Control Charts in Manufacturing
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Questions and Answers

What is the primary purpose of a control chart?

  • To predict the future performance of the process without analyzing the data
  • To make frequent adjustments to the manufacturing process
  • To eliminate all sources of variation in the process
  • To detect variations in the processing and warn of departures from specified tolerance limits (correct)

    • What is the condition of a process when it is currently under control?

  • Unstable with variation coming from sources external to the process
  • Stable with variation coming from sources common to the process (correct)
  • Unstable with variation coming from sources common to the process
  • Stable with variation coming from sources external to the process

    • What should be done when a process is in control?

  • Stop the process and restart with new parameters
  • Take a review of the situation and take corrective steps
  • Adjust the process control parameters
  • Leave the process alone and prevent unnecessary adjustments (correct)

    • What is the purpose of a control chart when both upper and lower values are specified for a quality characteristic?

    <p>To take a review of the situation and take corrective steps when products show variation outside the tolerances</p> Signup and view all the answers

    What is the result of adjusting process control parameters for a process that is in control?

    <p>Degraded process performance</p> Signup and view all the answers

    What does a control chart help to differentiate?

    <p>Significant change from the natural variability of the process</p> Signup and view all the answers

    What is the benefit of using control charts?

    <p>It helps in detecting undesired variations and removing their causes</p> Signup and view all the answers

    What is the basis of control charts?

    <p>Statistical sampling theory</p> Signup and view all the answers

    What is the main difference between variable control charts and attribute control charts?

    <p>Variable control charts measure job dimensions, while attribute control charts only differentiate between defective and non-defective items.</p> Signup and view all the answers

    What is a control chart primarily used for?

    <p>To judge whether statistical control has been attained or not</p> Signup and view all the answers

    What is the main purpose of Acceptance Sampling?

    <p>To make a decision as to whether to accept or to reject lots based on inspection of sample(s)</p> Signup and view all the answers

    In a single sampling plan, what happens if the number of defectives exceeds the acceptance number?

    <p>The lot is rejected.</p> Signup and view all the answers

    What is the purpose of the center line in a control chart?

    <p>To indicate the mean of the statistic</p> Signup and view all the answers

    What is the advantage of Acceptance Sampling over 100 percent inspection?

    <p>It is less expensive</p> Signup and view all the answers

    What is the primary purpose of sequential sampling plan?

    <p>To make a decision on acceptance or rejection after a stated number of samples.</p> Signup and view all the answers

    What is the advantage of attribute control charts?

    <p>They provide a quick summary of various aspects of product quality.</p> Signup and view all the answers

    In a double sampling plan, what happens if the number of defectives is less than or equal to the acceptance number (C1)?

    <p>The lot is accepted.</p> Signup and view all the answers

    What is the primary difference between control charts for variables and attributes?

    <p>Variables are measured, while attributes are counted</p> Signup and view all the answers

    What is the purpose of acceptance sampling?

    <p>To decide whether to accept or reject a production lot of material.</p> Signup and view all the answers

    What is the Operating Characteristic Curve (OC Curve) used for?

    <p>To show the probability of accepting lots with various levels of quality</p> Signup and view all the answers

    What is a characteristic of sequential sampling plan?

    <p>It involves higher administrative costs.</p> Signup and view all the answers

    What is the disadvantage of attribute control charts?

    <p>They require a larger sample size.</p> Signup and view all the answers

    What is the purpose of upper and lower warning limits in a control chart?

    <p>To provide an early warning of potential problems in the process</p> Signup and view all the answers

    What is the Acceptance Quality Level (AQL)?

    <p>The maximum percentage of defective items that can be considered satisfactory as a process average for sampling inspection</p> Signup and view all the answers

    In a sequential sampling plan, what happens if by the end of fourth sample, the lot is neither accepted nor rejected?

    <p>A sample n5 is drawn.</p> Signup and view all the answers

    What is the purpose of the standard error in a control chart?

    <p>To calculate the upper and lower control limits</p> Signup and view all the answers

    What is the Lot Tolerance Percent Defective (LTPD)?

    <p>The level of quality that is routinely rejected by the sampling plan</p> Signup and view all the answers

    What is the advantage of variable control charts?

    <p>They are more sensitive than attribute control charts.</p> Signup and view all the answers

    What is true about double sampling plan?

    <p>It may draw one or two samples.</p> Signup and view all the answers

    What is the purpose of inspection for acceptance?

    <p>To decide whether to accept or reject a production lot of material.</p> Signup and view all the answers

    What is the producer's risk?

    <p>The risk of rejecting a good lot</p> Signup and view all the answers

    What is the primary advantage of using control charts for attributes?

    <p>They require less time to monitor than variables</p> Signup and view all the answers

    In a single sampling plan, what is the number of defectives that determines the acceptance or rejection of the lot?

