Every machined part is composed of points, lines, and surfaces, and after manufacturing, each element inevitably carries deviations from its ideal geometry. These deviations include both form errors and positional errors. In industrial design and mechanical engineering, such errors are controlled through geometric tolerances (GD&T) and surface roughness requirements. For RF connector design, all required tolerances must be clearly marked using standardized symbols so that machining, inspection, and performance remain consistent. Understanding these tolerance principles is essential for high-precision mechanical design and production.

Form tolerance and position tolerance

Form Tolerance

Form tolerance controls the shape of a feature.
It is independent of a datum, meaning it can be applied to features without referencing another surface or axis.

Position Tolerance

Position tolerance controls the location of a feature relative to a datum.
A datum must be specified beforehand, making positional tolerance dependent on a defined coordinate system.

Classification and symbol

# The feature control frame of form tolerance does not include a datum frame, while the feature control frame of position tolerance must have a datum corresponding to it.

Straightness

Straightness applies to line elements and controls how closely a line follows a perfect straight path.
It is a form tolerance and does not require a datum.

Example: The axis of a cylinder must lie within a tolerance cylinder of 0.1 mm diameter.

Flatness

Flatness ensures that all points on a surface lie within two parallel planes.

• Flatness control: limits the allowable unevenness of a surface.

• Tolerance zone: two parallel planes separated by the tolerance value.

• Example: If flatness is 0.2 mm, the surface must lie within two planes 0.2 mm apart.

Verticality

Perpendicularity controls how much a line, plane, or axis can deviate from a perfect 90° orientation relative to a datum.

Types include:

• surface to surface

• line to surface

• axis to datum plane

Coaxiality

Coaxiality defines how closely two cylindrical axes align.
It is a position tolerance and requires a datum axis.

Example: The controlled axis must lie within a cylinder of 0.03 mm diameter centered on datum axis A.

Positional degree

Positional tolerance defines a feature’s exact location relative to datums and specifies allowable deviation from its theoretical position.

Circular runout

Circular runout controls variations of a circular surface when rotated around a datum axis.

Example: During one full rotation, radial deviation must not exceed 0.03 mm.

Total runout

Total runout controls the cumulative variation of an entire surface when rotated.

Example: Every point on the cylindrical surface must remain within 0.03 mm of the ideal surface across full rotation.

Surface roughness

Surface roughness describes small-scale irregularities (peaks and valleys) on a machined surface. These micro-errors usually have a spacing below 1 mm and directly affect:

• assembly precision

• sealing performance

• fatigue life

• conductivity (important for RF connectors)

A smaller roughness value indicates a smoother surface.

Surface Roughness Evaluation Parameters

Ra — Arithmetic Mean Deviation

Average value of surface profile deviations within sampling length.
Most commonly used parameter.

Rz — Average Maximum Height

Average of the five highest peaks plus the five lowest valleys within sampling length.

Ry — Maximum Height of the Profile

Distance between the highest peak and lowest valley in sampling length.

If Ra/Rz/Ry are not specified, the value defaults to Ra.

Surface Roughness Symbols

a) General surface (any machining method)
b) Surface requiring material removal
c) Surface produced without material removal

These symbols define how the surface should be produced and evaluated.

Examples of RA

Examples of Rz、Ry

Marking Methods for Surface Roughness

a. The tip is perpendicular to the machined surface, pointing from the outside of the material to the material surface, and the long stroke is always on the edge;

b. Digital dimension: the same as dimension;

c. All surfaces shall be marked only once; It can be marked on contour line (or extension line) and dimension limit；

d. exceptional case：

1）For the most frequently used code: if Ra is all the same, the code will be marked in the upper right corner; If most of Ra are the same, they can be uniformly marked on the upper right corner of the drawing with the word "Others" added. In both cases, the figure is magnified by 1.4 times；

2）Continuous surfaces are marked only once；

3）The discontinuous same surface shall be connected with a thin solid line and marked only once。、

Symbol and direction

Metric and English Comparison Table of Surface Roughness