Form Metrology: screw thread measurement

1. Screw Thread
Measurement

2. Screw thread- definition
a 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 or a cone.
A screw thread formed on a cylinder is known as straight or
parallel screw thread, while the one formed on a cone is
known as tapered threads.

3. Types of thread
External thread: a thread formed on outside of a
work piece is known as external thread. Example:
on bolts or studs etc.
Internal thread: a thread formed on inside of a
work piece is known as internal thread. Example:
on a nut or female screw gauge.

5. Screw thread - use
Screw threads are used:
• To hold parts together-act as fastners (ex: V-threads)
• To transmit motion & power (Square, Acme threads)

6. Screw Thread terminology
Pitch
Crest
Root
Flank
Thread
Angle
Pitch line
Axis of thread
Axial thickness
Addendum
Dedendum
Flank
angle
Major dia Pitch dia Minor dia
EXTERNAL THREAD TERMINOLOGY

7. Screw Thread terminology
⚫Pitch: The distance from a point on a screw thread to a
corresponding point on the next thread measured parallel to the
axis.
⚫Lead The distance a screw thread advances in one turn. For a
single start threads, lead=pitch,
For double start, lead=2xpitch, & so on.
⚫Thread Form: The cross section of thread cut by a plane
containing the axis.
⚫Major Diameter: This is the diameter of an imaginary cylinder, co-
axial with the screw, which just touches the crests of an external
thread or roots of an internal threads. It is also called as ‘Nominal
diameter’

8. Screw Thread Terminology
⚫Minor diameter: This is the diameter of an imaginary
cylinder, co-axial with the screw which just touches the roots
of an external thread or the crest of an internal thread. This
is also referred to as ‘root’ or ‘core diameter’.
⚫Effective diameter or Pitch diameter: It is the diameter of
an imaginary cylinder coaxial with the axis of the thread and
intersects the flanks of the thread such that width of the
threads & width of spaces between threads are equal.
⚫Flank: It is the Thread surface that connects crest with root.
Depth of thread: It is the distance between crest and root
measured perpendicular to axis of screw.

9. Screw Thread terminology
Angle of thread: Included angle between sides of thread
measured in axial plane.
Helix angle: Angle that the thread makes with plane
perpendicular to thread axis.
Flank angle: It is half the included angle of the thread.
Addendum: It is the distance between the crest and the
pitch line measured perpendicular to axis of the screw.
Dedendum: It is the distance between the pitch line &
the root measured perpendicular to axis of the screw.

10. Bench Micrometer

11. 1. Major diameter

12. 1. Major diameter

13. Measurement by Bench micrometer:

14. 2. Minor diameter

15. The threaded work piece is mounted between the centres of the
instrument and the V pieces are placed on each side of the work
piece and then the reading is noted.
After taking this reading the work piece is then replaced by a
standard reference cylindrical setting gauge.
Measurement of Minor
diameter

16. 3. Effective diameter –
using Thread Micrometer

17. Measurement of effective
diameter
Effective diameter measurement is carried
out by following methods.
1 two wires method
3. three wires method.
4. Micrometer method.

18. Two wire method:
⚫The effective diameter can not be measured directly but
can be calculated from the measurements made.
⚫Wires of exactly known diameters are chosen such that
they contact the flanks at their straight portions.
⚫If the size of the wire is such it contacts the flanks at the
pitch line, it is called the ‘best size’ of wire which can be
determined by geometry of screw thread.
⚫The screw thread is mounted between the centers &
wires are placed in the grooves and reading M is taken.
⚫Then the effective diameter E =T+P
where T =M-2d, & P is a value which depends on diameter
of wire, pitch & angle of the screw thread.

19. 3. Effective diameter –
using two Wire Method
E=T+P
Where, P is a
constant, which
depends upon the
diameter (d) of
wire and pitch (p)
of the thread.
P=0.866p - d

20. Two wire method:

21. Two wire method:

22. Three Wire method
The three-wire method is the accurate method.
 In this method three wires of equal and precise
diameter are placed in the groves at opposite sides of
the screw.
In this one wire on one side and two on the other side
are used. The wires either may held in hand or hung
from a stand.
 This method ensures the alignment of micrometer
anvil faces parallel to the thread axis.

23. Three Wire method

24. Three Wire method
⚫This method is more accurate than two wire method
as it ensures alignment of micrometer faces parallel
to the thread axis.
⚫Here, three wires of exactly known diameters are
used, one on one side & the two on the other side.
The wires may be held in hand or hung from a stand.
⚫From the fig, M=diameter over the wires
E= effective diameter (to be found)
d= diameter of wires, h=height of wire center above
the pitch line, r=radius of wire, H=depth of thread,
D=major diameter of the thread.

25. Three Wire method
E M
H
B C
D

A
h
E
M
Dia 'd'
P

E

26. Errors in Threads
1.Major diameter error – Cause interference
with the mating threads or reduction in the
flank contact or weakness by reduction of the
wall thickness.
2.Minor diameter error - Cause interference
with the mating threads or reduction in the
flank contact or weakness by reduction of the
cross section of the root.

27. Errors in Threads
3.Effective diameter error – Determine
the amount of slackness or
interference between the flanks of
mating threads.
4.Errors or Pitch and Thread angle –
Result in progressive tightening and
interference on assembly.

28. Pitch Errors
1. Progressive Error
2. Periodic Error
3. Drunken Error
4. Erratic or Irregular Error

32. Erratic Errors – Vary in an irregular manner along the
length of the thread. Their causes are difficult to
identify. Possible causes are:
1. Fault in the machine.
2. Irregular cutting action resulting from non-
uniformity in the material of the screw.
3. Disturbances in the machining setup.

33. Measurement of different elements
of Thread
1. Major diameter
2. Minor diameter
3. Effective diameter
4. Pitch

34. 4. Pitch

35. 4. Pitch – Tool Makers Microscope

36. 4. Pitch – Thread Pitch Gauge

37. Floating Carriage Dial Micrometer – Major,
Minor & Effective Diameters

38. Floating Carriage Dial Micrometer