Complete Guide to Machining Blueprint Symbols

nominal size dimension with a tolerance

Nominal Size

In the example shown, 24 is the nominal size. It is the size that the tolerance envelope is based on. A reference/starting point.

plus or minus symbol

Plus or Minus

Plus or minus is the most common tolerance type. They are less common, but tolerances can be all negative, all positive or +/- with enequal sizes such as +3/-2.

45 degrees

Degree Symbol

The symbol used to callout angle requirements.

Reference dimensions are shown in parentheses. They are provided for informational use only. They are not size requirements but often help make the print easier to understand.

Symbols For Features

radius callout

Radii

Half of a circle. The distance from the center of a circle to the edge.

diameter GD&T symbol

Diameters

The distance all the way across a circle. Two times the radius. This symbol gets used with other symbols as well such as counterbores, countersinks, and true position callouts.

depth of GD&T symbol

Depth of

This symbol specifies the depth of a feature.

A counterbore is a flat bottomed, recessed hole.

Countersinks are a round, angled feature added to a hole. They are often used to allow a screw head or other fastener to sit flush with the surface.

A spotface is  a shallow counterbore that allows a fastener to sit straight. The symbol has a few variations that get used. Often the counterbore symbol portion is left out and the SF is used only.

Surface Texture

A symbol for defining the surface finish of a part. There are many variations of the surface texture symbol but most often it is used with a microinch or micrometer value callout that specifies the roughness of a surface.

Symbols Used In GD&T Callouts

Basic dimensions represent a  theoretically perfect feature or size. They are shown enclosed in a box and are the basis for many GD&T callouts.

Datums are features that are used to locate a part for measurement. 

Feature control frames contain a geometric dimensioning and tolerance callout.

The example shown reads, the feature is perpendicular to within 0.001 to datum A.

GD&T Symbols

Symbols For Roundness

circularity gd&t symbol

Circularity

Otherwise known as roundness. Circularity only applies at one location. You might check the roundness of a hole at a specific depth.

cylindricity gd&t symbol

Cylindricity

Cylindricity is roundness but over all locations of a feature. Cylindricity of a hole would mean the hole must be round at all points.

Symbols Used for Flat Things

straightness gd&t symbol

Straightness

A straightforward requirement. The feature must be a straight line within the specified tolerance.

flatness gd&t symbol

Flatness

Similar to straightness but over a whole surface. The high and low of a surface must be within the specified tolerance of each other.

Symbols Used to Control Angles

perpendicularity gd&t symbol

Perpendicularity

Two features must be 90 degrees to each other.

parallelism gd&t symbol

Parallelism

Two features must run together (180 degrees to each other).

angularity gd&t symbol

Angularity

Angularity is used when two features must be have a specific angle between them and the angle between the features is not 90 or 180 degrees.

Symbols Used for Profile

profile of a line gd&t symbol

Profile of a Line

Profile of a line controls the shape of a feature. Imagine it as controlling the outline of a feature.

profile of a surface gd&t symbol

Profile of a Surface

Profile of a surface controls the shape of a feature but instead of controlling it in one location like profile of a line, it applies to the entire feature surface.

GD&T Symbols That Control Location of a Feature

true position symbol

True Position

Controls location of a feature based on the variation from the basic dimensions.

concentricity gd&t symbol

Concentricity

A requirement that the centerline or axis of two features are located together. 

symmetry gd&t symbol

Symmetry

A requirement that a feature must be evenly located based on another feature. Centered based on a feature or location.

circular runout gd&t symbol

Circular Runout

Controls how even a surface is when spun in a circle. 

total runout gd&t symbol

Total Runout

The same as circular runout but at all locations on the feature. Circular runout and total runout have a relationship similar to circularity and cylindricity.

Symbols That Change Tolerance

m in a circle maximum material condition gd&t symbol

Maximum Material Condition

A modifier that can give bonus tolerance based how close or far the features are from nominal size.

l in a circle least material condition gd&t symbol

Least Material Condition

A modifier that can give bonus tolerance based how close or far the features are from nominal size.

p in a circle projected tolerance zone gd&t symbol

Projected Tolerance Zone

Used when a tolerance zone applies outside the boundaries of the physical part.

s in a circle regardless of feature size gd&t symbol

Regardless of Feature Size

The tolerances do no change based on the size of the features. This is the default spec and any tolerance is assumed to be regardless of feature size even if the symbol is not used.

u in a circle unequally disposed tolerance zone gd&t symbol

Unequally Disposed Tolerance Zone

Profile tolerance zones are normally centered on the nominal dimensions. When this is not the case, the unequally disposed tolerance symbol is used to move the tolerance zone based off the nominal dimensions.

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Check out our Beginner’s Guide to Blueprint Reading for more in depth information related to understanding your blueprints and drawings

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