Flatness Tolerance in GD&T

July 1, 2019 Barbara Osborne

flatness tolerance

Does flatness matter in small parts production?

Surface flatness is one of the Geometric Dimensioning and Tolerancing (GD&T) standards used to communicate how a part should be manufactured. It can also be an important consideration when optimizing for manufacturability.

Along with positioning, cylindricity, and other geometric tolerancing characteristics, flatness tolerance may be indicated on an engineering drawing to help ensure the metal parts that are produced will have both the correct form and the desired functionality.

What is surface flatness?

Flatness is a measure of a surface’s form compared against itself, indicating that all the points along the surface lie in the same plane. Symbolized in GD&T by a parallelogram, flatness allows you to limit waviness or variation in a surface without tightening its dimensional tolerance.

The characteristic of flatness is useful in helping to ensure even wear across a surface. It is also useful where two surfaces must meet flush and form a tight seal, as with two surfaces bolted together with a gasket in between.

What is flatness tolerance?

The flatness tolerance zone is the area between two parallel planes, into which the surface in question would need to fit in order to qualify as “flat.” The flatness tolerance would be the allowable distance between those parallel planes.

Looking at it another way, the tolerance zone is the highest and lowest (±) acceptable points across the plane of the surface that is being measured.

Flatness vs. straightness

Flatness, then, is a characteristic of a three-dimensional (3D) object as it exists between the two parallel planes that delineate its tolerance zone.

Surface straightness, on the other hand, is two-dimensional (2D). Its tolerance zone exists between two parallel lines and is an indicator of flatness along one line on a part’s surface.

There is also axis straightness, a characteristic we deal with often here at Metal Cutting Corporation. Axis straightness controls how much bow or curve is allowed in the axis of a part. (Read more about measuring straightness.)

Flatness vs. parallelism

Unlike flatness, which does not consider any other data or features, parallelism compares the orientation of opposite surfaces. While there is also such a thing as axis parallelism, in GD&T it is most often called out for controlling parallelism between two surfaces or features.

Parallelism can be used when two surfaces need to work in sync and at a constant distance from each other. It can also be used when you have a part that needs to fit between two planes that must reference each other.

Flatness vs. perpendicularity

Unlike flatness across the plane of a single surface, perpendicularity compares the orientation of adjoining surfaces. In GD&T, perpendicularity tolerance is usually used to control either two 90° surfaces or an axis in relation to a particular reference point on a cylinder. 

At Metal Cutting, we typically measure perpendicularity in terms of a part in relationship to its end cut, comparing the side of a rod, tube, or wire to the end cut we put on that part. (Read more about measuring perpendicularity.)

How is flatness measured?

Flatness is notoriously hard to measure. First of all, there really is no such thing as “perfectly flat” (or “perfectly” anything!).

Second, measuring the surface flatness of the cut end on very small parts — such as tubes with a diameter of 0.040″ (1.016 mm) and other tiny parts we typically produce here at Metal Cutting — is virtually impossible.

Third, the flatness of a part can’t be measured by placing it on a table and running a height gage over the part. That’s because the table would then be part of the data — and so, you would be measuring parallelism, not flatness.

Instead, you need to make sure the points along the surface of the part are within the flatness tolerance zone, comparing the surface to two imaginary parallel planes.

Coordinate measuring machines (CCMs) allow for a type of 3D measurement, creating virtual planes to which a part’s surface profile can be compared. So, a CCM can be used to measure points across the length and width of a part to check that the entire surface is within the flatness tolerance.

Alternatives to flatness in small metal parts

At Metal Cutting, where we produce thousands of rods, tubes, and wires cut to length every day, we don’t often work with flatness tolerance for the surfaces we cut, due to their very small sizes.

Instead, our customers typically are more concerned with parallelism, angles, and the perpendicularity of the end cut rather than surface flatness. Additionally, our customers are looking for end cuts that are burr-free, especially with parts that need to mate with other parts.

However, customers do on occasion ask us to help them achieve a specified flatness tolerance — for instance, on parts that have a larger surface area, that have been machined, or that were pre-made and supplied to us by customers for additional finishing.

In these instances, we make good use of our lapping equipment to attain smooth, flat surfaces that meet customer specifications.

How do we verify flatness tolerance?

At Metal Cutting, we typically fixture the part so that the surface to be measured is level; we confirm this by finding three opposite points on the part surface that are the same height.

We then put a height gage on various other points across the part to note any differences and check that the entire part is within specifications for flatness.

We can also use our OGP SmartScope system to measure multiple random points along the surface plane of a part to make sure the highest and lowest points are within the allowable tolerance range.

Are there standards for flatness tolerance?

Without a doubt, flatness tolerance is important for some applications.

In January 2019, Apple touted the tight flatness tolerance of its new iPad Pro models. The company said the new iPad Pro was manufactured to “[allow] for no more than 400 microns of deviation across the length of any side — less than the thickness of four sheets of paper.” 

Along with using CNC machining to create a stronger plastic and aluminum enclosure, the tighter flatness tolerance was part of Apple’s response to complaints about bending issues in various devices. 

In small parts manufacturing, ASTM International and other organizations publish standards for flatness tolerances for steel, aluminum, and other materials, in forms ranging from plates, sheets, and bars to wires, rods, and tubes.

Using GD&T for accurate specifications

For the complete range of GD&T features, from parallelism, perpendicularity, and angularity to circularity, straightness, and flatness, the goal at Metal Cutting is always to keep production cost effective while delivering the very best quality for our customers’ products.

By providing detailed and complete specifications up front — including clear and carefully thought-out tolerances for surface flatness and other important characteristics — you can play a truly vital role in optimizing for manufacturability and helping us manage both costs and quality.

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