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Quality control. 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. 100% Inspection. Every feature/part is inspected Disadvantages are: Very time consuming and expensive.
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Quality control • 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
100% Inspection • Every feature/partis 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.
Acceptance Sampling • 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
Acceptance Sampling • 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
Acceptance Sampling Typical Operating Characteristic 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
When Should Inspection Take Place • 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
Graphic Representation of Data • 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
Control Charts/Standard Deviation • 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.
Control Charts • 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
