Design for Assembly: DFA
A new methodology to facilitate development of efficient product
designs
Introduction
What is DFA?
DFA methodologies were developed to support the designer
by generating feedback on the consequences of design decisions on
product assembly. The aim is to help the designer to produce an
efficient and economic design. The application of DFA guides the
designer towards a product with an optimum number of parts, that
requires simple, cost-effective assembly operations and the most
appropriate manufacturing processes and materials for its
components. The three best-known and also the most well-documented
DFA methods are the Boothroyd Dewhurst System, the Lucas DFA
Methodology and the Hitachi Assemblability Evaluation Method. In
general, the designer is guided through the analyses, which are
presented in a series of assessment charts. The charts are based on
empirical data gathered by knowledge engineering exercises with
industrial experts and organised in an easy-to-use worksheet
format. During the evaluation, the designer is required to assess
component functionality, form, manufacturing processes and assembly
characteristics using values extracted from the charts according to
component properties. These numbers are then compiled in tabular
format, and calculations performed. In this way, the designer is
able to quantify the suitability of the design. The Lucas DFA
Methodology has been chosen for use within the Designers' Sandpit
project because of existing expertise within the group.
Why Use DFA?
1. The Cost of Design Changes
The competitive nature of the international market place has led to
short product lifecycles and reduced price margins. Therefore,
engineering companies are constantly seeking methods to improve the
product development process and reduce costs.
Figure 1 - Design Change vs.
Cost
Much of the cost is incurred during the manufacture and assembly of
a product. A significant part of this cost can be attributed to the
labour-intensive activities associated with assembly.
2. Consequences of Component-Oriented
Design
There has been a trend towards automated assembly in order to
reduce labour costs. However, the potential benefits of assembly
technology are limited by the need for flexibility and the ability
to respond to product changes and short production runs. Within
this environment the most effective form of assembly is often
manual assembly. Therefore, increased automation of the product
assembly process is not necessarily the solution for reduced
product development costs.
In fact, the design process holds the key to reduction of
product development costs. As Figure 1 (above) shows, a large
proportion (approximately 80%) of the overall product development
costs are determined during the design stage. The high costs
associated with assembly are often due to an unnecessarily large
number of components in the product and the complex manufacturing
and assembly processes that are required due to the design of
inappropriate component interfaces. Studies have shown that often,
products are still designed with at least 50% excess of parts and
greater assembly content than is necessary.
Poor design, in terms of assembly, can be attributed, in part,
to the component-centred approach to design still prevalent in many
industries. Traditionally, different engineering departments
perform design, planning and manufacture of the product with no
integration or feedback and so assembly problems are identified
only at the later stages of production. In order to reduce lead
times and product costs effectively, manufacturing and assembly
issues must be detected and considered during design. This requires
the introduction of 'assembly-oriented design' so that product
development and assembly planning can be performed simultaneously
rather than consecutively .