Process Selection - From Design to Manufacture Part 2 docx

Figures 1.12–1.16 provide a general classification and guide to the range of materials and processes component manufacturing, assembly, joining and,bulk and surface engineering, respecti

A number of general rules have been developed to aid designers when thinking about themanufacture of the product:

. Holes in machined, cast, molded, or stamped parts should be spaced such that they can bemade in one operation without tooling weakness This means that there is a limit on howclose holes may be spaced due to strength in the thin section between holes

. Generalized statements on drawings should be avoided, like ‘polish this surface’ or marks not permitted’, which are difficult for manufacturing personnel to interpret Notes onengineering drawings must be specific and unambiguous

‘tool-. Dimensions should be made from specific surfaces or points on the part, not from points inspace This greatly facilitates the making of gauges and fixtures

. Dimensions should all be from a single datum line rather than from a variety of points toavoid overlap of tolerances

. The design should aim for minimum weight consistent with strength and stiffness ments While material costs are minimized by this criterion, there also will usually be areduction in labor and tooling costs

require-. Wherever possible, design to use general-purpose tooling rather than special dies, form cutters,etc An exception is high-volume production, where special tooling may be more cost-effective

. Generous fillets and radii on castings, molded, formed, and machined parts should be used

. Parts should be designed so that as many operations as possible can be performed withoutrequiring repositioning This promotes accuracy and minimizes handling

Figure 1.10 provides a number of specific design rules and objectives associated with effective DFM

As mentioned previously, selecting the right manufacturing process is not always simple andobvious In most cases, there are several processes that can be used for a component, andselection depends on a large number of factors Some of the main process selection drivers areshown in Figure 1.11 The intention is not to infer that these are necessarily of equalimportance or occur in this fixed sequence

The problem is compounded by the range of manufacturing processes and wide variety ofmaterial types commonly in use Figures 1.12–1.16 provide a general classification and guide

to the range of materials and processes (component manufacturing, assembly, joining and,bulk and surface engineering, respectively) that are widely available (All, except the latter, of

Fig 1.10 DFM rules and objectives.

these processes are discussed in detail in Part II of the book.) To be competitive, theidentification of technologically and economically feasible process and material combinations

is crucial The benefits of picking the right process can be enormous, as shown in Figure 1.17for a number of components and processing routes

The placing in the product design cycle of process selection in the context of engineering formanufacture and assembly is illustrated in Figure 1.18 The selection of an appropriate set ofprocesses for a product is very difficult to perform effectively without a sound Product DesignSpecification (PDS) A well-constructed PDS lists all the needs of the customers, end users andthe business to be satisfied It should be written and used by the Product Team and provide areference point for any emerging design or prototype Any conflict between customer needsand product functionality should be referred back to the PDS

The first step in the process is to analyze the design or prototype with the aim of simplifyingthe product structure and optimizing part-count As shown earlier, without proper analysisdesign solutions invariably tend to have too many parts Therefore, it is important to identifycomponents that are candidates for elimination or integration with mating parts (Everycomponent part must be there for a reason and the reason must be in the PDS.) This must

be done with due regard for the feasibility of material process combinations and joiningtechnology A number of useful approaches are available for material selection in engineeringdesign For more information see references (1.10), (1.39) and (1.40)

The next steps give consideration to the problems of component handling and fittingprocesses, the selection of appropriate manufacturing processes and ensuring that componentsare tuned to the manufacturing technology selected Estimation of component manufactureand assembly costs during the design process is important for both assessing a design againsttarget costs and in trade-off analysis Overall, the left-hand side of Figure 1.18 is closelyrelated to DFA, while the right-hand side is essentially material/process selection and com-ponent design for processing, or consideration in DFM A reader interested in more back-ground information on DFA/DFM and materials and process selection in productdevelopment is directed to references (1.40–1.45)

Fig 1.11 Key process selection drivers.

Fig 1.12 General classification of materials.

Fig 1.13 General classification of manufacturing processes.

