Ra is the arithmetic average roughness of a surface, measured as the average height of peaks and valleys from a mean line, and expressed in microinches or micrometers. A lower Ra means a smoother surface: 125 microinches is a standard machined finish, while 32 microinches or finer usually requires fine finishing or grinding.
Engineers call out surface finish to control friction, sealing, wear, fatigue life, and appearance. Buyers should understand Ra because finer finishes add cycle time, extra operations, and inspection, all of which show up in the machining quote.
What Ra measures
A profilometer drags a stylus across the surface over a set sampling length and records the vertical profile. Ra is the average absolute deviation of that profile from its mean line. Because it is an average, Ra does not describe the shape of the peaks and valleys, and two surfaces with the same Ra can behave differently. Where that matters, drawings may also specify Rz, Rmax, or lay direction.
Surface finish Ra chart and how each is produced
| Ra (microinch) | Ra (micrometer) | Typical description | Typical process |
|---|---|---|---|
| 250 | 6.3 | Rough machined | Roughing cuts, saw cut faces |
| 125 | 3.2 | Standard machined | Normal CNC turning and milling |
| 63 | 1.6 | Fine machined | Finish passes with sharp tools and controlled feed |
| 32 | 0.8 | Very fine machined or ground | Fine turning, fine milling, or grinding |
| 16 | 0.4 | Ground | Centerless or surface grinding |
| 8 and below | 0.2 and below | Lapped or polished | Lapping, honing, or polishing, often outside processing |
Values are typical. The finish a process produces depends on material, tool condition, feed, speed, and machine rigidity.
How surface finish affects cost
Moving from 125 Ra to 63 Ra often means an extra finish pass. Moving from 63 Ra to 32 Ra may mean slower feeds, a dedicated finishing tool, or a grinding operation. Below 16 Ra, grinding or lapping is usually required. Each step adds cycle time and may add a setup or an outside operation.
- Specify a default finish in the drawing notes, commonly 125 Ra.
- Call finer finishes only on sealing faces, bearing journals, sliding fits, and cosmetic surfaces that need them.
- Note whether the finish applies before or after plating or coating.
Material effects on surface finish
Free-machining steels and brass generally produce good finishes easily. Aluminum finishes well with sharp tools but can smear with the wrong tooling. Austenitic stainless steels and high-nickel alloys such as Inconel work harden and need rigid setups to hold fine finishes. Plastics can tear or fuzz, which calls for specific tool geometry. See our materials page for the alloys we machine.
How to call out surface finish on a drawing
Use the standard surface texture symbol with the Ra value above the check mark, in microinches for inch drawings or micrometers for metric drawings. Put a general note for the default finish, such as "All machined surfaces 125 Ra max unless otherwise specified," and apply the symbol to surfaces that need something finer. State if lay direction matters, for example on a dynamic seal surface.
How surface finish is inspected
Surface finish is measured with a profilometer, which traces the surface and calculates Ra over a set cutoff length. For routine checks, some shops compare parts visually and by touch against roughness specimens, but a profilometer reading is the objective record when finish is a critical characteristic. If finish must be reported, say so on the PO so measurement is planned. Where the drawing calls a finish on a surface a stylus cannot reach, such as a small bore, agree on the verification method before the order. Finish and tolerance are also related: a tight tolerance on a rough surface is hard to measure consistently, so finer tolerances usually call for finer finishes.
Surface finish at Wexmar
Wexmar produces standard and fine machined finishes on CNC lathes, Swiss-type machines, and horizontal machining centers, and runs a centerless grinder and a surface grinder for finer finishes on diameters and flat faces. Polishing and lapping to very low Ra values are arranged through qualified outside processors when required. Details on our CNC turning and capabilities pages.
Surface finish checklist
- Default Ra stated in the general notes.
- Finer finishes called only on functional surfaces.
- Units match the drawing units: microinches or micrometers.
- Finish stated as before or after coating.
- Lay direction specified if it matters.
Getting a quote
Send your drawing with finish callouts and Wexmar will quote the operations needed to achieve them. You can upload a drawing for an instant estimate or call (315) 400-2654.