INTRODUCTION TO MANUFACTURING PROCESS PLANNING AND DESIGN

INTRODUCTION

Manufacturing process planning is an important step in the product-realization process. It can be defined as ‘‘the function within a manufacturing facility that establishes which processes and param- eters are to be used (as well as those machines capable of performing these processes) to convert a part from its initial form to a final form predetermined (usually by a design engineer) in an engi- neering drawing’’ (Chang et al. 1998, p. 515). Alternatively, it can be defined as the act of preparing detailed work instructions to produce a part. The result of process planning is a process plan. A process plan is a document used by the schedulers to schedule the production and by the machinist

/ NC part programmers to control / program the machine tools. Figure 1 shows a process plan for a part. The process plan is sometimes called an operation sheet or a route sheet. Depending on where they are used, some process plans are more detailed than others. As a rule, the more automated a manufacturing shop is, the more detailed the process plan has to be.

To differentiate the assembly planning for an assembled product, process planning focuses the planning on the production of a single part. In this chapter, when a product is mentioned, it refers to a discrete part as the final product. One important step in process planning is process selection, which is the selection of appropriate manufacturing processes for producing a part. When none of the existing processes can produce the part, a process may have to be designed for this purpose. Process design can also be interpreted as determining the parameters of a process for the manufacture of a part. In this case, process design is the detailing of the selected processes. Thus, process planning and process design are used for the same purpose—determining the methods of how to produce a part.

In this chapter, process planning and design are discussed. Techniques employed for process planning and process design are also introduced. Due to the vast number of manufacturing processes, it would be impossible to cover them all in this chapter. Only machining processes are focused upon here. However, early in the chapter, casting, forming, and welding examples are used to illustrate alternative production methods.

The Product-Realization Process

Manufacturing is an activity for producing a part from raw material. In discrete product manufac- turing, the objective is to change the material geometry and properties. A sequence of manufacturing processes is used to create the desired shape. The product-realization process begins with product design. From the requirements, an engineering design specification is prepared. Through the design process (the details of which are omitted here), a design model is prepared. Traditionally, the design model is an engineering drawing (drafting) either prepared manually or on a CAD system. Since the

1990s, solid model for engineering design has gained popularity for representing design models. A design model must contain the complete geometry of the designed part, the dimensions and toler- ances, the surface finish, the material, and the finished material properties. The design model is a document, or contract, between the designer and the manufacturing facility. The finished product is checked against the design model. Only when all the specifications on the design model are satisfied is the product accepted.

There are several steps in the product-realization process (Figure 2): design, process planning, manufacturing, and inspection. Process planning is a function linking the design and the manufac- turing activities. The objective of manufacturing is to produce the product at an acceptable quality (instead of the best quality), in a desired time frame (not necessarily the shortest time), and at the lowest cost (lowest cost is always desirable). Because manufacturing follows the process plan, the quality of the process plan is critical to the success of manufacturing and thus product realization. In the following section, the more detailed steps of product realization are discussed.

From Design to Process Planning to Production

Before a product is materialized, it has to be designed and manufactured. Following are the major steps in this product realization process.

Selection of Materials

Materials are selected based on the functionalities of the part being made. Most parts are made from a single material. Material selection may not be the first step in design. However, it is an important decision to be made. Often, several materials all satisfy the functional requirements of the part. For example, one may choose steel, aluminum, of composite material for the part. Although the physical, mechanical, and electrical properties all satisfy the design requirements, the material and processing costs might be very different. The material cost is easily estimated (Table 1), but estimating the processing cost is more involved. For example, steel-part manufacturing is very different from composite-part manufacturing. Totally different machines and material-handling methods are needed. A good designer will take manufacturing issues into consideration. Design for manufacturing should begin with the proper selection of materials for manufacturing. In some cases, due to the material property, the geometry of the part will need to be changed.