1.5 Process selection strategy

In considering alternative design solutions for cost and quality, it is necessary to explorecandidate materials, geometries and tolerances, etc., against possible manufacturing routes.This requires some means of selecting appropriate processes and estimating the costs ofmanufacture early on in product development, across a whole range of options In addition,the costs of non-conformance (1.46) need to be understood, that is appraisal (inspection andtesting) and failure, both internal (rework, scrap, design changes) and external (warrantyclaims, liability claims and product recall) Therefore, we also need a way of exploringconformance levels before a process is selected For more information on this importantaspect of design, the reader is directed to Reference 1.32

The primary objective of the text is to provide support for manufacturing process selection

in terms of technological feasibility, quality of conformance and manufacturing cost Thesatisfaction of this objective is through:

. The provision of data on the characteristics and capabilities of a range of importantmanufacturing, joining and assembly processes The intention is to promote the generation

of design ideas and facilitate the matching and tuning of a design to a process, and

. The provision of methods and data to enable the exploration of design solutions forcomponent manufacturing and assembly costs in the early stages of the design and devel-opment process

To provide for the first point, a set of so-called manufacturing PRocess Information MAps(PRIMAs) have been developed In a standard format for each process, the PRIMAs presentknowledge and data on areas including material suitability, design considerations, qualityissues, economics and process fundamentals and process variants The information includes

Fig 1.14 General classification of assembly systems.

Fig 1.15 General classification of joining processes.

Fig 1.16 General classification of bulk and surface engineering processes.

not only design considerations relevant to the respective processes, but quite purposefully, anoverview of the functional characteristics of each process, so that a greater overall under-standing may be achieved Within the standard format, a similar level of detail is provided oneach of the processes included The format is very deliberate Firstly, an outline of the processitself – how it works and under what conditions it functions best Secondly, a summary ofwhat it can do – limitations and opportunities it presents – and finally an overview of qualityconsiderations including process capability charts for relating tolerances to characteristicdimensions.

To provide for the second point, techniques are put forward that can be used to estimate thecosts of component manufacture and assembly for concept designs It enables the effects ofproduct structure, design geometry and materials to be explored against various manufactur-ing and assembly routes A sample data set is included, which enables the techniques to beused to predict component manufacturing and assembly costs for a range of processes andmaterials The process of cost estimation is illustrated through a number of case studies, andthe scope for and importance of application with company specific data is discussed

Fig 1.17 Contrast in component cost for different processing routes.

Part II begins with the strategies employed for PRIMA selection, where attention is focused

on identification of candidate processes based on strategic criteria such as material, processtechnology and production quantity Having identified the possible targets, the data in thePRIMAs are used to do the main work of selection The PRIMAs include the main fivemanufacturing process groups: casting, plastic and composite processing methods, forming,machining and non-traditional processes In addition, the main assembly technologies and themajority of commercially available joining processes are covered In all, sixty-five PRIMAsare presented, giving reference to over one hundred manufacturing, assembly and joiningprocesses

Fig 1.18 Outline process for design for manufacture and assembly.

Part III of the text concentrates on the cost estimation methodologies for components andassemblies, their background, theoretical development and industrial application In practice,Part II of the work can be used to help select the candidate processes for a design from thewhole range of possibilities Part III is concerned with getting a feel for the manufacturing andassembly costs of the alternatives The book finishes with a statement of conclusions and a list

of areas where future work might be usefully directed

Design considerations are provided to enable the designer to understand more about thetechnical feasibility of the design decisions made The process quality considerations givevaluable information on process conformance, including data on process tolerance capabilityassociated with characteristic dimensions A good proportion of the PRIMAs is taken up withquality considerations No excuse is made for this Non-conformance often represents a largequality cost in a business As touched on earlier, such losses result from rework, orderexchange, warranty claims, legal actions and recall In many businesses, these losses accountfor more than 10 per cent of turnover (2.1) The goal is to provide data which enables theselection of processes that have the capability to satisfy the engineering needs of the applica-tion, including those associated with conformance to quality requirements.

Each PRIMA is divided into seven categories, as listed and defined below, covering thecharacteristics and capabilities of the process:

. Process description: an explanation of the fundamentals of the process together with adiagrammatic representation of its operation and a finished part

. Materials: a description of the materials currently suitable for the given process

. Process variations: a description of any variations of the basic process and any special pointsrelated to those variations.

