InspectionThe means by which poor quality is detected and good quality is assuredTraditionally accomplished using labor-intensive methodsSampling inspection is commonManual inspection is usually performed after parts are already madeIf defective product is produced, it is too late to correct during regular processingDefective parts that are already made must be scrapped or reworked

New QC Approaches to Address these Problems100% automated inspection rather than sampling inspection using manual methodsOn-line sensor systems for inspection during or immediately after processingFeedback control of sensor data to manufacturing processAdvanced inspection and sensor technologies (e.g., CMMs, machine vision)

Types of InspectionInspection for variables One or more quality characteristics of the part or product are measured Requires appropriate measuring instrumentationInspection for attributes Part or product is inspected to decide whether it conforms to the accepted quality standardSometimes based on judgment of inspectorSometimes uses a gageSometimes involves counting number of defects

Inspection ProcedurePresentation item is presented for examinationExamination consists of measuring or gaging a quality characteristic, or searching for and counting defects Decision to accept or reject the item? Action the item is accepted or rejected If rejected, can the item be reworked?Additional action may include adjustments in the manufacturing process

Which Features to Inspect? Key CharacteristicsIn general, inspecting every feature is unnecessaryCertain features are more importantKey characteristics (KCs) include:Matching dimensions of assembled componentsSurface roughness on bearing surfacesStraightness and concentricity of high-speed rotating shaftsFinishes of exterior surfaces on consumer products such as cars

Inspection ErrorsErrors can occur in the inspection procedure during the examination and decision stepsType I error when a good item is classified as defectiveA false alarmType II error when a defective item is classified as goodA miss

Why Errors Occur in Manual InspectionComplexity of the inspection taskInherent variations in the inspection procedureJudgment required by inspectorMental fatigueInaccuracies in the measuring or gaging instruments

Why Errors Occur in Automated InspectionComplexity of the inspection taskResolution of the inspection sensor, as affected by gain or other sensitivity adjustmentsEquipment malfunctionsBugs in the computer program controlling the inspection procedure

Inspection vs. TestingInspection is used to assess the quality of a part or product relative to design specificationsTesting is used to assess the functional aspects of the productDoes the product operate the way it is supposed to?Will it operate in environments of extreme temperature and humidity?In QC testing, the item is observed during actual operation or under conditions that might be present during operation

Sampling InspectionVariables sampling the mean value of the quality characteristic of interest is compared to an allowed valueThe batch is rejected if the mean value compares unfavorablyAttributes sampling if the number of defects in the sample is greater than the acceptance number the batch is rejectedThe allowed value or acceptance number is chosen so that the probability of rejecting the batch is small unless the quality level is indeed poor

100% Manual InspectionIn principle, the only way to achieve 100% acceptable quality is to use 100% inspectionTwo problems with 100% inspection when performed manuallyInspection cost per part is applied to every part in the batch rather than a small portion of the batch (the sample)Errors (types I and II) that accompany human inspection

Automated InspectionAutomation of one or more steps in the inspection procedureAutomated presentation of parts to human inspectorHuman inspector performs examination and decision stepsAutomated examination and decisionManual loading (presentation) and unloadingComplete automation of entire cycle (presentation, examination, and decision)

More on Automated InspectionFull potential of automated inspection is best achieved whenIt is integrated into the manufacturing process100% inspection is usedResults of the procedure lead to positive actionFeedback process control To allow compensating adjustments in the process to reduce variability and improve qualityParts sortation Defects are separated from process output

Action Steps in Automated Inspection

Feedback process control

Sortation into two or more quality levels

Off-Line and On-Line InspectionOff-line inspection performed away from manufacturing process, usually after a time delayOn-line inspection performed when the parts are made, either as an integral step in processing, or immediately afterwardOn-line/in-process inspection performed during the manufacturing processOn-line/post-process inspection performed right after the process

Off-Line Inspection

Performed away from the manufacturing processUsually a time delay between processing and inspection

On-Line/In-Process Inspection

The inspection procedure is performed during the manufacturing operationAllows for corrective action on current work unit

On-Line/Post-Process Inspection

Measurement or gaging procedure is accomplished immediately following the production processOn-line because it is integrated with the manufacturing workstation, and the results can immediately influence the production process for the next work part

