毕业设计论文

发布时间:2015-02-18 编辑:亮磊 手机版

Process Planning and Concurrent Engineering

The product design is the plan for the product and its components and subassemblies.To convert the product design into a physical entity ,a manufacturing plan is needed .The activity of developing such a plan is called process planning .It is the link between product design and manufacturing .Process planning involves determining the sequence of processing and assembly steps that must be accomplished to make the product .In the present chapter ,we examine processing planning and several related topics.

At the outset ,we should distinguish between process planning and production planning ,which is covered in the following chapter. Process planning is concerned with the engineering and technological issues of how to make the products and its parts. What types of equipment and tooling are required to fabricate the parts and assemble the product ? Production planning is concerned with the logistics of making the product .After process planning is concerned with ordering the materials and obtaining the resources required to make the product in sufficient quantities to satisfy demand for it.

Process Planning

Process planning involves determining the most appropriate manufacturing and assembly processes and the sequence in which they should be accomplished to produce a given part or product according to specifications set forth in the product design documentation.The scope and variety of processes that can be planned are generally limited by the available processing equipment and technological capabilities of the company of plant .Parts that cannot be made internally must be purchased from outside vendors. It should be mentioned that the choice of processes is also limited by the details of the product design.This is a point we will return to later.

Process planning is usually accomplished by manufacturing engineers .(Other titles include in industrial engineer.) The process planner must be familiar with the particular manufacturing processes available in the factory and be able to interpret engineering drawings .Based on the planner’s knowledge,skill,and experience ,the processing steps are developed in the most logical sequence to make each part .Following is a list of the many decisions and details usually include within the scope of process planning :

.Interpretation of design drawings. The part of product design must be analyzed (materials,dimensions,tolerances ,surface finished,etc.) at the start of the process planning procedure.

.Process and sequence. The process planner must select which processes are required and their sequence.A brief description of processing steps must be prepared.

.Equipment selection . In general , process planners must develop plans that utilize existing equipment in the plant .Otherwise ,the component must be purchased ,or an investment must be made in new equipment .

.Tools ,dies,molds,fixtures,and gages. The process must decide what tooling is required for each processing step.The actual design and fabrication of these tools is usually delegated to a tool design department and tool room ,or an outside vendor specializing in that type of tool is contacted.

Methods analysis . Workplace layout ,small tools ,hoists for lifting heavy parts ,even in some cases hand and body motions must be specified for manual operations .The industrial engineering department is usually responsible for this area.

.Work standards. Work measurement techniques are used to set time standards for each operation .

.Cutting tools and cutting conditions. These must be specified for machining operations ,often with reference to standard handbook recommendations.

Process Planning for parts

For individual parts,the processing sequence is documented on a form called a route sheet .(Not all companies use the

name route sheet ;another name is “operation sheet .”)Just as engineering drawings are used to specify the product design ,route sheets are used to specify the process plan .They are counterparts,one for product design ,the other for manufacturing .

A typical processing sequence to fabricate an individual part consists of : (1) a basic process,(2)secondary processes ,(3) operations to enhance physical properties,and (4)finishing operations.The sequence is shown in Fig.21.2. A basic process determines the starting geometry of the workpart.Metal casting ,plastic molding ,and roling of sheet metal are examples of basic processes.The starting geometry must often be refined by secondary processes,operations that transform the starting geometry (or close to final geometry ).The secondary geometry processes that might be used are closely correlated to the basic process that provides the starting geometry.When sand casting is the basic processes,machining operations are generally the second processes .When a rolling mill produces sheet metal,stamping operations such as punching and bending are the secondary processes.When plastic injection molding is the basic process ,secondary operations are often unnecessary,because most of the geometric features that would otherwise require machining can be created by the molding operation.Plastic molding and other operation that require no subsequent secondary processing are called net shape processes.Operations that require some but not much secondary processing (usually machining ) are referred to as near net shape processes.Some impression die forgings are in this category .These parts can often be shaped in the forging operation(basic processes)so that minimal machining (secondary processing )is required .

