In GD&T, a feature control frame is required to describe the conditions and tolerances of a geometric control on a part’s feature. The feature control frame consists of four pieces of information:
- GD&T symbol or control symbol
- Tolerance zone type and dimensions
- Tolerance zone modifiers: features of size, projections…
- Datum references (if required by the GD&T symbol)
This information provides everything you need to determine what geometrical tolerance needs to be on the part and how to measure or determine if the part is in specification.
Parts of the Feature Control Frame
- Leader Arrow – This arrow points to the feature that the geometric control is placed on. If the arrow points to a surface than the surface is controlled by the GD&T. If it points to a diametric dimension, then the axis is controlled by GD&T. The arrow is optional but helps clarify the feature being controlled.
- Geometric Symbol – This is where your geometric control is specified. See our page on GD&T symbols or the sidebar for a description of each symbol.
- Diameter Symbol (if required) – If the geometric control is a diametrical tolerance then the diameter symbol (Ø) will be in front of the tolerance value.
- Tolerance Value – If the tolerance is a diameter you will see the Ø symbol next to the dimension signifying a diametric tolerance zone. The tolerance of the GD&T is in whatever unit of measure that the drawing is written in. All of our examples on GD&T basics are metric units.
- Feature of Size or Tolerance Modifiers (if required) – This is where you call out max material condition or a projected tolerance in the feature control frame. See the Modifiers section of the GD&T Symbols page for further clarification on these features.
- Primary Datum (if required) – If a datum is required, this is the main datum used for the GD&T control. The letter corresponds to a feature somewhere on the part which will be marked with the same letter. This is the datum that must be constrained first when measuring the part. Note: The order of the datum is important for measurement of the part. The primary datum is usually held in three places to fix 3 degrees of freedom
- Secondary Datum (if required) – If a secondary datum is required, it will be to the right of the primary datum. This letter corresponds to a feature somewhere on the part which will be marked with the same letter. During measurement, this is the datum is fixated after the primary datum.
- Tertiary Datum (if required) – If a third datum is required, it will be to the right of the secondary datum. This letter corresponds to a feature somewhere on the part which will be marked with the same letter. During measurement, this is the datum is fixated last.
Special Condition: Size Dimension – the diameter or size of the feature that the geometric control is being called on. (See example below). It is optional to place the feature control frame next to the size dimension of the feature called. However, when a feature of size callout like max material condition is used the leader arrow will point to the dimension and the feature control frame must be right underneath.
Special Condition: Datum Control – If the feature being referred to by the feature control frame is also a datum, the datum symbol will show up off of the frame. This is not mandatory as the datum control can also be placed directly on the feature in the drawing as well.
Reading the Feature Control Frame
The feature control frame forms a kind of sentence when you read it. Below is how you would read the frame in order to describe the feature.
This feature control frame would be read in the following way:
Be sure to check out our individual pages for each GD&T symbol for examples and specific uses of the feature control frame.