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# The Notes When Choosing CMM

Aug 24, 2017

A. Measuring Range

This is the primary factor that selects the CMM. When we plan to purchase the coordinate measuring machine (CMM), we should first know the peripheral dimensions of the product, then choose the CMM size. When choosing bridge type CMM, for example, equipment price is proportional to the beam span, so we just can satisfy the measurement range, not the pursuit of unnecessary big range. If the work-piece to be measured is just a part of the entire product, then you need to choose the CMM that can also put the whole work-piece on the table, at the same time to consider the weight of the product, no more than the CMM bearing limits. The measurement range is also affected by the selected probe system, because the probe will take up a certain amount of measurement space.

1. Rotating probe: mainly used in the production type CMM

2.Fixed type probe: It is mainly used in the tri-CMM

There is no rotation error generated during rotation. Because the working principle of rotating probe and stationary probe differences, so in choosing measuring equipment when there is a difference between measuring range: rotating probe need larger measurement range. For the same work-piece, rotating probe in the rotation process will take up some space. According to our practical experience, in the choice of the CMM, if with rotational probe, consider more CMM range.

B. Accuracy

The CMM is the measuring instrument for measuring the geometrical size and shape position error of the work-piece, satisfying the precision requirement is the preferred target. When the CMM is selected, we can compare the measuring accuracy required of the measured work-piece with that of the measuring machine. Precision comparison is not a simple comparison process. In the technical specification of the measuring machine, two uncertainty calculation formulas of single axial length and space measurement are generally given, and the accuracy of repeated measurement is given. When the measurement is carried out, the measurement uncertainty of the measured parameters is limited to a certain range. In general measurements, many detection points need to be measured. In the measurement of shape bit tolerance, a large number of detection points are involved in the calculation, which will generate errors.

In summary, the user should select a slightly higher precision (including repetitive precision) CMM. This is not only due to the complexity of measuring work-piece, the error of the measuring point may be introduced is bigger than expected (by probe head change or longer extension bar will introduce greater error of measurement), and the precision of the measuring machine will be decreased with the using number increase.