Surface Finish Conversion Chart — Ra, Rz & N-Grade

What it is

Surface roughness is the fine texture left by machining. The most common measure is Ra, the arithmetic average of profile height deviations from the mean line. US drawings usually give Ra in microinches (µin); metric drawings give it in micrometers (µm). Older ISO drawings may use N grades, a step scale that doubles at each grade.

Rz is a different measurement: the average peak-to-valley height over several sampling lengths. Rz is always larger than Ra for the same surface, often several times larger, but there is no fixed conversion. Measure Rz when the drawing calls for Rz.

How to read it

Find the drawing value in the column that matches its unit. 1 µm = 39.37 µin, so the microinch values below are rounded to the customary numbers.

Ra and N grade equivalents

Typical process ranges (Ra, µm)

Key points

• Process ranges are typical, not limits. Tool condition, feed and rigidity move them a lot.
• In turning and milling, finish improves with a larger nose radius and a lower feed.
• The surface symbol on the drawing (ASME Y14.36 or ISO 1302) says which parameter and limit apply.

Common mistakes

• Treating Rz and Ra as interchangeable.
• Reading a µm value as µin, a 40-times error.
• Checking finish by eye or fingernail when the drawing calls for a measured value.

Governing standards: ASME B46.1 (surface texture); ISO 1302 (indication of surface texture on drawings); ISO 21920-1 (profile surface texture, indication). ASME Y14.36 for US drawing symbols.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.