--- title: "Measuring thread pitch, angle and diameters" subject: "Metrology" chapter: 12 key: "semester_3/metrology:toolmaker-microscope-thread-measurement" source: https://www.bctengineer.com/q/semester_3/metrology/toolmaker-microscope-thread-measurement site: BCT Engineering --- # Measuring thread pitch, angle and diameters Metrology · Chapter 12: Measurement of Screw Threads - Asked in: year not given, 5 marks - Video: [Thread Measurement Made Easy | Major Diameter, Minor Diameter & Microscope](https://www.youtube.com/watch?v=tZRyLBEJ6HI) — KN Prasad Academy (NITian) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/metrology/toolmaker-microscope-thread-measurement - Whole chapter: https://www.bctengineer.com/md/semester_3/metrology/chapter-12 ## Question Describe how the major diameter, minor diameter, pitch and thread angle of a screw thread are measured with a toolmaker's microscope. ## Short answer Major and minor diameters are measured by aligning the cross-wire on opposite crests or roots, taking micrometer head readings. Pitch is obtained by moving the table parallel to the axis until the same flank feature of the next thread aligns, reading the difference. Thread angle is measured by rotating the graduated eyepiece to align the cross-wire with each flank and noting the angle difference. ## Answer A **toolmaker's microscope** has a base with a table carrying the work, which can be moved in two perpendicular directions by micrometer heads (least count 0.01 mm or 0.005 mm), a microscope with a cross-hair eyepiece that rotates and has an angle scale, and a light source below the table that throws a sharp shadow of the thread profile. #### Major and minor diameters The thread is held on its axis on the table (between centres). The cross-wire is set on the edge of the crest on one side and the reading of the micrometer head is taken; the table is moved until the cross-wire touches the opposite crest, and the second reading is taken. The difference is the **major diameter**. The same is done at the roots to get the **minor diameter**. #### Pitch The cross-wire is set to a definite point (a flank) of a thread and the micrometer reading is noted. The table is moved parallel to the axis until the same point of the next thread (or after $n$ threads) comes in the cross-wire; the difference of readings is the pitch (or $n$ pitches, better for accuracy). Pitch error follows from comparison with the nominal pitch. The table must move parallel to the axis; the head is tilted by the helix angle to bring the flank into focus. #### Thread angle The eyepiece is turned until its cross-wire lies along one flank and the angle scale is read; then it is turned until it lies along the other flank and the scale is read again. The difference of the two readings gives the included angle (for a 60 degree thread, 60 degrees). The half-angles of each flank are measured separately to check symmetry. Limitations: the microscope only measures the projected profile, so errors arise if the helix angle is not compensated for, and the contrast depends on lighting. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is the major diameter measured with a toolmaker's microscope? A. Cross-wire is set on one crest, table moved to the opposite crest, and the micrometer readings are subtracted B. Thread angle readings are subtracted C. The table is moved along the axis by one pitch D. A wire is placed across the crests Answer: A. The difference of the two micrometer-head readings across the crests is the major diameter; the same at roots gives minor diameter. (Key point: Major diameter) 2. How is pitch measured? A. Table is moved across the axis between opposite flanks B. Table is moved parallel to the axis so the same point of the next thread (or n threads) reaches the cross-wire; difference of readings C. The eyepiece is rotated through the helix angle D. The shadow is compared with a gauge block Answer: B. Using n pitches improves accuracy; pitch error is found by comparing with nominal pitch. (Key point: Pitch) 3. How is the included thread angle measured? A. The table is moved along the axis between flanks B. A micrometer measures flank length C. The eyepiece cross-wire is aligned with one flank and then the other; the difference of the angle-scale readings D. Readings of the micrometer head are divided by pitch Answer: C. The rotating eyepiece with an angle scale gives the angle between flanks, e.g. 60 degrees; half-angles check symmetry. (Key point: Thread angle) ## 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 - [Effective diameter by three-wire method](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Athree-wire-method-effective-diameter) — Metrology, chapter 12 ## Related in other subjects - [Screw jack with square thread](https://www.bctengineer.com/question.md?key=semester_7%2Fmachine_design_ii%3Ascrew-jack-square-thread-numerical) — Machine Design II, chapter 8 - [Screw thread production and change gears](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Ascrew-thread-manufacturing-change-gears) — Manufacturing and Production Processes, chapter 11 - [Thread cutting on lathe and change gears](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Alathe-thread-cutting-change-gears) — Workshop Technology, chapter 6 - [Manometer reading in pipe with minor losses](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Amanometer-reading-pipe-minor-losses) — Hydraulics, chapter 2