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Design for Manufacture and Assembly

Xiaofan Xie

Dept. of Mechanical Engineering, University of Utah

ABSTRACT: Design for Manufacture and Assembly (DFMA) technique has been utilized extensively in industry field these days. This paper is a detailed review which will discuss the development history of DFMA, compare some other “DFMA-like” theories, and good stories about the successful implementations of DFMA in real-word product design.

1. DFMA and its development history 1.1 Traditional design paradigm In order to survive and develop in the market, the enterprises have to present their new product to the public rapidly and continuously. Meanwhile, they must keep their products with the low price and high quality. So, developing the new product to satisfy the costumes requirements and make them appear in the market as soon as possible becomes the key to share more part of the cake of the market. The tradition manufacturing method follows the developing cycle shown as figure 1. Following the conceptual sketches of the parts completed on the CAD workstation, the design engineers create the detail drawing and assembly engineer create the assembly plan. The workshop makes the prototype and gives the feedback, which accords to the sample test, to the design engineers and assembly engineers who will redesign the product, the assembly and the manufacturing process. The same cycle repeats again and again until arriving at competitive design. This kind of process makes the design and manufacturing been performed independently. “In this scenario, the designer designs a product and ‘tosses it over the wall’ to the manufacturer to produce. [1]” The design engineer and the manufacturing engineer works individually and what results is that the final products have many d isadvantages such as poor characteristics in manufacturing, assembly, maintenance; long developing period; high cost; and unguaranteed

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quality.

Fig 1. Traditional Manufacturing Process Some of the famous manufacture such as Boeing, Motorola, HP, have employed the new techniques during the period they develop the new products. One of the most widely used techniques is DFAM (Design for Manufacturing and Assembly) which combines the traditional manufacturing techniques with the modern computer techniques and considers all of the factors that will affect the final outputs in the early stage during the whole period of the products developing. By using this technique, they try to make out the successful products by just one time. 1.2 What is “DFMA”? The Term “DFMA” comes with the combination of DFA (Design for Assembly) and DFM (Design of Manufacturing). The basic concept of it is that the design engineers apply the DFMA paradigm or software to analyze the manufacturi ng and assembly problems at the early design stage. By this means, all of considerations about the factors that affect the final outputs occur as early as possible in the design cycle. The extra time spent in the early design stage is much less the time that will be spent in the repeatedly redesign. And meanwhile, the cost will be reduced. DFM is that by considering the limitations related to the manufacturing at the early stage of the design; the design engineer can make selection among the deferent materials, different technologies, estimate the manufacturing time the product cost quantitatively and rapidly among the different schemes. They compare all

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kinds of the design plans and technology plans, and then the design team will make revises as soon as possible at the early stage of the design period according these feedback information and determine the most satisfied design and technology plan. There are three goals in DFM: 1. Increase the quality of new produces during the developing period, including

design, technology, manufacturing, service and so on. 2. Decrease the cost, including the cost of design, technology, manufacturing,

delivery, technical support, discarding and so on. 3. Shorten the developing cycle time, including the time of design, manufacturing

preparing, and repeatedly calculation. DFA is considering and resolving the possible problems in the assembly process at the early stage of the design which can make sure the part will be assembled with high speed, low cost and productivity. DFA is a kind of design paradigm with which, the engineer use all kinds of methods such as analyze, estimating, planning and simulating to consider all the factors that will affect the assembly process during the whole design process; revise the assembly construc tions to satisfied the characteristics and functions of the final products; and meanwhile, lower the cost as most as possible. DFA is a kind of design method that can be used in two ways--- a tool for assembly analysis and a guide for assembly design. The former usage is that at the time after the beginning of the product design, the engineer makes estimation of assembly possibility by analyzing all the factors that can affect the assembly process, and give suggestions. The second one is that collecting the knowledge and experience from the assembly experts and recording them as design guides. By the help of these guides, the engineer can choose the design plan; determine the product construction such as under the guidance of those experts. 1.3 Development history of DFMA If we trace the history of DFMA, we can even find it before the Second World War. Ford and Chrysler use the DFM philosophy in their design and manufacturing process of the weapons, tanks and other military products. Dr. Geoffrey Boothroyd and Dr. Peter Dewhurst who founded the Boothroyd