Once the geometry has been established ,the next step for some parts is to improve their mechanical and physical properties .Operations to enhance properties do not alter the geometry of the part;instead,they alter physical properties .Heat treating operations on metal parts are the most common examples .Similar heating treatments are performed on glass to produce tempered glass.For most manufactured parts ,these property-enhancing operations are not required in the processing sequence.

Finally finish operations usually provide a coat on the work parts (or assembly )surface. Examples inclued electroplating ,thin film deposition techniques ,and painting.The purpose of the coating is to enhance appearance ,change color ,or protect the surface from corrosion,abrasion ,and so forth .Finishing operations are not required on many parts ;for example, plastic molding rarely require finishing .When finishing is required ,it is usually the final step in the processing sequence .Table 21-2 presents some typical processing sequences for common materials used in manufacturing .

Processing Planning for Assemblies

The type of assembly method used for a given product depends on factors such as : (1) the anticipated production quantities ;(2) complexity of the assembled product ,for example ,the number of distinct components ;and (3)assembly processes used ,for example ,mechanical assembly versus welding .For a product that is to be made in relatively small quantities ,assembly is usually performed on manual assembly lines .For simple products of a dozen or so components,to be made in large quantities ,automated assembly systems are appropriate .In any case ,there is a precedence order in which the work must be accomplished .The precedence requirements are sometimes portrayed graphically on a precedence diagram.

Process planning for assembly involves development of assembly instructions,but in more detail .For low production quantities,the entire assembly is completed at a single station .For high production on an assembly line ,process planning consists of allocating work elements to the individual stations of the line, a procedure called line balancing.The assembly line routes the work unit to individual stations in the proper order as determined by the line balance solution.As in process planning for individual components ,any tools and fixtures required to accomplish an assembly task must be determined ,designed,and built;and the workstation arrangement must be

laid out.

Make or Buy Decision

An important question that arises in process planning is whether a given part should be produced in the company’s own factory or purchased from an outside vendor ,and the answer to this question is known as the make or buy decision .If the company does not possess the technological equipment or expertise in the particular manufacturing processes required to make the part ,then the answer is obvious: The part must be purchased because there is no internal alternative .However ,in many cases ,the part could either be made internally using existing equipment ,or it could be purchased externally from a vendor that process similar manufacturing capability.

In our discussion of the make or buy decision ,it should be recognized at the outset that nearly all manufactures buy their raw materials from supplies .A machine shop purchases its starting bar stock from a metals distributor and its sand castings from a foundry .A plastic molding plant buys its molding compound from a chemical company.A stamping press factory purchases sheet metal either fro a distributor or direct from a rolling mill.Very few companies are vertically integrated in their production operations all the way from raw materials ,it seems reasonable to consider purchasing at least some of the parts that would otherwise be produced in its own plant.It is probably appropriate to ask the make or buy question for every component that is used by the company .

There are a number of factors that enter into the make or buy decision .We have complied a list of the factors and issues that affect the decision in Table 21-3 .One would think that cost is the most important factor in determining whether to produce the part or purchase it .If an outside vendor is more proficient than the company’s own plant in the manufacturing processes used to make the part ,then the internal production cost is likely to be greater than the purchase price even after the vendor has included a profit .However ,if the decision to purchase results in idle equipment and labor in the company’s own plant ,then the apparent advantage of purchasing the part may be lost .Consider the following example .Example 21.1 Make or Buy Decision

The quoted price for a certain part is $20.00 per unit for 100 units .The part can be produced in the company’s own plant for $28.00. The components of making the part are as follows :

Unit raw material cost = $8.00 per unit

Direct labor cost =6.00 per unit

Labor overhead at 150%=9.00 per unit

Equipment fixed cost =5.00 per unit

________________________________

Total =28.00 per uniit

Should the component by bought or made in-house?

