MANUFACTURING PROCESS PLANNING AND DESIGN:COMPUTER-AIDED PROCESS PLANNING SYSTEMS SELECTION CRITERIA

By -

COMPUTER-AIDED PROCESS PLANNING SYSTEMS SELECTION CRITERIA

Selecting a process-planning system for a company is not an easy task. Each machine shop has its own unique characteristics. Decision tables have been developed for selecting computer-aided process-planning systems. For example, Steudel and Tollers (1985) present a decision table for se- lecting CAPP solutions (the tables are also reprinted in Chang et al. [1991, chap. 13]). This approach assumes that certain decision rules can be written for each selection decision. However, often such rules are not available. Another approach is to compare the characteristics of the competing systems based on weights. Table 6 shows a comparison table for two process-planning systems. In the table are 12 major categories, most of which are divided into several subcategories. Weights can be assigned

Manufacturing Process Planning and Design-0244

to either each major category or subcategory. The total weights may be used to determine the final selection. The comparison is not limited to the pair comparison as it is in the example; more systems can be added to the table.

The meanings of the categories in Table 6 are defined below:

• Input data: The process-planning system must interface with the existing CAD design system. The system can either read the native CAD data or can input through an data-exchange format such as IGES or STEP.

• CAD file format: the design data file acceptable, e.g., Pro / E, CADDS, CATIA, IGES, STEP.

• Workpiece understanding:

• Shape analysis: feature recognition; converting design data into manufacturing feature data.

• Material analysis: identification of raw material shape, sizes, type (cast, bar, etc.), and weight.

• Specification analysis: extracting other manufacturing specifications from the design (e.g., paint, hardness, surface treatment).

• Tolerance analysis: extracting tolerance data from the design.

• Process selection:

• Feature hierarchy: process selection based on hierarchically arranged manufacturing features.

• Tolerance analysis: process selection based on the tolerance capabilities of processes vs. the design specifications.

• Process capability model: a computer model that captures the capability of a process. Process models are used in process selection. They may be customized.

• Specification analysis: understanding other manufacturing specifications and using them as the basis for selecting processes.

• Cost analysis: estimating and analyzing manufacturing cost and using it as the basis for selecting processes.

• Capacity analysis: machine selection with the consideration of the throughput of each machine.

• Machine tool management:

• Machine tool selection: selecting appropriate machine tools for the job.

• Machine capability model: a computer model that captures the process capability of a machine tool.

• Maintenance planning: maintenance schedule for machine tools.

• Environmental analysis: assessment of environmental requirements (temperature, pressure, hu- midity) necessary to achieve quality targets.

• Flow analysis: production flow analysis (i.e., throughput).

• Life-cycle analysis: the economic analysis, from cradle to grave, of asset costs to derive metrics like ROA.

• Supplier development: identifying supplier for the machine tool.

• Control specification: managing NC controller specifications for postprocessors.

• Facility planning: integration with the facility planning functions.

• Quality management

• Process control planning: generating control charts for process control.

• Gage planning: CMM programming, gage calibration, certification planning.

• Scrap and rework management: planning for management of the disposition of parts for scrap and rework

• Quality assurance: generating quality assurance plan for certification. Process plans may be used for this purpose.

• Process design

• Tool selection: selecting tools to be used for the machining of a part.

• Fixture design: designing the fixture necessary for the workpiece holding under a given setup.

• Gage design: designing the gage for inspecting the workpiece.

• Tool path generation: generating the NC tool path for the part.

• Operation sequencing: sequencing the NC tool path for the part

• Process parameters: selecting process parameters, e.g., feed and speed, for each tool / cut.

• Feature accessibility: analyzing the tool accessibility of each feature to be machined.

Manufacturing Process Planning and Design-0245

• Tolerance analysis: comparing the tolerance specification against the tolerance capabilities of the selected tools and machine tools; stacking up tolerance based on the setup and machine accuracy and setup error.

• Evaluation and validation:

• Target cost: estimating the manufacturing cost of that part.

