Quality Assurance. Chapter 42

Transcription

1 Quality (Part 8)

2 Quality Assurance Chapter 42

3 Quality Definitions Quality is defined as those products and/or service features that meet or exceed customer expectations Quality Assurance (QA) is proactive to quality planning, instituting system improvements, such as defects and reliability, while upholding the after-the-fact QC and audit functions Quality Control (QC): a system that identifies and responds to nonconformities, such as defects and is reactive in nature

4 Total Quality Management (TQM) A system that ensures that quality is the constant focus and continuous improvement never stops Primary goal is to achieve increasingly higher levels of customer satisfaction while continuously improving processes linked to business measurables such as cost and productivity Driven by customer needs and maintains a constant sense of urgency to improve

5 Quality Costs Poor quality ranges from 20-40% of sales, therefore, exceeding the profit margin Four categories: internal failure external failure appraisal prevention

6 Internal Failure Costs Costs of defects identified prior to the customer s receipt of the product or service Examples: rework, unrepairable scrap, sorting bad lots and decreasing selling price due to poor quality

7 External Failure Costs The monetary value assigned to defects found after product shipment Examples: warranty charges, complaint adjustments, and cost to return material

8 Appraisal Costs Expenditures related to determine the degree of conformance to product requirements Examples: incoming inspection and testing, final inspection and testing, quality audits of in-process and finished products, calibration of instruments, and measuring devices

9 Prevention Costs Incurred to reduce expenditure on failure and appraisal costs Examples: Quality and process planning, process control, evaluating suppliers and training

10 International Organization for Standards (ISO) 9000 Primary function is the development of international standards to facilitate international trade American National Standards Institute (ANSI) is the U.S. representative to (ISO) Published in U.S. as the ANSI/ASQ Q9000 series ISO commonly used as criteria for third party audits between a supplier of production parts, components, or equipment and in original equipment manufacturer (OEM) ISO 9000 certification implies that a quality system is in place

11 Malcom Baldrige National Quality Award (MBNQA) Created to improve quality and competitiveness from American companies Public Law , Passed in 1987 Instituted by the federal government in reaction to serious foreign encroachment in the domestic market Granted annually up to six winners in three categories: large manufacturing, large service companies, and small businesses

12 MBNQA Criteria 1. Leadership (9.5%) 2. Information and analysis (7.5%) 3. Strategic quality planning (6%) 4. Human resources development and management (15%) 5. Management of process quality (14%) 6. Quality and operational results (18%) 7. Customer focus and satisfaction (30%)

13 Statistical Methods of Quality Control Chapter 43

14 Inspection Inspection is the process of checking the conformance of a final product to its specifications Inspection must be a continuous activity because raw material machines and operators are all subject to variability

15 Quality Assurance Inspection of variables requires the quantitative measurement of characteristics such as dimensions and other physical or mechanical properties Attribute inspection offers the direct comparison of a dimension to a specification. Example: Go/No-Go Gage, presence of a flaw

16 Contemporary Approach: Is Preventing Rather than Identifying Defects Reducing the impact of variation through robust design Anticipating problems through design for manufacturability and assembly The use of mistake and error proofing and Attacking common-cause variation to push process-capability indices high enough to eliminate the need for inspection

17 Statistical Quality Control (SQC) Used to Detect Process Variability Natural Variability: is the inherent, uncontrolled changes that occur in the composition of material, the performance of the operator, and the operation of machines Assignable Variability: can be traced to a specific, controllable cause

18 Statistical Process Control A method to systematically detect assignable variability in a process

19 F 43-1 Distribution of Shaft Diameters

20 Continuous Distribution of Shaft Diameters F 43-2

21 Normal Distribution With Different Means and Dispersions F 43-3

22 Control Charts Purpose: to determine whether the quality characteristic is varying within acceptable limits for natural variability or whether the process is out of control Used to track down assignable causes for changes in process performance as a part of process improvement investigation

23 Most Used Control Charts X Chart shows the quality characteristic of the average of the sample R Chart shows the quality characteristic of the range of the sample

24 Control charts for Average Dimension and range F 43-6

25 X bar R Chart x grand average R average range and represents the populationmean and represents the typicalvariance x x x x 1 2 n n number of n samples R R 1 R2 R n n

26 R A x x LCL R A x x UCL x x x x X bar and R Control Limits D R R LCL D R R UCL R R R R

27 Control Chart Constants Sample Size A 2 D 4 D 3 d

28 Various Conditions Monitored on an X control chart F 43-9

29 Process Capability Analysis A method to determine or assure that a process can meet specifications A process is capable if it is able to consistently produce parts within specification Process capability ratios above 1.33 are capable of producing good parts

