Steel Surface Finishes | Precision Grinding, Inc.

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Steel Surface Finishes

About Steel Surface Finishes

Blanchard Ground finish

Classic Blanchard Ground finish

Steel surface finishes are often an important aspect of custom machine parts.  Sometimes steel surface finish is also referred to as surface texture or Surface Roughness.  The surface finish of a machine part may be important to reduce friction with other machine parts or to improve the aesthetics of the part.

Surface finish measurements (RMS, Ra, etc.) describe the microscopic deviations of a surface from a perfectly smooth ideal plane.  Note that surface finish is a different parameter than a machine part’s “flatness”.  Flatness refers to a machine part’s bending, bowing, cupping or twisting with respect to a reference plane such as a granite table top.

Finish of Surface Ground parts

Surface Finish of Surface Ground parts


Surface Roughness Metrology

Surface finish units include:

RMS = Root Mean Square in microinches

Ra = Roughness Average in micrometers or microinches

CLA = Center Line Average in microinches

Rt = Roughness Tool in microns

N = New ISO (grade) scale numbers

In our grinding shop, we most commonly discuss surface finish standards of 64RMS and 32RMS.  Our standard Blanchard Ground finish is 64RMS and our standard Surface Grinding finish is 32RMS. PGI standard machining surface finish is 125RMS unless otherwise specified.

Surface Roughness Chart

0.025 1 1.1 1 0.3 1
0.05 2 2.2 2 0.5 2
0.1 4 4.4 4 0.8 3
0.2 8 8.8 8 1.2 4
0.4 16 17.6 16 2 5
0.8 32 32.5 32 4 6
1.6 63 64.3 63 8 7
3.2 125 137.5 125 13 8
6.3 250 275 250 25 9
12.5 500 550 500 50 10
25 1000 1100 1000 100 11

Blanchard Ground Finish and Surface Ground Finish

Reciprocating grinding

Fine Blanchard Ground finish

Our Blanchard ground finish, whether accomplished in a grinder with a circular rotating table or a rectangular reciprocating table, has the characteristic arching ‘swirl’ marks.  A Blanchard ground finish is typically much smoother than it appears.  See photo above of reflection on Blanchard Ground part.   PGI can guarantee a 64Ra finish on Blanchard ground parts although occasionally achieving smoother finishes (see last photo on this page).

Our Surface ground finish may have the slightest indication of grinding marks along the long axis of the part.  See top photo on this page.  PGI can guarantee a 32Ra finish on surface ground parts although occasionally achieving smoother finishes.


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