Orientation controls include angularity, perpendicularity, and parallelism.
The orientation of features must be given in relation to datums. A table that is flat isn’t useful if you failed to control the angle of the tabletop in relation to the ground. When we ask if a table is level, we are asking if it is level in relationship to the floor, which we assume is level. The floor is acting as a datum feature simulator in this instance, allowing us to tell if the table is level, or parallel, to the floor. Without the datum, a table floating weightless in directionless space can’t be defined as level.
Orientation controls refer to either a surface, median plane, or axis called out with a basic angle in relation to a datum. The three controls that do this are angularity, perpendicularity, and parallelism.
Angularity is used to callout every angle except 90 and 180/0. When a surface or median plane is controlled with angularity, the tolerance zone is between two parallel planes that are nominally at the stated basic angle.
If this overhanging surface is called out at 45deg. from datum A with a .5mm tolerance, you can imagine a plane exactly at 45deg., and then and tolerance zone of .5mm centered on that plane. Every part of the angularity-controlled surface must fit within the .5MM TZ to pass inspection.
This is not the same as a +/- tolerance of an angle such as 45deg. +/- .5deg. As that would create a wedge shape of tolerance, instead of two parallel planes surrounding a nominal orientation. A +/- degree is not a GD&T method of tolerancing.
The same idea holds true for an angularity control for an axis, though the TZ for axis angularity can be slightly different than for other axial callouts. While most axis controls create a three-dimensional cylindrical tolerance zone in which the axis must fit, angularity can be used to create either a cylindrical or planar tolerance zone, depending on the drawing.
On this part the overhang is now a cylinder. If the cylinder is controlled with a normal angularity callout, the tolerance zone only applies in the orientation given in the drawing. Just like for the flat surface, the tolerance zone is defined by two parallel planes that are perpendicular to the drawing. Our 3D rendering of the part with the tolerance zone would look like this.
However, if the callout is changed to include the diameter symbol the callout now creates a cylindrical TZ at the basic angle, in which the entire axis of the pin or hole must fit. Our 3D rendering of the TZ now looks more like this.
Perpendicularity actually works the same way as angularity but at 90deg. angles, and Parallelism is the same, but at 0 or 180deg. angles. All of the TZs and principles stay the same and create TZs between two parallel lines or planes that are oriented in reference to a datum.
Keep in mind that orientation controls must reference a datum, and the drawing must give a basic angle of angularity callouts.
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1 авг 2024
