Surface Roughness (Ra)
The arithmetic average roughness of a surface in micrometres — the number specified when a bar is the finished running surface rather than stock to be machined.
Typical values by finish
Cold drawn bar comes in around 1.6-3.2 µm Ra, smooth turned at 1.6-3.2 µm, centreless ground at 0.4-0.8 µm and polished ground bar below 0.4 µm. Hard chrome plated hydraulic rod is finished to 0.2 µm or better, because that is what a gland seal needs to survive.
Specify it, don't assume it
Ra is not implied by a tolerance class. A bar can be dimensionally h8 and still too rough for a seal. If the surface runs against anything, put the Ra on the order.
Related terms
Where this applies
Stainless Bright Bars, Alloy Steel Bright Bars
Need help applying this?
Stancor Bright Bars engineers answer specification questions directly. Contact us.