Boothroyd and Dewhurst product design for assembly summary

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Boothroyd and Dewhurst product design for assembly summary

Boothroyd and Dewhurst product design for assembly
The previous sections have primarily described techniques that are applied to the design of a single component or part. Most products consist of several parts that must be assembled in order to provide functionality. Professors Geoffrey Boothroyd and Peter Dewhurst have directed extensive research in the areas of design for manufacture (DFM) as well as design for assembly (DFA). They have co-authored several books and papers in the manufacturing field, including the Product Design for Assembly Handbook [Boothroyd & Dewhurst, 89]. Their approach is based on product simplification through design for assembly (DFA), as the key to successful product design for manufacture.

DFA is intended to reduce assembly costs by simplifying the product structure. It has reduced part costs as well as assembly costs. In design for manufacture (DFM), different product design alternatives are considered and compared in order to find the most economical and efficient solution to the design problem. However, the designer usually does not have the tools for obtaining an early cost estimate of the different production alternatives until the product is fully designed and detailed. This comes too late for basic changes to be made, constraining the design that may not be cost effective to manufacture.

Boothroyd and Dewhurst's quantitative method is based on two basic steps: 1) to reduce the number of parts in a design, and 2) to estimate handling and assembly costs in the assembly process. To apply step 1 of the method, it is necessary to determine the number of essential parts in the assembly. This is referred to as the theoretical minimum number of parts. In order to be
indispensable to the design these parts have to satisfy one of three criteria [Boothroyd & Dewhurst, 1984]:

1. Is there relative motion between this part and all other parts already assembled?

Must the part be made of a different material or be isolated from all other parts already

assembled?

Must the part be separated from all parts already assembled because necessary assembly or

disassembly would otherwise be impossible?

In step 2 of the method, cost figures should be determined for the assembly process, therefore, an assembly process has to be selected. In "Design for Assembly: Selecting the Right Method", Boothroyd and Dewhurst present a procedure for selecting the right method to assemble a given product. The method selected using this procedure is the most economical assembly process that should be used. The selection is based on: projected market life, number of parts, projected production volume, and company investment policy.

There are three basic alternatives to choose from: manual (MA or MM), special purpose machine (AI or AF), and programmable-machine (AP or AR), see Table 1.1.

-MA    Manual Assembly
-MM   Manual assembly with Mechanical assistance
-AI      Automatic assembly machines with Indexing-transfer devices
-AF     Automatic assembly machines with Free-transfer devices
-AP     Automatic Programmable assembly machine
-AR    Automatic programmable assembly machine using Robot arms

Table 1.1 Assembly process types

Having chosen the assembly process to be used, the efficiency of the assembly operation and the estimated cost can be calculated. In [Boothroyd & Dewhurst, 1988] the authors present design for assembly rules, and procedures for estimating assembly costs and for evaluating efficiency indices of automatic, manual and robot-based processes. These DFA rules, if followed after selecting a process, can result in a manufacturing cost reduction of 20-40% and assembly productivity increases of 100 to 200% [Boothroyd & Dewhurst, 1983].

During the analysis, special attention is paid to the possibility of reducing the number of parts either by eliminating or combining them. This is indicated by assigning a theoretical minimum number of parts to the subassemblies (if there are any) and to the entire product. After obtaining the theoretical minimum number of parts the design efficiency can be calculated. In the case of manual assembly, for example, the following equation should be evaluated [Boothroyd & Dewhurst, 1983]:

Em = 3 Nm / Tm (1.3)

where: Em = manual-assembly design efficiency
Nm = minimum number of parts
Tm = total assembly time

This equation represents the ratio of the ideal assembly time/part (3s) to the actual assembly time/part, taking Nm as the actual number of parts. It has been assumed that each part is easy to handle and insert, and that one third of the parts are secured immediately after insertion. In the redesign stage, there is an opportunity to reduce the number of parts in the operations where the theoretical minimum number of parts is less than the actual number of parts (i.e., subassemblies). Also in this stage, excessive values in either the handling or assembly times should be studied carefully for improvement.

Boothroyd and Dewhurst's Quantitative Evaluation Method is a useful tool for Concurrent Engineering since the product/process design is optimized based on good assembly practices and analytical cost estimations are made based on practical data. Also, the assembly efficiency index represents a framework for comparisons among different assembly alternatives. The use of this method is the basis of the Product Design for Assembly Handbook, which includes practical procedures and data for designing products for ease of assembly.

To facilitate the use of this approach the authors have developed DFA software that by requesting the relationship between the parts, helps the designer determine an efficient assembly sequence for a new product starting from a sketch.

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