In the process of manufacturing product parts with CNC machine tools, there are many factors affecting the quality of parts, such as the accuracy of CNC machine tools, workpiece materials, workpiece heat treatment, processing technology, coolant, tools and so on. Among them, the accurate setting of tool parameters has been rarely concerned and valued by everyone. Here, we will focus on the characteristics and development trend of accurate tool setting on CNC machine tools.
? ? basic coordinate relations generally speaking, there are two coordinate systems commonly used: one is the machine coordinate system and the other is the workpiece coordinate system. The machine tool coordinate system is the inherent coordinate system of the machine tool. The origin of the machine tool coordinate system is called the machine tool origin or the machine tool zero point.
? ? in order to facilitate calculation and programming, we need to establish a workpiece coordinate system in the machine coordinate system. Take a certain point on the workpiece as the origin of the coordinate system (also known as the program origin) to establish a coordinate system, which is the workpiece coordinate system. In our daily work, we should try to make the programming benchmark coincide with the design and assembly benchmark.
1： Common tool setting methods of CNC machine tools and in machine tool setting instrument
? ? generally, the machine coordinate system of a machine tool is fixed, and several workpiece coordinate systems can be established according to the actual needs of the machining process. For example, g54 and G55 can select different workpiece coordinate systems.
? ? the purpose of tool setting: when performing NC machining, the path taken by the NC program is the movement path of the tool tip of the tool on the spindle. The motion path of the tool location needs to be accurately controlled under the machine coordinate system from beginning to end, because the machine coordinate system is the only benchmark of the machine tool. It is impossible for programmers to know the specific dimensions of tools of various specifications when programming. In order to simplify programming, it is necessary to use a unified benchmark when programming, and then offset the accurate length and radius of the tool with respect to the benchmark when machining with the tool, so as to obtain the accurate position of the tool tip. Therefore, the purpose of tool alignment is to determine the length and radius of the tool, so as to determine the accurate position of the tool tip in the workpiece coordinate system during machining.
Common tool setting method - tool setting outside the machine
? ? the tool presetter is a measuring instrument that can adjust and measure the length and diameter of the tool tip in advance. If the instrument forms a DNC network with the CNC machine tool, it can also input the tool length and diameter data into the tool parameters in the NC of the machining center remotely. The advantage of this method is that the tools are aligned outside the machine tool in advance, and can be used immediately after being installed on the machine tool, which greatly saves auxiliary time. However, the main disadvantage is that the measurement results are static values, and the tool wear or damage status cannot be updated in real time during the actual machining process, and the tool expansion and contraction caused by the thermal deformation of the machine tool cannot be measured in real time.
1. Knife setting by trial cutting method
? ? trial cutting method is to operate the machine tool in manual mode to cut the workpiece by a small amount before the workpiece is officially processed. The operator determines the current position of the tool tip based on his eyes and ears, and then carries out formal processing. The advantage of this method is that it does not require additional investment to purchase tools and equipment, and it is economical and practical. The main disadvantages are low efficiency, high technical requirements for operators, and easy to produce human errors. In actual production, there are many derivative methods of trial cutting method, such as gauge block method and color coating method.
2. Tool setting in the machine
?? this in-machine tool setting method uses a measuring device (tool setting instrument) set on the machine tool workbench to measure the tools in the tool library according to the preset program, and then compares it with the reference position or the standard tool to obtain the length or diameter of the tool and automatically update it to the corresponding NC tool parameter table. At the same time, the detection of the tool can also realize the identification of tool wear, damage or whether the installation model is correct.
Composition of tool setting instrument in the machine
? ? the tool setting instrument in the machine is generally composed of sensor, signal interface and tool setting macro program software.
? ? according to the working mode of the sensor, the in-machine tool setter can be divided into two types: contact tool setter and laser tool setter. The repetitive measurement accuracy of the contact tool setter is 1 μ m. According to the different signal transmission modes of the tool setter, it can be further divided into the following categories: cable type tool setter; Infrared tool setter; Radio tool setter.
? ? the cable type tool setting instrument has the best single cost performance ratio because it does not need the conversion part of the tool setting signal, so it is most common in work, but its disadvantage is the drag of the cable, which limits the application of the tool setting instrument, which is mostly suitable for small and medium-sized three-axis milling machines / machining centers.
