TABLE OF CONTENTS In many instances the geometry associated with tolerances is of significance and the geometry itself needs to be defined by tolerances such that parts fit, locate and align together correctly. Tolerances must therefore also apply to geometric features. The table in Figure 5.13 shows the commonly used geometric tolerance (GT) classes and symbols. These are a selection from ISO 1101:2002. The use of geometric tolerances is shown by three specific examples that are discussed in the detail in the following paragraph.
| Features and tolerance | Toleranced characteristics | Symbols | |
|---|---|---|---|
| Single features | Form tolerances | Straightness |
|
| Flatness | | ||
| Circularity | | ||
| Cylindricity | | ||
| Single or related features | Profile of any line | | |
| Profile of any surface | | ||
| Related features | Orientation tolerances | Parallelism | |
| Perpendicularity | | ||
| Angularity | | ||
| Location tolerances | Position | | |
| Concentricity & coaxiality | | ||
| Symmetry | | ||
| Run-out tolerances | Circular run-out | | |
| Total runout | |
Figure 5.14 shows the method of tolerancing the centre position of a hole. A 10mm diameter hole is positioned 20mm from a corner. The dimensions show the hole centre is to be 20,00 0,1mm (i.e. a tolerance of 100um) from each datum face. This means that to pass inspection, the hole centre must be positioned within a 200um square tolerance zone. However, it would be perfectly acceptable for the hole to be at one of the corners of the square tolerance zone, meaning that the actual centre can be 140um from the theoretical centre. This is not what the designer intended and GTs are used to overcome this problem.
