 What type of Inspection procedures are in use  Where in the process should inspection take place  How are variations in the process detected before they become defects

 Every feature/part is inspected  Disadvantages are:  Very time consuming and expensive.  repetitive nature can lead to inspectors losing concentration resulting in human error e.g. Wrong measurements being made, failure to identify defects etc.  Normally only employed when:  Failure of a component will result in significant risk of injury/death  Where fully automated inspection can be employed quickly and cost effectively and their is little chance of error.

 Close inspection of a randomly selected sample of materials or components from a batch.  Can be based on  Variables: where a specific value/s can be measured and recorded and can vary within prescribed tolerances e.g. Length, diameter, height etc.  Attributes: which are acceptable or unacceptable e.g. Colour, surface finish, size etc.(size is inspected by gauging with Go No-Go as apposed to specific using direct measuring equipment.  Decision whether to accept or reject the whole batch is based on mathematical statistical procedure using the results of this inspection.  This method is less time consuming however it does have certain risks for both the supplier and the consumer

 The most simple form of sampling is taking a sample(n) from a batch and accepting or rejecting the batch depending on defects found.  If defects found are equal or less than agreed limit batch is accepted, if number exceeds agreed limit batch is rejected.  This can be shown graphically by plotting its Operating Characteristic (OC) curve  In an ideal situation this graph would have a straight line as shown opposite where all batches with 5% defects or less (Acceptance Quality Level) AQL are accepted and all with a higher level are rejected Ideal operating characteristics Loop curve

 In practice this is never encountered as all processes have some degree of variability  The graph opposite shows a more typical OC curve where;  PAPD is the Process Average Percentage Defective, this usually coincides with the AQL, it is the percentage of defects produced when a process is considered to be operating at an acceptable level  LTPD is the Lot Tolerance Percentage Defective, this is the percentage of defects the customer would find unacceptable also know as consumer risk  AQL is based on type of defects e.g.  Critical (failure results in persons at risk)  Major (failure could seriously effect the function of item)  Minor (not likely to effect the function of the product)  Vendors can use this to rate suppliers Typical Operating Characteristic Curve

 Ideally inspection should take place at critical points in the production process to avoid further costly processes being carried out on already defective parts  Such points could be:  Prior to setting up and performing costly machining processes when a vital part is too small or large  Prior to a point of no return where rectification is impossible e.g. assembly of sealed parts  Before costly operations such as plating are carried out  Before painting which could mask defects  Prior to a process where failure of a part could result in costly damage to machinery

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 Consider a sample of 36 location pins selected randomly from a batch of 200.  The nominal diameter is 10mm, each pin is measured and the size and frequency is recorded on a histogram.  The frequency is plotted vertically and the size horizontally  The width of each bar is classified as the class interval

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 Control charts are based on the principle of variability of a process following a normal distribution curve.  There are a number of methods of producing control charts based on this variability or dispersion such as range, mean deviation and standard deviation.  Each has its own advantages but Standard deviation is the most satisfactory for control charts  Standard deviation is the distance from the mid-point on a distribution curve where it starts to change direction and move horizontally as in chart opposite.

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 Wheel DiameterFrequency 115 mm3 116 mm7 117 mm mm mm mm8 121 mm2 115 mm mm mm mm mm mm

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 115 mm mm mm mm mm mm mm

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 During production Sample batches are measured and the mean size is plotted on the graph this should remain between the UWL and LWL preferably around the normal size.  If they cross these lines then you need to consider making adjustment before they reach the UAL or LAL  If they cross the UAL or LAL the process is out of control and will start to make defective parts  In the chart opposite (bottom) the green line shows a gradual drift in size in a positive direction indicating tool wear over a period of time. The black line shows a gradual increase followed by a sharp increase going outside the UAL which shows a problem has occurred, possibly a broken tool

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