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Frequently Asked Questions

Here are some answers to questions related to GD&T that may be on your mind.
What is GD&T?

Geometric dimensioning and tolerancing (GD&T) is a widely used blueprint nomenclature. The GD&T language consists of a set of 14 geometric symbols, rules, definitions and principles that are used to precisely describe the size, form, orientation and location tolerances of part features.

It is a design tool that allows designers to communicate the functional requirements in a concise and unambiguous manner so that the manufactured product matches design intent.

The geometric controls applied to the print also helps to properly setup and inspect product conformance.

What is ASME Y14.5?

“The ASME Y14.5 is an international standard that established uniform practices for stating and interpreting dimensioning, tolerancing, and related requirements for use on engineering drawings and in related documents.” – American Society of Mechanical Engineers.

What geometric characteristic symbols are used to control the form of individual features?

There are four geometric symbols that can be applied to individual features to control form tolerances, and they are straightness, flatness, circularity and cylindricity.

Straightness tolerance zone consists of two parallel straight lines separated by the straightness tolerance value.  Each line element in the plane of view is verified separately.

Flatness tolerance zone consists of two parallel planes separated by the flatness tolerance value. All elements of the surface must line within the two parallel planes.

Circularity tolerance zone consists of two concentric circles separated by the circularity tolerance. All points on individual circular element must lie within the two concentric circles.

Cylindricity tolerance zone consists of two concentric cylinders separated by the cylindricity tolerance. All elements of the cylinder’s surface must lie within this tolerance zone.

 

 

What geometric characteristic symbols are used to control profile of individual or related features?

There are two geometric symbols that can be applied to individual or related features to control profile tolerances, and they are profile of a line and profile of a surface.

For profile of a surface, all elements of of the controlled surface must lie within the tolerance zone. Profile of a surface always controls 3D form and can also be used to control size, angles and location.

What geometric characteristic symbols are used to control orientation of related features?

There are three geometric symbols that can be used to control orientation of a features relative to datum(s), and they are angularity, perpendicularity and parallelism.

Orientation tolerance zone when applied to planar features consists of two parallel planes separated by the tolerance value, and controls flatness of the surface as well.

What geometric characteristic symbols are used to control location of related features?

There are three geometric symbols that can be applied to features that are related to datum(s) to control their location tolerances, and they are position, concentricity and symmetry. Position tolerance may be applied to features that are unrelated to any datum(s).

What geometric characteristic symbols used to control runout of related features?

There are two geometric symbols that can be applied to related features to control runout tolerances, and they are circular runout and total runout.

Runout tolerances are applied to features that are related to a datum axis.

If applied to a cylindrical surface, the tolerance zone consists of two concentric cylinders separated by the runout tolerance, and all elements of the surface must line within the tolerance zone.

When is GD&T used?

It is considered best practice to use geometric controls on all parts to precisely state the functional requirements by assigning dimensions and tolerances for form, orientation, location and profile of part features.

Who is this GD&T training for?
Knowledge of GD&T and the ASME Y14.5 standard is applicable (but not limited) to: 
Design engineerEngineering consultant
Manufacturing engineerInspector
Process engineerContract engineer
Quality engineerProject engineer
Tool or gage engineerTechnical specialist
Engineering managerProcurement engineer
User or programmer of CAD, CAM, CAE or other softwareEngineering Students
Drafter CheckerEducators
How does GD&T help save cost?

GD&T saves time and money by:

  • Detecting and correcting design errors early in the development process
  • Providing consistency in inspection verification
  • Reducing scrap and rework
  • Reducing drawing changes
  • Increasing production rates and reducing down-time
What are the benefits of an on-site training?

On-site trainings provide a curriculum tailored at your facility and offer the following advantages:

  • High degree of relevance
  • Customized using your products
  • Cost effective, elimination of employee travel, meals and lodging expenses
  • Less production disruption, training takes place on your schedule
  • Very targeted GD&T programs designed to the jobs and skill levels of your employees
  • Continued support via reduced rate refresher courses and instructor availability for follow up GD&T questions.
How much does it cost to implement GD&T?

This question maybe better addressed with another question – how much does it cost not to implement GD&T? Consider quantifying the time and money spent on rectifying:

  • Design errors produced by designers
  • Interpretation errors by manufacturing and inspection personnel
  • Drawing mistakes uncovered during the downstream process.
  • Miscommunication of design requirements between suppliers and manufacturers
  • Cost of poor quality – both internal and external failures.
How long does it take to learn and apply GD&T?

Much like learning any new language, it takes effort and time to thoroughly understand, select and apply geometric controls to drawings. Don’t expect to become an expert by attending the fundamentals and other advanced GD&T classes back to back.

The process of learning can be quicker when learning from knowledgeable instructors with real world experience using a curriculum that has been honed to be very practical.

Why use GD&T when many suppliers are not used to GD&T nomenclature?

Evolution is a continuous process and failure to adopt standardised engineering product definition practices can be detrimental in today’s advanced technology-based environment.

Moreover, it’s imperative to have clear communication between designers, manufacturing and quality when design and manufacturing are not co-located.

Suppliers from any location can quite easily learn to read and interpret geometric controls in a relatively short period as GD&T classes are offered worldwide.

Why do I need GD&T when I can use 3D CAD models for manufacturing and inspection?

Despite technological advancements, variations are inevitable in manufacturing and inspection equipment. GD&T is a technology that allow variations to be defined, measured and controlled to within acceptable limits for proper functioning of parts and products per the design intent.

Why use GD&T when co-ordinate dimensioning has been around for over 150 years?

GD&T offers the following advantages over the traditional dimensioning method:

  • Depicts functional design relationships accurately
  • New designs can be analysed correctly using tolerance stacks
  • Provides a common language for the production team
  • Makes maximum use of available tolerances
  • Insures parts interchangeability
  • Facilitates functional gaging
  • Offers clear rules and a quality-oriented design philosophy
  • Allows multiple source procurement
What is Geometric Dimensioning and Tolerancing Professional (GDTP) Certification Program?

This program provides the means to recognize proficiency in the understanding and application of the geometric dimensioning and tolerancing principles expressed in ASME Y14.5M-1994 and ASME Y14.5-2009. Those principles form an essential element of the language of engineering.

There are two levels of certification. The first level, Technologist GDTP, provides a measure of an individual’s ability to understand drawings that have been prepared using the language of Geometric Dimensioning and Tolerancing, as defined in the Y14.5 Standard. The second level, Senior GDTP, provides the additional objective measure of an individual’s ability to select and apply geometric controls to drawings. (Excerpted from ASME GDTP Application Information Handbook.)

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