## GD&T (ASTM Y14.5)

Geometric Dimensioning and Tolerances: Symbols (ASTM Y14.5-2009)

 Angularity – The condition of a surface, centerplane or axis being exactly at a specified angle
 Circularity – A condition where all points of a surface of revolution, at any section perpendicular to a common axis, are equidistant from that axis.
 Concentricity – The condition where the median points of all diametrically opposed elements of a cylinder (or a surface of revolution) are congruent with the axis of a datum feature.
 Cylindricity – A condition of a surface of revolution in which all points of the surface are equidistant from a common axis.
 Flatness – The condition of a surface having all of its elements in one plane.
 Parallelism – The condition that results when a surface, axis or centerplane is exactly parallel to a datum.
 Perpendicularity – The condition that results when a surface, axis, or centerplane is exactly 90° to a datum.
 Position – The zone within which the axis or center plane of a feature is permitted to vary from true (theoretically exact) position. Profile of a Line Control – A geometric tolerance that limits the amount of error for line elements relative to their true profile.
 Profile of a Surface Control – A geometric tolerance that limits the amount of error a surface can have relative to its true profile.
 Circular Runout – A composite control that affects the form, orientation, and location of circular elements of a part feature relative to a datum axis.
 Straightness (Axis or Centerplane) – The condition where an axis is a straight line (or, in the case of a centerplane, each line element is a straight line).
 Symmetry – The condition where the median points of all opposed elements of two or more feature surfaces are congruent with the axis or centerplane of a datum feature.
 Total Runout – A composite control affecting the form, orientation, and location of all surface elements of a diameter (or surface) relative to a datum axis.

Geometric Dimensionsing and Tolerances: Modifiers (ASTM Y14.5-2009)

 All-Around Symbol – A circle placed on the bend of the leader line of a profile control.
 Basic Dimension – A numerical value used to describe the theoretically exact size, true profile, orientation, or location of a feature or datum target.
 Between Symbol – A double ended arrow that indicates the tolerance zone extends to include multiple surfaces.
 Controlled Radius – A radius with no flats or reversals allowed. The symbol for a controlled radius is “CR.”
 Datum Feature – A part feature that contacts a datum.
 Datum Target – A symbol that describes the shape, size, and location of gage elements that are used to establish datum planes or axes.
 Feature Control Frame – A rectangular box that is divided into compartments within which the geometric characteristic symbol, tolerance value, modifiers, and datum references are placed.
 Least Material Condition – The condition in which a feature of size contains the least amount of material everywhere within the stated limits of size.
 Maximum Material Condition – The condition in which a feature of size contains the maximum amount of material everywhere within the stated limits of size.
 Projected Tolerance Zone – A tolerance zone that is projected above the part surface.
 Radius – A straight line extending from the center of an arc or circle to its surface.

Reference Specification ASTM Y14.5-2009

## Surface Finish of Machined Parts

When it comes to machined parts, most people are typically concerned only with average roughness of a part, Ra; however, there are many more roughness parameters which can be specified.

Surface Finish usually takes on the lay of its manufacturing process.  For example, if a part is saw-cut, it will have a linear lay following

the travel of the saw teeth.  On the other hand, if a part is rotary-ground it’s finish will have a circular lay.

There are several specification guiding the detailing of surface finish.  The Europeans use BS EN ISO 1302:2002 Geometrical product specifications (GPS).  In the US, ASME Y14.36M is commonly used.  The symbols in each standard are quite similar:

The metric-to-inch conversion is:

 R a-μm – 0.025 0.05 0.1 0.2 0.4 0.8 1.6 3.2 6.3 12.5 25 50 R a– μinch 1 2 4 8 16 32 63 125 250 500 1000 2000

Various machining operations have different typical surface finishes inherent to their respective manufacturing process, as indicated in the table below: