Surface Finish Ra and Why Polished Is Not a Specification

What Ra Actually Measures

Ra is the arithmetic average roughness of a surface, measured in micrometres. A lower figure means a smoother surface with shallower peaks and valleys. It is a measured number, which is exactly why it belongs on a purchase order and the word polished does not.

Why Roughness Drives Cleanability

Soil and microorganisms lodge in surface valleys. The rougher the surface the more of that shelter exists, and the harder the cleaning cycle has to work to remove it. This is why a line built from components with an unverified finish can fail a swab test while the CIP data looks correct.

Choosing Between 0.8, 0.6 and 0.4

Ra 0.8 um is the general hygienic standard and suits most food, beverage and dairy duty. Ra 0.6 um and Ra 0.4 um are worth specifying for high purity water, pharmaceutical product contact, and any surface that has repeatedly been the source of a cleaning problem. Finer finishes cost more and do not compensate for a badly drained layout.

Welds and Internal Surfaces

The finish only matters if it covers the whole product contact surface. Welded fittings in this catalogue are polished internally after welding so the bore does not carry a weld bead through the flow path. Ask about internal finish specifically rather than assuming it matches the outside.

Putting the Figure on the Order

Write the Ra figure on the line item. That is what makes it repeatable across a second and third order, and what gives you something to check the delivered parts against.