Quality Control
Dr. Richard Jerz
© 2010 rjerz.com1
Specifying Quality
• Tolerances• Fits
• Applies to manufacturing• Applies to service, too
© 2010 rjerz.com2
Purpose of Tolerances
• Impossible to make all parts to exact dimension• Interchangeability of parts• Avoid selective assembly• Different degrees of size (toys versus jets)• Quality depends upon repeatability and sizing• Loose and tight fit
© 2010 rjerz.com3
Fit Types (Parts)
• Clearance• Always a clearance
• Interference• Always interference• Press fits
• Transition• Clearance or interference depends upon specific parts
© 2010 rjerz.com4
Clearance Fit
© 2010 rjerz.com5
Standard Inch Fits
• Use of a symbols• Typically not shown on drawings, dimensions are typically shown
• Symbols: RC, LC, LT, LN, FN• Numbers – degree of fit• Complete Description ‐ Two letters and number• Example: RC2
© 2010 rjerz.com6
Design Suggestion
• Shafts rotating under 600 rpm with ordinary loads; >RC5
• Shafts rotating over 600 rpm with heavy loads; < RC5
• Shafts sliding freely; approx. LC• Push fits with keyed shafts and clamp, no fitting; LT• Parts assembled with some basic fitting; LN• Permanent assembly with no freely moving parts; FN1
• Permanent assembly with severe loading effects; FN3• Permanent assembly with press needed for assembly; FN5
© 2010 rjerz.com7
Tolerance Example1” RC7
© 2010 rjerz.com8
Table 17
Matching Specs to Processes
• Capability analysis
© 2010 rjerz.com9
Tolerances and Machining Processes
© 2010 rjerz.com10
© 2010 rjerz.com11
© 2010 rjerz.com12
© 2010 rjerz.com13
Example: Robot Repeatability Data
© 2010 rjerz.com14
Inspection Questions
• How Much/How Often• Where/When • Centralized vs. On‐site
© 2010 rjerz.com15
Inputs Transformation Outputs
Acceptancesampling
Processcontrol
Acceptancesampling
Inspection Costs
© 2010 rjerz.com16
Cos
t
OptimalAmount of Inspection
Cost of inspection
Cost of passingdefectives
Total Cost
Methods of Assuring Quality
© 2010 rjerz.com17
Acceptancesampling
Processcontrol
Continuousimprovement
Inspectionbefore/afterproduction
Correctiveaction duringproduction
Quality builtinto theprocess
The leastprogressive
The mostprogressive
Where to Inspect in the Process
• Raw materials and purchased parts• Finished products• Before a costly operation• Before an irreversible process• Before a covering process
© 2010 rjerz.com18
Examples of Inspection Points
© 2010 rjerz.com19
Type ofbusiness
Inspectionpoints
Characteristics
Fast Food CashierCounter areaEating areaBuildingKitchen
AccuracyAppearance, productivityCleanlinessAppearanceHealth regulations
Hotel/motel Parking lotAccountingBuildingMain desk
Safe, well lightedAccuracy, timelinessAppearance, safetyWaiting times
Supermarket CashiersDeliveries
Accuracy, courtesyQuality, quantity
Acceptance Sampling
• Form of inspection applied to lots or batches of items before or after a process, to judge conformance with predetermined standards (supplemental Chapter)
© 2010 rjerz.com20
Statistical Process Control
• Statistical evaluation of the output of a process during production• Define• Measure• Compare to a standard• Evaluate• Take corrective action• Evaluate corrective action
© 2010 rjerz.com21
What to Inspect?
• Variables
• Attributes
© 2010 rjerz.com22
Error Conditions
• Type 1• Type 2
© 2010 rjerz.com23
Mean
LCL UCL
/2 /2
Probabilityof Type I error
Type 1 and Type 2
© 2010 rjerz.com24
Statistical Process Control
• Variations and Control• Random variation: Natural variations in the output of process, created by countless minor factors
• Assignable variation: A variation whose source can be identified
© 2010 rjerz.com25
Process Capability
• Tolerances (design)• specifications
• Process variability• Natural variability in a process
• Process capability• Process variability relative to specification
© 2010 rjerz.com26
Control Charts
• Statistics• Normal distribution• Central limit theorem• “Out of control”
© 2010 rjerz.com27
© 2010 rjerz.com28
Normal Distribution
© 2010 rjerz.com29
Mean
95.44%
99.74%
Standard deviation
Control Limits
© 2010 rjerz.com30
Samplingdistribution
Processdistribution
Mean
Lowercontrol
limit
Uppercontrol
limit
Control Chart
© 2010 rjerz.com31
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
UCL
LCL
Sample number
Mean
Out ofcontrol
Normal variationdue to chance
Abnormal variationdue to assignable sources
Abnormal variationdue to assignable sources
Figure 10-8
Mean and Range Charts
© 2010 rjerz.com32
UCL
LCL
UCL
LCL
R-chart
x-Chart Detects shift
Does notdetect shift
(process mean is shifting upward)
SamplingDistribution
Control Charts
• Warning conditions• Two successive points near limit• Run of five above or below mean• Trend• Erratic behavior
© 2010 rjerz.com33
Industry Trends
• Continuous Improvement• In process inspection• 100% inspection• Use of statistical quality control• Deming concepts• Automate control of equipment
© 2010 rjerz.com34