While we have often talked about design being a creative process, the fact is that much of design is not very different from what has been done in the past. There are obvious benefi ts in cost and time saved if the best practices are captured and made available for all to use. Designing with codes and standards has two chief aspects: (1) it makes the best practice available to everyone, thereby ensuring effi ciency and safety, and (2) it promotes interchangeability and compatibility. With respect to the second point, anyone who has traveled widely in other countries will understand the compatibility problems with connecting plugs and electrical voltage and frequency when trying to use small appliances.
A code is a collection of laws and rules that assists a government agency in meeting its obligation to protect the general welfare by preventing damage to property or injury or loss of life to persons. A standard is a generally agreed-upon set of procedures, criteria, dimensions, materials, or parts. Engineering standards may describe the dimensions and sizes of small parts like screws and bearings, the minimum properties of materials, or an agreed-upon procedure to measure a property like fracture toughness.
The terms standards and specifications are sometimes used interchangeably. The distinction is that standards refer to generalized situations, while specifications refer to specialized situations. Codes tell the engineer what to do and when and under what circumstances to do it. Codes usually are legal requirements, as in the building code or the fire code. Standards tell the engineer how to do it and are usually regarded as recommendations that do not have the force of law. Codes often incorporate national standards into them by reference, and in this way standards become legally enforceable.
There are two broad forms of codes: performance codes and prescriptive codes. Performance codes are stated in terms of the specific requirement that is expected to be achieved. The method to achieve the result is not specified. Prescriptive or specification codes state the requirements in terms of specific details and leave no discretion to the designer. A form of code is government regulations. These are issued by agencies (federal or state) to spell out the details for the implementation of vaguely written laws. An example is the OSHA regulations developed by the U.S. Department of Labor to implement the Occupational Safety and Health Act (OSHA).
Design standards fall into three categories: performance, test methods, and codes of practice. There are published performance standards for many products such as seat belts, lumber, and auto crash safety. Test method standards set forth methods for measuring properties such as yield strength, thermal conductivity, or electrical resistivity. Most of these are developed for and published by the American Society for Testing and Materials (ASTM). Another important set of testing standards for products are developed by the Underwriters Laboratories (UL). Codes of practice give detailed design methods for repetitive technical problems such as the design of piping, heat exchangers, and pressure vessels. Many of these are developed by the American Society of Mechanical Engineers (ASME Boiler and Pressure Vessel Code), the American Nuclear Society, and the Society of Automotive Engineers.
Standards are often prepared by individual companies for their own proprietary use. They address such things as dimensions, tolerances, forms, manufacturing processes, and fi nishes. In-house standards are often used by the company purchasing department when outsourcing. The next level of standard preparation involves groups of companies in the same industry arriving at industry consensus standards. Often these are sponsored through an industry trade association, such as the American Institute of Steel Construction (AISC) or the Door and Hardware Institute. Industry standards of this type are usually submitted to the American National Standards Institute (ANSI) for a formal review process, approval, and publication. A similar function is played by the International Organization for Standardization (ISO) in Geneva, Switzerland.
Another important set of standards are government (federal, state, and local) specifi cation standards. Because the government is such a large purchaser of goods and services, it is important for the engineer to have access to these standards. Engineers working in high-tech defense areas must be conversant with MIL standards and handbooks of the Department of Defense.
In addition to protecting the public, standards play an important role in reducing the cost of design and of products. The use of standard components and materials leads to cost reduction in many ways. The use of design standards saves the designer, when involved in original design work, from spending time on fi nding solutions to a multitude of recurring identical problems. Moreover, designs based on standards provide a fi rm basis for negotiation and better understanding between the buyer and seller of a product. Failure to incorporate up-to-date standards in a design may lead to difficulties with product liability.
The engineering design process is concerned with balancing four goals: proper function, optimum performance, adequate reliability, and low cost. The greatest cost saving comes from reusing existing parts in design. The main savings come from eliminating the need for new tooling in production and from a signifi cant reduction in the parts that must be stocked to provide service over the lifetime of the product. In much of new product design only 20 percent of the parts are new, about 40 percent are existing parts used with minor modifi cation, while the other 40 percent are existing parts reused without modification.
Computer-aided design has much to offer in design standardization. A 3-D model represents a complete mathematical representation of a part that can be readily modifi ed with little design labor. It is a simple task to make drawings of families of parts that are closely related. A formal way of recognizing and exploiting similarities in design is through the use of group technology (GT). GT is based on similarities in geometrical shape and/or similarities in their manufacturing processes. Coding and classifi cation systems 15 are used to identify and understand part similarities. A computerized GT database makes it possible to easily and quickly retrieve designs of existing parts that are similar to the part being designed. This helps combat the tendency toward part proliferation, which is encouraged by the ease of use of a CAD system. The installation of a GT system aids in uncovering duplicative designs; it is a strong driver for part standardization. GT may also be used to create standardization in part features. For example, the GT database may reveal that certain hole diameters are used frequently in a certain range of parts while others are infrequently used. By standardizing on the more frequently used design features, simplifi cations and cost savings in tooling can be achieved. Finally, the information on manufacturing costs should be fed back to the designer so that high-cost design features are avoided.
Standards as a Limit to Technology Advancement
On balance, standards are necessary to the advancement of technology, but they can be an inhibiting factor as well. Consider the ASME Boiler and Pressure Vessel Code that has been adopted by all 50 states to regulate machinery using gases or liquids operating under pressure. Formulated during the early 1900s to prevent catastrophic failures and explosions, it spells out in detail the types of material that may be used and the performance specifi cations a new material must meet. The materials specifi cations are nearly the same as they were 50 years ago, despite the fact that much stronger, more fracture-resistant materials are now available. This is because the performance criteria are so stringent that it would take tens of millions of dollars of testing to qualify a new material. No one company can afford to underwrite such costs. But the costs of failure are so high that no one wants to risk changing the code without these tests.
An important aspect of standardization in CAD-CAM is in interfacing and communicating information between various computer devices and manufacturing machines. The National Institute of Standards and Technology (NIST) has been instrumental in developing and promulgating the IGES code, and more recently the Product Data Exchange Specifi cation (PDES). Both of these standards represent a neutral data format for transferring geometric data between equipment from different vendors of CAD systems. This is an excellent example of the role of, and need for, a national standards organization.
No comments :
Post a Comment