? ? infrared tool setter, signal transmission range is generally within 6m. Its advantage is that encoded HDR (high-speed data transmission) infrared technology is adopted to avoid the inconvenience and potential security threat caused by cable dragging. After tool setting, it can be removed from the workbench at any time without occupying processing space, and multiple machine tools can share a tool setting instrument to reduce the comprehensive cost. Its disadvantage is that the cost performance ratio is not high when used in small machining centers. Due to its characteristics, this kind of tool setting instrument is mostly used for medium-sized machine tools and large-scale CNC vertical lathe.
? ? radio tool setter, the radio signal transmission range is generally more than 10m. Its advantages are that the radio signal transmission range is large and it is not easy to be affected by the environment. After tool setting, it can be removed from the workbench at any time without occupying processing space. Moreover, it can share a tool setting instrument with multiple machine tools, which can reduce the comprehensive cost. This kind of tool setting instrument is mostly used for large / heavy machine tools.
?? laser tool setting instrument: the basic principle of this product is to use the focused laser beam as the trigger medium, and generate a trigger signal when the laser beam is shielded by the rotating tool. The essential difference between the laser tool setter and the contact type tool setter is that the laser tool setter uses non-contact measurement, and there is no contact force during tool setting, so it can measure extremely small tools without worrying about the damage of small tools due to contact force. For example, the straight diameter of the tool that can be measured by lts35.60 can be as small as 0.008mm (such as drill, tap or micro milling cutter), and its repeated measurement accuracy can reach 0.2 μ m。 At the same time, because the tool rotates at a high speed at the machining speed during measurement, the measurement state is almost identical to the actual machining state, which improves the practical accuracy of tool setting. Due to the use of laser technology, the tool alignment instrument can scan the shape of the tool to measure the contour of the tool, and can monitor the damage of a single edge of a multi-edge tool. The main disadvantage is that the structure is complex, requiring additional high-quality gas source to protect the internal structure, and the cost is high, which is mainly suitable for high-speed machining centers.
Common functions and advantages of in machine tool setting instrument
?? (1) automatic measurement and parameter update of tool length / diameter: dynamic measurement of tool length / diameter is carried out when the tool rotates. The measurement parameters include the end run out / radial run out error of the machine tool spindle, so as to obtain the "dynamic" offset value of the tool during high-speed machining; At the same time, the automatic measurement of tool parameters can be carried out at any time, which greatly eliminates the "change" of tool parameters caused by thermal deformation of the machine tool; The measurement results are automatically updated into the parameter table of the corresponding tool, completely avoiding the potential risks brought by manual tool setting and parameter input.
?? (2) automatic monitoring of tool wear / breakage: in the actual production process, when the tool is worn or broken (broken), it is difficult for the operator to find and correct it in time (especially for drill bits with smaller diameter), which will lead to more subsequent tool losses and even tool scrapping. The tool length can be automatically measured once after the tool is processed and before it is put back into the tool magazine by using the in machine tool setting instrument. In case of normal wear, the wear value can be automatically updated into the tool damage parameter. In case of excessive wear, it can be regarded as tool damage (break), so that the operator can choose to replace a new tool for processing the next workpiece or automatically stop the machine to give an alarm to prompt the operator to replace the tool. In this way, the product quality is improved and the tool loss or scrap rate is reduced.
?? (3) automatic compensation of thermal deformation of the machine tool: when the machine tool is in production and processing, the thermal deformation of the machine tool will occur at any time along with the change of the ambient temperature and the change of the working load, which will drive the change of the tool. As a result, the dimensional accuracy of the products processed by the same machine tool in the workshop will fluctuate greatly at different times in the morning / middle / evening. After using the tool setting instrument in the machine, the tool parameters can be automatically measured and updated at any time before or during machining. Each measurement is made under the current state of thermal deformation of the machine tool, thus greatly reducing the error caused by thermal deformation of the machine tool.
?? (4) measurement and monitoring of tool contour: in special processing, such as forming tools, it is time-consuming and complex to use an external tool setter to measure the tool contour and judge the tool status. At the same time, it also has high requirements for the operator's tool setting skills. At this time, if the in-machine laser tool setter is used, the laser beam can be used to scan, measure or monitor the tool contour at any time, and the corresponding parameters can be automatically updated as needed.
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