What is Perpendicularity in GD&T?

Perpendicularity is among the fundamental symbols in Geometric Dimensioning and Tolerancing (GD&T) that enforces an exact 90-degree (perpendicular) orientation of a feature with a reference datum.

Perpendicularity tolerance plays an essential role in components where precise alignment is critical. This includes arrangements such as shaft-hole pairs or load-bearing components where clear and uniform transfer of force is necessary.
A well-defined perpendicularity geometric tolerance helps eliminate problems like misalignment, wobbling, and tilting that affect the performance of the part.

Perpendicularity Symbol

The perpendicularity symbol is a must-know for GD&T professionals. The illustration below shows the perpendicularity symbol as well as a sample feature control frame with the tolerance value and datum definition.

Types of Perpendicularity Tolerances

There are multiple ways to define perpendicularity tolerance on parts since several types of features can be perpendicular to each other. For instance, surfaces and axes.

The following sections dive into the two types of perpendicularity definitions with appropriate explanations and examples.

Surface Perpendicularity

Surface perpendicularity applies to 2D surfaces. It ensures that a flat surface remains perpendicular to a datum plane. In this case, the tolerance zone comprises two planes, which are parallel to each other and perpendicular to the reference datum. The flat surface must lie between these two planes, as shown below.

Surface perpendicularity tolerance is quite important in components like brackets or flanges, where the mating surfaces need to mount flush against each other in order to get a tight assembly. Out-of-tolerance parts can lead to gaps or stress concentrations in the mating components, causing issues like leakage, deformation, and early failures.

Surface Perpendicularity Example

Let us learn further strengthen our understanding of surface perpendicularity by using the example below.

The engineering drawing shows a perpendicularity symbol callout on the vertical face of the component. The feature control frame indicates a tolerance value of 0.1 and the reference datum to be datum A.

The 3D visualization on the right shows how to imagine the tolerance zone. The datum surface is placed on a flat surface. Then, the tolerance zone is defined by two parallel planes, 0.1 units apart, which are perfectly perpendicular to the datum surface A.

All points on the control surface should fall in between these two planes in order for the component to pass the quality checks.

Axis Perpendicularity

Axis perpendicularity controls the central axis of a round feature with respect to a reference surface or datum. This includes cylindrical features like holes and pins that are a part of an engineering fit.

Common applications of axis perpendicularity include fastener holes, bearing housing, or shafts, where non-perpendicular features can negatively impact the fit, mechanical load, or motion.

Axis Perpendicularity Example

This section shares an example to explain how axis perpendicularity works. The image below shows that the perpendicularity symbol controls a cylindrical feature, with the tolerance zone having a diameter of 0.1 and the datum A being the reference surface.

In this case, as can be seen, the tolerance zone is cylindrical in shape, with the axis of this cylinder being perfectly perpendicular to the datum surface. The axis of the part needs to be fully within this cylindrical tolerance zone to avoid rejection.

How to Measure Perpendicularity Tolerances?

The quality control phase is a critical phase of the production lifecycle. Perpendicularity measurement is thus equally important as its definition and fabrication.

The following sections discuss the various methods engineers use to measure the perpendicularity tolerance on mechanical components.

CMM

Coordinate Measurement Machines (CMMs) are the modern, most accurate way for perpendicularity measurement. A CMM machine is precise and programmable, allowing measurements to be taken from complex geometries and hard-to-reach areas via a probe.

The machine collects data from multiple points on the feature and compares its orientation to the datum plane or axis. Afterward, CMM software calculates how much they deviate from a perfect 90° angle, providing reliable results to check perpendicularity tolerances.

Dial Indicators

Dial indicators are a relatively simpler but quite popular method for conducting a perpendicularity measurement. The dial indicator, or dial gauge, mounts on a fixture or a surface plate (serving as the datum), and its needle then traverses the feature on which the perpendicularity symbol is called on.

Engineers observe the variations in the dial indicator readings during this process to see how much the surface deviates from its intended perpendicular form.

Dial indicators are robust and provide repeatable measurements to an acceptable degree of accuracy. This is the reason they are a common sight in machine shops across the globe.

Angle Blocks

Angle blocks, or precision squares, are another method to perform manual perpendicularity measurement. The angle block provides a completely flat surface with a 90 ° angle between two of its faces.

Quality experts place the test part against the block and look for gaps between the control surface and the angle block. Generally, deviations from perfect form are verified visually, by shining a light through the gaps, or passing a thin wire through the gap, indicating both the non-perpendicularity and its extent.

Perpendicularity vs. Other GD&T Symbols

The perpendicularity symbol sits high in the list of the most relevant GD&T symbols, up there with other tolerances like parallelism, straightness, and position.

Let us see how perpendicularity compares with some of these other GD&T symbols:

• Parallelism: Parallelism and perpendicularity are quite opposite to each other, but they work in similar ways. Parallelism enforces two features to be parallel to each other within a defined tolerance zone. For example, the two side faces of bearings often have high parallelism to aid easy assembly and avoid imbalanced loads.
• Straightness: Straightness controls how perfectly straight a feature (like an axis) is. It only affects the feature’s form and does not need any reference, as with perpendicularity.
• Angularity: Angularity controls the angle between two features. It differs from perpendicularity tolerance in the sense that the reference angle is not 90 degrees, but instead a different value.

Perpendicularity in Practical Applications

GD&T perpendicularity is utilized in a wide range of industrial applications. The following list summarizes a few of its key applications:

• Mounting surface in brackets: Brackets structures support high mechanical loads. Perpendicularity is a core quality criterion for them to ensure proper alignment and load distribution.
• Shaft-bearing-housing pairs: Shaft-bearing systems require high precision to avoid bearing damage, imbalance loading, and smooth motion. Keeping the bearing faces perpendicular to the housing fixtures is necessary for this.
• Jigs and fixtures: Fixturing allows machinists to mount workpieces in specific orientations, including perpendicular to reference entities. Therefore, jigs and fixtures require tight perpendicularity tolerance on their locating faces for high machining accuracy.