We have started discussion on GD&T symbols in our previous blog Know GD&T symbols start your design engineering career with a bang! Part-1. Here we will continue exploring the next set of GD&T symbols for orientation and location.
Parallelism
It is desired characteristics between two surfaces, one surface and one axis or two axes. It is important due to the level of accuracy expected during the motion of the concerned parts or fitment of the concerned parts
- Opposite faces of a fixture plate or guide way surfaces of a bench or vice
- Lead screw axis and bed of a lathe
- Axis of spindle and table surface of a milling machine
- T slots of milling machine table or two guide pillars of a draw die
Application:
Parallelism is indicated by means of a symbol consisting of two parallel lines with 60 degree slant. To indicate permissible deviation at parallelism for any feature; some opposite datum feature is essential. The permissible deviation for the parallelism is always smaller than permissible size
In the diagram, upper surface B is expected to be parallel to the datum feature (surface) A within 0.2mm
Perpendicularity
Many a times, components call for perpendicularity or squareness between two surfaces, one axis and one surface or two axes to be within some reasonable limit.
- Upper and frontal surfaces of a cylinder block
- Cylinder bore axis with respect to the upper face of cylinder block
- Axis of the post locator with respect to its resting face
- Loss at perpendicularity can lead to improper alignments. Loss at the utility, poor location and resting of the components during the processing and use.
Application:
Perpendicularity is indicated by a symbol with two perpendicular lines as shown in the feature control frame. To indicate perpendicularity a datum is very essential. Depending upon the requirement functional surface, some axis or some center plane can be a datum.
Angularity
Some features of component are required to have some angular orientation like 30, 50, 60 degrees with respect to some datum within the component. It can be a case of some surface at some angle with some datum surface or some axis at some angle with some datum surface.
- Dovetail slot or dovetail guide
- Dipstick hole of a cylinder block with respect the bottom face
- Such angular orientations are bound to have some deviations and the feature control frame for angularity is expected to specify the limit.
Application:
The symbol for angularity is an angle of 60 degrees between intersecting lines as shown in the figure. The permissible deviations are never mentioned in terms of angular units. It is mentioned in terms of its linear equivalence.
Concentricity
Two or more outer diameters on the same axis or two or more bored holes on the same axis required to be reasonably concentric.
- Bore on opposite walls of the gear box
- Two bearing diameters on the axis of a crank shaft
Application:
The symbol for concentricity appears as two concentric circles. There is no dot in the center. A zone descriptor symbol “Ø” is used along with the tolerance magnitude indicating a circular zone for the deviation.
Symmetry
Symmetry is a characteristic which asks some feature on one side of a center line to have mirror image counterpart on the other side of the center line.
- Milling fixture to be kept on the table of horizontal milling machine
- Dovetail guide fitment over a dovetail slot
Application:
Symmetry is shown as three parallel lines. The deviation of symmetry shall be always along the concerned dimensions. Hence no zone descriptor like “Ø” is admissible here.
Position
The GD&T symbol for Position (sometimes referred as true position) with respect to an axis, point or plane defines the permissible variation from a specified exact true location. True position or the location is most often specified with two or three datums or references to help exactly locate the reference position.
- Milling fixture to be kept on the table of horizontal milling machine
- Dovetail guide fitment over a dovetail slot
Application:
It is specified using symbol of bull’s eye. Many times, LMC or MMC is also specified.
As designer you must be good in understanding of all these 14 symbols of GD&T. You must learn them in complete details with their applications. Learning significance of these symbols from a veteran like CADCAMGURU can make your career. Refer to the next blog on remaining 4 symbols to master all of them and come out with flying colors in the interview for design engineer position.
