Metrologi Industri & Kontrol Kualitas#10 Control Charts for VariablesRusnaldy, PhDLab. Metrologi IndustriTeknik Mesin Undip2010Teknik Mesin Undip

Histograms do not take into account changes over time.Control charts can tell us when a process changes

VARIATIONNo two objects are ever made exactly alikeNo two natural items in any category are the sameIf two items appear to have the same measurement, it is due to to the limits of our measuring instruments VariationLargeVery Small: height of human being: weight of fiber tipped pensItems are identicalPercision instruments will show differences

Three categories of variationsWithin-piece variationsurface roughnessPiece-to-piece variationoccure among pieces produced at the same timeTime-to-time variationthe difference in product produced at different times of the day

Equipment-tool wear, machine vibration, electrical fluctuationMaterial-tensile strength, ductility, thickness, porosity, moisture contentEnvironment-temperature, light, radiation, particle size, pressure, humidityOperator-method, physical and emotional, personal problemInspection-faulty inspection equipment, incorrect application of a quality standard, too heavy a pressure on a micrometerSource of variation

THE CONTROL CHART METHODIs a means of visualizing the variations that occur in the central tendency and dispersion of a set of observations

ChartsX chart : central tendencyR chart: dispersion

OBJECTIVES OF VARIABLE CCFor achieving quality improvementTo determine the process capabilityFor decisions in regard to product specificationsFor current decisions in regard to the production processFor current decisions in regard to recently produced items

CONTROL CHART TECHNIQUES

Individual Values compared to Averages

Control Limits and SpecificationsControl Limits : are established as a function of the averages. Control charts can not determine if the process is meeting specificationsSpecifications: are the permissible variation in the size of the parts and are for individual values. Established by design engineer

Process Capability (6s)(1) 6s < U - L

(1) 6s = U - L

(1) 6s > U - L