Machine tool compensation, a critical technology, is needed to correct machining errors and ensure machining accuracy. Below are the main types of machine tool compensation.

Because machine tools have systematic mechanical-related deviations that can be recorded by the system. However, due to environmental factors such as temperature or mechanical load, these deviations may still occur or increase during subsequent use.

Backlash Compensation

When transmitting force between the moving components of a machine tool and its driving components—such as ball screws—interruptions or delays can occur. This is because a completely gap-free mechanical structure would significantly increase machine tool wear and is also difficult to achieve from a technical standpoint. Mechanical gaps cause deviations between the motion path of the axis/spindle and the measurements from the indirect measurement system. This means that once the direction changes, the axis will move either too far or not far enough, depending on the size of the gap. The worktable and its associated encoder are also affected: if the encoder position leads the worktable, it reaches the commanded position prematurely, meaning the actual distance moved by the machine tool is shortened. During machine operation, by using the backlash tool compensation function on the corresponding axis, the previously recorded deviation is automatically activated when the direction changes, and this deviation is added to the actual position value.

 

Screw Pitch Error Compensation

The measurement principle of indirect measurement in CNC control systems is based on the assumption that the pitch of the ball screw remains constant over its effective travel range. Therefore, in theory, the actual position of the linear axis can be derived from the motion information of the drive motor.。

However, manufacturing errors in the ball screw can lead to deviations in the measurement system (also known as screw pitch errors). Measurement deviations (depending on the measurement system used) and installation errors of the measurement system on the machine tool (also referred to as measurement system errors) may further exacerbate this issue. To compensate for these two types of errors, an independent measurement system (such as laser measurement) can be used to measure the natural error curve of the CNC machine tool. The required tool compensation values are then saved in the CNC system for tool compensation.

Friction Compensation (Quadrant Error Compensation) and Dynamic Friction Compensation

Quadrant error compensation (also known as friction compensation) is suitable for all the aforementioned scenarios to significantly improve contour accuracy when machining circular profiles. The reason is as follows: During quadrant transitions, one axis moves at the maximum feed rate while the other axis remains stationary. As a result, the different friction behaviors of the two axes can lead to contour errors. Quadrant error tool compensation effectively reduces this error and ensures excellent machining results. The density of tool compensation pulses can be set based on an acceleration-related characteristic curve, which can be determined and parameterized through roundness testing. During roundness testing, deviations between the actual position of the circular contour and the programmed radius (especially during direction changes) are quantified and graphically displayed on the human-machine interface.

In newer versions of the system software, the integrated dynamic friction tool compensation function dynamically compensates for friction behavior at different machine speeds, reducing actual machining contour errors and achieving higher control accuracy.

 

Sag and Angular Error Compensation

If the weight of individual components of the machine tool causes displacement or tilting of moving parts, sag tool compensation is required because it can lead to sagging of relevant machine tool parts, including the guiding system. Angular error tool compensation is used when moving axes are not correctly aligned with each other (e.g., not perpendicular). As the offset from the zero position increases, the positional error also increases. Both types of errors are caused by the machine tool’s own weight or the weight of the tool and workpiece. During commissioning, the measured tool compensation values are quantified and stored in the SINUMERIK system in a form such as a compensation table, corresponding to specific positions. During machine operation, the positions of the relevant axes are interpolated based on the stored tool compensation values. For each continuous path movement, there is a base axis and a compensation axis.

 

Temperature Compensation

Heat can cause expansion in various parts of the machine tool. The extent of expansion depends on factors such as the temperature and thermal conductivity of each part. Different temperatures may cause changes in the actual positions of the axes, which can negatively impact the accuracy of the workpiece being machined. These changes in actual values can be offset through temperature tool compensation. Error curves for each axis at different temperatures can be defined. To ensure accurate compensation for thermal expansion, temperature compensation values, reference positions, and linear gradient parameters must be continuously transferred from the PLC to the CNC control system via functional blocks. Unexpected parameter changes are automatically corrected by the control system to prevent machine overload and activate monitoring functions.

 

Volumetric Compensation System (VCS)

The positions of rotary axes, their mutual tool compensation, and tool orientation errors can lead to systematic geometric errors in components such as turrets and rotary heads. Additionally, small errors may occur in the guiding systems of feed axes in every machine tool. For linear axes, these errors include linear position errors, horizontal and vertical straightness errors; for rotary axes, pitch, yaw, and roll errors may arise. Other errors, such as perpendicularity errors, can occur when aligning machine components. For example, in a three-axis machine tool, this can result in up to 21 geometric errors at the tool center point (TCP): six error types per linear axis multiplied by three axes, plus three angular errors. These deviations collectively form the total error, also known as the volumetric error.

 

The volumetric error describes the deviation between the actual TCP position of the machine tool and the TCP position of an ideal, error-free machine tool. SINUMERIK solution partners can determine volumetric errors using laser measurement equipment. Measuring errors at a single position is insufficient; errors across the entire machining volume must be measured. Typically, measurement values for all positions are recorded and plotted as curves, as the magnitude of errors depends on the position of the relevant feed axis and the measurement location. For example, deviations in the x-axis may vary when the y-axis and z-axis are in different positions—even at nearly the same x-axis position. With “CYCLE996 – Motion Measurement,” rotary axis errors can be determined in just a few minutes. This allows for continuous monitoring of machine tool accuracy and, if necessary, corrections can be made even during production.

 

Deviation Compensation (Dynamic Feedforward Control)

Deviation refers to the discrepancy between the position controller and the standard during the movement of a machine tool axis. Axis deviation is the difference between the target position and the actual position of the axis. Deviation causes speed-related unnecessary contour errors, especially when the contour curvature changes, such as in circles, squares, or other shapes. Using the NC advanced language command FFWON in the part program, speed-related deviations can be reduced to zero during path movement. Feedforward control improves path accuracy, resulting in better machining outcomes.

FFWON: Command to activate feedforward control

FFWOFF: Command to deactivate feedforward control

 

Electronic Counterbalance Compensation

In extreme cases, to prevent damage to the machine tool, tool, or workpiece caused by axis sagging, the electronic counterbalance function can be activated. In load axes without mechanical or hydraulic counterbalances, a vertical axis may unexpectedly sag once the brake is released. After activating the electronic counterbalance, unintended axis sagging can be compensated. Upon releasing the brake, a constant balancing torque maintains the position of the sagging axis.