Form Tolerance (Form Deviation)

Form tolerance is a basic geometric tolerance that determines the form of the target (part). None of the characteristics of form tolerance require a datum—forms can be independently determined.

Straightness

Straightness

The straightness requirement specifies how perfectly straight a target should be. It is applied to lines and not planes, and represents a curve in the center line or generatrix. Therefore, straightness is used to indicate the warpage tolerance of long objects.

Sample indication
Straightness
Explanation of drawings
If a tolerance frame is connected to the size indicating the diameter of a cylinder, the axis of that cylinder must be within a cylinder with a diameter of 0.1 mm.

Flatness

Flatness

The flatness requirement specifies the evenness of a surface, or how accurately flat a target plane should be. The most protruding part and the most concaved part must be at a specific distance between two planes that are separated vertically.

Sample indication
Flatness
Explanation of drawings
This surface must be between two parallel planes separated by only 0.3 mm.

Roundness

Roundness

The roundness requirement specifies how perfectly circular a target—the circular cross-section of a shaft, bore, or cone—should be.

Sample indication
Roundness
Explanation of drawings
The outer circumference of any cross-section of a shaft cut perpendicularly should be between two concentric circles just 0.1 mm apart on the same plane.

Cylindricity

Cylindricity

The cylindricity requirement specifies how accurately circular and straight a target cylinder is. The value represents any distortion in a cylinder.

Sample indication
Roundness
Explanation of drawings
The target plane must be between two coaxial cylinders just 0.1 mm apart.

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