. Economic considerations: a list of several important points including production rate, mum production quantity, tooling costs, labor costs, lead times, and any other points whichmay be of specific relevance to the process

mini-. Typical applications: a list of components or assemblies that have been successfully factured or fabricated using the process

manu-. Design aspects: any points, opportunities or limitations that are relevant to the design ofthe part as well as standard information on minimum section, size range and generalconfiguration

. Quality issues: standard information includes a process capability chart (where relevant),typical surface roughness and detail, as well as any information on common processfaults

A key feature of the PRIMAs is the inclusion of process capability charts for themajority of the manufacturing processes Tolerances tend to be dependent on the overalldimension of the component characteristic, and the relationship is specific and largely non-linear The charts have been developed to provide a simple means of understanding theinfluence of dimension on tolerance capability The regions of the charts are divided by twocontours The region bounded by these two contours represents a spectrum of tolerance-dimension combinations where Cpk
1.33* is achievable Below this region, tolerance-dimension combinations are likely to require special control or secondary processing if

Cpk¼ 1.33 is to be realized

In the preparation of the process capability charts it has been assumed that the geometry iswell suited to the process and that all operational requirements are satisfied Where thematerial under consideration is not mentioned on the charts, care should be taken Anyadverse effects due to this or geometrically driven component variation should be taken intoconsideration For more information the reader is referred to reference (1.32) The data used

in the charts has been compiled from contacts in industry and from published work Althoughattempts have been made to standardize the data as far as possible, difficulties were faced inthis connection, since it was not always easy to obtain a consensus view Consequently, asmany as twenty different data sources have been used in the compilation of the individualprocess capability charts to provide an understanding of the general tolerance capability rangeoffered by each manufacturing process

Different manufacturing technologies such as primary shape generating processes, joiningtechniques, assembly systems and surface engineering processes require that selection takesplace based on the factors relevant to that particular technology For example, the selection

of a joining technique may be heavily reliant on the ability of the process to join dissimilar

* C pk – process capability index If the process characteristic is a normal distribution, C pk can be related to a parts-per-million (ppm) defect rate C pk ¼ 1.33 equates to a defect rate of 30 ppm at the nearest limit At C pk ¼ 1, the defect rate equates to approximately 1350 ppm (see reference 2.2 for more information about process capability indices).

materials and materials of different thickness This is a particular requirement not sarily defined by the PDS, but one that has been arrived at through previous designdecisions, perhaps based on spatial or functional requirements Whereas assemblysystem selection may simply be dictated by a low labor rate in the country of manufactureand therefore manual assembly becomes viable for even relatively large productionvolumes.

neces-Although there may be many important selection drivers with respect to each processtechnology, a simple and effective strategy for selection must be sought for the generalsituation and for usability Selection strategies can be developed by concentrating on severalkey economic and technical factors which are easily interpreted from the PDS or otherrequirements Put in a wider context, the selection strategies, together with the informationprovided in the PRIMAs, must complement business strategy and the costing of designs, inorder to provide a procedure that fully justifies the final selection A flowchart is shown

in Figure 2.1, relating all the factors relevant to the process selection strategies discussed

in detail

2.3.1 Manufacturing process selection

Manufacturing processes represent the main shape generating methods such as casting,molding, forming and material removal processes The individual processes specific to thissection are classified in Figure 1.13 The purpose of this section is to provide a guide for theselection of the manufacturing processes that may be suitable candidates for a component.The manufacturing process selection strategy is given below, but points 4, 5 and 6 apply toall selection strategies:

1 Obtain an estimate of the annual production quantity

2 Choose a material type to satisfy the PDS

3 Refer to Figure 2.2 to select candidate PRIMAs

4 Consider each PRIMA against the engineering and economic requirements such as:

. Understand the process and its variations

. Consider the material compatibility

. Assess conformance of component concept with design rules

. Compare tolerance and surface finish requirements with process capability data

5 Consider the economic positioning of the process and obtain component cost estimates foralternatives

6 Review the selected manufacturing process against business requirements

The principal intention is that the candidate processes are selected before the componentdesign is finalized, so that any specific constraints and/or opportunities may be borne in mind

To this end, the manufacturing process PRIMA selection matrix (see Figure 2.2) has beendevised based on two basic variables:

. Material type– Accounts for the compatibility of the parent material with the ing process, and is therefore a key technical selection factor A large proportion of thematerials used in engineering manufacture have been included in the selection methodology,from ferrous alloys to precious metals, as classified in Figure 1.12

manufactur-PRIMA selection strategies 21