ISO 21920-1 surface texture symbols and the Ra series

How surface texture is indicated on a drawing: the graphical symbol, the parameter and its limit value, such as Ra 1,6.

Graphical symbols

ISO 21920-1:2021, 6.2 and Figure 4. ISO 21920-1 symbols carry a short horizontal bar above the short leg, which distinguishes them from the ISO 1302 symbols and from areal surface texture symbols (ISO 21920-1:2021, 6.2, NOTE 2).

Minimal indication

Graphical symbol, then the parameter symbol and its tolerance limit value, separated by a single blank, e.g. Ra 1,6 written under the horizontal line of the long leg (ISO 21920-1:2021, 6.1, 6.3.2 and Figure 5).

Parameters with default values: Rz Ra Rp Rv Rq Rmax Rt Pt (ISO 21920-1:2021, 6.3.2, NOTE 1 (defaults in ISO 21920-3:2021, Tables 3 to 6))

Symbol proportions

ISO 21920-1:2021 gives the proportions in Annex A (normative), which was not in the preview. Until it is seen, MechPilot draws the symbol with legacyProportions below and marks it as such.

ISO 1302:1978, Annex B, B.1.1 and B.3 (withdrawn edition). Symbol line thickness d' = h/10, h = height of the dimension lettering on the drawing (B.1.1).

Ra series

ISO 1302:1978, 4.1.5, Table 1: Ra values with roughness grade numbers N1 (0,025) to N12 (50). Kept as the list of values MechPilot offers first; any value may be typed.

Notes from the data file

• Ra values are micrometres by convention of the standard; raSeriesUnit says so explicitly. Every length on the symbol is in mm.
• Whether the N grade numbers survive in ISO 1302:2002 or ISO 21920 was not checked; MechPilot does not write N numbers on drawings.
• Complete indications (tolerance type, acceptance rule 16 % / max, filter types and nesting indices, evaluation length, manufacturing process, lay) follow ISO 21920-1:2021, 6.4 and Figures 7 to 9; MechPilot's first version writes only the minimal indication plus an optional process text.