Three Coordinate Measuring Machine Selection Precautions

1. Measuring Range

This is the main factor in choosing a CMM. When we plan to buy a coordinate measuring machine (CMM), we should first know the surrounding size of the product, and then choose the CMM size. For example, when choosing a bridge coordinate measuring machine, the equipment price is proportional to the beam span, so we only need to meet the measurement range, do not pursue an unnecessary large range. If the workpiece to be measured is only a part of the entire product, then you need to choose a coordinate measuring machine that can also put the entire workpiece on the table, while considering the weight of the product, do not exceed the bearing limit of the coordinate measuring machine. The selected probe system also affects the measurement range, as the probe takes up a certain amount of measurement space.

2. Probe Selection

1) Rotary probe: mainly used for production CMM

2) Fixed probe: mainly used for coordinate measuring machines

There is no rotation error during rotation. Because the rotating probe works differently from the stationary probe, there is a difference in the measurement range when selecting the device: the rotating probe requires a larger measurement range. For the same workpiece, the rotating probe will occupy a certain space during the rotation process. According to our practical experience, in the selection of coordinate measuring machine, if the rotary probe is used, more coordinate measuring machine range should be considered.

Coordinate measuring machine (CMM) is a measuring instrument to measure the geometrical size and shape position error of the workpiece. When choosing a coordinate measuring machine, we can compare the measuring accuracy of the workpiece to be measured with the measuring accuracy of the measuring machine. Precise comparison is not a simple comparison process. In the technical specification of measuring machine, two kinds of uncertainty calculation formulas for single-axis length and space measurement are generally given, and the accuracy of repeated measurement is also given. When measuring, the measurement uncertainty of the measured parameter is limited to a certain range. In general measurement, many measuring points need to be measured. When measuring the shape tolerance, a large number of measurement points are involved, which will cause errors.

In summary, users should choose a coordinate measuring machine with slightly higher accuracy (including repeat accuracy). This is not only because of the complexity of the measuring workpiece, the introduced measuring point error may be larger than expected (by changing the measuring head or lengthening the extension rod will introduce a greater measurement error), but also with the increase in the number of uses, the accuracy of the measuring machine will also decrease.

From the working form of the probe to the accuracy of the measurement results, there are contact trigger probes, contact scanning probes and non-contact optical probes. When selecting the probe, the measurement speed and frequency of the device probe are selected according to the measurement requirements.

The grating scale is the reading basis of CMM, so its thermal expansion coefficient is required to be as low as possible to ensure the reading accuracy. Now the material development of grating ruler is also centered on reducing the coefficient of thermal expansion. Measuring machines are divided into manual (or mobile) and CNC(automatic control). In the selection, the size of the test object should be measured from the aspects of batch size, automation degree, operator skill level and 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.

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