Measurement characteristics

In the equability axis test, an axis is moved at constant velocity completely over the defined measurement path.
The alternating components of the load-side force with respect to the axis positions are determined from the measured motor torque:
The alternating components of the load-side force with respect to the axis positions are determined from the measured motor torque. The characteristic is the maximum force during travel in both directions.

Causes of changes

If the maximum force increases, the following causes are possible:

• Insufficient lubrication of the axis

• Mechanical damage to the axis

• Jammed cover segments

• Chip contamination

If the force curve is substantially changed compared with the reference point approach, this could be due to the following:

• Alignment or parallelism errors on the axis (e.g. as a result of a collision)

Possible effects

If the maximum force or force characteristic deviates too much, the following effects are possible:

• Reduced workpiece quality: length and surface defects on the workpiece

• Damage to the machine if the causes are not rectified in good time

Calculation

The axis is traversed at constant velocity along the defined measuring range.

The friction test supplies three different measurement results:

• Friction dry (stiction)

• Friction viscous (speed-dependent friction components)

• Friction distribution

The friction distribution provides information on how the axial friction is distributed between the spindle nut or the guidance. In this way it is possible to estimate which component of the drive train is responsible for the increase in friction as total friction increases. If the increased friction results in increased compression between the measuring systems, then the friction on the guide side has increased. Otherwise the friction has increased near the motor - at the spindle nut or upstream gear.

Causes of changes

• Increase in friction distribution - spindle nut

  • Mechanical defect in spindle nut

  • Insufficient lubrication of spindle nut / ball screw

• Increase in friction distribution - guidance

  • Mechanical defect on the guide carriage running surface

  • Insufficient lubrication of the guide

  • Insufficient lubrication of the cover

• Increase in the friction viscous/friction dry parameters: Insufficient lubrication

Possible effects on the machine

• Positioning errors caused by overshoot

Stiffness describes the overall stiffness of the drive train between the two measuring systems and can also be understood as the axial stiffness of the entire drive train of an axis. It results from a series connection of the individual stiffness values of the components. In drive trains with ball screw, the axial stiffness of the screw nut is usually the point with the lowest stiffness. Overall stiffness decreases as the distance from the locating bearing increases since the stiffness of the ball screw (tension/compression and rotation) decreases with increasing length.

Causes of changes

The stiffness of the axes of a machine tool can decrease over time. Stiffness does not normally increase. Possible causes are:

• Diminishing ball screw pre-tension

• Wear on the ball bearings in the guide

• Damage to bearing

Possible effects

If the stiffness diminishes too sharply, the following effects are possible:

• Reduced workpiece quality: length and surface defects on the workpiece

• Fall in the lowest natural frequency of the drive train, which can lead to stability problems in the position control loop

Calculation

The axis is accelerated at various positions. The individual, discrete measuring points are then described with polynomial interpolation.

Backlash is a fault in positioning that occurs when the direction of force is reversed. It is caused by play and low levels of stiffness in the drive train. Backlash also affects bidirectional repeat accuracy.

Causes of changes

Backlash normally increases over time. Possible causes are:

• Wear in the guide grooves of the ball screw/spindle nut and thus increasing play

• Consequence of a collision: Ball bearings in the ball screw plastically deformed

Possible effects

If backlash increases too sharply, the following effects are possible:

• Positioning errors

• Surface defects (relief cutting during milling)

• Vibration of the machine during traversing movements with fast and frequent changes of direction

Calculation

Backlash is calculated from the difference between the two encoders after traversing with different μm-paths. Traversing is first in one direction at the measuring point and then in the opposite direction until the axis moves again. The backlash is determined in the middle between the upper and lower limits of the measuring range. For example, if a measuring range between 0 mm and 100 mm is specified, the backlash is determined at 50 mm.

The signature indicates periodic synchronous positioning errors due to location-dependent faults in the drive train. The signature is ascertained from the frequency ranges of several different constant speeds. If an order occurs in at least three different speeds, it can be excluded as a fault, e.g. due to an excited natural frequency. The order is adopted as a parameter.

The number of orders essentially indicates how often the motor must rotate until the periodic fault occurs again. Defective components in the drive train can be identified based on the signature. In such a case the comparison of the signature measurement with the reference measurement shows a new order, which was not present in the reference measurement.

Causes of changes

• Amplitude of one or more orders from the reference measurement increases

• New frequencies ("damage frequencies") suddenly become visible in comparison to the reference measurement

  • Bearing damage

  • Damage to the ball screw (e.g. plastic deformation in the guide grooves)

  • Loss of tension in a toothed belt

Possible effects

• Surface defects caused by vibrations

• Positioning errors

Calculation

The signature is determined by order analysis during the equability test of an axis at three different constant speeds. The signature of Analyze MyMachine /Condition is only evaluated as a real order if an order is measured at three different speeds.

A quadrant error can be regarded as a one-dimensional roundness test and refers to only one axis. A quadrant error occurs when the direction of the axis is reversed and is chiefly attributable to static friction effects of the axis.

Causes of changes

The quadrant error normally increases over time. Possible causes are:

• Change in the static friction of the drive train

• Incorrect setting of friction compensation

• Increased backlash

Possible effects

• Positioning errors

• Surface defects in mold making

Calculation

Quadrant errors are determined by a sine run and comparison with the ideal sine.

The standard measuring function of the SINUMERIK control calculates the mechanical behavior of the axis from the perspective of the drive in the frequency range. The result is displayed in the usual Bode diagram. The natural frequencies are determined automatically. The drive train is completely monitored by monitoring of high-frequency natural modes, such as couplings or belts whose effects are usually lost in the main stiffnesses.

Causes of changes

Possible causes are:

• Shifting natural frequencies due to wear in the drive train

• As a result of stiffness losses

Possible effects

• Possible incorrect parameterization of the existing controller settings, e.g. attenuation filters for resonance of the coupling, or parameterized gain factors no longer match the mechanical system

• Machine is no longer traversed with optimum controller parameters: Reduced traversing velocity or even quality defects on the workpiece

Calculation

The frequency response is determined using the "Auto Servo Tuning" (AST) measuring function of the SINUMERIK control. The measuring function is called via "AST from part program".