DFA Analysis Overview
Design for Assembly (DFA) is a team based product design
evaluation tool.
General Description
In current industrial practice DFA techniques are rarely, if
ever, applied at any point earlier than the final design stage.
Generally, 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. 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 Sandpit project because of existing
expertise within the group. There are four distinct stages to the
evaluation as shown below. Select any of the evaluation sections
for further details.
Figure 01. DFA Analysis Flowchart
The application of DFA has proven to be a significant influence
in achieving efficient manufacture and assembly through optimised
design. On average, the application of DFA techniques yields a
20%-30% reduction in assembly costs and a 10%-15% reduction in
manufacturing costs. This results in a more competitive product
with lower costs, yielding higher profits. Redford identifies
optimising part count as the single most significant aspect of DFA
for all types of assembly, responsible for at least 50% of the cost
savings.
TYPICAL DFA ANALYSES
Functional Analysis
The Functional Analysis facilitates part count reduction by
evaluation of each component in order to determine whether it is
essential for the performance of the product. Individual components
are assessed in terms of their relative motion, material type and
the need for removal for replacement or repair, according to nine
questions presented on the evaluation sheet. Thus, the designer is
able to identify parts that may be eliminated, component clusters
that may be replaced by single integrated pieces and opportunities
for subassembly partitioning. The evaluation classifies parts as
functional (A) or non-functional (B) components. The Design
Efficiency (E) is calculated as the ratio of functional to
non-functional parts as shown below :
Equation 01. Design Efficiency.
When developing a new product, a design efficiency as high as
possible should be achieved with 60%-70% as a recommended threshold
based on a study of 'good' designs.
Manufacturability Analysis
The Manufacturing Analysis determines the relative cost of
producing each component based on the manufacturing processes used.
This processing cost is determined using a basic processing cost
per annum (Pc) for an ideal design (independent of design
features), a design-dependent relative cost (Rc) and the cost of
the material used (Mc). This requires the engineer to consider
component properties such as shape complexity, tolerances and
surface finish and the material suitability and usage, for the
particular manufacturing processes chosen.
Handling Analysis
The Handling Analysis evaluates the suitability of a component
for manual handling and automated feeding to the point of assembly.
The evaluation considers component shape characteristics, size,
weight, orientation and mechanical properties. Careful selection of
manual handling operations and feeding technology leads to
improvements in safety and reduces the likelihood of component
damage or incorrect insertions. The main benefits include reduced
capital spend on equipment and improved assembly times.
Assembly Analysis
The Assembly Analysis is used to highlight problems and
inefficient operations associated with the build sequence and
component interfaces, and to identify the tooling requirements of
the design. The assembly analysis scores the difficulty associated
with gripping each component and inserting it into the assembly for
both manual and automated operations. Ease of insertion is
dependent upon the position of components in the assembly sequence
and hence DFA encourages the engineer to consider design from an
assembly point of view. As a consequence of this, the success of
any DFA evaluation is dependent upon the assembly sequence used as
the basis for the analysis. The designer is required by the
methodology to construct an assembly sequence and the graphical
notation used is illustrated by the example shown below.
Figure 01. Elements of a Proactive DFA Methodology (Click image to
enlarge)
Product Group Support
Product Group Support aims to help the designer to identify
possibilities for establishing product family themes at the
commencement of product design and development. By identifying
common features and similarities with existing products it provides
the opportunity for rationalisation and standardisation of parts
and assembly operations. It is essential that the use of any
standard components and features be based on a satisfactory DFA
history. It is proposed that DFA experience and case histories form
the core of the Product Group support to encourage the designer to
use common technologies, common assembly features and joining
methods, and common parts and modules.
Product Structure Support
In order to capture the essence of assembly oriented design it is
crucial to allow the designer to evaluate alternative product
structures and assembly sequences. The proposed Proactive DFA
methodology incorporates a simple interactive method for analysing
assembly sequences and the necessary tools for considering
geometric and assembly constraints during the evaluation process.
An essential aspect of generating a successful product structure
and assembly sequence is to optimise the part count. In order to do
this the functional requirements of individual components must be
assessed so that component clusters may be replaced with single
integrated pieces. In order to assist the designer, information
should be provided containing relevant data such as process
descriptions, material suitability and design considerations.
Component Design Support
The final stage of the proposed proactive DFA methodology provides
assistance to the designer for detailed component design. In order
to help the designer to generate designs that are suitable for
manufacture and assembly, advice on assembly insertion and
fastening processes is required. Process capability data and cost
models for other assembly and manufacturing processes are also
necessary. This information provides the designer with the
capability to evaluate the effects of design decisions on cost and
practicality of manufacture. To ensure that cost models are
appropriate for each working environment the system should allow
users to customise the data.
In summary, the Proactive DFA methodology described above defines
an interactive tool to perform evaluation and offer advice to the
designer throughout product development, with varying levels of
detail and support. The system would allow utilisation of
traditional DFA capabilities whilst removing many of the drawbacks.
By incorporating new or modified analyses it would enable design
optimisation decisions to be made prior to the complete definition
of component geometry, and thereby Proactive DFA would be able to
support the design process from the earliest stages.