Pre-electropolish Surface Refinement with Coated Abrasives
Can you give us finishing guidance and a reference of what abrasive belt and disc grits will achieve a given Ra finish?
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.
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