Inspection Technologies

Inspection MetrologyMeasurement - a procedure in which an unknown quantity is compared to a known standard, using an accepted and consistent system of unitsThe means by which inspection by variables is accomplishedMetrology the science of measurementConcerned with seven basic quantities: length, mass, electric current, temperature, luminous intensity, time, and matterFrom these basic quantities, other physical quantities are derived

Characteristics of Measuring InstrumentsAccuracy how closely the measured value agrees with the true valuePrecision a measure of the repeatability of the measurement process Rule of 10 the measuring instrument must be ten time more precise than the specified toleranceResolution the smallest variation of the variable that can be detectedSpeed of response how long the instrument takes to measure the variableOthers: operating range, reliability, cost

Analog vs. Digital InstrumentsAnalog measuring instrument output signal varies continuously with the variable being measuredOutput signal can take on any of an infinite number of possible values over its operating rangeDigital measuring instrument can assume any of a discrete number of incremental values corresponding to the variable being measuredNumber of possible output values is finiteAdvantages:Ease of reading the instrumentEase of interfacing to a computer

Two Basic Types of Inspection TechniquesContact inspectionMakes contact with object being inspectedNoncontact inspectionDoes not make contact with object being inspected

Contact Inspection TechniquesUses a mechanical probe that makes contact with the object being measured or gagedPrincipal techniques:Conventional measuring and gaging instruments, manual and automatedCoordinate measuring machinesStylus type surface texture measuring machines

Noncontact Inspection TechniquesUses a sensor or probe located a certain distance away from the object being measured or gagedTwo categories:Optical uses light to accomplish the inspectionNonoptical - uses energy form other than lightAdvantages of noncontact inspection:Avoids possible damage to surface of objectInherently faster than contact inspectionCan often be accomplished in production without additional part handlingIncreased opportunity for 100% inspection

Conventional Measuring and Gaging TechniquesMeasuring instruments - provide a quantitative value of the part feature of interestExamples:Steel rules, calipers, micrometer, dial indicator, protractorGages - determines whether a part feature falls within a certain acceptable rangeExamples:Snap gages for external dimensions, plug gages for hole diameters, thread gages

Coordinate MetrologyConcerned with the measurement of the actual shape and dimensions of an object and comparing these with the desired shape and dimensions specified on a part drawingCoordinate measuring machine (CMM) an electromechanical system designed to perform coordinate metrologyA CMM consists of a contact probe that can be positioned in 3-D space relative to workpart features, and the x-y-z coordinates can be displayed and recorded to obtain dimensional data about geometry

Coordinate Measuring Machine

CMM ComponentsProbe head and probe to contact workpart surfacesMechanical structure that provides motion of the probe in x-y-z axes, and displacement transducers to measure the coordinate values of each axisOptional components (on many CMMs):Drive system and control unit to move each axisDigital computer system with application software

Contact Probe Configurations

(a) (b)(a) Single tip and (b) multiple tip probes

CMM Mechanical StructureSix common types of CMM mechanical structures:CantileverMoving bridgeFixed bridgeHorizontal armGantryColumn

CMM Structures

(a) (b)(a) Cantilever and (b) moving bridge structure

CMM Structures

(c) (d)(c) Fixed bridge and (d) horizontal arm (moving ram type)

CMM Structures

(e)(f)(e) Gantry and (f) column

CMM Operation and Controls Four Main CategoriesManual drive CMM human operator physically moves the probe and records x-y-z- dataManual drive and computer-assisted data processing can perform calculations to assess part featuresMotor-driven CMM with computer-assisted data processing uses joystick to actuate electric motors to drive probeDirect computer control (DCC) operates like a CNC machine tool and requires part program

DCC ProgrammingManual leadthrough Operator leads the CMM probe through the various motions in the inspection sequence, indicating points and surfaces to be measured and recording these into control memoryOff-line programming Program includes motion commands, measurement commands, and report formatting commands and is prepared off-line

CMM SoftwareThe set of programs and procedures used to operate the CMM and its associated equipmentExample: part programming software for DCC machinesOther software divide into following categories:Core software other than DCC programmingPost-inspection softwareReverse engineering and application-specific software

