METROLOGY CONCEPT

Metrology | Introduction

Metrology or industrial inspection involves the inspection of components produced from production, a unit to check whether the dimensions fall within the acceptable limits assigned by the consumer. inspection is divided into active and passive types. inactive inspection, parts are checked when it is being produced in any production system. It is also called online inspection i.e. if something goes wrong, correction can be taken. but in passive inspection parts are checked when it is already being produced in any production system. It is also called online inspection i.e. if something goes wrong, correction can be taken. But in passive inspection parts are checked when it is already being produced and inspection is meant to segregate of the components as good or bad. in industries active inspection is encouraged but depends upon the type of application.

Parts that are produced from any manufacturing system follow a normal distribution. machine environment can be considered as an infinite universe and machined components are considered as random data collection. when data is collected randomly from an infinite universe, the data follows a normal distribution as shown in Fig.

Metrology | Introduction

let us take an example to understand the concept. there is a lathe machine aimed at producing shafts of 25.0 mm in diameter. As the shafts are coming out of the machine, no two dimensions will match and there will be some variation in the dimensions like 25.002 mm, 24.985 mm, etc. it is because these dimensions are following a normal distribution. that is why it is said that whatever components you have made from your production systems, no two components will match and you cannot duplicate the past. In the above normal distribution X is the mean or targeted value of the machine and σ is the process’s dispersion or standard deviation. following are the factors that may change the processes or targeted mean (X)  value in a typical machine environment:

  1. tool and die wear.
  2. Excessive machine vibrations
  3. wear or minor failure or loosening of machine parts.
  4. change of machine and process.
  5. measurement error or change the standard deviation of the process.

THE FOLLOWING FACTORS CHANGE THE STANDARD DEVIATION OF THE PROCESS.

  1. The carelessness of the operator.
  2. New or inexperienced operator.
  3. deterioration in the readjustment of the process.
  4. Frequent resetting or readjustment of the process.
  5. Increase variability of the material.

99.73% area of the normal distribution takes between ± 3σ limits of this normal distribution. It means 99.73% of components will be produced in the 6σ range. This 6σ is called the process capability of the machine. To move further with inspection let us understand normal distribution with the following examples.

(for more reading )

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