What is the difference between Ra and Rz?

Ra is the arithmetic mean deviation of the profile (the average roughness), while Rz is the average maximum height between the highest peaks and deepest valleys over the sampling lengths. Rz reacts more strongly to isolated peaks and scratches, so two surfaces with the same Ra can have very different Rz.

How do I convert Ra to Rz?

There is no exact formula. As a rough rule of thumb Rz is about 4 times Ra for typical machined surfaces, but the ratio commonly ranges from roughly 3 to 7 depending on the process and profile shape. Always measure both if the drawing specifies both.

What do the old triangle finish symbols mean?

The old JIS triangle marks indicate finish quality by the number of triangles: more triangles mean a smoother surface. Roughly, one triangle is a rough machined finish (around Ra 12.5 to 25), two is medium (Ra 3.2 to 6.3), three is fine (Ra 0.4 to 1.6) and four is a precision finish (Ra 0.2 or finer). A wavy line means as-cast or unmachined.

What are ISO N roughness grades (N1 to N12)?

N grades are a shorthand for Ra values defined in ISO 1302: N1 is Ra 0.025 micrometres and each step roughly doubles, up to N12 at Ra 50 micrometres. They are a quick way to state a roughness class without writing the Ra number.

How do I read an ISO 1302 surface finish symbol?

The basic check-mark symbol carries the roughness value, usually written as Ra in micrometres (for example Ra 1.6). A bar added to the symbol means material removal is required; a circle means it must not be removed. Process, lay direction, sampling length and a second limit such as Rz can be added around the symbol.

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Reference

Surface Roughness Ra / Rz Reference

Approximate relation between Ra and Rz, ISO N1–N12 roughness grades, old triangle finish symbols and typical machining processes. Enter a value to find the nearest row.

Scale

Value

GradeISO N	Raµm	Raµin	Rzµm (approx.)	Old symbolJIS ▽	Typical process