Tool setting error of CNC machining center and its improvement measures

1 Machining errors generated by CNC machining centers

)1) Programming error: It is mainly the interpolation error generated by the CNC system during CNC programming, which mainly occurs when avoiding the straight line segment or arc segment approaching the part contour. This is one of the important factors affecting the machining accuracy of parts. Although it can be solved by increasing the number of interpolation nodes, the programming workload will increase. (Refer to the detection and solution of new CNC cutter failures).

2) Cutting tool tip arc error: When cutting internal holes, external factors or end faces, the tool tip arc does not affect its size and shape, but when machining conical surfaces or arcs, it will be affected by the tool tip arc, forming bite edges or bite edges. This error can be eliminated by measuring the arc radius of the tool tip and using the tool radius compensation function.

3) Measurement error: It is mainly affected by the measurement accuracy and the measurement method of the measurer, and the measured size is inaccurate. This error can be compensated.

4) Tool setting error: This error mainly occurs in the tool setting instrument. When the tool moves to the tool setting instrument position, it is affected by the feed compensation scale value of the operating system. The solution is to reasonably select the feed compensation ratio. Especially when the tool approaches the starting position, the minimum stopper feed compensation is used to accurately position the tool at the starting position.

5) Error of the machine tool system: It is the geometric tolerance caused by the main body of the machine tool, which is generally not adjustable; The repeated positioning error caused by the servo unit and the drive device is mainly affected by the size, uniformity and transmission path of the machine tool pulse equivalent; Because these errors are small and stable, only precision machining should be considered.

2 Tool setting method

The CNC machining tool setting instrument, its processing quality directly affects the accuracy of the machined parts and also affects the operation of the CNC machine tool. The tool setting instrument refers to placing the tool tip at the starting point (tool setting instrument) in the workpiece coordinate system under the control of the CNC program. In this way, the machining surface cut by the tool has an accurate dimensional relationship relative to the positioning reference, which ensures the machining accuracy of the part. In CNC machining, the basic methods of tool setting instruments include test cutting method, tool setting instrument, ATC tool setting instrument, automatic tool setting instrument, etc.

2.1 Trial cutting method

According to the different position detection devices used in CNC machine tools, trial cutting methods are divided into relative and absolute types. There are three methods for tool setting based on relative trial cutting method. One method is to use a measuring device (such as steel plate size, etc.) to directly measure and make the size of the tool setting consistent. This method is simple but inaccurate; the second is to pull the tool to the size of the tool setting after the tool point is consistent with the working surface of the positioning block. The accuracy of the tool setting in this method depends on the accuracy of the consistency between the blade point and the working surface of the positioning block. The third method is to first cut the machining surface of the workpiece, measure the size of the workpiece, and indirectly calculate the size of the tool setting. This method is the most correct. The absolute value method of tool setting requires the use of a reference blade, and measures the deviation between the blade point of other tools and the reference blade by direct or indirect methods as the set blade compensation value of other tools. The above trial cutting method adopts the tool setting method of "trial cutting-measurement-adjustment (correction)". Although the machine tool takes up a lot of time and has low efficiency, the method is simple and requires less auxiliary equipment, so it is widely used in economical low-level CNC machine tools. 2.2 Tool setting instrument

Tool setting instruments are divided into in-machine tool setting instruments and out-machine tool setting instruments. In-machine tool setting instruments are to install the tool directly at a fixed position of the machine tool, directly on the tool holder on the lathe, or on the tool holder through a bracket. This method is mostly used for turning CNC machine tools. On the other hand, the company's external tool setting instrument must be installed on the tool holder through the tool holder. (Lathe), together with the tool holder, pre-calibrated outside the machine tool, and then the tool can be installed on the machine tool for use. This method is currently mainly used for boring machine CNC machine tools, such as machining centers. The use of tool setting instruments requires additional auxiliary equipment for tool setting instruments, which is costly and time-consuming and labor-intensive to disassemble and assemble tools, but it can save the tool setting time of the machine tool and improve the accuracy of the tool setting instrument. It is generally used for CNC machine tools with high accuracy requirements.

