--- title: "Micrometers, height gauge and depth gauge" subject: "Metrology" chapter: 4 key: "semester_3/metrology:types-of-micrometers-and-errors" source: https://www.bctengineer.com/q/semester_3/metrology/types-of-micrometers-and-errors site: BCT Engineering --- # Micrometers, height gauge and depth gauge Metrology · Chapter 4: Linear Measurement - Asked in: year not given, 8 marks - Video: [L 11 Vernier Height, Depth Gauge and Micrometers | Mechanical Measurement and Metrology | Mechanical](https://www.youtube.com/watch?v=Z0kSxsY-llA) — MECHANICAL & AUTOMOBILE ENGINEERING_LJIET - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/metrology/types-of-micrometers-and-errors - Whole chapter: https://www.bctengineer.com/md/semester_3/metrology/chapter-4 ## Question Describe the construction and working of an external micrometer. Briefly describe the internal micrometer, depth micrometer, vernier height gauge and vernier depth gauge with one application each. List the common errors of a micrometer. ## Short answer External micrometer has a 0.5 mm pitch screw and 50-division thimble (LC 0.01 mm). Internal micrometer measures bores with extension rods; depth micrometer measures depths; vernier height gauge does layout work; vernier depth gauge measures hole depths. Common errors: zero error, wear, non-parallelism, pitch error, variable force, temperature, parallax. ## Answer #### External micrometer It works on the principle of a screw and nut. A precision screw (spindle) of pitch 0.5 mm turns inside a fixed nut in the barrel (sleeve). A thimble attached to the spindle is divided into 50 parts, so one division moves the spindle $0.5/50 = 0.01$ mm. ```text anvil spindle sleeve thimble ratchet |==| |=========|-----|=|||||||||=[R] | |___| | |__________frame__________| ``` Parts: U-shaped frame, fixed anvil, spindle, locking nut, sleeve with datum line and the main scale (0.5 mm marks), thimble, and a ratchet stop that gives constant measuring force (about 5 to 10 N) to avoid error due to excessive pressure. Reading: sleeve reading + thimble division on datum line $\times$ 0.01 mm. Used for outside diameters and thicknesses to 0.01 mm (vernier micrometers read 0.001 mm). #### Other instruments | Instrument | Principle / use | |---|---| | Internal micrometer | Same screw principle, with jaws or rods (extension rods) that touch the bore wall; measures hole diameters, e.g. cylinder bores | | Depth micrometer | A base (bridge) rests on the work face and a rod projects from the spindle; measures depth of slots, holes, steps | | Vernier height gauge | A vernier caliper mounted vertically on a heavy base with a scriber; marks and measures heights on a surface plate, used in layout work | | Vernier depth gauge | A beam with a vernier scale and a base; measures depth of holes and recesses with least count 0.02 mm | #### Common errors of a micrometer 1. Zero error (anvil and spindle in contact do not read zero). 2. Wear of anvil and spindle faces and of the screw thread; backlash. 3. Non-parallelism or flatness error of measuring faces. 4. Error in the pitch of the screw and thimble graduation. 5. Variable measuring force if the ratchet is not used; deflection of the frame. 6. Temperature effects, e.g. holding the frame in the hand, and parallax in reading. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. An external micrometer with pitch 0.5 mm and 50 thimble divisions has a least count of: A. 0.01 mm B. 0.02 mm C. 0.05 mm D. 0.001 mm Answer: A. Least count = pitch / thimble divisions = 0.5/50 = 0.01 mm. (Key point: Micrometer principle and least count) 2. What is the purpose of the ratchet stop on a micrometer? A. To lock the spindle at a reading B. To give constant measuring force and avoid error from excessive pressure C. To increase magnification D. To zero the thimble Answer: B. The ratchet slips at a set force (about 5 to 10 N), keeping the measuring force constant. (Key point: Purpose of ratchet) 3. A base resting on the work face with a rod projecting from the spindle describes which instrument? A. Internal micrometer B. Vernier height gauge C. Depth micrometer D. Vernier caliper Answer: C. The depth micrometer measures depth of slots, holes and steps using a base and projecting rod. (Key point: Depth micrometer use) 4. A vernier height gauge is best described as: A. A micrometer with extension rods B. A vernier caliper mounted vertically on a heavy base with a scriber, used for layout C. An optical comparator D. A bubble level on a base Answer: B. It is a vertical vernier scale on a base with a scriber for marking and measuring heights on a surface plate. (Key point: Vernier height gauge) 5. Which is a common micrometer error? A. Cosine error of the sine bar B. Wear of anvil/spindle faces, zero error and variable measuring force C. Error in wavelength of light D. Wringing film thickness Answer: B. Common errors include zero error, wear, non-parallel faces, pitch error, variable force, frame deflection, temperature and parallax. 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