# Easy Guide to GD&T: Parallelism [ Symbol, Tolerance, Measurement ]

GD&T Parallelism Symbol

## Definition of Parallelism

In GD&T, Parallelism can refer to either Surface Parallelism or Axis Parallelism.

Surface Parallelism is a tolerance that controls Parallelism between two surfaces or other features.  It is an orientation control , just like perpendicularity and angularity tolerances.

The feature is controlled by two parallel planes acting as the tolerance zone, like Flatness.

That said, it is important not to confuse the function of Parallelism and Flatness. For starters, Flatness does not reference a datum while Parallelism must.

Put another way, Flatness is a feature compared to itself, while parallelism is a feature compared to a datum. This should make the difference quite clear.

Axis Parallelism is a tolerance that controls how Parallel a specific central axis needs to be to a datum plane or axis. It is controlled by a cylindrical tolerance zone around a theoretically perfectly parallel axis.

Surface Parallelism is much more commonly used than Axis Parallelism, but be sure to be on the lookout for cases when you have axis parallelism because it’s different.

See our page on Perpendicularity to understand better how an axis may be controlled by a datum.

## Parallelism vs Other Callouts

#### Parallelism and Flatness

Parallelism and Flatness have similar tolerance zones.  Their measuring methods are identical.

However, there are some significant differences.

Parallelism must have a datum and flatness does not.  Also, flatness tolerance measures the surface against itself.  Therefore, it cannot measure tilt relative to another feature.  Parallelism can measure and limit tilt.

#### Parallelism and Angularity

Parallelism and Angularity are both orientation controls.  They are similar in every way except that angularity can maintain part features at any angle between 0 and 90 degrees.  Parallelism only maintains a 0 degree angle.  As such Parallelism may be regarded as a special case of angularity.

#### Parallelism and Perpendicularity

Perpendicularity is another special case of angularity, except for an angularity of 90 degrees.

## Parallelism Callout on Drawings

Surface Parallelism Callout…

The Surface Parallelism Callout above shows how Surface Parallelism can be called out on a drawing. Parallelism is relative to the “A” datum in the callout.  The feature control frame specifies the tolerance and which datum is the parallelism control.

## Parallelism Tolerance Zone

The Tolerance Zone for Surface Parallelism looks like two planes that are parallel to the datum and separated by the tolerance. Every point on the controlled surface must fall between the planes.

As you can see, the parallelism tolerance doesn’t create an angular tolerance zone to control the 0-degree alignment between the controlled surface and the datum plane.

Instead, it assumes a basic 0-degree angle for the tolerance zone, and the permissible variation is controlled by widening or tightening the two surfaces of the tolerance zone.  The greater the distance between the zone’s two planes, the more error in parallelism it can accommodate.

Axis parallelism, by contrast, creates a cylindrical tolerance zone.  It may be used to maintain the axis of a feature such as a cylindrical pin parallel to a datum (such as a hole).

Note that parallelism cannot control the location of the tolerance zone.  It is only concerned with the orientation of the zone.  That tolerance zone exists at the location of the surface or axis.

## Gaging and Measurement of Parallelism

Refer to the Tolerance Zone diagrams above and you can tell pretty quickly how to sweep to measure Parallelism tolerance. Most of the time it’s easy just to place the datum on a surface plate and sweep the top with an indicator at constant height to check Parallelism.

## Important to Remember

• Parallelism must always have an associated datum feature.
• The parallelism tolerance cannot be greater than the size tolerance.  This is due to the envelope principle (GD&T rule #1).
• Temperature becomes an important factor when measuring parallelism with extremely small tolerances.

Recently updated on February 19th, 2024 at 10:52 am