--- title: "Straight edges and testing straightness" subject: "Metrology" chapter: 6 key: "semester_3/metrology:straight-edge-and-flatness-check" source: https://www.bctengineer.com/q/semester_3/metrology/straight-edge-and-flatness-check site: BCT Engineering --- # Straight edges and testing straightness Metrology · Chapter 6: Surface Measurement - Asked in: year not given, 5 marks - Video: [Measuring a Straightedge with a Precision level](https://www.youtube.com/watch?v=zx2sjNvNlM0) — oxtoolco - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/metrology/straight-edge-and-flatness-check - Whole chapter: https://www.bctengineer.com/md/semester_3/metrology/chapter-6 ## Question Describe the types of straight edges used in the workshop and explain how a straight edge is used to test the straightness and flatness of a surface. ## Short answer Types: toolmaker's bevel-edge, wide-edge, cast iron I-section, angle. Straightness tested by light-gap method (colored light for small gaps) or blueing. Flatness checked along multiple directions including diagonals. Straight edges verified by reversal or three-straight-edge method. ## Answer A **straight edge** is a long, narrow, rigid bar with at least one very accurately straight working edge. It is used as a reference for checking the straightness and flatness of surfaces, including machine slideways. #### Types 1. **Plain (toolmaker's) bevel-edge straight edge**: a steel bar with a bevelled working edge, up to about 300 to 500 mm long; the narrow edge touches the work along a line. 2. **Wide-edge straight edge**: a flat bar with a wide working face, used with a surface plate for checking larger surfaces by the light gap. 3. **Cast iron I-section / box-section straight edge**: for lengths of 1 to 3 m, ribbed to give high rigidity and low weight. It is supported at the **Bessel points** (about 0.22 L from each end) so that its deflection under its own weight is a minimum. 4. **Angle (square-edge) straight edge**: an L-section edge used for checking the straightness of inside corners and the squareness of two faces. #### Testing - **Light gap method:** The straight edge is held with its edge on the surface and viewed against a bright background. If the surface is straight, no light passes; even a gap of a few micrometres shows as a line of light. The size of the gap can be estimated from the colour of the light: very narrow gaps show coloured (bluish) light, while wider gaps show white light. - **Feeler or blueing method:** Prussian blue is smeared on the edge and moved over the surface; the high spots are marked and removed by scraping. A feeler gauge may be pushed into any gap to measure it. - **Flatness:** The test is repeated in several directions on the surface (along the sides and the diagonals). Straight edges themselves are checked for accuracy by reversal against a surface plate or by the three-straight-edge method. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Long cast iron straight edges are supported at the Bessel points, about 0.22 L from each end, to: A. Keep the edge parallel to the plate B. Minimise deflection due to their own weight C. Allow easy handling D. Increase weight Answer: B. Supporting at Bessel points gives minimum deflection from self-weight. (Key point: Bessel points) 2. In the light gap method, a straight edge held on the surface shows no light gap when the surface is: A. Convex B. Straight at the line of contact C. Rough D. Magnetic Answer: B. No gap means the surface is straight; a gap of about 0.002 mm or more shows as light, blue at about 0.002 mm and white at about 0.0075 mm. (Key point: Light gap method) 3. What does the Prussian blue method show? A. The gap size by colour B. High spots on the surface, which can be scraped C. The surface roughness value D. The angle of taper Answer: B. Blue smeared on the edge marks the high spots, which are then scraped away. (Key point: Blueing method) 4. To test flatness (not just straightness) with a straight edge, one should: A. Test in one direction only B. Repeat the test in several directions, along the sides and diagonals C. Use the edge only at the centre D. Rotate the surface plate Answer: B. Flatness is checked by repeating the test along sides and diagonals of the surface. (Key point: Flatness testing) ## Related in Metrology - [Optical flat and flatness interferometer](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Aflatness-interferometer-and-optical-flat) — Metrology, chapter 8 - [Fringe measurement of flatness error](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Ainterference-fringes-flatness-numerical) — Metrology, chapter 8 ## Related in other subjects - [Slab three edges continuous, one short edge discontinuous (5x4 m, 250 beam)](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aslab-three-edges-continuous-one-short) — Design of RCC Structure, chapter 9, asked 3× - [Testing of DC machines: retardation vs Swinburne's test](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Adc-testing-retardation-vs-swinburne) — Electrical Machines I, chapter 4 - [Camber elevations, straight and parabolic](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-65c-camber-320) — Transportation Engineering I, chapter 3 - [Slab continuous over three edges, 5x4 m](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aslab-continuous-over-three-edges-5x4) — Design of RCC Structure, chapter 9, asked 2×