Post-Inspection SoftwarePrograms applied after the inspection procedureStatistical analysis used to accomplish various statistical analysesProcess capabilityStatistical process controlGraphical data representation displays data collected during CMM inspection in a graphical or pictorial way, permitting easier visualization of form errors and other data

Reverse Engineering and Application-Specific SoftwareReverse engineering CMM explores part surface and constructs 3-D model Application-specific software:Gear checkingThread checkingCam checkingAutomobile body checking

Advantages of using CMMs over Manual InspectionReduced inspection cycle time translates to higher throughput rateEspecially with DCC, approximately 90% reduction in certain tasksFlexibility CMMs are general-purpose machinesReduced operator errors in measurement and setupGreater inherent accuracy and precisionAvoidance of multiple setups in general all measurements of a given part can be made in one setup

Inspection Probes on Machine ToolsMounted on toolholdersStored in the tool drumHandled by the automatic tool-changer the same way cutting tools are handledInserted into the machine tool spindle by the automatic tool-changer When mounted in the spindle the machine tool is controlled very much like a CMMSensors in the probe determine when contact is made with part surface so that required data processing is performed to interpret the sensor signal

Portable CMMsIn the conventional application of a CMM, parts must be removed from the production machine and taken to the inspection department where the CMM is locatedNew coordinate measuring devices allow the inspection procedures to be performed at the site where the parts are madeExample: Faro gage, a.k.a. Personal CMM, is a six-jointed articulated armAt the end of the arm is a touch probe to perform coordinate measurements, similar to a CMM

Machine VisionAcquisition of image data, followed by the processing and interpretation of these data by computer for some useful applicationAlso called computer vision2-D vs. 3-D vision systems:2-D two-dimensional image adequate for many applications (e.g., inspecting flat surfaces, presence or absence of components)3-D three-dimensional image requires structured light or two cameras

Operation of a Machine Vision SystemImage acquisition and digitizationImage processing and analysis Interpretation

Image Acquisition and DigitizationWith camera focused on subject, viewing area is divided into a matrix of picture elements (pixels)Each pixel takes on a value proportional to the light intensity of that portion of the scene and is converted to its digital equivalent by ADCIn a binary system, the light intensity is reduced to either of two values, white or blackIn a gray-scale system, multiple light intensities can be distinguishedEach frame is stored in a frame buffer (computer memory), refreshed 30 times per second

Dividing the image into a Matrix of Picture Elements (Pixels)(a) The scene(b) 12 x 12 matrix superimposed on the scene(c) Pixel intensity values, either black or white, in the scene

Illumination Techniques

(a) (b) (c)(a) Front lighting, (b) back lighting, (c) side lighting

More Illumination Techniques

(d) Structured lighting using a planar sheet of light

Image Processing and AnalysisSegmentation techniques to define and separate regions of interest in the imageThresholding converts each pixel to a binary value (white or black) by comparing the intensity level to a defined threshold valueEdge detection determines location of boundaries between an object and its background, using the contrast in light intensity between adjacent pixels at the boundary of an objectFeature extraction determines an objects features such as length, area, aspect ratio

InterpretationFor a given application, the image must be interpreted based on extracted featuresConcerned with recognizing the object, called pattern recognition - common techniques:Template matching compares one or more features of the image object with a template (model) stored in memoryFeature weighting combines several features into one measure by weighting each feature according to its relative importance in identifying the object

Machine Vision ApplicationsInspection:Dimensional measurementDimensional gagingVerify presence or absence of components in an assembly (e.g., PCB)Verify hole locations or number of holesDetection of flaws in printed labelsIdentification for parts sorting or countingVisual guidance and control for bin picking, seam tracking in continuous arc welding, part positioning

Other Optical Inspection MethodsConventional optical instrumentsOptical comparatorConventional microscopeScanning laser systemsLinear array devicesOptical triangulation techniques

Scanning Laser Device

Linear Array Measuring Device

Noncontact Nonoptical Inspection TechniquesElectrical field techniquesReluctance, capacitance, inductanceRadiation techniquesX-ray radiationUltrasonic inspection methodsReflected sound pattern from test part can be compared with standard Parts must always be presented in the same position and orientation relative to the probe

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