Basic Annotation Principles

• Direction Consistency: The writing and reading direction of surface texture symbols must strictly align with the direction of dimension marks on the drawing. This ensures consistency in reading and avoids misunderstandings during production and inspection.
• Symbol Contact Requirement: Symbols must point from outside the material to the part surface and make direct contact without leaving any gaps. This clear directional indication helps workers accurately identify the target surface.
• Unified Representation for Common Surfaces: When most surfaces of a part have the same surface texture requirement, a unified annotation method is adopted. The symbol is placed near the drawing's title block, followed by a parenthesis containing a basic symbol with no additional annotations. This simplifies the drawing and reduces redundant marks.

Annotation Positions and Methods

• Direct Annotation Scenarios: Surface texture symbols can be directly marked on visible contour lines, dimension lines, or their extension lines. They can also be placed above dimension boundaries and geometric tolerance frames, which is suitable for upper and left surfaces of parts in typical drawings.
• Leader Line Annotation Scenarios: For lower and right surfaces of parts, leader lines are required for annotation. This method ensures clear marking when direct annotation on contour lines is inconvenient or would cause confusion.
• Avoiding Ambiguity: Annotations must be placed in positions that do not overlap with other drawing elements (such as dimensions, tolerances, or notes). This maintains the drawing's clarity and readability.

Parameter Interpretation and Practical Examples

• Common Ra Parameters: Ra (arithmetic mean deviation of the profile) is a key parameter for surface roughness. Common values in mechanical manufacturing include Ra0.8, Ra1.6, Ra3.2, and Ra12.5, representing different levels of surface smoothness. Lower values indicate a smoother surface.
• Specific Drawing Examples: In practical applications, the upper surface of a part might be directly marked with Ra0.8 on its contour line, indicating a high-precision smooth surface. The left surface could be annotated with Ra12.5, suitable for non-critical surfaces requiring lower processing accuracy. The lower and right surfaces, which are not easily marked directly, use leader lines to indicate Ra1.6, balancing processing requirements and annotation clarity.
• Default Parameter Specification: If the drawing states, "Except for the surface texture requirements marked on the drawing, the remaining surface texture requirements are Ra3.2", Ra3.2 becomes the default standard for all unmarked surfaces. This provides a unified reference for unspecified areas and ensures consistency in the entire part's processing.

Importance in Mechanical Manufacturing

• Guaranteeing Assembly Performance: Reasonable surface roughness ensures tight fitting between parts, reducing wear and extending the service life of mechanical equipment. For example, mating surfaces with Ra0.8 have better sealing and load-bearing capacity.
• Guiding Processing Technology: Different Ra values correspond to specific processing methods. Ra12.5 can be achieved through rough turning, while Ra0.8 requires precision grinding. Clear annotations help guide the selection of processing equipment and tools.
• Ensuring Quality Consistency: Standardized drawing specifications enable consistent understanding among designers, process engineers, and production workers, avoiding processing errors caused by ambiguous marks and improving product quality stability.