GlossaryKind of term: Standard and conventionSurface texture symbol
What is a surface texture symbol?
The surface texture symbol is the drawing sign that carries a surface texture requirement and states whether material must, may or may not be removed.
The surface texture symbol is a tick-shaped sign with two legs of unequal length, placed on the surface, on an extension line or on a leader to it. The basic symbol allows any process. With a bar across it, material removal is required; with a circle in the angle, no material may be removed and the surface stays as the previous process left it. The long leg carries the requirement, such as Ra 1.6, and where needed the specified process, the lay direction with a sign such as = or ⟂, and a machining allowance.
What does the standard say about the surface texture symbol?
The indication is set in ISO 21920-1, which replaced ISO 1302 in 2021; many drawings still use the ISO 1302 notation. In North America ASME Y14.36 applies. A value is only unambiguous with its parameter, filter, evaluation length and acceptance rule; whatever is not stated follows the defaults of the standard the drawing refers to, and those can differ between the old and the new series. A general symbol near the title block followed by a basic symbol in brackets means: this requirement for every surface except those with their own indication. Old notations still turn up, such as triangles from an earlier DIN standard and roughness grades N1 to N12, where N7 corresponds to Ra 1.6 µm.
How does the surface texture symbol work in practice?
The symbol says which requirement the surface must meet after the last process, not which process that is, unless the process is written on it. Whether a value comes from the last milling or turning pass or needs a finishing pass, grinding or polishing is decided by machine, tooling, material and setup together. A circle affects stock and setup: the surface has to be right as it arrives, rolled, drawn or cast, and you cannot take a skim cut to give yourself a better clamping face. The general requirement near the title block applies to every surface without its own symbol, including faces you might otherwise leave as roughed.
What is the surface texture symbol often confused with?
The symbol is not the roughness itself: the symbol is the callout, Ra or Rz with its conditions is the requirement. Nor does a bar prescribe a particular process: it requires material removal and leaves the process open unless one is stated. And it is not an edge requirement: ISO 13715 covers the shape of an edge, the texture symbol covers the surface.
How do you see the surface texture symbol in Blake?
The roughness requirement from the symbol sits with the part in Blake as a proposal from the drawing, together with material, tolerances, finishing and threads. Once you confirm or correct it, who did so and when stays visible, along with any note. More on requests.
Frequently asked questions
What does the surface texture symbol with a circle mean?
That no material may be removed from that surface. It stays as the previous process left it, for example rolled, drawn, forged or cast. Your stock therefore has to meet the requirement at that spot already.
What is N7 on an old drawing?
A roughness grade from an earlier edition of ISO 1302, corresponding to Ra 1.6 µm. N8 is Ra 3.2 µm and N6 is Ra 0.8 µm: each grade down halves the value. When in doubt, ask which standard and which measuring conditions the customer means.
Does the roughness beside the title block apply to every surface?
To every surface without a symbol of its own. A basic symbol or a value in brackets after it means: except the surfaces indicated separately. Those get their own value and sometimes their own final operation.