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GPS 123 - Datums and Datum Systems

Datums and datum systems are the tools you use to constrain a tolerance zone for one feature on the component to one or more other features.

Datums and datum systems are used within GPS to constrain tolerance zones. By referencing datums in a geometrical specification, you can lock one or more degrees of freedom for the tolerance zone.

A properly designed datum system will simulate how the component is assembled into the product. By referencing this datums system in all the relevant geometrical specifications for the component, you can ensure that all the tolerance zones are locked into a common coordinate system. This allows the tolerances to be as large as possible, because tolerance is not wasted by tolerance zones being able to move relative to each other.

The course builds to some extent on GPS 120 - Introduction to Geometrical Tolerancing, GPS 121 - Degrees of Freedom, and GPS 122 - Tolerance Zones, so having completed these courses will make the course more meaningful.

A number of considerations have to be made, when selecting datums and deciding on the sequence of datums in the datum system:

  • GPS and geometrical tolerances simulate component functions
  • Component functions are dependent on the relative location of the features in the component
  • By appointing some of the features as datum features, tolerance zones for other features can be attached these features.
  • The functionally correct datum system is identified by observing how the component is assembled into the overall product
    • Which features control the location of the component?

The course covers:

  • Indication of datums on a drawing.
    • Simple datums associated to one entire feature.
    • Datums associated to part of a feature.
    • Datum targets, where the datum is associated to a number of points or zones on a feature.
    • Common datums, where a datum is associated to several features together.
  • Datum association
    • How the geometrically perfect datum is defined from the real datum feature on the component.
    • The relationship between the datum and the datum feature.
  • Datum systems
    • How a sequence of datums can be used to constrain all degrees of freedom.
  • TEDs - Theoretically exact dimensions.
    • Dimensions used to define the location and orientation of tolerance zones within a datum system.

The course will give you an understanding of how datums work, how they are indicated and what the rules are for building datum system and attaching tolerance zones to them.


Your Instructor


Henrik S. Nielsen
Henrik S. Nielsen

Dr. Henrik S. Nielsen is the president of HN Metrology Consulting and HN Proficiency Testing, the first accredited proficiency testing provider catering to dimensional inspection and calibration laboratories in the US.

His areas of expertise include geometrical product specifications, dimensional metrology, uncertainty estimation and quality assurance for calibration laboratories. He teaches seminars on all of these subjects though various organizations.

Dr. Nielsen is the chairman of ISO Technical Committee 213 “Geometrical Product Specifications and Verification” and has been an expert serving on related committees since 1988. He has served as project leader and author of several ISO standards.

Prior to founding HN Metrology Consulting in 1998, Dr. Nielsen was the Technical Manager of Corporate Standards at Cummins Engine Co. in Columbus, Indiana. Dr. Nielsen holds a M. Sc. In Mechanical Engineering and a Ph. D. in Metrology, both from the Technical University of Denmark.


Frequently Asked Questions


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The course starts now and never ends! It is a completely self-paced online course - you decide when you start and when you finish.
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