Solution :Although the vendor’s quote seems to favor a buy decision ,let us consider the possible impact on plant operations if the quote is accepted.Equipment fixed cost of $5.00 is an allocated cost based on investment that was already made .If the equipment designed for this job becomes unutilized because of a decision to purchase the part ,then the fixed cost continues even if the equipment stands idle .In the same way ,the labor overhead cost of $9.00 consists of factory space ,utility ,and labor costs that remain even if the part is purchased .By this reasoning ,a buy decision is not a good decision because it might be cost the company as much as $20.00+$5.0+$9.00=$34.00 per unit if it results in idle time on the machine that would have been used to produce the part .On the other hand ,if the equipment in question can be used for the production of other parts for which the in-house costs are less than the corresponding outside quotes ,then a buy decision is a good decision .,

工艺规程制订与并行工程

产品设计是用于产品,及它的部件装配的计划。为了把产品设计转换成一个实际物体,这需要一个制造计划。而制订一个这样的计划的行动就叫做工艺规程制订。它是产品设计和制造之间的连接,工艺规程制订包括决定加工顺序和制造产品所必须完成的装配步骤。在以下文章中,我们将解释工艺规程制订和他的一些相关主题。

文章开始,我们应该区别在下列文章中被反复提到的工艺规程制订和生产计划。工艺规程制订与如何制造产品和它的零件等工程技术问题有关,制造零件和装配产品需要什么样的设备和工具?工艺规程制订与产品制造物流管理有关系。它在工艺规程制订后面与原料分类及获得满足制造充分数量产品要求的资源有关。

工艺规程制订

工艺规程制订包括决定最适当的制造及装配步骤和顺序,在这些顺序和步骤中他们必须根据所提出的详细的设计说明书规范完成给定零件或产品制造。 能够被计划的工艺范围和多样性通常由于公司车间可用设备和技术能力而受到限制。在公司内部不能够制造的零件必须到外部市场购买,工艺规程制订所提及的工艺选择同样也受到详细设计资料的限制,我们稍后将会回到这一点。

工艺规程制订通常是由制造工程师完成的,工艺制订者必须熟悉工厂中详细可用的制造流程并且能够说明工程图。基于制订者的知识、技术和经验,用于制造每个零件的工艺步骤以最合乎逻辑的顺序被发展制订。下列各项是在工艺规程制订范围里的许多决定和详细资料:

.设计图的说明. 在工艺规程制订的开始,产品设计的这一部分( 材料、尺寸、公差、表面处理等等)必须进行分析。

.工艺和顺序. 工艺制订者必须选择哪一个工艺是必需的及必需工艺的序列。此外还必须准备好一个简短的工艺步骤描述。

.设备选择. 大体上,工艺制订者必须逐步展开利用工厂现有机器的计划。另外,组件必须被购买或在新设备上的投资必须被制定。

.工具、冲模、铸模、夹具、量具. 工艺必须决定每个工序需要什么工具,这些工具的实际设计和制造通常通过委派工具设计部门和工具库或者联系专攻那种工具制造的外面厂商来完成。

.方法分析. 车间规划,小工具,提升重物的提升间。甚至在一些人工操作情景中的肢体动作也被指定。 .操作步骤. 工作测量技术被用来为每个操作设定时间标准。

.切削工具和切削条件. 这些必须对加工操作通过推荐标准手册来进行详细说明。

零件工艺规程制订

对于单个零件,加工顺序通过一种被称为进路表的表格来进行文件证明备份。就如工程图被用于详细说明设计产品一样,进路表被用于详细说明工艺计划。他们是类似的,一个用于产品设计,另一个用于制造。

制造单个零件的典型加工顺序包括:(1) 一个基本工序 (2) 二级工序 (3) 提高物质特性工序和(4) 最后工序。一个基本工序决定了工件的起始造型。金属铸件、塑料成型、金属精炼是基本工序中的实例。起始造型常常必须通过改变起始造型操作(或者接近于最终造型)的二级工序来精制。二级工序习惯于和基本工序一起提供起始造型,当砂型铸造是基本工序,车加工通常是二级工序。当轧钢厂制造金属片是基本工序,冲压操作像冲裁和弯曲通常是二级工序。当塑料注入成型是基本工序时,二级工序通常是不必要的,因为他的大多数几何特征制造通过别的方式如成型制造来完成。塑料成型和其他操作的二级工序被称为净成型工序的并发二级工序,需要一些但并不多的二级工序的操作就是所提到的近似成型工序。许多有印象的摸锻件就是这一类,这类零件能够经常在锻造(初级工序)阶段被成型,因此减少了必要的加工(二级工序)。

