Precautions for the selection of coordinate measuring machine
2022-04-22
In general, the selection of the CMM mainly pays attention to the following aspects:
1. Measuring range of CMM. This is the number one factor in choosing a CMM. When we plan to buy three-coordinate measuring equipment, we must first clearly grasp the peripheral dimensions of the product to be measured, and determine the three-coordinate equipment of the corresponding specifications according to the size. For example, when choosing a bridge structure CMM, the price of the equipment is proportional to the span of the beam, and the size in the length direction has little effect on the price. What the workpiece needs to be measured is only a part of the entire product. At this time, the selected CMM should also place the entire workpiece on the work surface, and at the same time, the weight of the product should be considered, which cannot exceed the load-bearing range of the CMM. The measurement range selected by the CMM is also affected by the selected probe system. Because the probe will occupy a certain measurement space, the influence of the selected probe must be considered when selecting the measurement range. At present, in terms of measurement range, two types of probes are commonly used: probes with rotating heads and fixed probes.
(1) Rotary probe: mainly used in production-type CMM.
(2) Fixed probe: It is mainly used in metrology-type three-coordinate measuring machines. It does not have the rotation error generated by the rotating probe during the rotation process. Because of the difference in the working principle of the rotating probe and the fixed probe, there is also a difference in the selection of the measuring range of the equipment: the rotating probe needs a larger measuring range, for the same workpiece, because the rotating probe will occupy some during the rotation process. Space, according to our practical experience, when choosing a CMM, if the supplier is equipped with a rotating probe, it is necessary to consider a larger CMM range.
2. Measurement accuracy. The coordinate measuring machine is a measuring instrument that detects the geometrical dimension and shape and position error of the workpiece, and meeting the accuracy requirements is the basic goal. When CMM is selected, it is generally possible to compare the detection accuracy required by the workpiece to be measured with the measurement uncertainty given by the measuring machine to see whether the accuracy of the measuring machine meets the requirements. Accuracy comparison is not a simple comparison process. In the technical specification of the measuring machine, only two uncertainty calculation formulas and repeated measurement accuracy values for uniaxial length measurement and spatial length measurement are generally given. However, in the specific measurement, the measurement uncertainty of the measured parameter needs to be limited within a certain range. In general measurement, many probe points need to be measured. In the measurement of shape and position tolerance, a large number of probe points are involved in the calculation and bring measurement errors. It is very difficult to calculate accurately. Therefore, from experience, in general measurement, the measurement uncertainty should be the dimensional tolerance zone of the workpiece to be measured. 1/5 to 1/3 of .
In short, the user should choose a coordinate measuring machine with slightly higher accuracy (including repeatability). This is not only due to the fact that when measuring complex workpieces, the error that the measuring point may bring is larger than expected (due to the change or lengthening of the probe rod will introduce greater measurement error), but also the accuracy of the measuring machine will increase with the number of uses. It can be selected when the uncertainty value of the general measuring machine is less than or equal to 1/2 of the required uncertainty of the measured size.
3. Probe form. From the perspective of the influence of the working form of the probe on the accuracy of the measurement results, the commonly used probes include contact trigger probes, contact scanning probes and non-contact optical probes. When selecting equipment probes, select the probe form according to the measurement requirements, measurement speed and measurement frequency.
4. Grating ruler. The grating ruler is the reading benchmark of the CMM, so its thermal expansion coefficient is required to be as low as possible to ensure the accuracy of the reading. Now the material development of the grating scale is also carried out around reducing the thermal expansion coefficient. According to the degree of automation, measuring machines are divided into two categories: manual (or motorized) and CNC (automatic control). When selecting, it should be weighed according to the batch size of the test object, the degree of automation, the technical level of the operator and the size of the capital investment. Of course, the CNC measuring machine has high precision and fast measurement speed, but the preparation time before measurement is relatively long, the technical requirements are relatively high, and the capital investment is relatively large.
1. Measuring range of CMM. This is the number one factor in choosing a CMM. When we plan to buy three-coordinate measuring equipment, we must first clearly grasp the peripheral dimensions of the product to be measured, and determine the three-coordinate equipment of the corresponding specifications according to the size. For example, when choosing a bridge structure CMM, the price of the equipment is proportional to the span of the beam, and the size in the length direction has little effect on the price. What the workpiece needs to be measured is only a part of the entire product. At this time, the selected CMM should also place the entire workpiece on the work surface, and at the same time, the weight of the product should be considered, which cannot exceed the load-bearing range of the CMM. The measurement range selected by the CMM is also affected by the selected probe system. Because the probe will occupy a certain measurement space, the influence of the selected probe must be considered when selecting the measurement range. At present, in terms of measurement range, two types of probes are commonly used: probes with rotating heads and fixed probes.
(1) Rotary probe: mainly used in production-type CMM.
(2) Fixed probe: It is mainly used in metrology-type three-coordinate measuring machines. It does not have the rotation error generated by the rotating probe during the rotation process. Because of the difference in the working principle of the rotating probe and the fixed probe, there is also a difference in the selection of the measuring range of the equipment: the rotating probe needs a larger measuring range, for the same workpiece, because the rotating probe will occupy some during the rotation process. Space, according to our practical experience, when choosing a CMM, if the supplier is equipped with a rotating probe, it is necessary to consider a larger CMM range.
2. Measurement accuracy. The coordinate measuring machine is a measuring instrument that detects the geometrical dimension and shape and position error of the workpiece, and meeting the accuracy requirements is the basic goal. When CMM is selected, it is generally possible to compare the detection accuracy required by the workpiece to be measured with the measurement uncertainty given by the measuring machine to see whether the accuracy of the measuring machine meets the requirements. Accuracy comparison is not a simple comparison process. In the technical specification of the measuring machine, only two uncertainty calculation formulas and repeated measurement accuracy values for uniaxial length measurement and spatial length measurement are generally given. However, in the specific measurement, the measurement uncertainty of the measured parameter needs to be limited within a certain range. In general measurement, many probe points need to be measured. In the measurement of shape and position tolerance, a large number of probe points are involved in the calculation and bring measurement errors. It is very difficult to calculate accurately. Therefore, from experience, in general measurement, the measurement uncertainty should be the dimensional tolerance zone of the workpiece to be measured. 1/5 to 1/3 of .
In short, the user should choose a coordinate measuring machine with slightly higher accuracy (including repeatability). This is not only due to the fact that when measuring complex workpieces, the error that the measuring point may bring is larger than expected (due to the change or lengthening of the probe rod will introduce greater measurement error), but also the accuracy of the measuring machine will increase with the number of uses. It can be selected when the uncertainty value of the general measuring machine is less than or equal to 1/2 of the required uncertainty of the measured size.
3. Probe form. From the perspective of the influence of the working form of the probe on the accuracy of the measurement results, the commonly used probes include contact trigger probes, contact scanning probes and non-contact optical probes. When selecting equipment probes, select the probe form according to the measurement requirements, measurement speed and measurement frequency.
4. Grating ruler. The grating ruler is the reading benchmark of the CMM, so its thermal expansion coefficient is required to be as low as possible to ensure the accuracy of the reading. Now the material development of the grating scale is also carried out around reducing the thermal expansion coefficient. According to the degree of automation, measuring machines are divided into two categories: manual (or motorized) and CNC (automatic control). When selecting, it should be weighed according to the batch size of the test object, the degree of automation, the technical level of the operator and the size of the capital investment. Of course, the CNC measuring machine has high precision and fast measurement speed, but the preparation time before measurement is relatively long, the technical requirements are relatively high, and the capital investment is relatively large.
