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 or 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 industrial engineer, production engineer. and process 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, find experience, the processing steps arc developed in the most logical sequence to make each part. Following is a list of the many decisions and details usually included within the scope of process planning.
Interpretation of design drawings. The part or product design must be analyzed (materials, dimensions, tolerances, surface finishes, etc.) at the start of the process planning procedure.
Processes and sequence. The process planner must select which processes are required and their sequence. A brief description of ail 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, flxtures, and gages. The process planner must decide what tooling i~ required for each processing step. The actual design and fabrication of these tools is usually delegated to a 1001design department and tool room, or an outside vendor specializing in that type of tool is contracted
Methoth analysis. Workplace layout, smaf 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 spec
ify the process plan. They are counterparts. one for product design, the other for manufacturing. A typical route sheet, illustrated in Figure 25.1, includes the following information: (1) all operations to be performed on the work part, listed in the order in which they should be performed; (2) a brief description of each operation indicating the processing to be accomplished, with references to dimensions and tolerances on the part drawing; (3) the specific machine, on which the work is to be done; and (4) any special tooling, such as dies, molds, cutting tools. jigs or fixtures, and gages. Some companies also include setup times, cycle time standards, and other data. It is called a route sheet because the processing sequence defines the route that the part must follow in the factory. Some of the guidelines in preparing a route sheet are listed in Table 25.1.
Decisions on processes to be used to fabricate a given part are based largely on the starting material for the part. This starting material is selected hy the pmd(lct designer Once the material has been specified. the range of possible processing operations is reduced considerably. The product designer's decisions on starting material are based primarily on functional requirements, although economics and manufacturability also play a role in the selection
TABLE 25,1 Typical Guidelines in Preparing a Route Sheet
•Operation numbers for consecutive processing steps should be listed as 10, 20, 30, etc, This allows new operations to be inserted if necessary.
•A new operation and number should be specified when a work part leaves one workstation and is transferred to another station
•A new operation and number should be specified if a part is transferred to another workholder (e.g., jig or fixture), even if it is on the same machine tool
•A new operation and number should be specified if the workpart is transferred from one worker to another, as on a production line.
A typical processing sequence to fabricate an individual part consists of: (I) a basic process, (2) secondary processes, (3) operations to enhance physical properties, and (4) finishing opera/jam. The sequence is shown in Figure 25.2. A basic process determines the starting geometry of the workpart. Metal casting. plastic molding, and rolling of sheet metal arc examples of basic processes. The starting geometry must often be refined by secondary processes, operations that transform the starting geometry into the geometry (or close to the final geometry).The secondary processes that might be used are closely correlated to the basic process that provides the starting geometry. When sand casting is the basic process, machining operations are generally the secondary 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 operations 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 process) 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 phsyical 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 example. Similar heating treatments are performed on glass to produce tempered glass. For most manufactured parts, these property enhancing operations arc not required in the processing sequence, as indicated by the alternative arrow path in Figure 25.2.
Finally ,finishing operations usually provide a coating on the workpart (or assembly) surface. Examples include 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 .>0 forth. Finishing operations are not required on many parts: for example, plastic moldings rarely require finishing. When finishing is required, it is usually the final step in the processing sequence.
Table 25.2 presents some typical processing sequences for common engineering materials used in manufacturing.
In most cases, parts and male rials arriving at the factory have completed their basic process. Thus, the first operation in the process plan follows the basic process that has provided the starnng geometry of the part. For example, machined parts begin as bar stock or
castings or forgings, which arc purchased from outside vendors. The process plan begins with the machining operations in the company's own plant. Stampings begin as sheet metal coils or strips that are bought from the rolling mill. These raw materials are supplied from outside sources so that the secondary processes, property enhancing operations, and finishing operations can be performed in the company's own factory.
In addition to the route sheet, a more detailed description of each operation is usually prepared. This is filed in the particular production department office where the operation is performed. It lists specific details of the operation, such as cutting conditions and tooling (if the operation is machining) and other instructions that may be useful to the machine operator. The descriptions often include sketches of the machine setup.
Process 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 III relatively small quantities, assembly is generally accomplished at individual workstations where one worker or a team 0: workers perform all of the assembly tasks. For complex products made in mediurn and high quantities. assembly is usually performed on manual assembly lines (Chapter 17). For simple products of a dozen or so components. to be made in large quantities, automated assembly systems arc appropriate. In any case, there is a precedence order in which the work must be accomplished, an example of which is shown in Table 17.4.The precedence requirements are sometimes portrayed graphically on a precedence diagram, as in Figure 17.5.
Process planning for assembly involves development of assembly instructions similar to the list of work elements in Table 17.4, hut 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 (Section 17.4.2). The assembly line routes the work units to individual stations in the proper order as determined by the line balancing solution, As in process planning for individual components, any tools and fixtures required to accomplish an assemhly 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 possess similar manufacturing capability.
In our discussion of the make or buy decision, it should be recognized at the outset that nearly all manufacturers buy their raw materials from suppliers. 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 from a distributor or direct from a rolling mill. Very few companies are vertically integrated in their production operations all the way from raw materials to finished product. Given that a manufacturing company purchases some of its starting 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 compiled a list of the factors and issues that affect the decision in Table 25.3. One would think that cost is the most important factor in determining whether 10 produce the part or puchase it. ifan outside vendor is more proficient than the company's own plant in the manufacturing processes used 10 make the part, then the internal production cost is likely 10 he greater. t~an 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 25.1 Make or Buy Cost 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 cost components of making the part arc as follows:
Unit raw material cost = $8.00 per unit
Direct labor cos! 6.00 per unit
Labor overhead at 150% = 9.00 per unit
Equipment fixed COS! == 5.00 per unit
Total = 28.00 per unit
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 an investment that was already made If the equipment designated 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 cost the company as much as $20.00 + $5.00 + $9.00 = $34.00 per unit if it results in idle lime 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
Make or buy decisions are not often as straightforward as in this example. The other factors listed in Table 25.3 also affect the decision. A trend in recent years, especially in the automobile industry, is for companies to stress the importance of building close relationships with parts suppliers. We will return to this issue in our later discussion of concurrent engineering (Section 25.3).