Figure 16 - Pillow Block. This is how the remaining rear section would look. Diagonal lines cross-hatches show regions where materials have been cut by the cutting plane. Figure 17 - Section "A-A".

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It was noticed that though they were made to specifications, the parts did not assemble correctly. After the war, a committee was formed to tackle the defective parts problem. The objective of the committee was to find a way to ensure that parts would fit and function properly, irrespective of where they were manufactured.

Coordinate Tolerancing System Before Geometric Dimensioning and Tolerancing standards were introduced, the most popular system for determining tolerances was the coordinate tolerancing system.

It is a dimensioning system in which the X, Y, Z coordinates of the centres of features of size and surfaces are located by means of linear dimensions with given tolerances.

The shortcoming of this system over geometric dimensioning and tolerances is that it leaves out several important pieces of information about how to inspect the part. It also increases costs and causes safety or functional problems. Geometric dimensioning and tolerances is that language. It is a precise language of engineering symbols that clearly communicates the design intent of the part so that everyone is on the same page.

The result is an improvement in communication between design team and the production tem, and even between design teams. Ultimately, geometric dimensioning and tolerances result in improved part quality. When applied properly, the design engineer can concisely define a features location, size, shape and orientation on the part.

Geometric dimensioning and tolerancing is hence intended to be an addition to the coordinate dimensioning system, not as a complete replacement. The geometric dimensioning and tolerancing methodology takes into consideration the function of a part and how that part functions with related parts.

A properly toleranced drawing is a picture that not only communicates the size and shape of the part but also tells a story that explains the tolerance relationships between features. Geometric dimensioning and tolerances standards should be used when: Drawing delineation and interpretation need to be the same Features are critical to function or interchangeability of mating parts it is important to stop scrapping perfectly good parts and reduce drawing changes Without common specifications of dimensions and tolerances, engineering drawings would not be understood easily.

Two co-ordinate systems are established from these surfaces, one on each part. Relating features on each part to these datum references minimizes variation between related features on each part. While simple parts may have a single datum reference frame, complex parts may have many datum reference frames due to geometry or functional necessity. In such cases, each datum reference frame on a part must be related to the other datum reference frames on the part, either directly or indirectly.

Types of Geometric Dimensioning Tolerances Form A form tolerance specifies how far an actual surface or feature is permitted to deviate from the desired form specified in a drawing. It includes flatness, straightness, circularity, profile of a surface and profile of a line. It includes perpendicularity, angularity and parallelism. Profile Profile geometric dimensioning and tolerance is a 3D tolerance that defines a uniform boundary around a surface within which the elements of the surface must lie.

It is usually used on parts with complex outer shape and a constant cross-section such as extrusions. Location and Runout The location tolerance specifies how far an actual feature is permitted to deviate from the perfect location given in a drawing as related to datum or other features. It includes position, concentricity and symmetry. The runout tolerance specifies how far an actual surface or feature is permitted to deviate from the desired form given in a drawing during full rotation of the part on a datum axis.

There are two types of runout - circular and total. In order to apply geometric dimensioning and tolerances properly, the design engineer should know the manufacturing process that will be used to manufacture the part.

All dimensioning processes provide a certain degree of geometric control. In addition, geometric dimensioning and tolerance could be used to ensure consistency of datum references between design, manufacture and verification operations, and also when computer tools are used during design and manufacture.

Tolerance is crucial not only for the functionality of the component, but also for the economy of manufacturing. The success of a program like DCS Variation Analyst lies in the intelligent specification of tolerances for design and manufacturing.


Geometric Dimensioning & Tolerancing (GD&T)



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