30 Relationship of Specified Limits and Natural Limits F 43-10

31 Process Capability C p USL LSL 6 USL upper specified limit LSL lower specified limit process standard deviation ˆ R d 2

32 Cpk Process capability (Cp) does not account for where the process distribution is relative to the center of the tolerance. A process may have a high capability but produce bad parts if it is centered towards one of the specifications. Cpk indicates where the process is located with respected to the upper and lower limits of the tolerance. Cpk NearestSpec 3 ˆ X

33 Acceptable Sampling Used to determine if parts that have already been produced are acceptable Less than 100% of produced parts inspected. Example: destructive testing Creates the risk of rejecting good lots and accepting bad parts Does not guarantee that all parts in a lot are good

34 Types of Sampling Plans Lot Tolerance Percent Defective (LTPD) plan specifies a certain quality level with respect to a percent defect at a given risk of being accepted by the customer Acceptable Quality Level (AQL) plan specifies a certain quality level with respect to a percent defective at a given risk of being accepted by the customer

35 Dimensional Metrology Chapter 44

36 Terminology For Dimensional Metrology Accuracy is the closeness to the true answer Precision refers to the dispersion of measurements or fineness of the readings Reliability is the probability of achieving a desired outcome Repeatability of a group of measurements taken with the same instruments taken with the same instrument on the same part is the extent to which they are in agreement

37 Terminology Continued Sensitivity is the minimum input required to produce a noticeable output Resolution, in reference to dial indicators and other similar instruments, is the ratio of one scale division to the width of the dial hand Discrimination refers to the fineness of an instrument s scale divisions

38 Precision and Accuracy F 44-1 (Wick and Veilleux, 1987)

39 Types of Measurements Absolute or direct measurement Comparative measurement

40 Absolute or Direct Measurement Instruments Metric (mm) U.S. (in) Rules and scales.5.01 or 1/64 Slide calipers.5.01 or 1/64 Vernier Caliper height gages Dial calipers Digital calipers (LCD) displays Micrometers: outside, inside, depth

41 One-Inch Micrometer F 44-2 What is the micrometer reading? Answer Inches

42 What is the micrometer reading? Answer Inches

43 What is the micrometer reading? Answer Inches

44 What is the micrometer reading? Answer Inches

45 Comparison Measurement Instruments (Greater Accuracy than Direct Measurement) Type A Spindle parallel to the dial face (Dial Indicator) Type B Spindle at right angle to the dial face (Dial Indicator) Type C Measuring contact is a lever (Test Indicator)

46 F 44-3 (Wick and Veilleux, 1987) Dial Indicator

47 Continuous and Balanced Indicators F 44-4

48 F 44-5 (Wick and Veilleux, 1987) Typical Test Indicator

49 Optical Comparators or Profile Projector A measuring microscope for small parts Magnifications up to 500X Only limitation is the size of the screen

50 Horizontal Shadow-Lighted Optical Comparator F 44-6 (Wick and Veilleux, 1987)

51 Coordinate Measurement A two or three-dimensional process to locate the position of holes, surfaces, centerlines and slopes Six sides of a cube shaped part may be inspected without repositioning

52 Elements of a Coordinate Measuring Machine (CMM) X-Y-Z positioning device machine structure Probing system to collect data Machine control and computer hardware Software for geometry analysis Configurations: cantilever, column, bridge, horizontal arm and gantry

53 Moving-Bridge Coordinate Measuring Machine F 44-7 (Wick and Veilleux, 1987)

54 Advantages of Using CMMs Flexibility Reduced set up time Reduced operator influence Improved accuracy Improved productivity

55 Types of Measurement Error Instrument Inherent accuracy limitations due to construction and calibration Parallax Reader to instrument position Bias Operator will unconsciously influence a measurement Technique Manipulate or using the instrument incorrectly Condition Temperature error

56 F 44-8 Snap and Plug Gage

57 Qualifying Gages Snap Gage: used for external dimensions (shafts) Plug gage: used for internal dimensions (holes) Gage design principle - It is better to reject a good part than declare a bad part to be within specifications

58 Gage Tolerances Gage Tolerance: the permissible variation in the manufacture of the gage. Typically 10% or less of the working tolerance Wear Allowance: compensates for the wear of the gage surface as a result of repeated use

59 Standard Gagemakers Tolerances F 44-1

60 Gage Block Types Class B Working Blocks Class A Reference Blocks Class AA Master Blocks

61 Sizes of Gage Blocks in 81-Block Set One Ten-Thousandth Series 9 Blocks One Thousandth Series 49 Blocks Fifty Thousandth Series 19 Blocks Inch Series 4 Blocks

62 , BLOCKS 4 BLOCKS