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 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
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 .,
.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。
.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。
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)。