What is Surface Finish?

Surface finish is the term used to describe the requirements for the texture of a surface most notably for mechanical interfaces such as seals or where components fit tightly together in both static and dynamic conditions.

Surface finish is most commonly used to specify a surface roughness, however surface finish can also be used to specify surface waviness and lay.

Surface Roughness

Surface roughness refers to small defects in the surface of a component and is typically measured perpendicular to the lay direction.

On technical drawings, roughness is characterised by a parameter and a numerical value (um). The most common parameters are Ra and Rz. Ra represents the average variation in roughness across a complete surface, where as Rz is slightly different and is based on maximum and minimum values. Rz is calculated by splitting a surface into sections and the height difference between highest and lowest points in each section is averaged.

Surface Waviness

Surface roughness and surface waviness are very similar in form however surface waviness accounts for much larger periodic imperfections in a component’s surface compared to surface roughness. Surface waviness will usually be a result of a specific manufacturing process whereas surface roughness can be the result of a number of factors such as chemical reactions at the surface and even the molecular structure of the material itself. Both surface waviness and surface roughness are considered small and frequent enough that they do no affect form features like straightness, flatness, roundness and cylindricality.

Lay

Lay is the term used to describe the dominant pattern and the orientation of that pattern on a finished surface. The lay pattern is produced by the manufacturing process, or example milling directions, and can be defined by a number of terms: parallel, across, crossed, multi-directional, concentric, radial or non-directional. These finishes can be defined on the surface finish symbol which we will explore later.

Surface finish is highly dependant on the manufacturing processes used to manufacture the part. Many fine tolerance, highly finished components will need additional processes such as grinding or polishing to achieve values stated on a technical drawing. Where possible it is important to state surface finishes within the capability of the manufacturing process to avoid unnecessary costs and lead times.

How to define Surface Finish

Surface finish is defined on a 2D technical drawing by a texture symbol and its variations of this symbol that provide instruction and requirements to the manufacturer. This symbol will usually rest on a surface or dimension leader on projected views or it can point to hatched areas on isometric views.

Symbol

The basic surface texture symbol indicates that the required surface finish may be produced by any method at the discretion of the manufacture.

A closed surface texture symbol indicates that material removal by machining is required to produce the surface finish required. This could mean that additional processes such as polishing, grinding or honing are required to produce the finished surface.

A surface texture symbol with a circle indicates that subsequent material removal to achieve the required surface finish is prohibited. This symbol is used for manufacturing processes such as casting, forging or injection moulding.

Parameters

The following shows the location for each parameter in accordance with BS EN ISO 1302. Note ISO and ASME standards do differ in the location of parameters in relation to the texture symbol.

Examples