1、STATISTICAL QUALITY CONTROL,Chapter Thirteen,Copyright 2014 by The McGraw-Hill Companies, Inc. All rights reserved.,McGraw-Hill/Irwin,Learning Objectives,LO1301: Illustrate process variation and explain how to measure it. LO1302: Analyze process quality using statistics. LO1303: Analyze the quality
2、of batches of items using statistics.,13-2,Statistical Quality Control (SQC),The quantitative aspects of quality management Processes usually exhibit some variation in their output,13-3,Measuring Variation,13-4,Variation and Specifications,Upper specification the maximum acceptable value for a chara
3、cteristic Lower specification the minimum acceptable value for a characteristic,Traditional View of Variability Costs,Taguchis View of Variability Costs,13-5,Process Capability,The ability of a process to consistently produce a good or deliver a service with a low probability of generating a defect
4、Specification limits range of variation that is considered acceptable by the designer or customer Process limits range of variation that a process is able to maintain with a high degree of certainty,13-6,Process Capability,Process control limits exceed specification limits process is not capable of
5、meeting requirements,13-7,Process Capability,Specification control limits exceed process limits (for improved process) process is capable of meeting requirements,13-8,Ratio of the range of values produced divided by the range of values allowed Shows how well the parts being produced fit into the ran
6、ge specified by the design specifications Cpk larger than one indicates process is capable When the two numbers are not close, indicates mean has shifted,Excel: Process Capability,13-9,Process Capability Index (Cpk),Example 13.1,13-10,The quality assurance manager is assessing the capability of a pr
7、ocess that puts pressurized grease in an aerosol can. The design specifications call for an average of 60 pounds per square inch (psi) of pressure in each can with an upper specification limit of 65 psi and a lower specification limit of 55 psi. A sample is taken from production and it is found that
8、 the cans average 61 psi with a standard deviation of 2 psi. What is the capability of the process? What is the probability of producing a defect?,Example 13.1: Solution,13-11,Process Control Procedures,Concerned with monitoring quality while the product or service is being produced Statistical proc
9、ess control - testing a sample of output to determine if the process is producing items within a preselected range Attributes - quality characteristics that are classified as either conforming or not conforming Variable - characteristics that are measured using an actual value,Excel: Statistical Pro
10、cess Control,13-12,Process Control Measurements p-Charts,Used when an item (or service) is either good or bad (a yes-no decision),13-13,Creating p-Charts,13-14,Process Control Measurements c-Charts,Used when an item (or service) may have multiple defects,13-15,Variable Measurement Process Control Ch
11、arts X-bar and R-Charts,Size of samples Preferable to keep small (usually 4 or 5 units) Number of samples Once chart set up, each sample compared to chart Use about 25 samples to set up chart Frequency of samples Trade-off between cost of sampling and benefit of adjusting the system Control limits G
12、enerally use z = 3,13-16,X-bar and R-Charts,13-17,Process Control Charts Interpretation,13-18,Acceptance Sampling,Performed on goods that already exist to determine what percentage of the products conform to specifications Executed through a sampling plan Results include accept, reject, or retest,13
13、-19,Acceptance Sampling Purposes,Determine quality level Ensure quality is within predetermined level,13-20,Acceptance Sampling,Risks of accepting “bad” lots and rejecting “good” lots Added planning and documentation Sample provides less information than 100 percent inspection,Economy Less handling
14、damage Fewer inspectors Upgrading of the inspection job Applicability to destructive testing Entire lot rejection (motivation for improvement),Disadvantages,Advantages,13-21,Acceptance Sampling Designing a Sampling Plan,Determine (1) how many units, n, to sample from a lot, and (2) the maximum number of defective items, c, that can be found in the sample before the lot is rejected,13-22,Operating Characteristic Curve,13-23,
