by Alex Krulikowski

Understanding GD&T, and using it correctly, is critical to good design. Here are some guidelines to get you started.

Is your company confused about where to start using geometric dimensioning and tolerancing? Do they wonder how much GD&T they should use on their drawings?

Some engineers fear using too much GD&T. In many cases, the GD&T skills outside of engineering are scarce. This raises concerns that adding GD&T to the drawings may cause confusion in the plants and at suppliers. This mind set causes a prolonged use of the antiquated tolerancing method known as coordinate tolerancing.

Some managers feel that they are getting along well using coordinate tolerances. They are afraid to upset the system by introducing the additional complexity of GD&T to drawings. In reality, the organization is often not “getting along” so well. There is often wasted effort trying to meet unrealistic tight tolerances, and the measurement of the parts is left up to assumptions instead of being explicitly defined on the drawing. There are often difficulties with parts from suppliers. In engineering, it is difficult — if not impossible — to debug field problems, because no one really knows what the tolerance is on the parts in production.

Although some may feel that coordinate tolerancing is simpler, it is not. Coordinate tolerancing looks easier to interpret, but it does not address three areas:

  1. Coordinate tolerancing does not define the part with a repeatable definition for inspection.
  2. The part tolerances are overly restrictive.
  3. The dimensioning does not represent how the part functions.

Using GD&T does add some complexity to the drawings, because it is more complete. GD&T fully defines the part requirements and, therefore, eliminates the shortcomings of coordinate tolerancing. If the same level of part requirement definition was attempted with coordinate tolerancing, many notes would have to be added, making the drawings much more complex and confusing. GD&T is the most efficient method for defining parts to the level needed for part production.

Using GD&T adds three important benefits to your designs:

  1. The manufacturing tolerances will be significantly larger.
  2. The sequence for addressing the part to gauge measurement is defined
  3. The drawings will clearly define parts that fit and function as intended by engineering.

The fear that the suppliers won’t understand the GD&T is not realistic. I have heard of several overseas companies that have requested their customers add GD&T to the drawings so they can clearly understand the part requirements and know how the part should be measured.

When you understand the language of GD&T, you want to realize its benefits as much as possible. Still, some managers are concerned that they don’t want to use too much GD&T on their designs. Is there such a thing as too much clarity, too much tolerance for manufacturing, or too much measurement repeatability on drawings? The question becomes — how much GD&T needs to be specified?

Here are some guidelines that can help an organization use GD&T appropriately:

  1. All parts (except very simple parts, like a pin or a sphere) must have assembly (or mounting) datum features identified.
  2. The interrelationship between the datum features (orientation and location) must be specified with GD&T.
  3. All mating surfaces must be oriented and located with GD&T.
  4. All critical functional relationships must be oriented and located with GD&T.
  5. Form tolerances for datum features, mounting features, and mating features must be specified with GD&T.
  6. Non-critical part features may be defined with coordinate tolerances or general tolerances.

Each GD&T specification on a drawing will be a direct result of one or more of the above guidelines. Here are the guidelines with rationale for their use.

Guideline Reason for using GD&T
1. All parts (except a very simple part, like a pin or a sphere) must have assembly (or mounting) datum features identified. Datums are the origin for measurement. Every part (or at least 99% of them) needs at least one set of datums. The surfaces that orient and locate the part in the assembly are used as datum features.
2. The interrelationship between,the datum features (orientation and location) must be specified with,GD&T. Since datums are the origins for,measurement, the datum feature boundaries need to be clearly defined for,datum simulation. Using GD&T to define the interrelation between datum,features allows the datum simulator boundaries to be easily calculated.
3. All mating surfaces must be,oriented and located with GD&T. Part mating surfaces are important,features for tolerancing functional relationships on a part. Mating surfaces,are often related to the part assembly datum(s). Therefore, they need to be,dimensioned in a repeatable, measurable method.
4. All critical functional,relationships must be oriented and located with GD&T. These are the tolerance-sensitive,relationships that make your product a success. To avoid any confusion in,these sensitive areas, GD&T should be used to establish one setup for,inspection and to ensure clear definition of the tolerance zones.
5. Form tolerances for datum,features, mounting features, and mating features must be specified with,GD&T. The form of a surface dictates how,much contact it will have with its mating surfaces. Using form controls on,datum features establishes the part-to-gage contact. Using form tolerances on,mating surfaces establishes the part-to-part contact. Using GD&T is the,clearest method for specifying form requirements on drawings.
6. Non-critical part features may,be defined with coordinate tolerances or general tolerances. Non-critical features are those with,no effect on part-to-gage setup, mating relationships, or critical functional,relationships. A few examples are wall thickness, fillets, tangent radii,,hole depths, and non-mating surfaces. These may be defined with coordinate,tolerances or general tolerances.

Using these GD&T guidelines will ensure that all the important part features are fully defined for measurement, allow maximum tolerance for manufacturing, and protect the part function. The drawing will clearly communicate the engineering requirements for the part.

Some plants and suppliers will embrace the use of GD&T. Some suppliers may resist using GD&T. The question you need to ask is, should we make our drawings with less tolerance, less clarity, and less ability to analyze problems in order to accommodate an unskilled supplier, or should we find a better, more skilled supplier? If your suppliers decide against using email or CAD technology, will you stop using these tools to accommodate them? You expect your suppliers to keep abreast of technology, so using GD&T should be a requirement.

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