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This document was created by Reata, Engineering and Machine Works, and repurposed with permission. 

When you request prototype or production machining services, it’s essential to call out the part specifications accurately in your print. Inaccurate information could result in a part that doesn’t function as intended or extends your lead time as you and your contract manufacturer go back and forth to get the details right. 

Parallelism and flatness are two of the most common areas of confusion customers run into on their prints, often mistaking one for the other. In this latest installment of our blog series spotlighting Design for Manufacturing (DFM) best practices, we’ll help to clear up this confusion. 

Understanding Parallelism and Flatness

First, let’s look at the basic definitions of parallelism and flatness so that we’re on the same page: 

Flatness Callout

• Parallelism. Parallelism describes a parallel orientation of one referenced feature of a part to a datum surface or line.
• Flatness. Flatness references how flat a surface is regardless of any other datums or part features.

When considering parallelism vs. flatness, it’s helpful to imagine a table standing on a floor. If you carefully place a ball on the table, you may notice that the ball rolls across the table without you pushing it. 

Does this action indicate that the table surface isn’t flat? Not necessarily. 

The table surface could be entirely flat, but perhaps it was mounted to the table’s legs at a slight angle. In this case, the table and floor surfaces aren’t parallel to one another, causing the ball to roll.

Clearing Up Points of Confusion Regarding Parallelism and Flatness

Parallelism Callout

Incorrectly calling out parallelism or flatness can compromise the cost and quality of your part, so it’s essential to clarify what you’re asking for. Generally speaking, parallelism is the correct callout when you’re talking about how two different planes should relate to one another. Flatness is the proper callout when you are only referencing one surface, and how flat that surface is relative to itself.

One trick is to consider the number of surfaces your part will mount to. If multiple mounting surfaces are involved, you can view them as datums. Parts that mount to only one surface can have flatness callouts and parallelism callouts, while parts that mount to more than one surface should have parallelism callouts.  Many people use these callouts interchangeably, they are not.  In most cases, Flatness is more expensive than Parallel.

If you don’t call out flatness and parallelism in your print, we’ll hold these dimensions within the standard tolerances implied by the thickness of your material and general tolerances of your part.

An Alternative to Calling Out Parallelism and Flatness

A simple alternative to calling out parallelism and flatness in your print is to call out the profile of the surface instead. 

This specification describes the 3D tolerance zone around the surface of the part. Take the curved surface of a football, for example. A profile callout would specify that the entire surface where the radius is must fall within the tolerance zone. In many cases, profile callouts simplify measuring a part and still ensure your parts meet the specs needed for proper functionality.