Understanding Surface Texture in Manufacturing

Surface Texture

• Surface texture encompasses roughness, waviness, lay, and flow
• Manufacturing processes and part structures affect surface characteristics
• Multiple methods exist for interpreting surface roughness plots
• Common methods include:
  • Distance between peak to centerline
  • Average distance between peaks and valleys for sample length
  • Sum of average of first 5 peaks and valleys
  • Maximum peak to valley height
  • Maximum peak height
  • RMS of peak and valley average
• Variations in surface roughness methods exist worldwide, exceeding 128 conventions
• Common method is interpreting average roughness (Ra), where R stands for roughness and a for average.
• Other methods include Rz, Rt, and Rmax
• Ra (Roughness Average) is the most common indicator of surface roughness and is indicated on technical drawings
• Surface roughness is now indicated using symbols, superseded the older triangle symbols, though some companies may still use triangle symbols
• A table correlates Ra values with roughness grades and corresponding symbols
• Basic symbol for surface texture involves two unequal length lines angled approximately 60 degrees relative to the line representing the surface
• Thin lines are used for the symbol
• If machining removes material, an additional bar is added
• If material removal is not allowed, a circle is added
• Special surface characteristics can be illustrated using additional lines on the symbols
• Numerical values defining roughness criteria are added to the symbols, including maximum and minimum limits
• Production method(s) can be noted on an extension line or other extensions of the symbol
• Unique methods are indicated when needed
• Surface texture values are typically specified once per surface, ideally on the same view as size and location of the surface
• Symbols are oriented for readability (bottom/right side of the drawing)
• Surface texture requirements can be indicated on dimension lines or tolerance frames; this applies when no risk of misunderstanding is present

Symbols of Lay

• Parallel: Symbol is parallel to projection plane of drawing view
• Perpendicular: Symbol is perpendicular to projection plane of drawing view
• Crossed: Symbol is crossed in two slant directions relative to projection plane of view
• Multidirectional: No specific direction, multidirectional
• Circular: Approximately circular relative to centre of surface
• Radial: Approximately radial relative to surface centre

Graphical Symbol for "All Surfaces Around a Workpiece Outline"

• A circle is added to the complete graphical symbol if all surfaces have the same texture around a work piece outline.
• The outline represents all surfaces
• Texture requirements are shown once per surface, in the location where the size or position of the surface is shown
• Texture specifications are indicated on the same view to minimize confusion

Positioning and Orientation of Graphical Symbols

• Symbols and supplementary information should be on the bottom or right side of the drawing
• Texture requirements are connected to surfaces using reference lines and arrowheads, or placed on extension lines
• Reference lines point to the relevant surface

On Dimension Line/Feature-of-Size Dimensions

• Surface texture requirements can be noted on dimension lines when no risks of misinterpretation are present

Tolerance Frame for Geometrical Tolerances

• Surface texture requirements may be noted on tolerance frames

Cylindrical and Prismatic Surfaces

• On a cylindrical surface, surface texture info may be placed once on a centerline if all indicated surfaces have the same texture info
• Distinct texture info is needed on each prismatic surface if there are different textures, as shown in the provided figures
• All prismatic surfaces must have texture info specified uniquely

Indication of Two or More Manufacturing Methods

• Separate note or Figure is included if two or more surface texture specifications are needed (i.e., before and after treatment)