In a drawing, wherever two surfaces are shown to be parallel, the size dimension of the surfaces controls the parallelism.
This method is satisfactory is some cases only but contains two short comings:
- Parallelism is the same value as the size dimension.
- No datum references.
DEFINTION OF A PARALLELISM CONTROL
The condition in which the surface, axis or center plane is exactly parallel to a datum is known as parallelism.
A geometric tolerance which limits the surface, axis or center plane to vary from being parallel is known as parallelism control.
PARALLELISM TOLERANCE ZONES
The two common tolerance zones for parallelism are as follows:
- Two parallel planes.
- A cylinder.
- Application of parallelism to a surface.
- Application of parallelism on a cylindrical FOS.
The most common application of parallelism is when it is defined to a surface.
The following conditions are applicable.
- Two parallel planes are the tolerance zones which are parallel to the datum.
- Tolerance zone is within the size dimensions.
- The distance between the tolerance zones is the tolerance value.
- All the elements must be within the tolerance zone.
- Flatness is limited by the parallelism tolerance.
In the below example, a parallelism control is stated to a cylindrical FOS which contains the MMC modifier.
Parallelism applied to a surface
The function of assembly is ensured by this type of geometric control.
The conditions which are applicable are stated as follows:
- A cylinder is the tolerance value which is parallel to the datum plane.
- The diameter of tolerance zone cylinder is the tolerance value.
- Axis must be within the tolerance zone.
- Bonus tolerance is allowed.
- The worst case boundary is affected.
Parallelism (MMC) applied to a diameter
For verifying the parallelism, a gage is used which is shown in the above figure.
The gage has a surface which acts as a simulated datum feature.
As pin is present in the gage which verifies the parallelism of the hole.
The virtual condition of hole is the gage pin diameter (10.2-0.1=10.1).
The worst case boundary of the toleranced surface is not affected when a parallelism control is applied to a surface.
When it is applied to a FOS, then the worst case boundary is affected.
PARALLELISM WITH THE TANGENT PLANE MODIFIER
Application of parallelism is also with a tangent plane.
- Two parallel planes is the tolerance zone.
- The tangent plane formed by the high points of the surface must be in the limit of 0.1 parallelism tolerance zone.
- Flatness is not controlled by parallelism control.
Parallelism with a tangent plane modifier
INDIRECT PARALLELISM CONTROLS
There are a number of geometric controls which can indirectly affect the parallelism of a part feature.
Tolerance of position, total runout and profile can limit parallelism.
Indirect parallelism controls are not mostly inspected.
If the part feature needs to be inspected, then parallelism control should be specified.
The tolerance value must be less than any indirect parallelism controls that are applicable when a parallelism control is used.
LEGAL SPECIFICATION TEST FOR PARALLELISM CONTROL
Following conditions must be satisfied for the parallelism control to be a legal specification.
- In the feature control frame, one or more datums planes, datum axis or center plane must be referenced.
- The tolerance value of a parallelism control must be lesser than any other geometric tolerance value.
- If the parallelism control is applied to a surface, then the modifiers such as (MMC, LMC or diameter) are not required to be mentioned in the feature control frame.
In the below example, let’s see how to verify parallelism control.
The part is placed on the plate.
A dial indicator is used to verify the surface elements are within the specified limits of parallelism control.
We can use dial indicator to check the maximum or minimum variation.
This variation will help us to identify the parallelism error.