High-level process planning identifies the manufacturing operations that can be performed by candidate partners to manufacture the design under consideration. Given the critical design attributes stored in the OOGT information model, the system generates high-level process plans, which identify alternative processes that can be used for the product manufacture and candidate manufacturing plants that are capable of performing these processes. The plans do not describe the detailed instructions necessary for performing each process. Associated with each alternative process-plant are the product features that the plant could fabricate using the corresponding process. In contrast to other methods, plant-specific process planning supports agile manufacturing by considering the specific process capabilities and performance of the candidate partners.
The high-level process planning procedure treats strictly mechanical products and electromechanical products differently. The process plans for mechanical products include a primary process, a secondary process, and a tertiary process. Primary processes are net-shape processes such as casting, forging, and injection molding. Secondary processes are material removal processes such as machining. Tertiary processes such as grinding are finishing operations that do not affect the product's shape. The process plans for electromechanical products consist of through-hole drilling and plating, artwork generation, machining, assembly and soldering, and testing (see Figure).
Our high-level process planning approach generates feasible process alternatives at each step by process selection and plant selection. Process selection is a plant-independent procedure which retrieves all candidate processes (from the process database) associated with key design attributes and discards processes which are globally infeasible (i.e. infeasible at any plant). For example, primary process selection depends upon the product material [1] and the expected production quantity, secondary and tertiary process selection depends upon feature type and the assembly process (manual or assembly) depends upon the component mounting methods and the production quantity.
Plant selection uses plant information from the manufacturing plant model to identify which candidate partners are capable of performing the process to generate the corresponding attributes of the product design. In general, a globally feasible process is feasible at a candidate partner plant if the process is available at the plant and the plant's capabilities satisfy the design specifications. For example, sand casting is a feasible process if the product envelope dimension is within specific dimensions, and it can manufacture features that do not form a thin section and do not have a large section ratio. For other processes, feature feasibility may depend upon tool accessibility, the presence of undercuts, or tolerances. If a process or plant is infeasible, the system provides the designer with the reason, which may allow the designer to modify the product design appropriately.
A process planning data structure (PPDS) was developed to capture the feasible process alternatives generated for the given product design. Figure shows the PPDS for mechanical products. It contains three types of nodes: the root node, process nodes, and plant nodes. The root node contains high-level product information and manufacturing constraints imposed by the designer. Process and plant nodes are used for each process type in the process plan. A process node contains the required the subprocesses and the list of plants that can perform these subprocesses. A plant node contains the list of features that are not manufactured by the process at that plant. This list is composed of features which are either infeasible or unpreferable to manufacture. This list is an important component of the process plans for mechanical products: although a secondary process can produce features and some surface finishes, other features may be realized by a primary process and other surface finishes may require tertiary processing. Note that a NULL process node at a certain level indicates that processing at that level was not required. A NULL plant node follows a NULL process node to store the list of features that remain due to lack of processing.
Machining, grinding, thru-hole plating, artwork generation, and assembly each comprise distinct subprocesses. For example, machining is accomplished by milling and drilling, and thru-hole plating requires drilling and plating steps. In our approach, a plant can feasibly perform a process only if it can perform all required subprocesses. Additional details of the feasibility assessment approach can be found in Gupta et al. [12].
Following the generation of alternative high-level process plans, the manufacturability assessment procedure evaluates the cost, quality, and lead time of each process-plant combination in the process planning data structure. The procedure uses process-specific knowledge, expressed as rules and formulas, and data about the performance of the processes in the specific plants. (The data are found in the manufacturing plant model.)
The lead time associated with each process is the queue time for the process, the setup time for the entire production quantity and each batch, and the total run time of all subprocesses. For example, the sand casting lead time includes time for making the patterns and cores, pouring sand, jolting, removing patterns and arranging cores, pouring molten metal, solidifying the metal, removing the part, cleaning, and inspecting. The queue time, which is plant-dependent, is found in the manufacturing plant model. Process-specific procedures calculate the process setup and run times based on design characteristics, plant capabilities, and process knowledge. The system includes procedures for sand casting, investment casting, forging, milling, drilling, surface grinding, internal grinding, plating, etching, automated assembly, automated soldering, manual assembly, and testing. For example, the milling setup time is the total recurring setup time (for loading, unloading, and cleaning) and the non-recurring setup time. The milling run time includes the actual cutting time for all features (roughing and finishing) and the tool approach time (during rapid and slow travel).
The cost of the process is the setup cost and direct labor cost of the process. The costs are the plant-specific setup and labor rates multiplied by the setup and run times and a plant-specific overhead rate. The quality of a process is the process capability ratio Cp (where appropriate) and a plant-specific yield otherwise. The Cp for etching is the quotient of the minimum artwork tolerance (the minimum of the line width tolerance and the line spacing tolerance) and six times the plant's etching standard deviation. If a process consists of subprocesses, the procedure determines the performance of each subprocesses and aggregates them to calculate the process performance. (In this case Cp are converted to yields, multiplied, and transformed again to a composite Cp.) When this step is completed, the PPDS contains the feasible processes and plants and the cost, quality, and lead time of each combination, which is required for the comparison of high-level process plans and selection of partners.
Figure: Electrical Process Planning
Figure: Process planning data structure (PPDS)