√ Surface texture symbol
Surface texture symbol · Estado superficial · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
The √ symbol indicates a surface finish requirement; the values next to it (Ra, Rz, process, etc.) define how smooth the indicated area must be.
What controls
A symbol that carries all the surface finish requirements for a face: the roughness parameter and its limit value, whether or not material removal is required, and optionally the mandatory manufacturing process, the lay direction and the machining allowance. Each position around the symbol has a fixed meaning, so the same number placed elsewhere means something else. It controls the fine texture of the surface, not its form or location: a face can meet the required finish and be warped or out of position, because that is governed by geometric tolerances.
How to verify
A contact profilometer (or non-contact on delicate surfaces) set to the parameter the symbol calls for and with the cutoff length appropriate to the expected finish; a wrongly chosen cutoff changes the result without the part changing. The stylus must travel perpendicular to the machining marks, which is where roughness is highest. Several readings are taken in representative areas of the face, avoiding edges, dings and isolated marks, and evaluated against the drawing limit.
Workshop guideline values
- General
- Ra 3.2 to 6.3 µm on functional faces from ordinary milling and turning.
- Accuracy
- Ra 0.8 to 1.6 µm on seats, slideways and close-fitting bearing surfaces.
- High precision
- Ra 0.1 to 0.4 µm for fine grinding, honing and dynamic sealing surfaces.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Reading the number on the symbol as if it were always Ra: the parameter is whatever the symbol states, and Rz or Rq are interpreted differently.
- Ignoring the process specified on the symbol when the roughness value is achieved another way: if it is written there, it is mandatory.
- Confusing finish with geometry: meeting the roughness says nothing about flatness, perpendicularity or position.
How to get it in the workshop / if it comes out of tolerance
Read the whole symbol before choosing the strategy: if it specifies a process (grinding, turning, honing), that process is mandatory and cannot be replaced by another one that gives the same number. A value with no further letters is understood as the upper limit of the specified parameter. Remember that the finish is decided on the final pass: feed, nose radius, cutting edge condition and setup rigidity matter more than the machine.
If the finish comes out worse than required, work first on feed and nose radius in turning, and on stepover and speed in milling. A worn edge or one with built-up edge makes the finish worse even with correct parameters. If visible waviness appears, the problem is chatter: reduce tool overhang or change the speed. And if the drawing requires a specific process, don't look for shortcuts: even if the number comes out right, the process requirement is not met.
Other symbols in Surface texture
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