Surface Finish Symbols

By Ollie Cockman | 17/03/2022

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Understanding surface finish symbols is vital in ensuring the success of any component outlined within an engineering drawing. Separate to surface roughness and its associated symbols, surface finish symbols refer directly to the process(es) used to achieve the desired finish of a part. 

Surface finish explained

Surface finish refers to a surface’s texture. A vital component in ensuring the successful creation of manufactured parts, surface finishes are outlined within engineering drawings to ensure that parts fit together tightly. For example, for a seal to be airtight, parts manufactured must have compatible surface finishes to prevent the occurrence of any friction or damage between the multiple surfaces. 

Surface finishing is also sub-categorised into three separate categories - waviness, lay, and roughness: 

Waviness 

The title is given to imperfections within the surface of a component, often caused by warping due to excessive heating and cooling, and machining defects such as those caused by chatter. Waviness is measured over an evaluation length, with waviness spacing a calculation of the peak-to-peak spacing of waves, with the average height meanwhile defined by average waviness. 

Lay 

Produced by the manufacturing process undertaken to manufacture the component in question, lat is the dominant pattern of a part’s surface. It can be either parallel, perpendicular, circular, isotropic, multi-directional, or cross-hatched. 

Surface roughness

Minor irregularities in the geometry of a component’s surface. Many engineer’s who reference surface finish will be referring exclusively to surface roughness, as this is the category most commonly measured. 

See below the diagram demonstrating the relationships between waviness, lay, and surface roughness: 

Waviness, Lay and Surface Roughness

To denote different surface finishes within different components, a list of surface finish symbols is required to reference contrasting finishes in a universally understood way. See below for a full list of all surface finish symbols, their meaning, and how they should be applied:

Basic surface texture symbol

Any method may be employed to produce the desired surface unless the circle or bar are specified, as illustrated within the symbols below. 

Material Removal By Machining is Required Symbol

Material removal by machining is required

This symbol indicates that material removal by machining is necessary for the desired surface finish to be achieved. A horizontal black bar is added to the basic surface texture symbol to indicate this. 

Material Removal Prohibited Symbol

Material removal prohibited

Used to indicate that processes such as injection moulding, cold finishing, die casting, casting, forging, hot finishing or powder metallurgy must be used to acquire your desired surface finish. A black circle is added to the basic surface texture symbol to denote this. 

Surface Texture Symbol

Surface texture symbol

Applicable when a surface is produced via any possible method, excluding when a bar or circle is specified or a method is specified above the horizontal line. 

Parallel To Plane Of Projection Symbol

Parallel to plane of projection 

An edged tool is used during machining to create creases parallel with the surface in the diagram in which the symbol is written. 

Perpendicular to Plane of Projection Symbol

Perpendicular to plane of projection 

The creases made by the edged tool during machining are parallel to the surface captured in the diagram in which the symbol is written.

Crossed in Two Oblique Directions to Plane of Projection Symbol

Crossed in two oblique directions to plane of projection 

An edged tool is used during machining to create creases at an angle to the plane of projection, crossing one another on the surface on which the symbol is written. 

Mulit-directional Symbol

Multi-directional 

The edged tool employed during machining creates creases that cross multiple times or have no set direction. 

Approx Circular Symbol

Approx circular 

An edged tool is used during machining to create creases in the form of concentric circles in relation to the centre of the surface upon which the symbol is denoted. 

Approx Radial Symbol

Approx radial 

An edged tool is used during machining to create creases forming mostly a radial pattern in relation to the centre of the surface on which the symbol is written.

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Ollie Cockman

Head of Content. Football fan ⚽️

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