Design for X

The term 'Design for manufacture' was originally coined by Boothroyd and Dewhurst, to encompass their approach to ensuring a product is both manufacturable and simple to assemble. Since then, the expression 'design for x' has emerged to encompass a wide range of approaches to product design and a diverse collection of tools, techniques and philosophies. These approaches typically aim to link customer requirements to some quality criteria, such as robustness, serviceability, reliability and environmental impact. There are literally dozens of different 'Design for X' methodologies, which are often of greatest interest in specific markets or for particular types of product. However, 'Design for manufacture' and 'design for assembly' remain the most important as they have a direct and recognisable impact on product costs.

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Process selection
Involves the analysis of both material and processing method for individual components, based on basic assumptions:

• Critical performance criteria (conductivity, strength, friction, thermal properties etc)
• Tolerancing requirements
• Component complexity requirements
• Set up and tooling costs - apportionment of fixed and variable costs
• Production volume
• Expertise and capability

Reducing the number of process stages

• Eliminating unnecessary process stages, through a combination of alternative strategies:
• Component minimisation
• Elimination of finishing processes
• Combining processes
• Single direction processing or machining to reduce set up requirements

Designing for the process - guidelines
There are many design guidelines which aim to ensure optimum detailed design of components to satisfy the constraints of specific production processes. These guidelines help designers to exploit the benefits and recognise the limitations of processes whilst also preventing basic errors. They can be viewed as capturing 'good practice' for individual processes and are often 'common sense' heuristics, which generally hold true - although there are always exceptions.

Many guidelines are publicly available, although companies frequently develop their own rules to support their own production facility. Typical examples include: machining guidelines, casting guidelines, injection moulding guidelines, sheet metalwork guidelines, welded joint guidelines, adhesive guidelines etc.

In practice, guidelines can be inaccessible and difficult to use. There are literally hundreds of different guidelines, for all conceivable processes. They are frequently book based, although increasingly, computerised guidelines are available. It can be much more advisable to directly consult the process experts when designing individual components.

Design for Assembly
Design for Assembly (DfA) can be viewed as a major subset of the DfM approach, and supports the DfM goal of minimising the total number of components. In addition, DfA techniques aim to maximise the ease with which parts can be moved, held, located and joined. There are two basic approaches to considering DfA:

DfA - guidelines
Like the guidelines for DfM, there are a wide collection of DfA guidelines. These guidelines tend to take basic design rules and expand upon them with graphical examples of 'bad designs' and suggestions of improved 'good designs'. Typical rules include minimise part count, designing out wires and cables, design out adjustment, maximise part symmetry, insert parts from the same direction, eliminate fasteners and do not assemble in enclosed spaces. Like the guidelines for DfM, they suffer from inaccessibility and can be difficult to use in practice

DfA - Systematic approaches
Systematic approaches aim to provide a structure to analyse an assembly and focus the decision making process. There are various methods, but best known are the Boothroyd & Dewhurst method developed in the 1970s and the Lucas Engineering Systems method developed in the 1980s. Typically, a systematic approach begins with an analysis of the assembly to determine if parts can be eliminated or reduced, based upon some simple rules:

• Is there relative movement between one part and another?
• Does the material need to be different for functional purposes?
• Does the part need to be replaced or maintained?

The second stage is to map the assembly sequence and rigorously assess each component for difficulty of handling (or feeding for automated assembly), insertion and fitting (locating or securing). This assessment is based on tables of data which provide relative measures depending upon the design of the components.

Implications of DfM and DfA approaches
It is impossible to make sensible DfM and DfA decisions without first knowing the desired unit cost and making a rough estimate of the unit cost of proposed design solutions. This requires a knowledge of manufacturing volumes in order to balance fixed and variable costs.

While many of the DfM and DfA principles can be applied irrespective of production volume, the basic philosophy of minimising component count can have implications for products which are produced in low volume. A logical impact of component reduction is that there are fewer, more complicated parts. These parts may ultimately have a lower piece part price, but at the expense of higher tooling costs. It can often be more appropriate to have more parts with correspondingly lower tooling costs for low volume production.

A further complication is the potential conflict between DfM and DfA approaches and product platform or modularity strategies. In order to achieve a modular product range, the interfaces between modules need to be clearly defined. This often results in additional components, costs and hence complexity. It is often not possible to both minimise component count and deliver a modular solution.

Minimising complexity of design
It can be useful to view the act of designing a new product as adding logistic complexity to the business. Every new component, assembly or process results in time, effort and cost to the business. Thus, when designing a new product, a useful metric to monitor is the level of complexity introduced. This can be assessed through considering the number of new purchased parts, manufactured parts, processes, vendors and tools required. This 'complexity scorecard' can assist in both assessing alternative designs, as well as focusing attention on component and process reuse.

Interesting link
Check out the online  IBM Proprinter Case Study

Further information

• Baxter M, (1999), Product design: practical methods for the systematic development of new products, Stanley Thornes, UK
• Bralla, Design for manufacturability handbook

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