2.3.3 Concurrent Engineering and Design for Manufacturing
2.3.3 Concurrent Engineering and Design for Manufacturing
Concurrent engineering refers to an approach used in product development in which the functions of design engineering, manufacturing engineering, and other functions are integrated to reduce the elapsed time required to bring a new product to market. Also called simultaneous engineering, it might be thought of as the organizational counterpart to CAD/CAM technology. In the traditional approach to launching a new product, the two functions of design engineering and manufacturing engineering tend to be separated and sequential, as illustrated in Figure 2.3.5(a). The product design department develops the new design, sometimes without much consideration given to the manufacturing capabilities of the company. There is little opportunity for manufacturing engineers to offer advice on how the design might be altered to make it more manufacturable. It is as if a wall exists between design and manufacturing. When the design engineering department completes the design, it tosses the drawings and specifications over the wall, and only then does process planning begin.
By contrast, in a company that practices concurrent engineering, the manufacturing engineering department becomes involved in the product development cycle early on, providing advice on how the product and its components can be designed to facilitate manufacture and
assembly. It also proceeds with the early stages of manufacturing planning for the product. This concurrent engineering approach is pictured in Figure 2.3.5(b). In addition to manufacturing engineering, other functions are also involved in the product development cycle, such as quality engineering, the manufacturing departments, field service, vendors supplying critical components, and in some cases the customers who will use the product. All of these functions can make contributions during product development to improve not only the new product's function and performance, but also its produceability, inspectability, testability, serviceability, and maintainability. Through early involvement, as opposed to reviewing the final product design after it is too late to conveniently make any changes in the design, the duration of the product development cycle is substantially reduced.
Concurrent engineering includes several elements: (1) design for manufacturing and assembly, (2) design for quality, (3) design for cost, and (4) design for life cycle. In addition, certain enabling technologies such as rapid prototyping, virtual prototyping, and organizational changes are required to facilitate the concurrent engineering approach in a company.
(1) Design for Manufacturing and Assembly
It has been estimated that about 70% of the life cycle cost of a product is determined by basic decisions made during product design.
These design decisions include the material for each part, part geometry, tolerances, surface finish, how parts are organized into sub-assemblies, and the assembly methods to be used. Once these decisions are made, the ability to reduce the manufacturing cost of the product is limited. For example, if the product designer decides that a part is to be made of an aluminum sand casting but which possesses features that can be achieved only by machining (such as threaded holes and close tolerances), the manufacturing engineer has no alternative except to plan a process sequence that starts with sand casting followed by the sequence of machining operations needed to achieve the specified features. In this example, a better decision might be to use a plastic molded part that can be made in a single step. It is important for the manufacturing engineer to be given the opportunity to advise the design engineer as the product design is evolving, to favorably influence the manufacturability of the product.
Terms used to describe such attempts to favorably influence the manufacturability of a new product are design for manufacturing (DFM) and design for assembly (DFA), Of course, DFM and DFA are inextricably linked, so let us use the term design for manufacturing and assembly (DFM/A). Design for manufacturing and assembly involves the systematic consideration of manufacturability and assemblability in the development of a new product design. This includes: (1) organizational
changes and (2) design principles and guidelines.
Organizational Changes in DFM/A. Effective implementation of DFM/A involves making changes in a company’s organizational structure, either formally or informally, so that closer interaction and better communication occurs between design and manufacturing personnel. This can be accomplished in several ways: (1) by creating project teams consisting of product designers, manufacturing engineers, and other specialties (e.g., quality engineers, material scientists) to develop the new product design; (2) by requiring design engineers to spend some career time in manufacturing to witness first-hand how manufacturability and assemblability are impacted by a product’s design; and (3) by assigning manufacturing engineers to the product design department on either a temporary or full-!roe basis to serve as producibility consultants.
