Geometric Dimensioning and Tolerancing (GD&T)
GD&T characteristics and how they are measured.
What is Geometric Dimensioning and Tolerancing?
Geometric Dimensioning and Tolerancing (GD&T) is the language used in design, manufacturing and quality inspection to communicate information on engineering drawings.
GD&T consists of a set of symbols, rules and principles to ensure that everyone has exactly the same information and understanding of a feature or how a part is defined.
The design engineer provides a GD&T drawing to manufacturing department, and they will use this common technical language for communicating what must be done without ambiguity.
Quality engineers will use the drawing to check if components manufactured meet the desired tolerances for each characteristic. GD&T is a universal system to regulate communication.
We offer GD&T fundamentals training sessions to guide those working in design and manufactureing to gain understanding for their individual role.
Form
What is Form?
Form tolerance dictates the geometric shape, types of form include straightness, roundness, flatness, and profile of the part. All form tolerances are independent of their position within a coordinate frame. Form tolerances are often inferred from a dimensional tolerance.
How to Measure Form
Some form tolerances can be measured with hand tools, for example you could measure flatness with Surface plates used as reference with a dial test indicator (DTI). There are systems that are more versatile such as form testers and CMMs that will offer automatic measurement and data collection.
Systems Used to Measure Form
GD&T Symbols
These characteristics include the following, click for more details:
Straightness
Flatness
Circularity
Cylindricity
Profile
What is Profile?
It straddles between a form tolerance and a location tolerance. It is valid to measure against a datum feature or independent of any reference. The same measurement with and without a datum could give a different result. Profile is evaluating how close a surface or line is to it’s nominally defined shape. The location of this shape may also be important in which case you would use a datum reference.
How to measure profile
The evaluation of primitive features such as lines, planes, circles, cylinders can be done using first principles. You can also use hand tools to evaluate complex shapes as long as discrete points are defined on a drawing/CAD model. Complex shapes are more often measured on CMMs and 3D scanners.
Systems Used to Measure Profile
GD&T Symbols
These characteristics include the following, click for more details:
Profile of a line
Surface profile
Orientation
What is Orientation?
This tolerance defines the orientation of a shape in reference to a specific datum feature. This is often confused as simply the angular dimension but is often more complex as it may infer a form tolerance. For example an angularity tolerance between two planes would also infer a flatness tolerance. Orientation includes the measurement of parallelism, perpendicularity, and angularity.
How to measure orientation
First principles measurement of orientation requires the use of reference standards such as a surface plate, precision squares or angle blocks in conjunction with DTIs to evaluate. A more efficient and commonly used method is using a coordinate measurement machine (CMM) or vision measurement machine VMM to build features virtually and evaluate tolerances using a computer.
GD&T Symbols
These characteristics include the following, click for more details:
Angularity
Perpendicularity
Parallelism
Location
What is Location?
Location tolerance establishes the precise location of a feature with respect to a reference feature. It is impossible to measure a location tolerance without a datum feature being specified.
Location tolerances do not evaluate the shape of the feature but the shape can influence the final result.
How to measure location
In order to measure location in first principles you would also need to utilise mathematical formulae. Often high production parts are measured using jigs and fixtures which simulate the evaluation physically. Fundamentally location is almost universally measured using digital methods like CMMs, VMMs throughput permitting.
GD&T Symbols
Location derived from median points. These characteristics include the following, click for more details:
Position
Concentricity
/Coaxiality
Symmetry
Runout
What is Runout?
Runout measures form and position of a feature in reference to a datum. As an analogy runout can be thought of as combining roundness and concentricity in to the same measurement. Total runout could be thought of as combining cylindricality and coaxiality in to the same measurement. Runout tolerances refer to circular features so the datum also refers to an axis. These tolerances are most commonly found on precision shaft products like camshafts and crankshafts.
How to measure runout
A part rotated around its reference axis with a DTI measuring the feature will give you total indicated runout TIR. This can be done automatically when using a shaft measurement system or CMM.
GD&T Symbols
These characteristics include:
Runout
Total runout
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The Sempre Group Ltd
The Lodge
37 Barnett Way, Barnwood
Gloucester, GL4 3RT
The Lodge
37 Barnett Way, Barnwood
Gloucester, GL4 3RT
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