# geometric dimensioning and tolerancing (gd&t)

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Geometric Dimensioning and Tolerancing (GD&T). What is Geometric Dimensioning and Tolerancing and why use it?. - PowerPoint PPT Presentation

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Geometric Dimensioning and Tolerancing is the process of applying standard, international symbols rules and conventions to engineering drawings to control the Form, Profile, Orientation, Position or Run out of a feature or features of a component in addition to limits of size.

Reasons for use are: Ensures Interchangeability of mating parts. Saves time during manufacturing/Assembly process. Aids the inspection process with standardised easy to

understand drawings Alleviates the need for additional notes on drawings

Considered a mathematical language and very precise Describes components in three dimensions similar to

Cartesian coordinate system developed by Rene Descartes

Rene Descartes established three precepts about the way we should examine things

Most important being “Never accept anything for true which you do not clearly see to be so”.

This led to idea of examining everything in relation to what it should be “Exact and Perfect”

Cartesian Coordinate system (looking at coordinate planes to describe position of objects) was developed from this.

Looking at components from this perspective led to standards used today

ASME (American Society of Mechanical Engineering ASME Y14.5 and ISO (International Organisation for Standards ISO 1101

Straightness is two dimensional tolerance.

The edge must remain within two imaginary parallel lines to meet tolerance.

The distance between the lines is determined by the tolerance size

Rectangular parts usually have a straightness tolerance, it can also be applied to the axis or edge of a cylinder

Flatness is a three dimensional version of straightness tolerance.

The surface must remain within two imaginary perfectly flat perfectly parallel planes.

Only the surface not the entire thickness is referenced to the planes.

If used as a primary datum flatness must be specified

Circularity is a two dimensional tolerance.

Any two dimensional cross section of a round feature must remain within the tolerance zone created by two concentric circles.

Is Applied to cylinders, cones and Spheres

Cylindricity specifies the roundness of a cylinder along its entire length.

All cross-sections of the cylinder must be measured together, so cylindricity tolerance is only applied to cylinders.

Circularity and cylindricity cannot be checked by measuring various diameters with a micrometer.

Part must be rotated in a high-precision spindle. Best method would be to use a Coordinate Measuring Machine (CMM).

The profile of a line is a two dimensional tolerance.

It requires the profile of a feature to fall within two imaginary parallel lines that follow the profile of the feature.

Profile of a Surface is three-dimensional version of the line profile.

Often applied to complex and curved contour surfaces such as aircraft and automobile exterior parts.

The tolerance specifies that the surface must remain within two three dimensional shapes.

A three dimensional tolerance.A three dimensional tolerance. Shape of the tolerance zone Shape of the tolerance zone

depends on the shape of the depends on the shape of the feature.feature.

If applied to flat surface, If applied to flat surface, tolerance zone becomes two tolerance zone becomes two imaginary planes, parallel to imaginary planes, parallel to the ideal angle.the ideal angle.

If applied to a hole, it is If applied to a hole, it is referenced to an imaginary referenced to an imaginary cylinder existing around the cylinder existing around the

ideal angle and center ideal angle and center of the hole must stay within of the hole must stay within that cylinder.that cylinder.

Three-dimensional Three-dimensional tolerances that use tolerances that use the same tolerance the same tolerance zones as angularity.zones as angularity.

Difference is that Difference is that parallelism defines parallelism defines two features that two features that must remain parallel must remain parallel to each other, while to each other, while perpendicularity perpendicularity specifies a 90-specifies a 90-degree angle degree angle between features.between features.

Position is one of the most common location tolerances.

Is a three dimensional related tolerance. Usually involves more than one datum to

establish position of feature. Does not rely on size shape or angle is

concerned with position. In the case of holes, the tolerance involves In the case of holes, the tolerance involves

the center axis of the hole and must be the center axis of the hole and must be within the imaginary cylinder around the within the imaginary cylinder around the intended true position of the hole.intended true position of the hole.

If toleranced feature is rectangular, the If toleranced feature is rectangular, the zone involves two imaginary planes at a zone involves two imaginary planes at a specified distance from the ideal true specified distance from the ideal true position.position.

Concentricity and Concentricity and Symmetry are both three-Symmetry are both three-dimensional tolerances.dimensional tolerances.

Concentricity is not Concentricity is not commonly measured.commonly measured.

It relates a feature to one It relates a feature to one or more other datum or more other datum features.features.

This shaft is measured in This shaft is measured in multiple diameters to multiple diameters to ensure that they share a ensure that they share a common center-axis.common center-axis.

Symmetry is much like Symmetry is much like concentricity.concentricity.

Difference is that it controls Difference is that it controls rectangular features and rectangular features and involves two imaginary flat involves two imaginary flat planes, much like parallelism.planes, much like parallelism.

Both symmetry and Both symmetry and concentricity are difficult to concentricity are difficult to measure and increase costs of measure and increase costs of inspection.inspection.

When a certain characteristic, When a certain characteristic, such as balance, is important, such as balance, is important, these tolerances are very these tolerances are very effective.effective.

Circular and Total Runout are three-Circular and Total Runout are three-dimensional and apply only to cylindrical dimensional and apply only to cylindrical parts.parts.

Both tolerances reference a cylindrical Both tolerances reference a cylindrical feature to a center datum-axis, and feature to a center datum-axis, and simultaneously control the location, form simultaneously control the location, form and orientation of the feature.and orientation of the feature.

Circular runout can only be inspected Circular runout can only be inspected when a part is rotated.when a part is rotated.

Calibrated instrument is placed against Calibrated instrument is placed against the surface of the rotating part to detect the surface of the rotating part to detect the highest and lowest points.the highest and lowest points.

The surface must remain within two The surface must remain within two imaginary circles, having their centers imaginary circles, having their centers located on the center axis.located on the center axis.

Total Runout is similar to Total Runout is similar to circular runout except that it circular runout except that it involves tolerance control along involves tolerance control along the entire length of, and the entire length of, and between, two imaginary between, two imaginary cylinders, not just at cross cylinders, not just at cross sections.sections.

By default, parts that meet total By default, parts that meet total runout tolerance automatically runout tolerance automatically satisfy all of the circular runout satisfy all of the circular runout tolerances.tolerances.

Runout tolerances, especially total Runout tolerances, especially total runout, are very demanding and runout, are very demanding and present costly barriers to present costly barriers to manufacturing and inspection.manufacturing and inspection.

The diagram below demonstrates how variation of form can affect the fit of a component at maximum material condition.

Components can have many datum's each considered to have perfect geometric form.

Datum’s can be: Straight lines Circles/Holes Flat planes Spheres Cylinders Cones or a single point some

are shown opposite. Utilising datum's as a

reference gives tolerances a new meaning

Engineering, manufacturing, and inspection all share a common “three plane” concept (Cartesian Coordinate system )

These three planes are:◦ Mutually perpendicular◦ Perfect in dimension and

orientation◦ Positioned exactly 900 to

each other. This concept is referred to as

the Datum Reference Frame (DRF)

The Diagram opposite shows how a rectangular part fits into the corners represented by the inter-section of the three datum planes.

The most important concept to grasp is that when the part is placed into inspection apparatus, it must make contact with the apparatus planes in the order specified by the feature control frame. (Primary, then secondary, then tertiary). This is the only way to assure uniformity in the measurement of different parts.

A cylindrical part rests onthe flat surface of the primaryplane and the center of the cylinder aligns with the vertical datum axis createdby the intersection of the planes.

In this case, it becomes veryimportant to be able to establishthe exact center of the part,whether it is the center of a solid surface, or the center of a hole.

Cylindrical parts are more difficult to measure.