Design Principles and Guidelines. DFM/A also relies on the use of design principles and guidelines for how to design a given product to maximize manufacturability and assemblability. Some of these are universal design guidelines that can be applied to near1y any product design 2.3 Process Planning and Concurrent Engineering 12
situation, such as those presented in Table 25.4. In other cases, there are
design principles that apply to specific processes, for example, the use of
drafts or tapers in casted and molded parts to facilitate removal of the part
from the mold. We leave these more process-specific guidelines to texts
on manufacturing processes.
The guidelines sometimes conflict with one another. For example,
one of the guidelines in Table 2.3.4 is to “simplify part geometry; avoid unnecessary features.” But another guideline in the same table states that “special geometric features must sometimes be added to components” to
design the product for foolproof assembly. And it may also be desirable
to combine features of several assembled parts into one component to minimize the number of parts in the product. In these instances, design
for part manufacture is in conflict with design for assembly, and a suitable compromise must be found between the opposing sides of the conflict.
Interpretation and Advantages TABLE 2.3.4 General Principles and
Guidelines in DFM/A Guideline
Minimize number of components Reduced assembly costs.
Greater reliability in final product.
Easier disassembly in maintenance and
field service.
Automation is often easier with reduced
part count.
Reduced work-in-process and inventory control problems.
Fewer parts to purchase; reduced ordering costs.
Use standard commercially available components Reduced design effort.
Fewer part numbers.
Better inventory control possible. Avoids design of custom-engineered components.
Quantity discounts possible.
Use common parts across product lines Group technology can be applied.
Quantity discounts are possible.
Permits development of manufacturing
cells.
Design for ease of part fabrication Use net shape and near net shape
processes where possible.
Simplify part geometry; avoid
unnecessary features.
Avoid surface roughness that is
smoother than necessary since additional
processing may be needed.
Design parts with tolerances that are within process capability Avoid tolerances less than process capability.
Specify bilateral tolerances. Otherwise, additional processing or sortation and scrap are required.
Design the product to be foolproof during assembly Assembly should be unambiguous. Components designed so they can be assembled only one way.
Special geometric features must sometimes be added to components.
Minimize flexible components These include components made of
rubber, belts, gaskets, electrical cables;
etc.
Flexible components are generally more
difficult to handle.
Design for ease of assembly. Include part features such as chamfers
and tapers on mating parts.
Use base part to which other
components are added.
Use modular design (see following
guideline).
Design assembly for addition of
components from one direction, usually
vertically; if mass production, this rule
can be violated because fixed
automation can be designed for multiple
direction assembly.
(2) Other Design Objectives
To complete our coverage of concurrent engineering, let us briefly discuss the other design objectives: design for quality, cost, and life cycle.
Design for Quality. It might be argued that DFM/A is the most important component of concurrent engineering because it has the potential for the greatest impact on product cost and development time. However, the importance of quality in international competition cannot
be minimized. Quality does not just happen. It must be planned for during product design and during production. Design for quality (DFQ) is the
term that refers to the principles and procedures employed to ensure that
the highest possible quality is designed into the product.
Design for Product Cost. The cost of a product is a major factor in determining its commercial success. Cost affects the price charged for the product and the profit made by the company producing it. Design for product cost (DFC) refers to the efforts of a company to specifically
identify how design decisions affect product costs and to develop ways to
reduce cost through design.
Design for Life Cycle. To the customer, the price paid for the product may be a small portion of its total cost when life cycle costs are considered. Design for life cycle refers to the product after it has been manufactured and includes factors ranging from product delivery to product disposal.