一旦模型被建立,许多零件的下一步是改良它们的机械物理性能。提高特性工序并不改变零件模型,然而,它却能改变零件的物理特性。金属零件的热处理操作就是最普通的实例。类似的如玻璃通过热处理来制造钢化玻璃,对于大多数零件的制造来说,这些特性加强工序在加工工序中并不需要。

最后工序通常对零件(或装配体)的表面提供一个涂层。例如电镀、薄膜沉积技术、涂漆。表面处理的目的是改善外观,改变颜色或者表面保护防止腐蚀和磨损等等。在很多零件中最后工序是并不需要的。例如:塑料成型就很少需要最后程序。当必须需要最后程序,他通常是加工顺序的最后一步。

装配工艺规程制订

一个既定产品的典型装配方法由以下因素决定的:(1)预期产品数量(2)装配产品的复杂性。例如:不同组件的数量和(3)常用装配工艺。例如:机械定位焊接、对于小数量产品,通常在人工装配线上进行装配。对于大量制造的一打或这样组件的简单零件,要采用适当的自动化装配线。无论如何这里有一个工作必须被完成的优先顺序,这个优先需求经常用一个优先表来进行图表描绘。

装配工艺规程制订包括装配指令的发展,但是更详细地对于小批量生产。在一个岗位完成整个装配,对于一个装配线上的大批量生产,工艺规程制订由一种分配工作条件到装配线个别工位并被叫做人工投入线性平衡法的程序组成。这种装配线按照装配线平衡解决方案决定的顺序发送工作单元到个别工位,在个别组成,任意工具或夹具的工艺规程制订时,一条装配线的决定、设计和制造必须被完成,并且工作站的必须被列出来。

制造或购买决定

在工艺制定过程中出现的一个重大问题是一个特定零件应该在公司内部的工厂内生产还是从外部销售商处购买,并且这个问题的答案被认为是制造或购买决定。如果公司没有技术设备或制造零件所必须的详细制造工艺中的专门技术,那么答案就很明显了。因为没有其他选择零件必须购买。然而,在很多例子中零件既可以在利用现有设备在内部制造或者可以从外部拥有相似制造能力的生产销售商处购买。

在我们的关于制造或购买的决定的讨论中,他应该认识到在开始几乎所有的制造者从供应商那里购买原料。一个机械加工厂从一个金属经销商购买他的起动柄原料或从一个铸造厂购买他的砂型铸件。一个塑料成型厂从一个化工厂购买他的模塑料。一个冲压厂可以去经销商或直接从轧钢厂购买金属片。很少的公司能够在操作中从原料一直进行垂直整合,这看来至少购买一些也许在他的工厂可以另外制造的零件是合理的。也有可能为公司使用的每一个组成要求制造或购买决定。

这里有许多影响制造或购买决定的因素,一个人可能认为成本是决定是购买还是制造零件的最重要的因素。如果一个外部经销商比公司工厂更精通于制造零件的工艺,因而公司内部生产成本可能比经销商赚取成本后的价格还要高。可是,如果购买决定导致公司工厂设备和劳动的闲置,购买零件的表面优势就会丧失。考虑以下例子制造或购买决定。

为一个特定零件被引述的价格是100个单位的每单位$20.00。制造零件的成分如下所示:

单位原料成本=每单位$8.00

直接劳动成本=每单位$6.00

劳动加班150%=每单位$9.00

设备修理成本=每单位$5.00

___________________

总计=每单位$28.00

这个组成应该被购买还是在内部制造?

解决方案:尽管经销商的引证似乎支持购买决定,让我们来考虑如果引证被接受可能在生产操作中的冲突。$5.00设备维修成本是已经被制定的投资成本,如果设备设计因为购买零件的决定而变的没有利用价值,那么这个固定成本仍然继续尽管设备闲置着。同样,如果零件被购买由工厂空间,效用和劳动成本组成的$9.00的劳动间接成本仍然继续。通过这种推理,如果应该已用于生产零件的设备闲置的购买决定并不是一个好决定因为他可能花费公司将近$20.00+$5.0+$9.00=$34.0每单元。另一方面,如果正在讨论的设备可以被用于生产其他零件并且内部生产成本低于外部联系报价,那么一个购买决定就是一个好决定。

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