Computation Section
Process Flow Analysis
 - Subassemblies
It may be that several products are constructed using common subassemblies. We have adapted the add-in to recognize subassemblies as raw materials. This may reduce substantially the data required for some systems.

To illustrate we consider product A used earlier for illustration. The product receives its inputs from two separate manufacturing lines, the SMT line and the TH line. The process is shown below.

Since products B and C are the same as product A except receiving different proportions of inputs from the two lines, it may be more efficient to describe the processes as below. Here we have identified a new SMT product and a new TH product. These are subassemblies that are inputs to operations 1 and 2 respectively in the product A process. We use SMT and TH as the product names. We use SMT_L and TH_L to identify the process resources.

Products B and C differ only in the proportions coming from the two lines.

It is more convenient to represent the system as a collection of subassemblies because the details the SMT and TH lines need not be repeated.


Defining the Subassemblies


The figure below shows the Excel worksheet describing process A. Note that operations 1 and 2 link to the SMT and TH subassemblies. The link is provided by the names in column K. The subassemblies SMT and TH are listed as the raw materials for the first two operations.


The portion of the worksheet defining the SMT and TH subassemblies appears below. The flows through the subassemblies are not automatically linked to the processes defined for A, B and C. Linking equations could be placed in cells B49 and B60, however, the program does not provide them. The linking occurs on the Project worksheet.


The definitions for B and C also use SMT and TH as raw materials.



Project Worksheet


Selecting the Process Economics item from the OM/IE menu, creates the Project worksheet. It is similar to the worksheet without subassemblies, however, new features are added to link the processes to the subassemblies. We see in the Product portion of the worksheet the table describing the economics of production. The contents are driven by the Sales entered in column I, 200, 500 and 1000, for products A, B and C respectively. Note that SMT and TH have no sales. Column J, however, shows the production of these subassemblies that are required by their inclusion in the three primary products. Column J is computed from the Assembly matrix starting in column O. The assembly matrix is constructed by the add-in from data defining the processes.

Column K shows the sum of the resource operation cost and raw material cost for each product. For products A, B and C, however, this cost does not include the cost of the subassemblies. Column L computes the total cost including subassemblies. Column M is the net operating profit computed as the unit revenue less the unit total cost.


Further down the Project worksheet we find the matrices defining material and resource use. Note that SMT and TH are listed both as materials and products in these matrices.



Mathematical Programming Models


The mathematical programming models must have additional variables and additional constraints to represent the link between products and subassemblies. The product mix model is shown below. An additional variable is provided to allow each product to also act as a subassembly. There is also an additional constraint for each product to link sales to subassembly use. The solution shows the optimum sales of each of the products and the associated subassembly requirements.

The other mathematical programming models are adjusted in a similar manner.

  Whenever one or more products is listed as a raw material, the add-in adjusts the analysis and mathematical programming worksheets as indicated on this page. When no products appear as raw materials, the subassembly adjustments are not made.


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Operations Management / Industrial Engineering
by Paul A. Jensen
Copyright 2004 - All rights reserved

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