About Steel Surface Finishes
Blanchard Ground Finish and Surface 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 photos above of reflection on Blanchard ground parts. 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. PGI can guarantee a 32Ra finish on surface ground parts although occasionally achieving smoother finishes.
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.
Surface Roughness Metrology
Surface finish units include:
RMS = Root Mean Square in micro inches
Ra = Roughness Average in micrometers or micro inches
CLA = Center Line Average in micro inches
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
RA (MICROMETERS) RA (MICROINCHES) RMS CLA (MICROINCHES) RT N
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