L5 measurement of screw thread

Measurement of Screw thread parameters

1. Department of Mechanical Engineering JSS Academy of Technical Education, Bangalore-560060 MECHANICAL MEASUREMENTS AND METROLOGY (Course Code:18ME36B )

2. TEXT BOOKS Mechanical Measurements , Beckwith Marangoni and Lienhard , Pearson Education, 6th Ed., 2006 . Instrumentation, Measurement and Analysis , B C Nakra , K K Chaudhry, 4th Edition, McGraw Hill. Engineering Metrology , R.K. Jain, Khanna Publishers, Delhi, 2009 REFERENCE BOOKS: Engineering Metrology and Measurements , N.V.Raghavendra and L.Krishnamurthy , Oxford University Press.. Further Reference: National Programme on Technology Enhanced Learning ( NPTEL ) http://nptel.ac.in/courses/112104121/1

3. MECHANICAL MEASUREMENTS AND METROLOGY CHAPTER 5: Measurement of screw thread

4. Learning Objectives To understand basic principle of measurement of screw threads.

5. Explain the measurement principles of major diameter , minor diameter , effective diameter, pitch, angle, and form of screw threads. Outcomes

6. Module 2 Terminology of screw threads, Measurement of major diameter, Minor diameter, Pitch, Angle Effective diameter of screw threads by; 2-wire 3-wire methods. Best size wire. Screw thread gauges Tool maker’s microscope

7. SCREW THREAD TERMINOLOGY Definition (Screw thread) As per American Society of Tool and Manufacturing Engineers (ASTME) Screw thread is the helical ridge produced by forming a continuous helical groove of uniform section on the external or internal surface of a cylinder.

8. Forms of thread

9. SCREW THREAD TERMINOLOGY

10. SCREW THREAD TERMINOLOGY Forms of thread: Shape / contour of one complete thread , (in axial section). Example: BSW, American Standard, ISO, Sellers, Buttress, Acme, etc.) Axis of thread (pitch line): Imaginary line running longitudinally through the centre of the screw.

11. SCREW THREAD TERMINOLOGY Angle of thread: Angle between the flanks of a thread ( Also called as included angle.) Flank angle : Angle formed between a flank of the thread and the perpendicular to the axis of the thread. Fundamental triangle : Imaginary triangle formed when the flanks are extended, till they meet each other to form apex.

12. SCREW THREAD TERMINOLOGY Pitch: Distance between two corresponding crest or root. Lead : The axial distance moved by the screw, when the screw is given one complete revolution .

13. SCREW THREAD TERMINOLOGY Major (nominal) diameter: The largest diameter of a screw thread. Minor (core) diameter: The smallest diameter of a screw thread. Pitch diameter / Effective diameter:   The imaginary diameter of a cylinder, passing through the threads at the points where the thread width is equal to the space between the threads.

14. Measurement of Major Diameter Measurement of a major diameter by; Screw thread micrometer (Comparator) Bench micrometer

15. Measurement of Major Diameter Screw thread micrometer Micrometer is set over the master/standard cylinder having approx. dia (D) Micrometer is then set over the Major diameter (M) and reading is recorded (R) M = D ± (R s ~ R ) (D= Dia. of the master)

16. Measurement of Major Diameter 2. Bench micrometer

17. Measurement of Major Diameter 2. Bench micrometer Designed to remove the deficiencies of the hand micrometer. Fiducial micrometer ensures the readings are taken with uniform pressure. Used as a comparator. Procedure is same as of ordinary micrometer.

18. Measurement of Minor Diameter Measurement of a minor diameter by; Floating carriage micrometer

19. Measurement of Minor Diameter Floating carriage micrometer M = D ± (R s ~ R ) (D= Dia. of the master)

20. Measurement of Minor Diameter Floating carriage micrometer Minor diameter is measured by a comparative process . V-pieces are placed on each side of the screw. The small V-pieces makes contact at the root of the threads. Initial reading is taken by mounting a setting cylinder/ master. Then threaded workpiece is mounted between the centres and the reading is taken . The difference in the two readings directly gives the error in the minor diameter .

