Roughness average (Ra) · microinches
Surface finish reference
How smooth a diameter comes out, and how smooth it needs to be. These are the roughness averages each process reaches and the finishes we hold on the surfaces that matter, bearing seats and seal journals.
Surface finish is usually stated as Ra, the roughness average: the mean deviation of the surface from its centerline. In this shop we work in microinches (a millionth of an inch); lower is smoother. A standard finish turn lands in one band and a fine finishing pass reaches a tighter one, so the material and the pass set what is reasonable to ask for on a given diameter.
Finish is not cosmetic on a working shaft. A bearing seat that is too rough loses its interference as the peaks flatten under load. A seal journal that is too rough chews up the lip seal; one that is too smooth (mirror finish) can starve the seal of the micro-lubrication it needs, so seal diameters have a target window, not just a ceiling.
The process table below is a general reference for where each machining operation lands; we reach our own finishes by turning. The application table shows the Ra we aim for on the features that care, drawn from the same finish targets the configurator applies when you set a fit on a diameter.
Achievable finish by process
| Process | Ra range (µin) |
|---|---|
| Sawing (cutoff) | 125 to 1000 |
| Drilling | 63 to 250 |
| Milling | 32 to 250 |
| Turning / boring | 16 to 250 |
| Reaming | 32 to 125 |
| Broaching | 16 to 125 |
| Cylindrical grinding | 4 to 63 |
| Honing | 2 to 32 |
| Polishing | 1 to 16 |
| Lapping | 0.5 to 8 |
| Superfinishing | 0.5 to 8 |
| Electrolytic grinding | 4 to 32 |
Ra ranges in microinches. Ranges overlap because feed, tooling, and material all move the number.
Target finish by shaft feature
| Surface | Target Ra (µin) | Notes |
|---|---|---|
| Bearing seat | 32 | Rolling-element bearing inner-ring seat on a rotating shaft. |
| Seal journal | 16 to 32 | Rotary lip-seal running surface; a window, not just a ceiling. |
| Bushing / sleeve journal | 16 to 32 | Shaft running in a plain bushing or bearing. |
| Coupling / hub seat | 16 | Press or interference seat for a coupling, gear, or pulley hub. |
| Locating diameter | 16 to 32 | Non-rotating diameter that just has to position a part. |
Recommended Ra in microinches for common working surfaces, from the configurator finish targets.
Reading an Ra callout
A finish turn on a good machine reaches roughly 63 to 125 microinch Ra, and a fine finishing pass on the right material gets tighter still. We reach the Ra you specify by turning, so ask for the tightest finish only where a bearing or seal needs it, since each step down in Ra costs time.
Ra and diameter tolerance are independent callouts. A part can be right on size and too rough, or beautifully smooth and out of tolerance. Specify both on a critical diameter; the shaft fits guide covers the size side.
Frequently Asked Questions
These references are for specifying a part; every value here is one the configurator applies when you build one. Numbers are representative of the standard and confirmed against your part before machining. See all guides on the guides page.