(See related pages)
|
|
|
| Geometric dimensioning and tolerancing (GDT) is a method of defining parts based on how they function, using standard ASME/ANSI symbols. For many years, only a few companies used GDT to any extent, and even within those companies, there was disagreement on the actual meaning of the symbols and terms. Within the last fifteen years, there has been a resurgence of interest and training in GDT, mostly because of the increased use of statistical process control (SPC), and because GDT is a powerful tool in reducing the cost of precision parts. Statistical process control offers real advantages to companies that manufacture large numbers of parts, because they can reduce or even eliminate the inspection of some features. However, SPC does require that the actual feature be toleranced very efficiently in order to take advantage of the cost reductions. Often, the only way to tolerance such features so that SPC can be used properly is through the use of GDT. Another reason GDT has become so popular is the widespread acceptance of worldwide quality standards,such as the ISO 9000 series. These standards require that a manufacturer specify not only that something is controlled, but also how it is to be controlled. For example, a drawing may say that a feature is round. The questions would be: “How round?” and “How is that roundness inspected?” The standard that controls the dimensioning and tolerancing of parts is ASME Y14.5–1994, and it includes all of the GDT symbols and concepts. Size tolerances alone are sometimes not enough to meet the design needs of a part. Relationships between features may also need to be controlled. In such cases, notes are added to the drawing defining these relationships. For example, a table height from the floor to the top is given as 30 inches (Figure 16.1). Is the top of the table flat? It is, if the tolerance on the 30-inch height is, say, ±1 inch. The top could never be more than 31 inches or less than 29 inches. This means that the top must be flat within 2 inches. If the top must be flatter than that, a tighter tolerance would be, say, ±1/4 inch. Now the top would be flat to within 1/2 inch. However, the height tolerance would become too restrictive, causing the rejection of any table that is out of the height tolerance range, even if it is a good table. This is an example of trying to control the form of a part with a size tolerance. Without GDT, the only way to separate the height tolerance from the flatness tolerance is with notes. The note for the table could read something like this: NOTE 1. TABLE TOP TO BE FLAT WITHIN 1/2 INCH TOTAL. Using GDT, we could return to the ±1 inch tolerance and simply place a flatness control (total of .50 inch, in this example) on the top surface. This would solve the problem and would communicate the design needs to the manufacturer and the inspector. The symbols used in GDT create manufacturing and inspection definitions with a minimum of confusion and misinterpretation. The questions that should be asked continuously during the design phase are, What kind of part would be rejected with these tolerances? Will the rejected parts be unusable? Will we reject all of the parts we cannot use? For our table example, the answers are as follows:
|