WHAT IS A MEASUREMENT SYSTEM?

MEASUREMENT SYSTEM is a Process to assign a number or decision to a characteristic.

In this Process following factors are present: Appraiser, Checking Method, Instrument, Environment.
E.g, OD checking, Concentricity checking.

STATISTICAL PROPERTIES OF MEASUREMENT SYSTEM
IDEAL MEASUREMENT SYSTEM IS THE ONE: That would produce only correct measurements each time it was used. That would always agree with a standard. The Quality of a measurement system is determined solely by the statistical data produced over time.

A “GOOD” MEASUREMENT SYSTEM IS THE ONE: Adequate Discrimination and Sensitivity. The measurement system ought to be in Statistical control, only under the influence of common causes, not Special causes.

For Product control, the variability of the measurement system must be small compared to the specification limits. For Process control, the variability of the measurement system must be small compared to the manufacturing process variation.

CRITERIA FOR A MEASUREMENT PROCESS DESIGN SELECTION

1 Who should be part of the team for “ Need” analysis?
2 How the measurement will be used? Is it for control, sorting, Qualification, indication, etc.?
3 What level of sensitivity will be required?
4 What is product specification?
5 What is the expected process variability?
6 What type of information required to be provided along with the gage?
7 What basic operators training required? Who will do the training?
8 How the measurement will be taken? (Manual, Automatic, offline, conveyor, etc.,)
9 Contact or non-contact?
10 How to calibrate? Who will calibrate?
11 When and where the measurement will be taken?
12 Will the part is clean, oily, hot, etc.?
13 What will be the frequency of use?
14 What will be calibration frequency?
15 Will the temperature difference between master used for setting and the part measured is significant?

MEASUREMENT ISSUES

Measurement system errors are classified as following five categories:
· Bias
· Stability
· Linearity
· Repeatability
· Reproducibility

APPLICATIONS OF THE ABOVE STUDIES PROVIDE THE FOLLOWING:

· A criterion to accept the new measuring equipment/ system
· A comparison of one measuring system against another
· A basis for evaluating a gage suspected of being deficient
· A comparison for measuring equipment before and after repair