تويت
Process planning encompasses selecting the best process to be used in the most advantageous sequence, selecting the specific jigs, fixtures, gages, etc. to be used, and specifying the locating points of the special tools and the speeds, feeds, and depths of cut to be employed.
The processes that take place in transforming a part from chosen raw material to a finished piece include the following.
Basic Processes The first processes used in the sequence that leads to the finished design.
Secondary Processes Those operations required to transform the general form created by the basic process to product specifications. These include:
1. Critical manufacturing operations applied to areas of the part where dimensional or surface specifications are sufficiently exacting to require quality control or used for locating the workpiece in relation to other areas or mating parts.
2. Placement operations whose method and sequence are determined principally by the nature and occurrence of the critical operations. Placement operations prepare for a critical operation or correct the workpiece to return it to its required geometry or characteristic.
3. Tie-in operations, those productive operations whose sequence and method are determined by the geometry to be imposed on the work as it comes out of a basic process or critical operation in order to satisfy the specification of the finished part. Thus, tie-in operations are those secondary productive operations which are necessary to produce the part, but which are not critical.
4. Protection operations, those necessary operations that are performed to protect the product from the environment and handling during its progress through the plant and to the customer, and also those operations that control the product’s level of quality.
Effective process planning requires the consideration of a large number of manufacturing aspects. Today, the modern computer is able to make the many comparisons and selections in order to arrive at an economic plan that will meet quality and quantity requirements. With computerized planning, considerably less time is required and it can be completed by a technician having less factory experience than needed for manual planning.
Process Analysis
Process analysis is a procedure for studying all productive and non-productive operations for the purpose of optimizing cost, quality, throughput time, and production output. These four criteria are not mutually exclusive and they are not necessarily negatively correlated. High quality with few if any rejects can result in high production output with low throughput time and cost. All four of these criteria need to be addressed if a facility is going to be a world competitor producing a quality product. It is possible, for example, to have high productivity with efficient equipment producing good quality, but still fall short in the competitive world because of excessive throughput time. The high throughput time will cause poor deliveries and high cost due to excessive in-process inventory resulting from poor planning and scheduling.
In applying process analysis to an existing plant producing a product line, the procedure is first to acquire all information related to the volume of the work that will be directed to the process under study, namely, the expected volume of business, the chance of repeat business, the life of the job, the chance for design changes, and the labor content of the job. This will determine the time and effort to be devoted toward improving the existing process or planning a new process.
Once an estimate is made of quantity, process life, and labor content, then all pertinent factual information should be collected on operations; facilities used for transportation and transportation distances; inspections, inspection facilities, and inspection times; storage, storage facilities, and time spent in storage; vendor operations, together with vendor prices; and all drawings and design specifications. When the information affecting cost and method is gathered, it should be presented in a form suitable for study, e.g., a flow process chart. This chart presents graphically and chronologically all manufacturing information. Studies should be made of each event with thought toward improvement. The recommended procedure is to take each step in the present method individually and analyze it with a specific approach toward improvement, considering the key points of analysis. After each element has been analyzed, the process should be reconsidered with thought toward overall improvement. The primary approaches that should be used when analyzing the flow chart include (1) purpose of operation, (2) design of parts, (3) tolerances and specifications, (4) materials, (5) process of manufacture, (6) setup and tools, (7) working conditions, (8) materials handling, (9) plant layout, and (10) principles of motion economy. (See also Sec. 12.1, ‘‘Industrial Plants.’’)
Purpose of Operation
Many operations can be eliminated if sufficient study is given the procedural process. Before accepting any operation as necessary, its purpose should be clearly determined and checklist questions should be asked to stimulate ideas that may result in eliminating the operation or some component of it. Typical checklist questions are: Can purpose be accomplished better in another way? Can operation be eliminated? Can operation be combined with another? Can operation be performed during idle period of another? Is sequence of operations the best possible?
Design of Parts
Design should never be regarded as permanent. Experience has shown that practically every design can be improved. The analyst should consider the existing design to determine if it is possible to make improvements. In general, improvements can be made by (1) simplifying the design through reduction of the number of parts, (2) reducing the number of operations required to produce the design, (3) reducing the length of travel in the manufacture of the design, and (4) utilizing a better material in design.
Tolerances and Specifications
These frequently can be liberalized to decrease unit costs without detrimental effects on quality; in other instances, they should be made more rigid to facilitate manufacturing and assembly operations. Tolerances and specifications must be investigated to ensure the use of an optimum process.
Materials Five considerations should be kept in mind relative to both the direct and the indirect material used in the process: (1) finding a less expensive material, (2) finding materials easier to process, (3) using materials more economically, (4) using salvage materials, and (5) using supplies and tools economically.
