Overview:

Straightness — GD&T

1. What is Straightness?

Straightness is a form tolerance that controls how straight a feature must be.

Controls the straightness of a line or surface
Does not require a datum
Controls individual elements of a feature
Helps prevent unwanted bending or bowing

2. Straightness Symbol

Symbol: ─

The straightness symbol is a single straight line
It is specified in a Feature Control Frame (FCF)
Tolerance is usually given as a linear value

3. Straightness of a Surface

For a surface, straightness controls each individual line element.

Example:
Straightness | 0.05 mm

Each line element of the surface must lie within the specified tolerance zone.

4. Straightness of an Axis

For a cylindrical feature, straightness can control the axis.

Example:
Straightness | ⌀0.05 mm

The derived axis must remain within a cylindrical tolerance zone of Ø0.05 mm.

Summary:

Type: Form tolerance
Datum required: No
Controls: Line elements / Axis
Symbol: ─
Main purpose: Control how straight a feature is

Straightness — GD&T
- 1. What is Straightness?
1. Straightness in GD&T
2. Straightness Symbol
3. What Does Straightness Control?
4. Types of Straightness
5. Straightness of a Surface
6. Straightness Applied to an Axis
8. Example — Shaft Straightness
9. Straightness vs Flatness
10. Straightness vs Circularity
11. Straightness vs Cylindricity
12. Inspection of Straightness
13. Common Mistakes
14. Straightness — Interview Questions
Summary:

1. Straightness in GD&T

Straightness is one of the four form tolerances in GD&T.

The other form tolerances are:

Flatness
Circularity (Roundness)
Cylindricity

Straightness controls how much a line element or axis can deviate from a theoretically perfect straight line.

The key point is:

Straightness controls the straightness of a feature. It does not control its location or orientation unless applied in a specific feature-of-size context.

2. Straightness Symbol

The GD&T symbol for straightness is:

⏤

It looks like a simple horizontal straight line.

A typical Feature Control Frame may look like:

| ⏤ | 0.05 |

This means the controlled feature must satisfy a straightness tolerance of 0.05 mm.

3. What Does Straightness Control?

Straightness controls the amount by which an actual feature can deviate from a theoretically perfect straight line.

The actual feature cannot deviate beyond the specified straightness tolerance zone.

4. Types of Straightness

Straightness has two important applications:

Type 1 — Surface/line-element straightness

Controls the straightness of individual line elements on a surface.

Type 2 — Axis straightness

Controls the straightness of the derived median line/axis of a cylindrical feature of size.

These two applications are important because their tolerance zones are different.

5. Straightness of a Surface

When straightness is applied to a surface or line element, it controls each applicable line element independently.

For example:

| ⏤ | 0.05 |

may be applied to a surface.

The tolerance zone consists of two parallel lines separated by 0.05 mm for a line element in the specified direction.

The actual line element must remain between these two boundaries.

When straightness is applied to a surface, it controls:

Individual line elements of that surface.

It does not necessarily mean that the entire surface is flat.

This distinction is very important.

Example

A surface can have every individual line element within its straightness requirement while the overall surface still has variation in another direction.

Therefore:

Straightness ≠ Flatness

6. Straightness Applied to an Axis

Straightness can also control the axis of a cylindrical feature, such as:

Shaft
Hole
Pin
Cylindrical bore

For an axis, the tolerance zone is generally cylindrical.

Example:

| ⏤ | ⌀0.05 |

The diameter symbol ⌀ indicates a cylindrical tolerance zone.

The derived axis must remain inside a cylinder having:

Diameter = 0.05 mm

8. Example — Shaft Straightness

Suppose a shaft has:

Ø20.00 ± 0.05 mm

and its axis has:

Straightness | ⌀0.04 |

The shaft’s derived axis must remain within a cylindrical zone of:

Ø0.04 mm

along the applicable length of the shaft.

9. Straightness vs Flatness

This is one of the most common GD&T interview questions.

Straightness	Flatness
Controls straightness	Controls flatness
Can apply to a line element or axis	Applies to a surface
Line element → two parallel lines	Two parallel planes
Axis → cylindrical tolerance zone	No cylindrical zone
Does not require datum	Does not require datum
Can control shaft axis	Controls entire surface

Straightness = line

Flatness = surface

10. Straightness vs Circularity

Another common question:

Straightness	Circularity
Controls straightness	Controls roundness
Controls a line/axis	Controls circular cross-section
Line/axis concept	Circular profile concept
Can control shaft axis	Controls each circular section
Tolerance zone can be two parallel lines or cylinder	Two concentric circles

Straightness → Is it straight?

Circularity → Is it round?

11. Straightness vs Cylindricity

Straightness	Cylindricity
Controls a line element or axis	Controls entire cylindrical surface
Narrower requirement	More comprehensive
Can control an axis	Controls cylindrical form
Does not by itself control roundness	Controls complete cylindrical form
Usually simpler to inspect	More demanding

A cylindrical component can have a straight axis but still have poor cylindrical surface form.

12. Inspection of Straightness

Straightness can be inspected using different methods depending on the feature and required accuracy.

Common equipment includes:

Surface plate
Dial indicator
Height gauge
V-blocks
Precision straightedge
CMM
Optical measuring systems
Roundness/form measuring machines

Simple Shaft Inspection Method

For a shaft, one possible inspection setup is:

Place the shaft appropriately on suitable supports.
Rotate or translate it as required by the inspection method.
Measure the deviation using a dial indicator.
Compare the measured variation with the specified requirement.
Ensure the measurement method matches the applicable GD&T standard and feature definition.

For precision inspection, a CMM or dedicated form-measuring machine may be used.