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Dewhurst, Inc. (BDI) in 1982 are the first persons doing the research job in this new technology at the beginning in the early 1970’s. Actually, the “DFMA” is a trademark of their company. They created and developed the DFMA concept which is used in developing the products of their company --- DFMA software system. Currently these programs are used to help the design in almost all the industrial fields including circuit boards [2], with manual assembly, with robotic assembly, and with machining. They also do a lot of work examining the economic justification of each design revision [3]. 2. Alternative “DFMA-like” theories There are many “DFMA-like” paradigms available. Some of them are discussed below. Most of the theories mentioned here are based on the discussion of the paper by Sanjay Ramaswamy, Arizona State University. [4] (1) The Lucas DFA methodology The basic construction of Lucas DFA is very similar to the DFA of BDI, it is the result of the cooperation of Lucas Organization and the University of Hull in U.K. Now, the logic of Lucas DFA has been integrated in the engineering analysis software “TeamSet” which is the product of CCI (check it at http://www.teamset.com/ ). Lucas DFA separates the product design process into three stages: FcA (Function Analysis), HA (Handing Analysis) and FtA (Fitting Analysis). The relations of these three stages are shown as figure 2. Before the manufacturing and assembly process, the PDS (Product Design Specification) occurs which change the requirements of the customs into engineering specifications. After that, the design engineers perform the design job according to this information. This is a kind of process to change the engineering specifications into the real design and meanwhile, all the requirements should be satisfied. The Function analysis in Lucas DFA theory is to separates all the parts of the product into the essential parts and the non essential parts that employs very similar adjustment standard used by BDI DFA. Following the function analysis, comes the analysis of handing. Same as the function analysis, Lucas DFA separated the handing analysis into the automatic handing analysis and the manual analysis. During the fitting analysis, the sequence of parts assembly will

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be determined first, and then according to the assembly flow chat, analyze the gripping and the fitting process. After finishing the whole DFA analysis process mentioned above, the inadequate of the design will be highlighted, the revisal job occurs at this time.

Fig 2. Lucas DFA method

(2) CyberCut In summary, the technology used in CyberCut is DSG (Destructive Solid Geomety) developed at UC Berkeley. The mean feature of this system is that using a feature based design system in conjunction with a knowledge based process planner to promote design for manufacture [Wright and Dornfeld, 1998]. After using this technology, the complexity of planning is reduced by removing the non essential

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features from a solid block and leaving these features to the CNC machining process. The design tool is developed by Java language and the whole system is based on Internet. (3) The Nippondenso Method

Nippondenso is a big Japanese car parts provider that gets huge orders for various parts from many famous car producers. They divide their products into sub-assemblies and make them interchangeable. And one of their standards is that none of the products should have the jigs and fixtures.

(4) Producibility Measurement Tool

Producibility measurement tool uses Producibility Assessment Worksheets (PAWs) which is based on the knowledge and experience of the persons doing the evaluation. The worksheets use numeric values to determine the ease of producibility for the elements that make up the process which when averaged produce a measure of the probability of successful production, i.e., producibility. Depending on the total program being measured for producibility, tens or even hundreds of PAWs may be needed to produce a valid measurement of producibility.

(5) Feature Based Manufacturability Evaluation

This manufacturing evaluation method is built up by Gupta and Nau. The most important steps for it are shown as followed. First, generate all the possible features for the design part; each feature representing a machining operation [4], and get rid of the parts which are not machinable. And second, a FBM (Feature base model) which contains no redundant features is created. After that, the same process yields again and again, the best plan is pick up at last.

(6) Taguchi Method

Taguchi Method is an advanced statistical technique which used for design of experiment to find the best parameters or conditions in different manufacturing process. By employing Taguchi Method, manufacturers can upgrade stability and quality of their products. Taguchi method is useful for shortening the time to find the best conditions for making new products in large number.

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Except the technology mentioned above , there are a lot of “DFMA-like” theories such as:

l Failure-Mode and Effects Analysis (A method which is used for preventing of anticipated failures.)

l Group Technology (A method that it improves the design efficiency as well as product quality.[4])

l Quality Function Deployment (QFD) is a detailed planning technique used to translate the customs’ requirements into company specifications at each major stage of the product developing process.

l Design to Target Cost (DTC) is the managed process of monitoring and controlling manufacturing cost at the product specification stage..

3．Post-Boothroyd attempts at developing similar techniques After the huge success in developing the DFMA technology and software, Dr. Boothroyd and his fellows keep doing the research in this field. DFS (Design for Service) was released in 1994, which is used in the early stage of the product design process to check the cost of the maintenance and the maintainable ability. In many cases, an easy assembly product is not a easy maintenance. For example, a nut that is so easy to be assembled in less then 20 seconds is very hard to be released when it is rusted. Fig 3 DFS software [6] Cooperated with TNO and used the cost estimation model developed by this Holland research center, BDI released DFE (Design for Environment) in 1996. This product provides a method which estimates the cost of environment of the product at its early design stage. This software make it possible that a custom o has not the ecology background can consider the environment factors in the design process. DFE is one of the earliest software used in the analysis of environment cost.

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Fig 4. DFE software [6] There are several other software in the BDI’s series of products such as DFT (Design for Test), DFD (Design for Discarding). All of these computer tools are very useful in the product design. 4. Stories about the successful implementations of DFMA 4.1 Development of the BagEasy III. The BagEasy III is a manual resuscitator designed for “single patient use” (to be used multiple times on a single patient) by medical personnel in emergency rooms, ambulances and other treatment locations (Figure 3) [7].

