This information provided by Bryan R. Fischer, of TDP360 LLC.
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Orientation tolerances
There are three orientation tolerances in GD&T: angularity, parallelism, and perpendicularity. These tolerances are equivalent to one another, as they provide the same control over geometry. The only difference between these tolerances is the angle between the datum reference frame and the toleranced feature.
Angularity | Parallelism | Perpendicularity |
General explanation of orientation tolerances
Perpendicularity is used to control geometry that is nominally perpendicular to a primary datum plane or axis.
Parallelism is used to control geometry that is nominally parallel to a primary datum plane or axis.
Angularity is used to control geometry that is nominally at some other angle to a primary datum plane or axis.
ASME Y14.5-2009 and ASME Y14.5-2018 allow angularity to be applied to geometry that is at any angle to a datum reference frame. Thus, angularity can also be used to control parallel and perpendicular geometry. The designer is free to choose whether to use angularity or parallelism and perpendicularity, as applicable.
Relating orientation tolerances to a datum reference frame
Orientation tolerances must be related to a datum reference frame, such as A, A|B, A|B|C, D, D|E... Usually, we use the orientation tolerance that matches the relationship of the toleranced feature to the primary datum (e.g. if a feature is nominally parallel to the primary datum, we control its orientation with parallelism).
Orientation tolerance zones are only oriented to a datum reference frame – they are not located to a datum reference frame. Only rotational degrees of freedom are constrained between the tolerance zone and a datum reference frame – translational degrees of freedom are not constrained between the tolerance zone and a datum reference frame. Thus, orientation tolerances do not control location. If you also want to control location, use profile of a surface or position.
Type of control provided by orientation tolerances
Orientation tolerances control the orientation of features or center geometry (e.g. axis, center plane). In some cases, orientation tolerances also control the form of features. If an orientation tolerance is applied to a:
Flat surface | The tolerance controls the form and the orientation of the feature. The entire feature must be on or within the tolerance zone, thus the tolerance also controls flatness. |
Cylindrical surface | The tolerance controls the orientation of the axis (RFS default) or it controls the orientation of the surface (if an MMC or LMC modifier is specified). Note that the form of a cylindrical surface is usually controlled by a size tolerance and Rule #1. |
Orientation Tolerances Applied to Flat Surface | Orientation Tolerances Applied to Cylindrical Feature of Size |
MMC Virtual Condition
When an orientation or positional tolerance with an MMC or LMC modifier is specified for a feature of size, a virtual condition boundary is defined. Virtual condition is a calculated value. MMC virtual condition (MMC VC) is more common and thus more important, so we will focus on MMC VC here. The MMC VC is a theoretical boundary that represents the space available within an internal feature or occupied by an external feature in all its possible orientations or locations.
For an internal feature of size (e.g. hole), MMC is the smallest size. MMC VC represents the useful size of an MMC hole that may tilt or shift relative to mating surfaces. It is the smallest size of the hole minus its orientation or positional tolerance. To calculate the MMC VC for an internal feature, we subtract the orientation or positional tolerance applied at MMC from the MMC size of the feature.
MMC VC formula for internal feature
Orientation tolerance: | MMC VC = MMC size - orientation tolerance @ MMC |
Position tolerance: | MMC VC = MMC size - position tolerance @ MMC |
MMC VC calculation MMC Size = Ø.600 - .005 = Ø.595 Orientation @ MMC = Ø.010 MMC VC = Ø.595 - .010 = Ø.585 |
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For an external feature of size (e.g. pin), MMC is the largest size. MMC VC represents the useful size of an MMC pin that may tilt or shift relative to mating surfaces. It is the largest size of the pin plus its orientation or positional tolerance. To calculate the MMC VC for an external feature, we add the orientation or positional tolerance applied at MMC to the MMC size of the feature.
MMC VC formula for external feature
Orientation tolerance: | MMC VC = MMC size + orientation tolerance @ MMC |
Position tolerance: | MMC VC = MMC size + position tolerance @ MMC |
MMC VC calculation MMC Size = Ø.570 + .005 = Ø.575 Orientation @ MMC = Ø.010 MMC VC = Ø.575 + .010 = Ø.585 |
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Note that in the examples above, even if datum feature A on both parts makes flush contact, the MMC VC pin will fit into the MMC VC hole. MMC VC fit is functionally very important. This is a very common example… MMC VC calculations helps ensure that parts with mating features of size will assemble.