SURFACE FINISHES

The most common parameter for surface texture is Average Roughness (Ra). Ra is calculated by an algorithm that measures the average length between the peaks and valleys and the deviation from the mean line on the entire surface within five sampling sections. Ra averages all peaks and valleys of the roughness profile and then neutralizes the few outlying points so that the extreme points have no significant impact on the final results. It’s a simple and effective method for monitoring surface texture and ensuring consistency in the measurement of multiple surfaces.

SAMPLING SECTIONS

Rs µm

Section
mm

Total Length
mm

0.006 < Ra ≤ 0.02

 0.08 0.4
0.02 < Ra ≤ 0.1 0.25 1.25

0.1< Ra ≤ 2

 0.8 4

ASME BPE STANDARDS

Mechanically Polished

Surface
Designation

Ra-Max
µ-in

Ra Max
Outside

SF0

No Ra Req.

No Ra Req.

SF1*

20

0.51

SF2

25

0.64

SF3

30

0.76

Mechanically Polished & Electropolished

Surface
Designation

Ra-Max
µ-in

Ra Max
Outside

SF4*

15

0.38

SF5

20

0.51

SF6

25

0.54

*Most Common

Another parameter used for roughness is Mean Roughness depth (Rz). Rz is calculated by measuring the vertical distance from the highest peak to the lowest valley within five sampling lengths, then averaging these distances. Rz averages only the five highest peaks and the five deepest valleys—therefore extremes have a much greater influence on the final value.

The specification according to the ASME-BPE includes seven surface finishes that may be used when specifying surface finishes.

DIN 11865 STANDARDS

Mechanically Polished

Surface
Designation

Ra-Max
inside

Ra Max
Outside

H1

 < 1.6 μm

< 3.2 μm

H2

 < 0.8 μm

< 3.2 μm

H3

< 0.8 μm

< 0.8 μm

H4

< 0.4 μm

< 0.4 μm

H5

< 0.25 μm

< 0.25 μm

Mechanically Polished & Electropolished

Surface
Designation

Ra-Max
inside

Ra Max
Outside

H3

 < 0.8 μm < 0.8 μm

H4

 < 0.4 μm < 0.4 μm

H5

 < 0.25 μm

< 0.25 μm