---
title: "Effects of surface finish and its factors"
subject: "Metrology"
chapter: 10
key: "semester_3/metrology:surface-finish-effects-and-factors"
source: https://www.bctengineer.com/q/semester_3/metrology/surface-finish-effects-and-factors
site: BCT Engineering
---

# Effects of surface finish and its factors

Metrology · Chapter 10: Measurement of Surface Finish

- Asked in: year not given, 6 marks
- Video: [Factors Affecting Surface Roughness in Surface Finish Metrology](https://www.youtube.com/watch?v=9LDBZGrMDb4) — Horizon Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/metrology/surface-finish-effects-and-factors
- Whole chapter: https://www.bctengineer.com/md/semester_3/metrology/chapter-10

## Question

Explain the effect of surface finish on fatigue life, wear, bearing properties, corrosion and fit. List the factors that affect surface finish.

## 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.

## Answer

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
1. Cutting conditions: speed, feed (finish worsens as feed rises), depth of cut.
2. Tool geometry: nose radius, rake and clearance angles, sharpness and wear of the tool.
3. Tool and work material; formation of built-up edge.
4. Cutting fluid (lubrication and cooling).
5. Rigidity of machine, tool and work; vibration and chatter.
6. Machining process used: grinding and lapping give better finish than turning or milling.
7. Machine accuracy (spindle runout, slideway errors) and uniformity of the work material.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does a rough surface affect fatigue life?
   A. Peaks strengthen the surface and raise the endurance limit
   B. Valleys act as stress raisers where cracks start, so fatigue strength falls
   C. It has no effect since fatigue depends only on material
   D. Roughness relieves residual stress and extends life
   Answer: B. Tool-mark valleys act as notches; a fine finish raises the endurance limit. (Key point: Fatigue)

2. What is the best surface for wear and lubrication according to the answer?
   A. The roughest possible surface
   B. The smoothest possible mirror finish
   C. A controlled medium finish, since very rough peaks shear off and very smooth surfaces do not hold oil
   D. Any finish since wear is independent of roughness
   Answer: C. Peaks break in contact causing high initial wear, but a too-smooth surface cannot hold an oil film. (Key point: Wear)

3. How does roughness affect an interference (press) fit?
   A. Peaks increase the effective interference
   B. The fit becomes a clearance fit instantly
   C. Roughness only affects running fits
   D. Peaks flatten during assembly, so the effective interference is smaller
   Answer: D. Flattening of the peaks reduces the effective interference; in a running fit wear of peaks increases clearance. (Key point: Effect on fit)

4. How does feed rate affect the surface finish in machining?
   A. Finish worsens as feed increases
   B. Finish improves as feed increases
   C. Feed has no effect; only speed matters
   D. Finish is the same but tool life changes
   Answer: A. Cutting conditions are listed as a factor: finish worsens as feed rises. (Key point: Feed and finish)

## Related in Metrology

- [Thread errors and effect of pitch error](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Ascrew-thread-errors-and-pitch-effect) — Metrology, chapter 12

## Related in other subjects

- [Causes of deterioration; effects of steel corrosion](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acauses-deterioration-corrosion-effects) — Concrete Technology and Masonry Structure, chapter 6, asked 2×
- [Fatigue effect in concrete](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Afatigue-effect) — Concrete Technology and Masonry Structure, chapter 4
- [Tyre construction, types and factors affecting life](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Atyre-construction-bias-radial-life) — Automotive Technology, chapter 8
- [Bearing life, load rating and equivalent load](https://www.bctengineer.com/question.md?key=semester_6%2Fmachine_design_i%3Abearing-life-load-rating) — Machine Design I, chapter 6