21. Measurement of Effective Diameter A notional value . Cannot be measured directly . An indirect approach to be followed i.e. (Measurement by wire method) Principle Small, hardened steel wires (best-size wire) are placed in the groove and the distance over them is measured as part of the measurement process.

22. Measurement of Effective Diameter There are three methods of using wires: One-wire method Two-wire method Three-wire methods.

23. Measurement of Effective Diameter Two-wire method In this method, two steel wires of identical diameter are placed on opposite flanks of a screw, as shown in Fig. First the micrometer reading is taken over the standard . The standard is replaced by the screw thread and the r eading is recorded. Difference between reading over the standard and the screw thread is taken as a measurement of Effective diameter. Pitch line

24. Measurement of Effective Diameter Two-wire method T = M − 2d M= Dimension over wires d = diameter of the best-size wire Effective diameter (E); E = T + P T = Dimension under the wires P = Correction factor P depends on diameter of wire, pitch and thread angle ‘ P’ value can be calculated from the following formula; P = 0.86602 X p (ISO & Unified) P = 0.96049 X p = 1.16568 x d (BSW) Where, p = Pitch d = mean diameter of wires used. P = Difference between effective dia and dia under the wires.

25. Two-wire method If the size of the wire contacts the flanks at the pitch line, it is called the best size wire, can be determined by the geometry of screw thread;

26. Best Wire Size If the size of the wire contacts the flanks at the pitch line, called the best size wire, determined by the geometry of screw thread. P

27. Best Wire Size P Half of thread angle =   From triangle OAP, Sin POA = or Sin (90 - ) =   Wire diameter = 2r Best Wire diameter = D b = 2r = 2AP sec  

28. Best Wire Size P Since AP lies on the pitch line AP =   Best Wire diameter = D b = sec  

29. Measurement of Effective Diameter Diameter of Best-size Wire The best size wire to be selected as recommended in IS 6311. ‘ P’ value can be calculated from the following formula; P = 0.86602 X p (ISO & Unified) P = 0.96049 X p =1.16568 x d (BSW) Where p = Pitch d = mean diameter of wires used.

30. Measurement of Effective Diameter Three-wire method M = Distance over the wires E = Effective diameter d = diameter of best-size wires H = height of threads

31. Measurement of Effective Diameter Three-wire method h = is the height of the centre of wire from the pitch line. h = OC − BC.

32. Measurement of Effective Diameter Three-wire method Distance over wires, M = E + 2 h + 2 r r = radius of the wires Therefore, effective diameter

33. Measurement of Pitch Screw Pitch Gauge

34. Used to measure the pitch or lead of a screw thread. Not a precision measurement, only for reference. To determine the profile of the given thread and quickly categorize the thread by form and pitch . Screw Pitch Gauge

35. Measurement of Pitch Pitch Measuring Machine

36. Measurement of Pitch Pitch Measuring Machine Position the stylus between the flanks of the thread along the pitch line. Ensure the fiducial indicator reads 0 . Record the initial reading . Move the stylus into the adjacent flank and record the reading. Difference between the two readings is taken as a measurement of pitch.

37. Tool Maker’s Microscope A versatile instrument that, applies the principle of optics. It is designed for measurements of; Complex forms / Profiles. Distance between the centres. Linear measurements & Angular measurements.

38. Tool Maker’s Microscope Principle

39. Tool Maker’s Microscope Principle

40. Tool Maker’s Microscope A light from a source (fig.) is reflected by a mirror through 90°. It passes through a transparent glass plate. A shadow or profile of the workpiece passes through the objective lens of the optical head It is projected by system of three prisms to a glass screen. Measurements are made by means of cross lines engraved on the ground glass screen. The screen can be rotated through 360. Principle

41. End of Module