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How to choose and purchase the measurement tools?

You need to consider many factors when you select a measuring or gauging instrument or system for a particular manufacturing inspection operation. Generally speaking, there is the Rule of Ten can help you to one you want.. The Rule of Ten , often refer to as the Gage Maker’s Rule, states that inspection measurements should be better than inspections measurements by a factor of 10. If, for example, the tolerance on a shaft diameter is ±0.025mm(±.0010in.), then the increment of measurement on the inspection instrument should be as small as 0.025/10=0.0025mm(.00010 in.). Similarly, the increment of measurement for the calibration standard for that inspection instrument should be as small as 0.025/10=0.0025mm(.00010 in.)
Once the smallest increment of measurement for an instrument has been determined, the candidate instruments then need to be evaluated in terms of the degree of satisfaction they offer relative to the following performance criteria:
1. Accuracy : The accuracy of the output reading on an instrument over the actual input dimension.
2. Linearity: The accuracy of the measurements of an instrument throughout its operating range.
3. Magnification: The amplification of the output reading on an instrument over the actual input dimension.
4. Repeatability: The ability of the instrument to achieve the same degree of accuracy on repeated applications(often referred to as precision).
5. Resolution: The smallest increment of difference in dimension that can be read on an instrument.
6. Sensitivity: The smallest increment of difference in dimension that can be detected by an instrument.
7. Stability or drift: The ability of an instruments to maintain its calibration over a period of time.
8. There are other selection criterion too, such as the shape and size of the measured part or work piece, work piece material, and capabilities of the metrology laboratory.
Consideration of these factors, along with cost and operation convenience, should help in selecting an appropriate measurement, most instruments are or can be equipped with discrete digital readout devices. Most of these can be connected to microprocessors or computers for data recording and analysis.