As discussed in our previous post, form features are a great way in establishing a strong foundation to build our GD&T requirements from however they cannot control the size or the orientation of our components. This brings us on to:
Parallelism
Perpendicularity
Angularity
In all three cases; parallelism, perpendicularity and angularity can be defined in two ways. Most commonly they are applied to control the deviation in a planar surface with respect to a surface datum. In this instance, the tolerance zone is bounded by two parallel planes.
Alternatively, parallelism, perpendicularity and angularity can be applied to a hole or cylindrical axis, where the tolerance zone is defined as a 3D cylindrical tolerance zone defined by a line feature relative to a planar or axial datum.
Orientation features can be controlled using basic or theoretically exact dimensions (TED) which are denoted as a boxed dimension on the drawing. TED’s refer to the theoretically perfect position of the defined feature, therefore, they have no tolerance associated to them. More importantly, they must relate back to the respective datum called out in the control frame.
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“The basic dimension sets the perfect position, while the geometric tolerance defines the size and shape of the tolerance zone”
Parallelism
In this example we have used flatness to control our surface datum 'A'. The upper surface is defined to be parallel to surface datum 'A' as shown in the control frame, to the value specified.Note the use of the diameter symbol in the control frame. This defines the tolerance zone as a cylindrical tolerance zone.
Perpendicularity
It is important to note; the angle range for a specific tolerance zone can be calculated with simple trigonometry. The value '0.4' stated controls the width of the tolerance zone, not the angle of the surfacePerpendicular measurement of an axis to a datum surface can be measured by running a dial indicator along the length of the diameter. An Angle Plate can be used as a supplementary Datum 'A'
Angularity
Tolerance zone defined by two parallel planes offset to datum surfaceBoth datums 'A' and 'B' are required in order to constrain the hole axis in all directionsAll information on this website is provided in good faith, ‘The Engineering Notebook’ nor its parent company ‘RYM Engineering Ltd’ make no representation or warranty of any kind for information held on this website. Under no circumstances shall we have any liability for loss or damage of any kind as a result of the use of information from this website. The use of this website and any information on this website is solely at your own risk.
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