Have you ever wondered what the difference is between the terms perpendicular and perpendicularity?
“Perpendicular” is an elementary geometry term that conveys two lines, objects, or planes intersecting at a 90-degree angle. “Perpendicularity” is a geometric dimensioning and tolerancing (GD&T) term that has a very specific, and slightly different, meaning. GD&T terms are frequently used in contract manufacturing to define and communicate design intent and part function.
In this latest installment of our Design for Manufacturing (DFM) series, we dig into GD&T perpendicularity, what it is, and why it matters when designing your precision machined parts.
What Is Perpendicularity?
Perpendicularity establishes the tolerance zone in which a feature must lie to be accepted as perpendicular. It’s a type of orientation control that is always relative to a datum (i.e.,the main or primary feature on a part).
In part drawings, engineers note perpendicularity to specify the acceptable tolerance to which the orientation of a right-angled part feature may vary. But here’s what often trips up engineers: perpendicularity, like parallelism, is measured in length, not degrees.
For example, suppose you hire a crew to install a flagpole in your front yard. You designate a specific circular patch (the tolerance zone) where they could install the flagpole. If the flagpole is installed anywhere within that patch, you’ll be satisfied—or so you think.
The crew installs the flagpole inside your tolerance zone. The bottom of the flagpole (which connects to the ground, or the “datum” in this case) is within the tolerance zone. The top of the flagpole is also within the tolerance zone. But the flagpole itself looks like the Leaning Tower of Pisa!
You gave the crew a specific tolerance, which they followed. But if you wanted to guarantee that flagpole was roughly straight, you also needed to provide perpendicularity.
If a straight edge was placed against the bottom of the flagpole, your perpendicularity measurement would be the acceptable distance that the top of the flagpole could deviate from that straight edge.
Perpendicularity Makes Parts Manufacturable
Why not just tell your flagpole crew that you want the flagpole installed at a perfect 90 degree angle? For a flagpole, that instruction might be good enough. But when you’re dealing with precision parts holding extremely tight tolerances, you need to be more specific.
Holding an exact 90 degree angle (90.000) without any variation is nearly impossible. Perfect right angles very rarely appear in nature, and although they’re very easy to design, they’re extremely difficult to achieve with even the most advanced machining equipment. Tool speed, tool pressure, tool wear, and even the age of the oil in the machine can all create miniscule variations in a part’s exact angle.
Perpendicularity accounts for this reality and allows you to identify an acceptable range of variation for manufacturing that still allows your part to function as intended.
Perpendicularity vs Angle in Constraint
We often see designs requesting “a perpendicularity of .5 degrees,” but that reference is like mixing apples and oranges. What a customer usually means is that they want their contract manufacturer to hold a 90 degree angle, ± 0.5 degrees. That callout is actually holding a specific angle in constraint. If you want to hold an angle in constraint, note the required angle with an ideal ± tolerance.
Although you can call out an angle in constraint, keep in mind that perpendicularity, when used correctly, will constrain the angle and the variation of form of the specified feature. Perpendicularity is also a three-dimensional constraint and can address the feature from multiple views, whereas an angle in constraint is two-dimensional and has to be called out one angle at a time.
Specificity = Success
One of the reasons we love GD&T is that it allows us to talk to other engineers using a shared vocabulary. When used wisely, GD&T can express the details of design intent and help your contract manufacturing partner understand your requirements.
Perpendicularity is one of those callouts that can convey a lot of information up front—or cause back and forth communication that may delay getting your quote back to you. If you’d like to learn more about GD&T, check out the resources we’ve pulled together on this topic.
And please reach out if you ever have questions or need clarification on a GD&T callout! Our team is committed to being your precision machining partner and making the parts you need. See what we can do for you—request a quote for your next project today.