Two important geometric tolerances in GD&T are straightness and flatness. Both control the form of a surface or feature without relating it to other design elements. However, straightness and flatness apply to different geometric entities – straightness controls the linearity of edges or axes, while flatness controls the evenness of surfaces.
Proper application and interpretation of straightness and flatness tolerances are critical to ensure manufactured parts fit and function as intended. In this blog, we discuss the key differences between these two GD&T controls: what each symbol represents, how they are annotated on technical drawings, common tolerance values, measurement methods, and their distinctive applications to lines, axes, or surfaces. A clear understanding of straightness versus flatness is essential for designers to communicate tolerances accurately, and manufacturers to evaluate and validate feature geometries.
What is Straightness in GD＆T?
In Geometric Dimensioning and Tolerancing (GD&T), straightness is a tolerance that specifies that a line or surface must be perfectly straight within a specified tolerance zone. It ensures that the feature does not deviate beyond this zone, allowing for proper alignment and functionality in assembly or mating applications. Straightness is the deviation from a geometrically straight line of a linear element such as a axis or edge. There are no size limitations in straightness, unlike other tolerances.
Straightness Symbol
The straightness symbol in geometric dimensioning and tolerancing (GD&T) is a straight line. When using the ASME Y14.5 or ISO 1101 standards, straightness is indicated by annotating a feature on a technical drawing with this symbol within a feature control frame.
The feature control frame for straightness typically consists of the following:
The straightness symbol (a straight line).
The tolerance value that specifies the allowable deviation from perfect straightness.
Optionally, additional information may include datum references if the straightness needs to be related to specific datum planes or points.
Here’s how a feature control frame for straightness might look on a technical drawing:
┌───────┐
│ ─ │
│ 0.05 │
└───────┘
In this example, the straight line in the top section of the frame is the symbol for straightness, and “0.05” in the middle section represents the tolerance value in the same units as the drawing (e.g., millimeters or inches). There is no datum reference in this example, indicating that the straightness is to be evaluated independently of other features.
When placed on a drawing, this feature control frame would be connected to the feature that it controls with a leader line or would be placed directly beside the dimension of the feature to which it applies.
Straightness Drawings
To provide an example of how straightness might be indicated on a technical drawing, let’s take the case of a straight shaft. The straightness tolerance would ensure that the axis of the shaft doesn’t deviate from a straight line by more than the specified tolerance.
Here are some examples of how to show the straightness on the technical drawing:
Straightness of a Cylindrical part
For a cylindrical part, straightness is specified on the outer contour. In the figure below, the straightness of the reference axis is indicated to be 0.02 or less. In this case, the tolerance zone is between two parallel planes with an interval of the tolerance value (0.02). Measurement needs to be within this tolerance of 0.02mm.
Straightness of the top surface of a plate
For the top surface of a plate, straightness can be specified in both the longitudinal (x) direction and transverse (y) direction. In the figure below, straightness is indicated as 0.05 or less in the long direction and 0.02 or less in the short direction. This allows accurate reflection of design intents that are difficult to express in tolerance design. The tolerance zone is between two parallel straight lines with an interval of the tolerance value t (x direction 0.05, y direction 0.02) that are parallel to the projected surface.
Straightness of a hole axis
Straightness can be specified in two perpendicular directions of x and y axes for a hole axis. In this figure, the tolerance value is indicated as 0.06 in the X axis direction and 0.03 in the Y axis direction.
Straightness of a cylindrical part axis with no direction specified
For a cylindrical part, straightness can be specified without defining the direction of the axis. As shown in the figure below, straightness is specified opposite to the dimensional lines indicating the diameter of the axis. However, unlike straightness of a hole axis, there are no restrictions in two perpendicular directions, and it is a cylindrical tolerance zone with no direction specified.
Straightness Tolerances
The straightness tolerance is often specified on a technical drawing, using the standard symbols and notation from the ASME Y14.5 or ISO 1101 geometric dimensioning and tolerancing (GD&T) standards.
Straightness tolerances will vary depending on the size of the feature and the precision required for the particular application. Here is a simplified table that might be found in a general engineering handbook or standards manual, illustrating common straightness tolerances for different ranges of feature sizes.
| Feature Size Range (mm) | Straightness Tolerance (mm) |
| 0 to 10 | 0.02 |
| 10 to 30 | 0.05 |
| 30 to 100 | 0.1 |
| 100 to 300 | 0.2 |
| 300 to 1000 | 0.3 |
| 1000 and up | 0.5 or greater |
Please note that these values are for illustrative purposes only. Actual tolerances would be determined based on the specific requirements of the part, its function, the manufacturing process, and the industry standards applicable to the particular field (e.g., aerospace, automotive, medical devices).
When specifying straightness in a technical drawing, the tolerance would be annotated alongside the feature it applies to, often using a feature control frame under the relevant dimension or as a note.
For precise applications, it is always best to refer to the actual standards and engineering specifications relevant to the project you are working on. The values provided above are general and may not be suitable for all applications.
Straightness Measurement Methods
Measurement can be done using height gauges, dial gauges, micrometers etc. Simple measurement in the field can be done using a feeler gauge. In recent years, three-dimensional measuring machines have become common, allowing easy and accurate measurement. For measuring large machine tables or vehicle chassis, optical measuring instruments called autocollimators are used.
