--- title: Surface Finish (Ra): What the Number Means | Wolf Machine url: https://wolfmachinenv.com/glossary/surface-finish-ra/ business: Wolf Machine & Fabrication, Inc. address: 4812 Longley Ln, Reno, NV 89502 phone: (775) 359-5570 hours: Shop Monday-Friday 7:00 am-3:30 pm; closed Saturday and Sunday type: Metal and plastic fabrication and machining (custom and CNC machining, welding, metal and plastic fabrication, gearbox and part repair, align boring, metal spray) --- [Photo: Stock photo: a milling cutter taking a finishing pass on a steel block, coolant line beside it] - [Home](https://wolfmachinenv.com/) - [Glossary](https://wolfmachinenv.com/glossary/) - Surface finish (Ra) # Surface finish (Ra) Ra is the arithmetic average roughness of a surface: the average height of the tiny peaks and valleys left by the cutting process, measured along a short line. It is written in microinches (millionths of an inch) in the US and micrometers elsewhere, and a smaller number means a smoother surface. Stock photo: a milling cutter taking a finishing pass on a steel block, coolant line beside it Look at a turned shaft under a lamp and you can see fine lines. Those lines are the tool marks, and their depth is what a finish callout puts a number on. A print might say 63 Ra on a bearing journal and leave a rough note on everything else, because only some surfaces need to be smooth. ## Microinches and micrometers One micrometer is about 40 microinches, so 32 microinches Ra is roughly 0.8 micrometers. Prints from US customers usually show microinches with a small check-mark symbol, while prints from metric drawings show micrometers. Either way, the number on the print is a ceiling: the surface may be that smooth or smoother. Ra is an average, so it does not show a single deep scratch. For that reason a seal surface may carry an extra note, such as no spiral lead or no cross-scratches. The older RMS figure averages the same profile a different way and is covered with the other short entries on the [glossary page](https://wolfmachinenv.com/glossary/). ## Textbook ranges by process These are general ranges for the processes, taken from standard machining references. They describe what the processes tend to produce in principle. They are education, not a statement of what any one shop holds, and the callout on your print is what sets the finish. A rough turned or milled surface commonly falls in the low hundreds of microinches. A sharp tool and light feed commonly give tens of microinches. Grinding goes smoother still, and lapping or polishing smoother again. Each step down takes more time and care. ## Where finish matters on a part Finish matters most where one part touches or seals against another: bearing journals, seal diameters, sliding faces and sealing faces. A seal that runs on a surface that is too rough wears quickly, and a surface that is too smooth can fail to hold oil. When a worn seal area has been rebuilt by [metal spray](https://wolfmachinenv.com/services/metal-spray/) or a sleeve, the surface is machined or ground back to the finish on the print. Put the finish callout on the drawing, or tell us the shaft or bore it goes with, when you send the [quote request](https://wolfmachinenv.com/request-a-quote/). A finish tighter than the part needs adds cost without adding function, as the [tolerance](https://wolfmachinenv.com/glossary/tolerance/) entry also explains. ### Sources - [Surface roughness (Ra)](https://en.wikipedia.org/wiki/Surface_roughness) (2026-10-18) - [ASME Y14.5-2018, Dimensioning and Tolerancing](https://www.asme.org/codes-standards/find-codes-standards/y14-5-dimensioning-tolerancing) (2026-10-18) ### Talk to the shop [(775) 359-5570](tel:(775)%20359-5570) | Monday to Friday | 7:00 am - 3:30 pm | |---|---| | Saturday and Sunday | Closed | [Request a quote](#quote) ### Keep reading - [What to put on a drawing before you ask for a quote](https://wolfmachinenv.com/guides/what-to-put-on-a-drawing/) - [Metal spray](https://wolfmachinenv.com/services/metal-spray/) ## See also [Tolerance](https://wolfmachinenv.com/glossary/tolerance/) [GD&T (geometric dimensioning and tolerancing)](https://wolfmachinenv.com/glossary/gd-and-t/) [Interference fit](https://wolfmachinenv.com/glossary/interference-fit/) ## Where you will see it on this site [Blanchard grinding](https://wolfmachinenv.com/services/blanchard-grinding/) [Titanium](https://wolfmachinenv.com/materials/titanium/) [What to put on a drawing before you ask for a quote](https://wolfmachinenv.com/guides/what-to-put-on-a-drawing/) [Photo: Stock photo: a lathe turning a bar, hot chips flying] ## Request a quote Tell us what you need, attach a drawing or photos of the part, and we will get back to you during shop hours. Free consultations; prompt estimates. [Call (775) 359-5570](tel:(775)%20359-5570) - From your specifications, drawings or samples. We can also assist you in designing your product or part. - Free consultations and prompt estimates - Stands behind the work: Our rates are very competitive and our quality is second-to-none. Wolf Machine stands behind the work we perform. - Company accounts: all major credit cards, or we can open an account for your company Shop hours Monday to Friday, 7:00 am to 3:30 pm. Send your request any time and we will get back to you during shop hours. Stock photo: a lathe turning a bar, hot chips flying