Surface Finish Ra Callouts and Cost Impact

Core Principle

The smoother the required surface finish (lower Ra value), the more machining time and special processes are needed.

Cost by Surface Finish

• Ra 3.2 μm (125 μin): Standard as-machined finish - 1x Cost
• Ra 1.6 μm (63 μin): Slower finishing passes - ~1.5x Cost
• Ra 0.8 μm (32 μin): Secondary operations like grinding - ~3x Cost
• Ra 0.4 μm (16 μin): Precision grinding/honing - ~5x+ Cost

Application Guidelines

• Specify low Ra only on critical surfaces: seals, sliding contacts, bearing fits
• For non-contact surfaces (housings, brackets): Ra 3.2 μm is sufficient
• Consider functional requirements vs. cost impact