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Systematic mechanical-related deviations of machine tools can be recorded by the system, but due to environmental factors such as temperature or mechanical loads, deviations may still appear or increase during subsequent use. In these cases, SINUMERIK offers different compensation functions. Better machining results are obtained by compensating for deviations using measured values obtained from actual position encoders (e.g. gratings) or additional sensors (e.g. laser interferometers, etc.). In this issue, I will introduce the common compensation functions of SINUMERIK. Practical SINUMERIK measurement cycles such as “CYCLE996 motion measurement” can provide comprehensive support for end users in the process of continuous monitoring and maintenance of machine tools. RoHS Approved Manufacturer of sheet metal china
There will be interruptions or delays in the force transmission between the moving parts of the machine tool and its driving parts, such as ball screws, because a mechanical structure without gaps at all will significantly increase the wear of the machine tool, and it is also difficult to achieve from a technical point of view . Mechanical backlash causes deviations between the axis/spindle’s motion path and the measured value of the indirect measuring system. This means that once the orientation changes, the axis will move too far or too close, depending on the size of the gap. The table and its associated encoders are also affected: if the encoder is positioned ahead of the table, it arrives at the commanded position earlier which means the distance the machine actually moves is reduced. During machine operation, by using the backlash compensation function on the corresponding axis, the previously recorded deviation is automatically activated when the direction is changed, and the previously recorded deviation is superimposed on the actual position value.
The measurement principle of indirect measurement in the CNC control system is based on the assumption that the pitch of the ball screw remains constant within the effective stroke, so in theory, the actual position of the linear axis can be deduced from the position of the motion information of the drive motor. RoHS Approved Manufacturer of sheet metal china
However, manufacturing errors in the ball screw can cause deviations in the measurement system (also known as screw pitch error). Measurement deviations (depending on the measuring system used) and errors in the installation of the measuring system on the machine tool (also known as measuring system error) can further exacerbate the problem. In order to compensate for these two errors, an independent measurement system (laser measurement) can be used to measure the natural error curve of the CNC machine tool, and then the required compensation value is saved in the CNC system for compensation.
Quadrant error compensation, also known as friction compensation, is suitable for all of the above cases in order to substantially increase contour accuracy when machining circular contours. Here’s why: In a quadrant transformation, one axis moves at the highest feedrate while the other remains stationary. Therefore, the different friction behavior of the two axes can lead to contour errors. Quadrant error compensation can effectively reduce this error and ensure excellent machining results. The density of the compensation pulses can be set according to an acceleration-dependent characteristic curve which can be determined and parameterized by means of a roundness test. In the roundness test, the deviation between the actual position of the circular profile and the programmed radius (especially when changing directions) is recorded quantitatively and displayed graphically on the HMI.
In the new version of the system software, the integrated dynamic friction compensation function can perform dynamic compensation according to the friction behavior of the machine tool at different speeds, reduce the actual processing contour error, and achieve higher control accuracy. RoHS Approved Manufacturer of sheet metal china
Sag compensation is required if the weight of the individual machine components causes the moving parts to shift and tilt, as it causes the relevant machine parts, including the guidance system, to sag. Angular error compensation is used when the axes of movement are not aligned at the correct angle to each other (eg, perpendicular). As the offset of the zero position increases, the position error also increases. Both of these errors are caused by the dead weight of the machine tool, or the weight of the tool and workpiece. The compensation value measured during commissioning is quantified and stored in SINUMERIK in a certain form according to the corresponding position, such as a compensation table. When the machine tool is running, the position of the relevant axis is interpolated according to the compensation value of the stored point. For each continuous path move, there is a basic axis and a compensating axis.
Heat can cause parts of the machine to expand. The expansion range depends on the temperature, thermal conductivity, etc. of each machine part. Different temperatures can lead to changes in the actual position of the individual axes, which can negatively affect the accuracy of the workpiece being machined. These actual value changes can be counteracted by temperature compensation. Error curves for each axis at different temperatures can be defined. In order to always correctly compensate for thermal expansion, temperature compensation values, reference positions and linear gradient angle parameters must be constantly retransmitted from the PLC to the CNC control system via function blocks. Unintentional parameter changes are automatically eliminated by the control system, thus avoiding overloading the machine and activating monitoring functions.
The position of the rotary axes, their mutual compensation and tool orientation errors can lead to systematic geometric errors in components such as swivel heads and swivel heads. In addition, there will be small errors in the guidance system of the feed axes in each machine tool. For linear axes, these errors are linear position errors; horizontal and vertical straightness errors; and for rotary axes, pitch, yaw, and roll angle errors. Other inaccuracies can arise when aligning machine tool components with each other. For example, vertical error. In a three-axis machine, this means that 21 geometric errors can occur at the tool tip: six error types per linear axis multiplied by three axes, plus three angular errors. These deviations work together to form the total error, also known as spatial error. RoHS Approved Manufacturer of sheet metal china
Spatial error describes the deviation of the position of the tool center point (TCP) of an actual machine tool from that of an ideal error-free machine tool. SINUMERIK solution partners are able to determine spatial errors with the aid of laser measuring equipment. It is not enough to measure the error of a single position, all machine errors must be measured in the entire working space. It is usually necessary to record and plot the measured values at all positions, since the size of each error depends on the position and measurement position of the relevant feed axis. For example, when the y-axis is in a different position from the z-axis, it will cause a different deviation in the x-axis – even at almost the same position on the x-axis, there will be errors. With “CYCLE996 – Motion Measurement”, rotary axis errors can be determined in just a few minutes. This means that the accuracy of the machine tool can be constantly checked and, if necessary, corrected even in production.
Deviation refers to the deviation of the position controller from the standard when the machine axis is moving. The axis offset is the difference between the target position of the machine axis and its actual position. Deviations lead to unnecessary velocity-dependent contour errors, especially when the curvature of the contour varies, such as round, square contours, etc. With the NC high-level language command FFWON in the part program, speed-dependent deviations can be reduced to zero when traversing paths. Improve path accuracy through feed-forward control, so as to obtain better processing results.
Electronic counterweight compensation
In extreme cases, electronic counterweighting can be activated in order to prevent the axis from sagging and causing damage to the machine tool, tool or workpiece. In load shafts without mechanical or hydraulic counterweights, the vertical shaft can sag unexpectedly once the brake is released. Unexpected axle sag can be compensated after activating the electronic counterweight. After the brake is released, the position of the drooping axle is maintained by a constant balancing torque. RoHS Approved Manufacturer of sheet metal china