Two groups of tolerances are important to know of when talking about mechanical drawings: dimensional tolerances and geometrical tolerances. Dimensional tolerances are the classic group of tolerances that could be specified on a drawing. They are also known as ±tolerances. These tolerances have been around since the 1920s and have basically not changed since then. It was not until the 40s that the geometrical tolerances came about and not until the middle of the 90s that their development really took off.
To specify as well as interpret dimensional and geometrical tolerances on a mechanical drawing, the GPS-system (Geometrical Product Specification) is used, which can be described as a symbolic language for drawings. This language should not be mistaken as the satellite navigation system “Global Positioning System”. About 150 ISO standards collectively define GPS.
Other than GPS, there are other symbolic languages for drawings that are in use out in the world. The largest one among them is the American one, which is defined by the American standard ASME Y14.5. However, GPS is the national standard in the European countries and in many of the other ISO member countries.
In this article, the GPS language is applied and the benefits of using geometrical tolerances versus dimensional tolerances within this language are presented.
Why not only use dimensional tolerances?
Dimensional tolerances are versatile and with them you can specify the tolerance of many different kinds of geometrical features of a part, but not all of its features. How concentric two concentric holes should be relative to each other is one example of this. Moreover, certain applications of these tolerances can be interpreted in more than one way. Below is one example of such an application: