Geometric Dimensioning & Tolerancing: Symbols & Design

Instructor: Joseph Komor

Joseph has taught High School Engineering and is a consultant for the Space Program. He has a BS in mechanical engineering, a BS in general business, and is working towards his master's degree.

Explore geometric dimensioning and tolerances along with their symbols. Discover how they are properly used in different applications and how they uphold function within a system.

How Inexact Can We Be?

In manufacturing, parts can never be made as precise as they are intended on engineering drawings; thus, engineers and designers must take this into consideration to ensure the integrity of the system is not compromised. Geometric Dimensioning and Tolerancing (GD&T) is a way to specify a range of deviation from a nominal geometric shape.

Geometric Dimensioning and Tolerancing is specified on engineering drawings and are for the manufacturers to ensure their parts come to reality as they were originally intended. It is the job of the engineer to ensure geometric tolerances are utilized when appropriate.

They must look at the function of the part, and then deem what geometric tolerances are necessary. All geometric tolerances must include a specified range that the manufacturer must adhere to.

Types of Geometric Dimensioning and Tolerancing

Let's go through a few of the types of geometric dimensioning and tolerancing.

Straightness, Flatness, Circularity

Straightness is a tolerance that conforms a surface or line to be a true straight line. This is mainly used when the part needs to be properly aligned with another. This can be seen in combustion engines in cars. The cylinders that pistons fire need to be straight to ensure efficiency.

Flatness specifies a deviation of true flatness of a surface. This is important when dealing with mechanical motion to ensure two surfaces do not interfere.

For an example, if two gears are turning very close side by side, it is very important that they do not touch. Flatness would be a critical tolerance to call out in this instance.

Circularity specifies deviation from a true perfect circle. If you were designing a piston to fire inside of a chamber, their diameters and true circularity must match in order for them to not interfere. After all, an oval inside of a circle cannot work as functioned.

Symbols for straightness, flatness, and circularity

Cylindricity, Line Profile, Surface Profile

Cylindricity specifies deviation from a true cylinder. This is important when dealing with circularity among a distance. This specifies that it must have a true circle profile for the entire length of the cylinder.

For example, if you were building an outer ring of a bearing to an inner ring of a bearing, you would want their cylindrical profiles to be the same.

Profile of a line and profile of a surface are tolerances that specify deviation from a curved or non-straight line. These are usually called out in drawings to ensure their profile stays the same.

These are important when two parts must fit together within a fashion of curves. Picture an oval fan. The cage around the fan must match the shape of the blades, otherwise the blades would interfere with the cage.

Symbols for cylindricity, line profile, and surface profile

Perpendicularity, Angularity, Parallelism

Perpendicularity specifies deviation between two surfaces, lines, or planes that must be perpendicular to each other, in other words, at 90 degrees. Think of a part must fit into another and be exactly 90 degrees above it.

Angularity specifies a deviation between two features (surface, line, planes) and a specified angle between them. Say, a steering wheel that must insert into a car at 45 degrees.

Parallelism specifies deviation between two features (surface, line, plane) and their parallelism. Parallel is defined as two planes or surfaces being the same distance apart at any part of them. Gears in watches, for example, must be at a true parallel in order to avoid interference.

Symbols for perpendicularity, angularity, parallelism

Symmetry, Position, Concentricity

Symmetry specifies deviation between two symmetrical features on a part. It ensures that two surfaces are the same shape and therefore truly symmetrical across a plane. This can be very important if you want a part to be able to flip either way in your application. Assembly of systems is easier when parts can be flipped around with no effect.

Position specifies a deviation between a true location on a part and the drawing. For an example, if a hole needs to be exactly in the center of a gear.

Concentricity specifies a deviation between the centers of two circles. Two circles that are concentric means their center axis are at the same point. This is important to specify when dealing with two cylinders that must be perfectly spaced all around, like a pipe that must fit at a specified distance inside another.

Symbols for symmetry, position, and concentricity

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