Manufacturing tolerance existed from the presence of the first engineering drawing. The manufactured tolerance of a part can be measured as the deviation from perfection. The measurements for the variation from precision must be within defined limits. The assigned degree of tolerance depends on the primary function of the part.

Understanding Tolerancing Principles

Properly assigning tolerance ensures a part fits together with other components. The main objective of manufacturers is to determine a balance between cost and tolerance. Constituents with a precise tolerance have a high manufacturing cost due to the precision required to build the part. The opposite is true for elements with a wide tolerance.

Introductory values of tolerance were simple because they had a +/- tolerance value. With the development of engineering practices, manufacturing parts became a complicated process. The need for more definitive measurements became apparent. We now use geometric dimensioning & tolerancing (GD&T) to implement tolerances practically.

What Is Geometric Dimensioning & Tolerancing?

GD&T uses relatively simple tools to implement comprehensive and consistent tolerances. For designers, “design intent” is the main focus that allows designers to communicate the essential purpose of the design. The design takes into consideration the “function” of the part. A system, more correctly, a “universal” terminology is required to reveal the design intent to anybody who encounters a design.

The engineering department (designers), the manufacturing department (production and fabrication), and the quality assurance department (measurement and inspection) are the first teams to encounter designs. These three faculties must speak the same language, and GD&T eliminates any uncertainty from the equation. GD&T employs a set of rules and symbols to specify the geometric limits of mechanized parts. The rules and symbols developed by The American Society of Mechanical Engineers (ASME) are a standard for a design to serve its purpose and prescribe established conventions.

To communicate design intent geometric dimensions must not transcend certain limits based on these standards. The prescribed limits of measurement are a result of empirical research (mostly trial and error), engineering intelligence, and experience. An engineering drawing facilitates communication between different departments by implementing these mandatory standards. An engineering drawing contains crucial information because it can also serve as a legal contract.

How To Implement GD&T Standards?

The main goal of GD&T is to communicate and specify the dimensions of a part for it to function properly. It’s that easy, right? Not so fast, as with all standards, certain requirements must be met. When we talk about specifying the dimensions of a part, how do we do that?

The measurements of SLOF (Size, Location, Orientation, and Form) are the four fundamental factors of an engineering drawing. Size refers to the physical size of the final part and this includes detailed dimensions of each side. Location refers to the object’s location in context to the X, Y, and Z axes in 3D space. The orientation refers to the object’s orientation on u, v, and w vectors in 3D space.

The form of an object refers to the shape of the features of the final product. Manufacturing costs of an item increase as the precision of each of these measurements increases.