Geometry Creation

The shape of a product can be determined by functional or aesthetic considerations. Individual parts in an assembly must fit together to form the assembly. They use the geometric shape to carry out a specific function. For example, an angle bracket is used for mounting a machine, a hole is used to

From S. Kalpakjian, Manufacturing Engineering and Technology, 3d Ed., © 1995. Reprinted by permission of Prentice-Hall, Inc., Upper Saddle River, NJ.

fit an axle, and a T-slot is used to hold a bolt. The designer must create the appropriate geometry for the part in order to satisfy the functional requirements. The ultimate objective of the designer is to create a functionally sound geometry. However, if this becomes a single-minded mission, the designed part may not be economically competitive in the marketplace. Cost must also be considered.

Manufacturing processes are employed to shape the material into the designed geometry. A large number of unrelated geometries will require many different processes and / or tools to create. The use of standard geometry can save money by limiting the number of machines and tools needed. For example, a standard-size hole means fewer drill bits are needed. Design for manufacturing also means imposing manufacturing constraints in designing the part geometry and dimension.

The designed geometry is modeled on a CAD system, either a drawing or a solid model (see Figure 3). More and more designs are modeled using 3D solid modelers, which not only provide excellent visualization of the part and assembly but also support the downstream applications, such as functional analysis, manufacturing planning, and part programming. The key is to capture the entire design geometry and design intents in the same model.

Function Analyses

Because the designed part must satisfy certain functional requirements, it is necessary to verify the suitability of the design before it is finalized. Engineering analyses such as kinematic analysis and heat transfer are carried out from the design. Finite element methods can be used, often directly from a design model. The more critical a product or part is, the more detailed an analysis needs to be conducted.

Design Evaluation

The task of design evaluation is to separate several design alternatives for the final selection of the design. Cost analysis, functionality comparison, and reliability analysis are all considerations. Based on the predefined criteria, an alternative is selected. At this point the design is ready for production.

Process Planning

Production begins with an assembly / part design, production quantity, and due date. However, before a production order can be executed, one must decide which machines, tools, and fixtures to use as well as how much time each production step will take. Production planning and scheduling are based on this information. As noted earlier, process planning is used to come up with this information. How to produce a part depends on many factors. Which process to use depends on the geometry and the material of the part. Production quality and urgency (due date) also play important roles. A very different production method will definitely be appropriate for producing a handful of parts than for a million of the same part. In the first case, machining may be used as much as possible. However, in the second case, some kind of casting or forming process will be preferable.

When the due date is very close, existing processes and machines must be used. The processes may not be optimal for the part, but the part can be produced in time to meet the due date. The cost

will be high—one pays for urgent orders. On the other hand, when there is plenty of lead time, one should try to optimize the process. When the production quantity justifies the cost, it might be necessary to design new processes or machines for the part. One good example is the use of a transfer line for engine block production. Machines (stations) in a transfer line are specially designed (or configured) for a part (e.g., an engine block). Production is optimized. Lower cost and higher quality can be expected. Table 2 shows the recommended production systems for different production quan- tities and production lead times.

Production Planning and Scheduling

After process planning is complete, production is ready to begin. Production planning and scheduling are important functions in operating the manufacturing facility. Because multiple products or parts are being produced in the same manufacturing facility, the resource allocation must be done appro- priately in order to maximize the production output. When a transfer line or production line (assembly line) is the choice, the line is balanced (equal numbers of tasks are allocated to each machine station)

and designed. After a line is installed, the operation of the line is simple. Upon the workpiece being launched at one end of the line, the product is built sequentially through the line. However, in a shop environment, production scheduling is much more complex. For each planning horizon (day or week), what each machine should process and in which sequence must be decided. Predefined objectives such as short processing time, maximum throughput, and so on are achieved through proper sched- uling. Scheduling uses information provided by the process plan. A poorly prepared process plan guarantees poor results in production.

Consideration of Production Quantity in Process Planning

As noted above, production quantity affects the manufacturing processes selected for a part. If the production quantity is not considered in process planning, the result may be an expensive part or a prolonged production delay. Large quantities make more specialized tools and machines feasible. Small-quantity production must use general-purpose machines. Although the total quantity may be high, in order not to build up inventory and incur high inventory cost, parts are usually manufactured in small batches of 50 parts or less. Therefore, production batch size is also a factor to be considered. There is decision making loop. The process plan is the input to production planning; production planning determines the most economical batch size; batch size, in turn, changes the process plan. A decision can be made iteratively.