The document discusses measurement and metrology of screw threads. It begins with definitions of screw thread terminology such as major diameter, minor diameter, pitch, angle, and forms of threads. It then describes methods for measuring the major diameter, minor diameter, effective diameter, and pitch of screw threads. The key measurement methods discussed are using micrometers, pitch gauges, and a tool maker's microscope. The goal is to understand principles and techniques for measuring characteristics of screw threads.

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

11. 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.

12. 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.

13. 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.

14. 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.

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

16. Measurement of Major Diameter
1. 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 ± (Rs ~ R ) (D= Dia. of the master)

17. Measurement of Major Diameter
2. Bench micrometer

18. 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.

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

20. Measurement of Minor Diameter
1. Floating carriage micrometer
• M = D ± (Rs ~ R ) (D= Dia. of the master)

21. Measurement of Minor Diameter
1. 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.

22. 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.

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

24. 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 reading is recorded.
• Difference between reading over the standard and the screw thread is taken as a
measurement of Effective diameter.
Pitch line

25. 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.

26. 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;

27. 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

28. Best Wire Size
P
Half of thread angle =
𝜃
2
From triangle OAP, Sin POA =
𝐴𝑃
𝑂𝑃
or
Sin (90 -
𝜃
2
) =
𝐴𝑃
𝑂𝑃
Wire diameter = 2r
Best Wire diameter = Db = 2r = 2AP sec
𝜃
2

29. Best Wire Size
P
Since AP lies on the pitch line
AP =
𝑃
4
Best Wire diameter = Db =
𝑃
2
sec
𝜃
2

30. 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.

31. 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

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

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

34. Measurement of Pitch
Screw Pitch Gauge

35. • 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

36. Measurement of Pitch
Pitch Measuring Machine

37. 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.

38. 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.

39. Tool Maker’s Microscope
Principle

40. Tool Maker’s Microscope
Principle

41. 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

42. End of Module