Tolerance and Stack Analysis Manual and Advance approach

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CHAPTER 2 
Manual Approach 
2.1 Introduction 
In production planning for quality runs, parts cannot always be machined 
dimensionally as shown on the blueprint. Manufacturing datum surfaces must be set up 
based on the selection of locating surfaces (setup datums) for fixturing and on cutting 
tool design layout. Hence, certain blueprint dimensions can only be obtained indirectly. 
As a result, the tolerance stackup problem is encountered. The principle of tolerance 
stackups is illustrated in Figure 2.4; the underlying assumption for this principle is that 
individual dimensions are independent of each other. Therefore, addition or subtraction 
of length dimensions is always characterized by the adding of tolerances on the 
individual lengths (Wade, 1983).When a blueprint dimension cannot be obtained directly, 
it needs to be calculated based on a dimension chain. A dimension chain is "a 
consecutive series of interrelated dimensions which form a closed loop" (Wang and Li, 
1991, p. 86). It is also referred to as a tolerance chain by many authors. In a dimension 
chain, there is one and only one dimension that is determined by the other dimensions. 
This is the dimension to be calculated and is referred to as a concluding link. The other 
dimensions in the chain are referred to as component links. Component links can be 
further classified as being increasing or decreasing. The increase of an increasing link 
results in the increase of the concluding link; while the increase of a decreasing link 
results in the decrease of the concluding link. Figure 2.1c shows a dimension chain. 
Recall the machining process for the example part shown in Figure 2.1a. Dimensions 
A1, A2, and A3 are obtained directly (measured dimensions). A4 is the blueprint 
dimension, which is obtained indirectly. Dimensions A1, A2, A3, and A4 form a