Pre-electropolish Surface Refinement with Coated Abrasives

Q. We have a 7- to 16-Ra surface finish specification on 304 stainless prior to electropolishing. Can you give us finishing guidance and a reference of what abrasive belt and disc grits will achieve a given Ra finish? —S.K.

A. Abrasive finishing combines a harder-than-workpiece abrasive mineral with a bonded (grinding wheel) or coated (belts and discs) product that is rubbed and/or moved with pressure across the workpiece surface.
Abrasive processes for surface refinement work well and are commonly used. Abrasive removal and refinement rates are affected by a number of factors, including:

Abrasive grit size
Abrasive mineral used
Machine or tool used
Pressure applied
Use of lubricants
Surface feet speed (speed of the abrasive moving by the workpiece)

When refining your stainless, start your finishing with the finest abrasive grit possible. This reduces further steps. Once the initial scratch is performed, the rest of the work is to refine that initial scratch. Refining surfaces requires perpendicular finishing to the previous operation. Removing scratches in the same direction does not efficiently remove the depth of the previous scratch line.

Below is a chart on abrasive grit size and the approximate microinch finish they produce. This is based on half-life abrasive (or halfway worn abrasive). Use of lubricants will lower the microinch finishes. The various metal alloys will also have an effect on the finish estimates.

Abrasive Grit Size/Ra, RMS Comparison Estimate Chart

Grit Size (abrasive belt polishing)	Ra Finish Capability (in µ inches)	RMS Finish Capability (in µ inches)
24	154-164	170-180
36	132-141	145-155
60	83-88	91-97
80	66-71	72-78
120	47-51	52-56
150	35-40	39-44
180	25-30	27-33
220	14-19	15-21
320	7-11	8-12
400	4.5-8.0	5-9
600-1200	1.0-4.5	1-5

Below is a surface feet speed recommendation chart for various materials. Note that grinding wheels will have different recommendations.

Surface Feet Speed Recommendations for Polishing with Coated Abrasives

Material	Surface Speed (in sfm*)
Aluminum	6,500-8,000
Brass	7,200-7,900
Composite	6,000-7,000
Glass	5,000 (7,500 with diamonds)
Stainless	6,500-6,900
Steel	6,300-6,700
Titanium	2,400-3,100

*sfm = surface feet per minute (wheel diameter in inches × rpm × 0.2618)

Running inefficient surface feet speeds can reduce production rates and cause premature abrasive wear. The wrong speed can also cause excessive heat and galling of the part, which will affect Ra surface finishes.