--- title: PH Tool Reference Standards Blog | Surface Finish description: Surface Finish | PH Tool Reference Standards Blog --- [![PH Tool | PH Tool Reference Standards - The Standard of Excellence](https://content.phtool.com/hubfs/Phtool%20November2017/images/logo.png)](https://www.phtool.com/) [Order Online](http://www.phtool.com/store2/) [267-203-1600](tel:2672031600) - [Select Language](https://customers.phtool.com/language/) - [English](https://customers.phtool.com/) - [Deutsch](https://customers.phtool.com/language/deutsch/) - [Chinese](https://customers.phtool.com/language/chinese/) - [Français](https://customers.phtool.com/language/francais/) - [Dutch](https://customers.phtool.com/language/dutch/) - [Italiano](https://customers.phtool.com/language/italiano/) - [Japanese](https://customers.phtool.com/language/japanese/) - [Korean](https://customers.phtool.com/language/korean/) - [Portuguese](https://customers.phtool.com/language/portuguese/) - [Español](https://customers.phtool.com/language/espanol/) - 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[PH Tool Holiday Hours](https://customers.phtool.com/contact/ph-tool-holiday-hours/) - [Hotel Accommodations](https://customers.phtool.com/contact/recommended-accommodations-for-your-visit/) # PH Tool Reference Standards™ ## [The Importance of Surface Finish when Making NDT Reference Standards and Test Blocks](https://content.phtool.com/blog/he-importance-of-surface-finish-when-making-ndt-reference-standards-and-test-blocks) Posted by [PH Tool](https://content.phtool.com/blog/author/ph-tool) on May 17, 2018 10:04:00 AM - [Tweet](https://twitter.com/share) [![GAR_RT2-1](https://content.phtool.com/hubfs/GAR_RT2-1.jpg)](https://content.phtool.com/blog/he-importance-of-surface-finish-when-making-ndt-reference-standards-and-test-blocks) Surface finish describes the roughness or irregularity of a surface using a measurement system to quantify the results.  This system covers a wide range from extremely rough-textured surfaces all the way to those that are highly polished, mirror-like surfaces.  The predominant parameter is Roughness Average, denoted by “Ra,” which is the arithmetic mean deviation of a profile.  The higher the number, the rougher the finish.  Roughly milled surfaces will be in the 250 to 500 Ra µin (6.3 to 12.5 µm) range.  Smooth surfaces machined by successive light passes on a surface grinder with flood coolant will yield finishes in the 16 to 32 Ra µin (0.4 to 0.8µm) range.  The span of 16 to 500 Ra µin (0.4 to 12.5 µm) covers the majority of machined tools and parts in industry today.  There are extremes beyond that range of course, but for purposes of Non-Destructive Testing (NDT) standards, they are rare. [Read More](https://content.phtool.com/blog/he-importance-of-surface-finish-when-making-ndt-reference-standards-and-test-blocks) 0 Comments [Click here to read/write comments](https://content.phtool.com/blog/he-importance-of-surface-finish-when-making-ndt-reference-standards-and-test-blocks#comments-listing) Topics: [Surface Finish](https://content.phtool.com/blog/tag/surface-finish), [Roughness Average](https://content.phtool.com/blog/tag/roughness-average), [Surface Roughness](https://content.phtool.com/blog/tag/surface-roughness) [All posts](https://content.phtool.com/blog/all) [![Click to Download Your Nominal Pipe Size Chart](https://no-cache.hubspot.com/cta/default/3809397/4aa36967-656d-43bf-a81d-0173a41dc936.png)](https://cta-redirect.hubspot.com/cta/redirect/3809397/4aa36967-656d-43bf-a81d-0173a41dc936) --- ### Subscribe to Email Updates ### Recent Posts ### Posts by Topic - [Calibration Blocks (1)](https://content.phtool.com/blog/tag/calibration-blocks) - [EDM notch shapes (1)](https://content.phtool.com/blog/tag/edm-notch-shapes) - [Flat-bottom holes (1)](https://content.phtool.com/blog/tag/flat-bottom-holes) - [Pipe Calibration (1)](https://content.phtool.com/blog/tag/pipe-calibration) - [Roughness Average (1)](https://content.phtool.com/blog/tag/roughness-average) - [Surface Finish (1)](https://content.phtool.com/blog/tag/surface-finish) - [Surface Roughness (1)](https://content.phtool.com/blog/tag/surface-roughness) - [What is EDM? 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