2.3 ATC tool setting instrument

AIC tool setting instrument is a method of automatically calculating the tool length using a tool setting instrument microscope on a machine tool. Due to the operation of the tool setter, the process of operating the tool setter, and the visual tool setter, the tool setter has errors. Compared with the tool setter, it is simpler to only disassemble and install the tool. The automatic tool setter refers to the use of the tool detection function of the CNC device to automatically and accurately detect the length of the tool in each coordinate direction, automatically correct the tool compensation value, and directly process the workpiece without stopping. Compared with the previous tool setting method, this method reduces the error of the tool setter and improves the accuracy and efficiency of the tool setter, but requires an automatic tool setting system composed of a tool setter and a part detection system, and the CNC system requires an auxiliary function of automatic tool detection

2.4 Automatic Preset

The automatic tool setter is a function that uses the tool detection of the CNC device to automatically correct the tool compensation value. It can automatically and accurately detect the length of the tool in each coordinate direction, and can directly process the workpiece without stopping. The automatic tool setter is also called the tool tip inspection function:

After the workpiece is installed in the machining center once, multiple tools in the tool magazine need to be used to process multiple surfaces of the workpiece. In order to improve the accuracy and efficiency of the tool setter, methods such as tool setters, ATC tool setters, and automatic tool setters are generally used. Among them, tool setting gauges are generally widely used in mid-range milling machining centers. When using a tool setting gauge, in order to determine the coordinate origin of the workpiece, the reference spindle is first aligned with the workpiece surface, and then a surface that is convenient for the tool setting gauge is selected, and a dynamic tool setting method is adopted.

3 Causes and countermeasures of machining errors

In summary, the causes of tool setting gauge errors are:

(1) When pre-cutting is performed using the trial cutting method, the pre-cutting error is mainly caused by the measurement error after the trial cutting of the workpiece and the visual error during operation.

2) When using tool setting gauges, tool setting gauges, and automatic tool setting gauges, the error is mainly caused by the manufacturing, installation, and measurement errors of the equipment. If the equipment is used improperly, it may also cause errors. (3) The tool measurement is static, but the machining process is dynamic, and is affected by cutting force and vibration external forces. Therefore, the machining size is inconsistent with the preset size. The size of the error is determined by the quality and dynamic stiffness of the tool.

4) The "machine tool reference point reset" operation is often performed in the tool setting gauge. During this process, zero drift may occur, resulting in zero drift, thereby causing tool setting errors.

5) There is a measuring device inside the machine tool, and the size of the minimum measuring unit is also related to the error. Generally speaking, the measuring device with a large minimum measuring unit has a large error, and the measuring device with a small minimum measuring unit has a small error.

The main measures to reduce the error of the tool setting instrument are:

(1) When using the trial cutting method to open the tool, be careful. After the tool setting instrument, it is necessary to correct the tool compensation value according to the actual size of the part processed by the tool and the error of the programmed size. In addition, it is also necessary to consider the impact of the machine tool's repeat positioning accuracy on the tool setting accuracy, and the impact of the installation height of the tool setting point on the tool setting accuracy.

2) When using the instrument to bake the tool setting instrument, pay attention to the manufacturing, installation, and measurement accuracy of the instrument. You must master the correct method of using the instrument.

3) When selecting tools for the machining center, pay attention to the quality and dynamic rigidity of the tool.

4) Regularly check the zero drift of the CNC machining center machine tool and pay attention to timely adjustment of the machine tool.

By using the trial cutting method and the approximate value method to process parts, 5 parts were processed using different methods. Measure its axial dimension and one radial dimension (10) respectively, and find the average value of the absolute value of the dimensional deviation. The larger the average value, the larger the error of the tool setting instrument.

When the axial dimensional accuracy of the part is required, do not use the test tool setting instrument with manual measurement. Since human error during measurement is difficult to avoid, you can also use the test tool setting instrument method such as automatic measurement by the system, but please also confirm whether the measurement accuracy of the system and the dimensional accuracy that should be guaranteed exist. If the radius size of the part needs to be very high, it is difficult to accurately align the tool with the rotation center of the workpiece without using the approximate value method for preset. However, this method does not have a large error in the axial dimension of the tool setting instrument component. Because the axial dimension of the tool setting instrument in the machining center is automatically measured by the system, the intervention of human error can be avoided.