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ToleranceStackup
- 1. Page 1 Tolerance stackup analysis WinsonMao August 22, 2012
- 2. Page 2 Tolerance stackupanalysis Performed to determine the variation of a single untoleranced dimension or distance on a component or in an assembly.
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- 4. Page 4 Methods Arithmetic method Worstcase method (WC) Statistical method Root sum square method (RSS) Modified root sum square method (MRSS) Six sigma method (6S)
- 5. Page 5 Formulas WC RSSMRSS 6S Mean dimension Standard deviation N/A Bilateral tolerance Close dimension ∑∑ = − = −= n ki i k i i µµµ 1 1 ∑= = n i iTTWC 1 WCTD ±= µ ∑∑ = − = −= n ki i k i i µµµ 1 1 ∑∑ = − = −= n ki i k i i µµµ 1 1 σ3=RSST RSSTD ±= µ CT f MRSS ×= σ3 MRSSTD ±= µ ∑∑ = − = −= n ki i k i i µµµ 1 1 ∑= = n i Ti 1 2 ) 3 (σ σ×= aT t eS6 TD S6 ±= µ ∑ = = n i Ti 1 2 3 σ ∑ = = n i e C T pki i 1 2 3 σ
- 6. Page 6 Process capability σ6 LSLUSL Cp − =( )CCC puplpk ,min= σ µ 3 LSL Cpl − = σ µ 3 − = USL Cpu where , ,
- 7. Page 7 Coefficients ( ) () 1 1 5.0 + − − = nT TTC RSS RSSWC f • MRSS coefficient, • Limit size coefficient, Cf 6~3=at at
- 8. Page 8 Coefficients ( ) () 1 1 5.0 + − − = nT TTC RSS RSSWC f • MRSS coefficient, • Limit size coefficient, Cf 6~3=at at Limit sizes Process yield (%) Number of rejects per million component produced µ±1σ 68.2 317310 µ±2σ 95.4 45500 µ±3σ 99.73 2700 µ±3.5σ 99.95 465 µ±4σ 99.994 63 µ±4.5σ 99.9993 6.8 µ±5σ 99.99994 0.6 µ±6σ 99.9999998 0.002
- 9. Page 9 Arithmetic vs.statistical • Number of tolerances in the tolerance stackup. • The quantity of parts to be manufactures. • Manufacturing process controls. • Past company practices. • Willingness to accept risk.
- 10. Page 10 Step-by-step procedure Equalbilateral tolerancing Define direction of dimensions Ite m + Dim - Dim Tol 1 30 ±0.2 2 45.5 ±0.5 3 13.5 ±0.5 Total 45.5 43.5 ±1.2 25.435.45 =−=µ 2.1±=Tol Fill in table