Surface Roughness Chart — Ra, Rz, Rmax, N grades
Convert between Ra, Rz, Rmax, N grades and triangle finish marks, see typical roughness by machining process and how to write it on a drawing
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A conversion chart between Ra, Rz, Rmax, N grades, triangle finish marks (▽) and microinches with the ISO 4288 cutoff, typical roughness by process, and a maker for the ISO 1302 drawing symbol. For reading old drawings or choosing a process for a required finish.
How to use
- On "Conversion chart" pick the "Input" type and enter the "Value"; the nearest preferred row is highlighted with all equivalents.
- On "By process" enter the "Target Ra" to highlight the processes that reach it; narrow the list with "Search processes".
- On "Drawing symbol" choose "Material removal", "Parameter", "Value" and "Lay direction"; switch "Notation" for triangle marks.
- Download the symbol with "Save SVG" or "Save PNG" and print the chart with "Print".
How it works
- The Ra series is the preferred series of ISO 4287; the Rz · Rmax column and triangle marks are the same row of the JIS B 0601:1994 comparison table (identical in KS B 0161). N grades run from N1 (Ra 0.025) to N12 (Ra 50).
- Rz ≈ 4 × Ra and RMS ≈ 1.11 × Ra are rules of thumb; measured ratios vary between about 2 and 7. Cutoffs follow ISO 4288; process ranges come from the ANSI B46.1 chart in Machinery's Handbook.
- Reference values only. For contracts and inspections check ISO 21920 (formerly ISO 1302, ISO 4287, ISO 4288).
Questions and answers
- How many triangles is Ra 1.6?
- Three (▽▽▽). ▽▽▽▽ is Ra 0.2 or less, ▽▽▽ 0.4 to 1.6, ▽▽ 3.2 to 6.3, ▽ 12.5 to 25, ~ unmachined.
- How do I convert Ra to Rz?
- There is no exact conversion. The table pairs Ra 1.6 with Rz 6.3 and Rz ≈ 4 × Ra is the rule of thumb, but real ratios vary from 2 to 7, so state the parameter that will be measured.
- Which process gives Ra 0.8?
- Grinding, honing, lapping and electropolishing typically; fine turning, milling, reaming and broaching can reach it.
- What Ra is N7?
- N7 is Ra 1.6 µm. The grades start at N1 = 0.025 µm and double at each step up to N12 = 50 µm.
- Which cutoff do I use to measure Ra 1.6?
- λc 0.8 mm and ln 4 mm for Ra above 0.1 up to 2 µm (ISO 4288); Ra 3.2 uses 2.5 mm.