CHAPTER-5 MEASUREMENT SYSTEM ANALYSIS. Two case studies (case study-3 and 4) conducted in bearing

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1 6 CHAPTER-5 MEASUREMENT SYSTEM ANALYSIS 5.0 INTRODUCTION: Two case studies (case study- and 4) conducted in bearing manufacturing facility. In this industry the core process is bearing rings machining using processor controlled grinding machines. The rings dimensions must be close to the target values, other wise bearing life will be reduced. The rings inner diameter is measured with air gauges and outer diameter measured with dial gauges. As measurement system plays crucial role in the industry, MSA is conducted in Sujana Universal Industries Limited (SUIL). As the tolerances are very important for bearings, measurement system plays crucial role here. SUILs measurement system analysed with Gage-RR studies. When appraisers/operators do not measure a part consistently, the expense to a company can be great. Traditionally, the tool to address the appraiser/operator consistency is a gage repeatability and reproducibility(r&r) study, which is the evaluation of measuring instruments to determine capability to yield a precise response. Gage repeatability is the variation in measurements considering one part and one operator. Gage reproducibility is the variation between operators measuring one part. Mathematically, measurement system analysis involves the understanding and quantification of measurement variance, in relation to process variability. σ T = σ p + σ m

2 7 σ T = Total variance σ p = Process Variance σ m = Measurement Variance Repeatability is the variation due to the measuring device. Conditions for Gauge : RR- Repeatability Same Person Same thing being measured Same characteristic Same Instrument Same Set-up Same environmental conditions Reproducibility is the variation due to the measurement system. It is the variation found with different persons measuring the identical parts using the same device. Different Person Same part Same characteristic Same Instrument Same Set-up Same environmental conditions 5. Gage R&R CONSIDERATIONS In a gage R&R study the following characteristics are essential: The measurements must be in statistical control, which is referred to as statistical stability. Measurement system variation should be from common causes and not from assignable causes.

3 8 Measurement system variation must be low relative to the manufacturing process variation. (less than 0 % of process variability) 5. Gage R&R RELATIONSHIPS: σm = σe + σo σm = Measurement System standard Deviation σe = Gage Standard Deviation σo = Appraiser standard deviation The measurement system study or an independent process capability study determines the part standard deviation σp component of the total gage study variation in the equation σt = σp + σm The percentage of process variation contributed by the measurement system for repeatability and reproducibility (%R&R) is then estimated as % R&R = (σm/ σt)*00 Number of Distinct categories = (σp/ σm)*.4 The number of distinct categories must be at least 5 or more, for the measurement system to perform in an acceptable analysis of the process. The Crossed Gauge-RR : Design. Select parts to be measured. Select the persons to measure the parts.. Ask each operator to measure each part for selected no of times

4 9 Popular Gauge RR- Design Three Operators-0 parts- Three trials The criterion to judge R&R is: Very good < 0%, Good : 0 % to 0%, Unacceptable: > 0 % Bearing Rings Selected for Study: The company makes wide variety of bearings. 5 Bearings were selected for study based on the volume of production. Table 5. Bearing Ring Specifications Bearing No Ring Characteristic Specifications In mm Specifications In mm 60 Inner Bore Dia 4.99 to Outer Outer Dia to Inner Bore Dia to Outer Outer Dia to Inner Bore Dia to Outer Outer Dia to Inner Bore Dia to Outer Outer Dia to LM48548 Inner Bore Dia 4.95 to LM48548 Outer Outer Dia to

5 40 5. STATISTICAL STABILITY OF THE PROCESS: To check the statistical stability of the process, data collected and analysed using control charts like X-Bar and R-Charts. Table 5. : Control Charts Summary Bearing No Ring Characteristic Specifications Remarks In mm 60 Inner Bore Dia 4.9 to 5.00 Stable 60 Outer Outer Dia 4.89 to 5.00 Stable 6 Inner Bore Dia to Stable 6 Outer Outer Dia to Stable 608 Inner Bore Dia 9.88 to Stable 608 Outer Outer Dia to Stable Inner Bore Dia to Stable Outer Outer Dia 09.8 to 0.00 Stable LM48548 Inner Bore Dia 4.9 to 5.06 Stable LM48548 Outer Outer Dia to 65. Stable CONCLUSIONS FROM CONTROL CHARTS: For all the rings the values falls within the tolerance limits. Hence overall the process seems to be in statistical stable condition. 5.4 GAGE-RR STUDIES: To analze the GAGE-RR studies 0 parts were selected, three operators and three trials were performed. Measured dimensions were shown in table 5.

6 4 Table 5. Data to analyze Gage RR studies Bearing Ring Code:60 Machine 500 Process: IR -Bore Grinding Inner Ring Tolerances Gauge: Air Gauge 4.99 TO 5 MM OPERATOR- OPERATOR- OPERATOR- PART Results for: 60-IR.MTW Gage R&R Study - ANOVA Method Two-Way ANOVA Table With Interaction Source DF SS MS F P PART OPERATOR PART * OPERATOR Repeatability Total Alpha to remove interaction term = 0.5 Gage R&R %Contribution Source VarComp (of VarComp) Total Gage R&R Repeatability Reproducibility OPERATOR OPERATOR*PART Part-To-Part Total Variation Number of Distinct Categories = 8

7 4 Gage R&R (ANOVA) for RESPONES Gage name: Date of study : Reported by : Tolerance: Misc: 00 Components of Variation % Contribution RESPONES by PART % Study Var Percent Gage R&R Repeat Reprod Part-to-Part R Chart by OPERATOR PART RESPONES by OPERATOR 9 0 Sample Range Xbar Chart by OPERATOR _ UCL= R=0.000 LCL= OPERATOR OPERATOR * PART Interaction OPERATOR Sample Mean _ X= UCL= LCL= Average PART Figure 5. Gage RR- ANOVA for Responses Table 5.4 Summary of GAGE-RR Results Bearing No Ring Characteristic GAGE- RR No of Distinct Categories 60 Inner Bore Dia 8 60 Outer Outer Dia Inner Bore Dia Outer Outer Dia.5 6 Inner Bore Dia Outer Outer Dia. 9 Inner Bore Dia 7.9 Outer Outer Dia.95 8 LM48548 Inner Bore Dia LM48548 Outer Outer Dia.06

8 4 5.5 CONCLUSIONS ON GAGE-RR STUDIES: Except for one bearing ring(), other ring Gage-RR studies indicates that the measurement system is sound. Reasons for failure of Gage-RR for the bearing- could be excessive variability between pieces. Once the Measurement System is sound, quality improvement programs can be planned and executed. As the Gage-RR study here shows measurement system is performing satisfactorily, Six Sigma programs are planned and conducted. The details of Six Sigma projects are given in Chapter 7(Case Study-) and Chapter 8 (Case Study-4)