# Worst Case Tolerance and Statistical Tolerance Analysis

Pages: 5 (457 words) Published: December 5, 2013
﻿Worst Case Tolerance
And
Statistical Tolerance
Analysis

1.0 REVISION PAGE

REVISION        DATE                            DESCRIPTION

1                 11-17-2013                  Initial Release 2 11-19-2013 Final Release 2.0 TABLE OF CONTENT

1.0 Revision Page ……………………………………………………………………………… 2 2.0 Table of Content …………………………………………………………………………… 3 3.0 Introduction ………………………………………………………………………………….4 4.0 Drawing…………………………………………………………………………………...… 5 5.0 Data and Calculation …………………………………………………………………………6 6.0 Recommendation ……………………………………………………………….…………….8

3.0 INTRODCUTION
Objective
The following report demonstrates Worst Case Tolerance Analysis, WCTA, and Statistical Tolerance Analysis, STA. The part is failing due to clearance between circuit card from male part of assembly and a metal contacting device of female part of assembly. We need use WCTA, STA to find suitable tolerance limit to eliminate so as to reduce probability of making parts outside of extreme tolerance zone.

Assembly Overview
The circuit card in male part of assembly which transmits the electrical current is inserted between metal contacting devices attached to female part of assembly to provide power to main source.

4.0 DRAWING

The following engineering drawing was used to identify necessary dimensions and its tolerances.

5.0 DATA AND CALCULATION
Nominal Clearance/ interference on Top and Bottom of Circuit Card Dimension Number
Dimension(d)
Ti
1A
0.87
0.1
1B
1.75
0.08
2A
1.94
0.12
2B
1.13
0.12
2C
0.4
0.1

Nominal Assembly Dimension Calculation
Bottom Contact Surface
Dimension Used

Male
1B
1.75
Female
2A
1.94
Nominal Assembly  dimension () =  1.94-1.75 =    0.19
Top Contact Surface
Dimension Used

Male
1A,1B
2.62
Female
2A,2C
2.34
Nominal Assembly  dimension() =  2.62 - 2.34 = 0.28

Note: stands for Bottom Nominal Dimension and for Top Nominal Dimension. WORST CASE TOLERANCE ANALYSIS (WCTA)
Bottom Contact Surface

Dimension Used
Ti
Σ Ti

Max interference(+Σ Ti)
Max Clearance(-Σ Ti)
1B,2A
0.08, 0.12
0.2
0.19
0.39
-0.01
Top Contact Surface

Dimension Used
Ti
Σ Ti

Max interference(+Σ Ti)
Max Clearance(-Σ Ti)
1A,1B,2A,2C
0.08, 0.1, 0.12, 0.1
0.4
0.28
0.68
-0.12

STATISTICAL TOLERANCE ANALYSIS (STA)
Bottom Contact Surface

Dimension Used
Ti
[ΣTi2](1/2)

Max Interference
(+[ΣTi2](1/2))
Min Interference
(-[ΣTi2](1/2))
1B, 2A
0.08, 0.12
0.144222051
0.19
0.334222051
0.045777949
Top Contact Surface

Dimension Used
Ti
[ΣTi2](1/2)

Max Interference
(+[ΣTi2](1/2))
Min Interference
(-[ΣTi2](1/2))
1A,1B,2A,2C
0.08, 0.1, 0.12, 0.1
0.201990099
0.28
0.481990099
0.078009901

6.0 RECOMANDATION

The calculated nominal dimension interference for top and bottom part of circuit card is 0.19 and 0.28. If we assign statistical tolerances to these parts, we have 0 probability of making parts with clearance, in other words, male and female parts produced will always have contact. This means if we can reduce our worst case assembly tolerance (WCAT) to less than 0.19 and 0.28 respectively for top and bottom circuit board assembly, the rejection rates is 0. However, to achieve this goal, it is necessary to keep the production process stable and capable of producing these parts within these specifications. Statistically, we have very low probability of making these parts out of our extreme tolerance zone. The reason to fail is because the production process might be not. Therefore, we would recommend the following steps: 1. Monitor the production process to see if there are any assignable cause variation present first before modifying any design of this product. 2. Redesign the part with tighter tolerance to reduce the variation within the assembly....