How can you ensure a part is perpendicular to a reference surface using GD&T?

Let’s start by using the correct terms. 1. We do not evaluate perpendicularity relative to a reference surface. We can evaluate perpendicularity relative to a datum plane or we can evaluate relative to a datum axis. The words “reference surface” are confusing…and incomplete. 2. Datums can be points, but the referenced datum for perpendicularity cannot be a point. The referenced datum for perpendicularity must be an axis or a plane.

To add more terminology refinement discussion, I believe we should, & Y14.5 eventually must, say that perpendicularity can be evaluated relative to any datum reference frame (DRF) that has the necessary rotational degrees of freedom constrained, rather than just to a datum plane or datum axis. For a part with three spherical datum features as primary, secondary, & tertiary, there will be no datum planes or axes, only the 3 datum points that will be used in sequence to establish the DRF. The perpendicularity tolerance zone is then constrained to a plane or axis of that DRF, even though this application has no datum planes or datum axes. Defining a DRF as being a coordinate system constrained by one or more datums is also an eventual must.

To define perpendicularity, it is essential to initially identify the type, distinguishing between surface perpendicularity and axis perpendicularity. Secondly, the reference for this perpendicularity should be specified. Surface perpendicularity is regulated by two parallel planes, while axis perpendicularity serves as a tolerance to control the alignment of a particular axis in relation to the reference plane.

The first step in ensuring a part is perpendicular using GD&T is identifying the feature and the datum. The feature could be a surface, hole, pin, or any element with a specific orientation. The datum, serving as the reference, could be a plane, line, or point. For instance, if a hole needs to be perpendicular to the bottom surface of a part, the hole is the feature and the bottom surface is the datum.

The next step in ensuring perpendicularity using GD&T is applying the perpendicularity symbol and tolerance to the feature on the drawing. The symbol is a box with a right angle. The tolerance, which can be expressed as a distance or an angle, is the allowable deviation from perfect perpendicularity. For example, to ensure a hole is perpendicular to the bottom surface within 0.1 mm, write 0.1 inside the box and attach it to the hole with a leader line.

Specifying the Datum Reference Frame (DRF) is the third step in ensuring perpendicularity using GD&T. The DRF, a set of three mutually perpendicular planes or axes, establishes the part's orientation and location. Indicated by letters inside square brackets after the tolerance value, each letter corresponds to the datum defining each plane or axis. For instance, if the bottom surface is the datum for a hole, write [A] after the 0.1 in the box.

The final step in ensuring perpendicularity using GD&T is checking the feature's perpendicularity with a gauge or instrument. This instrument, aligned with the DRF, measures the deviation from the ideal right angle. The deviation is the difference between the actual position of the contact point or surface and the ideal position defined by the tolerance. For instance, to check a hole's perpendicularity, insert a dial indicator into the hole and rotate it around the axis. The deviation is the maximum reading on the dial minus the minimum reading.

An example would be to utilise a measurement system for example a co-ordinate measurement system (CMM) or a form measurement system. These would be ideal dependant of the level of accuracy required for the parts.

If the feature to be controlled is a diameter or series of diameters, you can get a combined check of Perpendicularity and True Position in a single Control Frame. Example of this would be a wheel hub. Doing this combines Fit and Function in 1 Virtual Condition Gage Check. As the Perpendicularity error increases, the Position error would have to decrease, but Fit and Function would be assured. In an application like this, a simple attribute gage would be all that is required.

Regarding how to measure and evaluate the perpendicularity of a feature, an alternative method can be carried out by obtaining the angular deviation to calculate its tangent value and multiply it by the height or coordinate of the functional point of the feature (this must be specified in the dimensional drawing or if this data does not exist, carry out a functional design evaluation to establish the interaction point). For an axle with a height of 100 mm. Formula example: (Tan (90° - 89.5°)) * 100 = 0.8726... mm. Finally the result must be compared with the tolerance defined on the perpendicularity feature control frame.