制造业的并行工程和设计
并行工程引用一种常用于产品发展的路径，通过它使工程设计功能、工程制造功能和其他功能综合起来以减少一种新产品投放市场所需要的共用时间，也被称为并发工程，他可能被认为是CAD/CAM技术的类似组织版本，按照传统路径来使一件产品投放市场。如图(1)a所示，工程设计功能和工程制造功能这两种功能是分开并且连续的，产品设计部门开展一项新的设计有时很少考虑到公司的制造能力，也很少有机会能够让制造工程师来提供如何使设计更容易制造的一些建议。他好像消除了在设计和制造之间的一堵墙，当设计部门完成设计，他投掷工程图和说明书越过这面墙，并且那时工艺规程制订也开始了。
图（1）比较 : (a) 传统产品发展周期和 (b) 并行产品的发展周期
通过比较，实行并行工程的公司，工程制造部门在早期就参与到产品发展周期。为如何使产品和他的组成能够被设计的更适于制造提供建议。他同样为产品提供制造计划继续进行的早期准备，这种并行工程的路径在图(1)b中被描绘出。除了工程制造以外其他功能同样被包括在产品发展周期中，如质量工程、制造部门、后勤服务、市场供应评定组成和一些情况下将使用这些产品的消费者。在产品发展阶段的所有这些功能不仅能改善新产品的功能和性能，同时也能改善他的可造性、自检性、易测性、服务能力和可维护性。通过早期功能改善，因为在最终产品设计之后的回顾太晚以至于不能对设计进行便利的修改的不利因素的消除，使产品发展周期的持续期大大减少。
并行设计包含以下因素：(1)一些制造和装配设计(2)质量设计(3)成本设计和(4)生命周期设计。另外，像快速成型、虚拟制造、和组织转变等辅助技术需要被用来促进公司的并行工程。
制造和装配设计
据估计一件产品的70%的生命周期成本是由在产品设计时所做的基本决定所决
定的，这些设计决定包括每个零件的材料、零件模型、公差、表面处理、零件是如何被组织装配的和常用装配方法。一旦这些决定被指定，减少产品制造成本的能力就会被限制。例如，如果产品设计者决定用铝砂型铸造法制造一个分开零件，但是这个零件的工艺特性只能通过加工来完成(如螺纹孔和配合公差)，制造工程师没有选择的余地，只能按照先砂型铸造在加工的方法来达到既定要求。在这个例子中，用一个在单独步骤所需要的塑料模制品也许是一个较好的决定。因此，当产品设计展开时给制造工程师一个忠告设计者的机会对产品的顺利可造性是非常重要的。
这种被用于尝试描述顺利改变一件新产品的可造性的条件是制造设计(DFM)和装配设计(DFA)。当然，DFM和DFA是紧密相连的，因此让我们用制造和装配设计(DFM/A)的形式来表达。制造和装配设计包括在一件新产品中的可造性和可装配性的综合考虑，这包括： (1)组织变化和(2)设计原理和指导方针。
.在DFM/A中的组织变化. DFM/A的有效执行包括公司组织机构的正式或非正式的变化，因此设计职工和制造职工之间有很好的交流和交互作用。这可以通过以下方法来完成：(1)通过成立由产品设计者制造工程师和其他员工(例如：质量工程师、材料专家)组成的攻关小组来进行产品开发；(2)通过要求设计工程师用一些事业时间在制造上，以能够掌握第一手可造性和可装配性是如何通过产品设计联系在一起的；(3)通过指派制造工程师到产品设计部门在一个临时的或专任的基础上做一个还原性顾问。
.设计说明和指导方针. DFM/A为了理解如何设计一个既定产品来使可造性和
可装配性最大化也依赖于设计说明和指导方针的使用，这些通用设计指导方针中的一些几乎适用于任何产品设计。在其他方面，一些设计原理只适用于特定工序，例如：轴或锥度在阶梯中的使用和利用模制品来切除模内零件，在制造过程中我们只把这些具体过程指导方针放在书本上。
指导方针有时互相矛盾，一条指导方针是“简化零件模型，避免不必要的特征”。但是在同一表格里的另一指导方针为了装配安全而规定在设计产品时“特殊几何特征必须不时加上他的组成”。而且他也许值得来结合个别装配件的特征来减少产品中零件的数量。在这些示例中零件制造设计与装配设计相冲突，在这个矛盾冲突的两边，一个适当解决方法必须被发现。