Fig 5 BagEasy III

A design team was founded and the goal of the team was to finish the design with

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a concept what would meet the product requirement and meanwhile, improve the manufacturability of the product. They used Boothroyd-Dewhurst DFMA as the framework during the whole design process. Throughout the whole design process, every member of the team shared the ideals with each other. They communicated every day and the team meeting happened anytime as needed. This resulted “breaking down the walls between functions” [7] and achieving parallel design method which focused the team on the object. Every member knew their product and what the product was going to be. Supplier took part in the team activities and answered the questions from the designer on how the parts could be produced. The feedback comes so quickly instead of long time waiting as usual. After the concept design finished, models generated in CAD was used for analysis. As details of the design created, the concept was turned into real-word models. The team members met the supplier at this time, reviewed the part design and developed a better one. The results of these efforts are that the new product is greatly simplified; the improvement of assembly is 84%, of assembly cost is 74%.

4.2 DFMA Applied to analysis of Swingline® Classic 747 stapler. One of the projects for my DFMA class in this semester is the manufacturing and assembly analysis of the Swingline® Classic 747 stapler. The group was made up by five people and our goals for this project is that by using DFMA paradigm in the real-world product analysis, we can be familiar with the DFMA method; know the advantages we can obtain by DFMA in product design. This product has been in the market for over 30 years and it seems that the design of every part is classic and require no change. However, during the period we applied DFMA paradigms in simulating the whole manufacture process for this product, we found we may make some changes in some of the parts. According to our analysis by DFMA means, these changes could either shorten the manufacturing time or reduce the tool cost. We focused our efforts on the analysis of material selection, assembly process,

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and the sheet metal working of the parts. Five parts were redesigned from a manufacture point of view which reduced the time and cost of manufacture. An added bonus to those changes was a time and cost saving at the assembly stage. However, most of the components, from the point of view of both material selection and manufacture, and of assumed assembly methods of the stapler, proved to be very acceptable. Figure 6 is the results we got from the calculation of sheet metal working by DFMA means.

Fig 6. Comparing of the manufacturing cost

There are many other stories about the applications of DFMA in all kind of industrial fields, such as “Design-cycle time for mass spectrometer slashed from 48 months to 16 months [8]”, “Application on the design of AH64 helicopter [9]” Interested reader may check the BDI’s website (www.dfma.com) to find more implementations of DFMA. 5．Conclusions

Following the preceding discussions, DFMA is a good method to increase the productiveness, decrease the cycle time and the cost of manufacture and assembly. Meanwhile, the DFMA software is helpful to finish these jobs faster and easier.

DFMA software combines the knowledge of product design, assembly process,

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material selection and manufacturing technology. An independent designer can use it to make the feasible decision instead of collection tons of related materials or consulting the experts. Every members of a product developing team can thinks the same problem at the same time. Comparing with the traditional design methods, DFMA shorten the manufacturing time, decrease the cycle time and keep the high quality of the products, and meanwhile, make design more creative. From the point of view of manufacturing, assembly and maintenance; DFMA software provides a program which can analyze the developed plan, design, and technology. Designers use the program to analyze the cost of manufacturing, assembly and maintenance in order to make the final products low-priced, reliable and easy assembly. However, three shortcomings should be highlighted of DFMA method. 1. Insufficient scientific base. As a matter of fact, what Boothroyd and his fellows

do is adding all kinds of data (such as the parameters of materials, the functions of calculations of cost and cycle time, the components that can or cannot be assembled and etc.) to the database of their software, and let the computer find out all the possible selections to a certain mission. By comparing all these results, an optimal plan is picked out and put into effect.

2. DFMA method has little efforts at the conceptual design stage, where most of

the product cost is fixed. 3. Another Insufficiency of this method is the inability to provide redesign

suggestions to modify shapes in the case of an unsuitable design [4]. BIBLIOGRAPHY [1] Boothroyd, G.., Knight, W., and Dewhurst, P. , Product Design For

Manufacture and Assembly, 2nd Edition, Marcel Dekker, Inc., 2002 [2] G. Boothroyd and W. Knight, ‘‘Manufacturing À La Carte: Efficiency:

Design for assembly,’’ IEEE Spectrum . Sept 1993, pp. 51-53. [3] G. Causey, “Elements of agility in manufacturing”, Ph.D. Dissertation

(Mechanical Engineering), CWRU, January 1999.

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[4] Sanjay Ramaswamy，”A Survey of DFM Methods”, Arizona State University

http://asudesign.eas.asu.edu/education/MAE540/sanjayweb.htm [5] BMP (http://www.bmpcoe.org), ‘‘Application of Producibility Measurement

Tool’’ [6] http://www.extech.com [7] Miles Parker, “Breathing easier with DFMA”. [8] Nelson O. Weber, “Product Development Teams and Tools Applied to the

Aircraft Industruial”, World Class Design to Manufacture, Vol. 1 No. 6, 1994, pp.20-26, MCB University Press, 1352-3047

[9] Alfredo Herrera, “Design for Manufacture and Assembly Application on the

Design of the AH64 Helicopter”, Presented at 12th International Forum on DFMA, June, 1997, Newport, RI.