• Tool path verification: graphically simulating the tool motion to ensure that the tool path has no collision with the fixture and the machine and produces correct part geometry.

• Workpiece quality: confirming that quality indices are met.

• Process reliability: reliability indexes for the processes selected for the part.

• Production times: estimating the time it takes to produce the part.

• Document generation:

• Tool / gage orders: job orders for tools and gages needed.

• Equipment orders: job orders for equipment used.

• Operation sheets: detailed description of each operation and operation parameters including gaging detail / instructions.

• Process routing: process routing sheet. Lists operation sequence and processes.

• Part programs: NC part program and part program format, e.g., RS 274, CL, BCL, APT source, COMPACT II source, postprocessed N-G code.

• Setup drawings: drawings of the workpiece with fixture for each setup.

• Machine domain: machine tools that can be modeled and planned by the system.

• Part domain: types of part that can be planned by the system (e.g., turned, prismatic, casting).

• Platform: computer platform on which the software can be used.

• Technical support: technical support provided by the vendor.

• Total: total composite score based on the criteria above.

A composite score may not be sufficient for making the final decision. It is necessary to record the specifications in each subcategory. Table 7 illustrates such a table for the same comparison as in Figure 6. With these two tables, an appropriate computer-aided process planning system can be selected.

Comments

Post a Comment

Popular posts from this blog

MATERIAL-HANDLING SYSTEMS:STORAGE SYSTEMS

By -
Image
STORAGE SYSTEMS Various types of racks have been developed to store loads of different sizes and shapes. One of the simplest storage systems is block stacking, where loads (which may or may not be palletized) are simply stacked on top each other with no separate support structure. While block stacking is inex pensive and provides dense storage (i.e., little or no space is lost due to air between the loads), its use is limited in practice because only certain types of loads (such as appliances or furniture packed properly in cartons) can be stacked without crushing / damaging the loads at the bottom and access to individual loads (i.e., load selectivity) is very limited. In fact, loads must be removed from a stack, starting with the load at the top. This leads to last-in-first-out (LIFO) stock rotation, which is not desirable for many storage systems. A fundamental trade-off that exists in storage systems design is that between storage density (how well the cubic volume is utilized) ...
Read more

NETWORK OPTIMIZATION MODELS:THE MINIMUM SPANNING TREE PROBLEM

By -
Image
THE MINIMUM SPANNING TREE PROBLEM The minimum spanning tree problem bears some similarities to the main version of the shortest-path problem presented in the preceding section. In both cases, an undi r ecte d and connected network is being considered, where the given information includes some mea- sure of the positive length (distance, cost, time, etc.) associated with each link. Both problems also involve choosing a set of links that have the shortest total length among all sets of links that satisfy a certain property. For the shortest-path problem, this property is that the chosen links must provide a path between the origin and the destination. For the minimum spanning tree problem, the required property is that the chosen links must provide a path between ea c h pair of nodes. The minimum spanning tree problem can be summarized as follows: 1. You are given the nodes of a network but not the links. Instead, you are given the po- tential links and the positive length for each ...
Read more

DUALITY THEORY:THE ESSENCE OF DUALITY THEORY

By -
Image
THE ESSENCE OF DUALITY THEORY Given our standard form for the primal problem at the left (perhaps after conversion from another form), its dual problem has the form shown to the right. Thus, with the primal problem in maximizatio n form, the dual problem is in minimization form instead. Furthermore, the dual problem uses exactly the same pa r amete r s as the primal problem, but in different locations, as summarized below. 1. The coefficients in the objective function of the primal problem are the right-hand sides of the functional constraints in the dual problem. 2. The right-hand sides of the functional constraints in the primal problem are the coef- ficients in the objective function of the dual problem. 3. The coefficients of a variable in the functional constraints of the primal problem are the coefficients in a functional constraint of the dual problem. To highlight the comparison, now look at these same two problems in matrix notation (as introduced at the beginn...
Read more