

However, do not conclude that “the tool must be dull” from the following error alone. The tool, the mechanical motion and the program conditions must be evaluated together.

3. Check the Axis Reference Status

After the machine is switched on, the axes may need to be referenced (homed).

If an axis is not referenced:

• the automatic program may not start,
• the machine coordinate display may be incorrect,
• software limits may not be active,
• some compensations may not work.

If a Referenced indicator is available on the Axis Service screen or the machine status display, check the reference status of the axis.

Siemens documentation states that some compensation and monitoring functions only become active after the reference point has been approached.

If the reference is lost, apply the machine’s standard referencing procedure.

4. Does the Axis Move?

Try moving the axis in JOG mode, in a safe direction and at low speed.

Check:

• Does the axis receive a command but not move?
• Does the axis fail to move in one direction only?
• Is there noise or vibration during movement?
• Is the axis movement jerky or intermittent?
• Does the speed fluctuate during movement?

If the axis receives a command but does not move, possible causes include:

• The servo enable signal is not active.
• The drive is not ready.
• The mechanical brake has not released.
• The axis may be at a limit.
• An alarm may be blocking axis movement.

Siemens documentation recommends checking the servo enable and controller mode information when an axis receives a motion command but does not move.

If the axis still does not move after these checks, call the service technician.

5. Compare the Position Setpoint with the Actual Position

On some SINUMERIK controls, the Diagnosis or Service Axis screen can display the following values:

• Position setpoint
• Actual position
• Following error
• Actual speed
• Speed setpoint

The position setpoint is the position the control wants to move the axis to.

The actual position is the real position reported by the encoder or measuring system.

According to the documentation, the actual position shows the real measured value in the machine coordinate system, while the position setpoint shows the target value sent from the interpolator to the position controller.

Simple interpretation

If the setpoint changes but the actual position does not, the axis is receiving commands but is not moving, or no measurement feedback is being received.

If the setpoint and the actual position both change but the difference between them keeps growing, the axis may be struggling to follow the commanded motion.

The operator should only observe these values and must not change servo parameters.

6. Following Error

The following error is the difference between the position requested by the control and the measured actual position of the axis.

According to the Siemens definition, the following error is the difference between the position setpoint and the actual position of the active measuring system.

There is no single “normal” following error value that applies to every machine.

The value can vary with:

• Axis speed
• Acceleration
• Servo tuning
• Machine structure
• Workpiece load
• Feedforward status

The operator should not attempt to change the numerical value.

What should the operator look at?

• Does the value only rise during cutting?
• Is it also high during air moves (no cutting)?
• Does it only increase during direction reversals?
• Does it occur on one specific axis?
• Does the behaviour change when the feed override is reduced?

This information should be passed to the service technician for a correct diagnosis.

7. Check the Actual Speed and the Speed Setpoint

The Service Axis or spindle screen may show the following information:

• Speed setpoint
• Actual speed
• Spindle speed
• Override

The speed setpoint is the speed demand sent to the drive.

The actual speed is the real speed measured by the encoder.

According to the documentation, the actual speed shows the real speed measured by the encoder, while the speed setpoint shows the speed demand sent to the drive.

What should the operator observe?

• Does the spindle speed rise when an S1000 command is given?
• Does the speed remain stable?
• Does the actual speed fluctuate continuously?
• Does the spindle fail to rotate despite the command?
• Has the override been left at a very low value by mistake?

If the spindle override or feed override has been left low, the machine may run slower than expected.

8. Check the Override Setting

The feedrate override and spindle override switches reduce or increase the programmed speeds as a percentage.

Example:

Programmed feedrate: F1000
Feed override: 50%
Effective feedrate: approximately 500 mm/min

The Override line on the Service screen may show the active percentage.

When diagnosing a problem, record the override percentage together with the programmed F value.

9. Be Careful with the PRESET Function

On some SINUMERIK systems, PRESET is not a simple operation that only changes the displayed value.

According to the documentation, software limits become active after the reference point has been approached and can be deactivated after a PRESET operation.

For this reason, after a PRESET operation do not start an automatic program before checking:

• the reference status,
• the machine coordinate,
• the software limits,
• the absolute encoder status.

If the machine builder’s procedure is not known, do not use PRESET.

10. Settings the Operator Must Not Change

The operator must not change the following settings without service approval:

• Servo gain
• Following error tolerance
• Contour tolerance
• Encoder parameters
• Backlash compensation
• Temperature compensation
• Sag (deflection) compensation
• Feedforward machine data settings
• Motor and drive parameters

Increasing tolerance values just to clear an alarm can hide the real problem.

When Should the Service Technician Be Called?

In the following situations, stop the machine and call the service technician:

• The axis struggles even during air moves
• The axis receives commands but does not move
• The encoder value changes while the axis is stationary
• The following error keeps rising even without cutting
• The axis knocks during direction reversals
• A servo or drive alarm keeps repeating
• The reference is repeatedly lost
• The spindle cannot reach the commanded speed
• The same alarm reappears after RESET

Information the Operator Should Give the Service Technician

Before calling the service technician, prepare the following information:

• Machine make and model
• Control unit model
• Alarm number
• Alarm text
• The axis that triggered the alarm
• Program block number
• Programmed feedrate
• Feed override percentage
• Spindle speed
• Whether the problem occurs during cutting or during air moves
• Tool and workpiece condition
• The direction in which the problem occurs
• A photo of the screen, if possible

This information helps the fault to be diagnosed much faster.

Disclaimer

This content was prepared to help operators understand basic observation and first-check procedures. Servo, drive, encoder, machine data and compensation settings must only be adjusted by authorized service personnel. The same codes and functions may behave differently on different control versions and on machines from different builders; the machine builder’s manual always takes priority.