What is Flatness in GD＆T?
Flatness in Geometric Dimensioning and Tolerancing (GD&T) is a 2D surface tolerance that ensures a surface does not deviate from a perfectly flat plane by more than a specified amount. Indicated by two parallel lines in a feature control frame on a technical drawing, flatness tolerance means that the entire defined surface must lie between two parallel planes whose separation distance equals the flatness tolerance value. This tolerance does not relate to any datum plane and is critical for surfaces requiring a high degree of flatness for functional reasons, such as sealing surfaces or contact faces for assembly.
Straightness Symbol
The flatness symbol in GD&T (Geometric Dimensioning and Tolerancing) is represented by two parallel lines. When this symbol appears on a technical drawing, it specifies that the feature associated with the symbol must lie between two parallel planes where the distance between these planes is the flatness tolerance.
Here’s how the flatness symbol generally appears within a feature control frame on a technical drawing:
┌───────┐
│ │
│ // │ <── Flatness Symbol
│ 0.05 │
└───────┘
In the feature control frame shown above, the flatness symbol (//) is located at the top, and the tolerance value (e.g., 0.05 mm or inches) is indicated below it. This frame is typically linked to or placed near the surface on the drawing to which the flatness tolerance applies.
Flatness on Drawing
To show flatness on a technical drawing, you would use a feature control frame (FCF) and the flatness symbol, which consists of two parallel lines. This FCF is attached to the surface for which the flatness tolerance is specified.
Here are some examples of how to show the straightness on the technical drawing:
Flatness of a block
The figure below indicates flatness specified on the top surface of a plate. In this case, the tolerance zone is between two parallel circular planes with an interval of the tolerance value. The actually measured surface must be within this 0.08 interval of the two parallel planes.
Flatness of an axial part
The figure below indicates flatness specified on the plane of a stepped axial part. In this case, the tolerance zone is between two parallel circular planes with an interval of the tolerance value. The axial portion not subject to flatness tolerance is excluded from the tolerance zone. The measured surface must be within the 0.08 interval of the two parallel circular planes.
CZ
CZ stands for Common Zone. Without CZ, each plane is considered independently. But with CZ, the two planes must be parallel to each other and within a common zone.
NC
NC stands for Not Convex. Convex means raised or domed shaped. By putting Not in front, it means the center cannot be higher. The circular area at the center must always be lower than the four islands outside.
Not Concave
Concave means recessed or concave shaped. By putting Not in front, it means the center cannot be lower. The circular area at the center must always be lower than the four islands outside.
Common Tolerances for Flatness
Common tolerances for flatness in machining and manufacturing processes can vary widely depending on the material, the size of the surface, the type of process used, and the part’s intended application. There is no one-size-fits-all table for flatness tolerances because they are highly dependent on specific industry requirements and the capabilities of the manufacturing process being used.
However, for illustrative purposes, below is a generic table giving a rough idea of flatness tolerances that might be expected for machined surfaces. These figures are for general guidance only and may not be appropriate for all situations.
| Surface Size (Length/Width) | Flatness Tolerance (Typical Range) |
| Up to 100 mm (4 in) | 0.005 – 0.05 mm (0.0002 – 0.002 in) |
| 100 to 300 mm (4 to 12 in) | 0.02 – 0.1 mm (0.0008 – 0.004 in) |
| 300 to 1000 mm (1 to 3 ft) | 0.05 – 0.25 mm (0.002 – 0.01 in) |
| Over 1000 mm (Over 3 ft) | 0.1 mm and up (0.004 in and up) |
For high-precision applications (like aerospace or medical devices), tolerances may be tighter, while for less critical applications (like construction or commercial products), tolerances may be more relaxed.
Always consult relevant manufacturing standards, material specifications, and design requirements when selecting the appropriate flatness tolerance for a given part. For critical applications or when in doubt, it’s best to work directly with experienced engineers or refer to industry-specific handbooks and guidelines.
Flatness Measurement
To measure flatness, a dial gauge is fixed to measure the height of each surface. In recent years, advanced measuring tools like three-dimensional measuring machines and non-contact laser interferometers also exist.
Difference Between Flatness and Straightness
Flatness and straightness are both types of geometric tolerances used in GD&T (Geometric Dimensioning and Tolerancing) to control the form of a feature without reference to any other feature. However, they apply to different aspects of a part’s geometry:
Flatness:
- Applies to a surface.
- Ensures that all points on a specified surface lie between two parallel planes.
- The tolerance zone is defined by two parallel planes within which the entire surface must lie.
- It is a 2D tolerance and does not pertain to an axis.
- Flatness tolerance does not depend on the part’s orientation or any datum features.
Straightness:
- Applies to a line or axis.
- Ensures that a feature or line is straight within a specified tolerance.
- The tolerance zone can be a cylindrical zone within which the entire line element or axis must lie.
- It can be a 2D or 3D tolerance, controlling the straightness of a line on a surface (2D) or the straightness of an axis (3D).
- When applying straightness to an axis, it may or may not relate to datums, depending on whether it’s controlling the median line (no datum reference) or the feature’s orientation (datum reference might be used).
Flatness is concerned with the overall plane being even and flat, while straightness is concerned with an individual line or axis being straight. An easy way to remember the difference is that flatness is about a surface (like a table top), and straightness is about a line or edge (like a ruler or shaft).
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