- Practice · 6 marks
Effects of surface finish and its factors
Short answer
A rough surface reduces fatigue life by stress concentration, causes high initial wear, impairs bearing load distribution, promotes corrosion, and reduces effective interference in fits. Factors: cutting conditions (speed, feed, depth), tool geometry and wear, workpiece material, cutting fluid, rigidity, machining process (grinding vs turning), and machine accuracy.
Explain the effect of surface finish on fatigue life, wear, bearing properties, corrosion and fit. List the factors that affect surface finish.
Surface finish is the smoothness of the machined surface. Every machined surface has peaks and valleys; the valleys act as notches and the peaks carry the load.
| Property | Effect of a rough surface |
|---|---|
| Fatigue life | Valleys act as stress raisers; cracks start at the roots of the tool marks, so fatigue strength falls. A fine finish (ground or polished) raises endurance limit |
| Wear | Peaks are sheared and broken in contact (high initial wear); a very smooth surface also does not hold oil. The best is a controlled medium finish |
| Bearing properties | Peaks carry the load on a small real area, giving high local pressure and quick wear of the bearing and journal; oil film breaks. Smooth surfaces give better load-carrying area and lubrication |
| Stress | High stress concentration at the valleys (notch effect), residual tensile stress may be left on a rough machined surface |
| Corrosion | Valleys retain moisture and corrosive agents; larger surface area makes corrosion faster. Smooth surface is more resistant |
| Fit | In a press fit, the peaks are flattened during assembly so the effective interference is smaller; in a running fit, wear of the peaks increases clearance and the fit loosens |
Factors affecting surface finish
- Cutting conditions: speed, feed (finish worsens as feed rises), depth of cut.
- Tool geometry: nose radius, rake and clearance angles, sharpness and wear of the tool.
- Tool and work material; formation of built-up edge.
- Cutting fluid (lubrication and cooling).
- Rigidity of machine, tool and work; vibration and chatter.
- Machining process used: grinding and lapping give better finish than turning or milling.
- Machine accuracy (spindle runout, slideway errors) and uniformity of the work material.
✓ Answer checked by Claude Opus 5.5 (high effort)
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