    <p>Acceptance number.</p> Signup and view all the answers

    What is the disadvantage of variable control charts?

    <p>They are more expensive than attribute control charts.</p> Signup and view all the answers

    What is the consumer's risk?

    <p>The risk of accepting a defective lot</p> Signup and view all the answers

    What is the purpose of division into zones in a control chart?

    <p>To provide additional rules for governing frequencies of observations</p> Signup and view all the answers

    What type of chart is used for control of number of defects per unit?

    <p>C chart.</p> Signup and view all the answers

    What is the purpose of the second sample in a double sampling plan?

    <p>To make a final decision on the lot acceptance or rejection.</p> Signup and view all the answers

    What is the primary purpose of a control chart?

    <p>To detect unusual variations in a process</p> Signup and view all the answers

    What is a sampling plan?

    <p>A plan for acceptance sampling that precisely specifies the parameters of the sampling process and the acceptance/rejection criteria</p> Signup and view all the answers

    Study Notes

    Control Charts

    • Control charts are used to determine when to leave a process alone and prevent unnecessary adjustments.
    • They are graphical representations based on statistical sampling theory.
    • Control charts detect variations in the processing and warn if there is any departure from specified tolerance limits.
    • They help in detecting the cause of undesired variations and removing them.
    • Control charts are used to predict the future performance of a process if it is currently under control.
    • They can help determine the sources of variation and eliminate them to bring the process back into control.

    Purpose and Advantages of Control Charts

    • Control charts indicate whether a process is in control or out of control.
    • They determine process variability and detect unusual variations.
    • They ensure product quality level.
    • They warn in time, reducing scrap or percentage rejection.
    • They provide information about the selection of process and setting of tolerance limits.
    • Control charts build up the reputation of the organization through customer satisfaction.

    Components of a Control Chart

    • Points representing a statistic (e.g., mean, range, proportion) of measurements of a quality characteristic in samples.
    • The mean of the statistic using all the samples.
    • A center line drawn at the value of the mean of the statistic.
    • Upper and lower control limits (natural process limits) indicating the threshold at which the process output is considered statistically 'unlikely'.
    • Optional features: upper and lower warning limits, division into zones, and annotation with events of interest.

    Types of Control Charts

    • Control charts can be used to measure any characteristic of a product.
    • Characteristics can be divided into two groups: variables and attributes.
    • Control charts for variables are used to monitor characteristics that can be measured and have a continuum of values.
    • Control charts for attributes are used to monitor characteristics that have discrete values and can be counted.

    Comparison of Control Charts for Variables and Attributes

    • Variables charts involve the measurement of job dimensions and an item is accepted or rejected based on tolerance limits.
    • Attribute charts differentiate between defective and non-defective items without measuring dimensions.
    • Variables charts are more detailed and contain more information.
    • Attribute charts require a bigger sample size and are less expensive.
    • Variables charts are more sensitive and can alert to quality problems before they occur.

    Commonly Used Charts

    • (X-Bar) and R charts for process control.
    • P chart for analysis of fraction defectives.
    • C chart for control of number of defects per unit.

    Acceptance Sampling

    • Acceptance sampling is concerned with the decision to accept or reject a mass of manufactured items based on standards of quality.
    • It uses statistical sampling to determine whether to accept or reject a production lot of material.
    • Acceptance sampling is usually done as products leave the factory or within the factory.
    • The decision to accept or reject is made by determining the number of defective items in a sample from the lot.
    • The lot is accepted if the number of defects falls below the acceptance number or otherwise rejected.

    Operating Characteristic Curve

    • The Operating Characteristic Curve (OC Curve) shows the probability of accepting lots with various levels of quality.
    • It is the working plan of acceptance sampling.
    • AQL (Acceptance Quality Level) is the maximum % defective that can be considered satisfactory as a process average for sampling inspection.
    • RQL (Rejectable Quality Level) is the % defective, also known as the Lot Tolerance Percent Defective (LTPD).

    Risks in Acceptance Sampling

    • Producer's risk: the risk of rejecting a good quality lot due to a bad sample.
    • Consumer's risk: the risk of accepting a bad quality lot due to a good sample.

    Acceptance Sampling Plans

    • A sampling plan specifies the parameters of the sampling process and the acceptance/rejection criteria.
    • Variables to be specified include the size of the lot, the size of the sample inspected, the number of defects above which a lot is rejected, and the number of samples that will be taken.
    • Types of sampling plans include single sampling, double sampling, and sequential sampling.
    • Single sampling plan: decision is made after drawing a single sample from the lot.
    • Double sampling plan: a second sample is drawn if the number of defectives in the first sample lies between two acceptance numbers.
    • Sequential sampling plan: additional samples are drawn until an inference can be made, with a maximum number of samples specified.

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    Description

    Learn about control charts, which are used to monitor and control manufacturing processes, preventing unnecessary adjustments and detecting variations in processing.

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