This document discusses screw threads, including their classification, measurement, and sources of errors. It defines a screw thread as a helical ridge on the external or internal surface of a cylinder or cone. Threads are classified by their location (external or internal) and direction of rotation (right-handed or left-handed). Methods to measure the major diameter, effective diameter, and pitch are described using instruments like micrometers and wires. Sources of errors in threads include issues with the major diameter, minor diameter, pitch, and form that can lead to interference or lack of contact between threads.

1. Mechanical Measurement And Metrology
ENGINEERING COLLEGE. TUWA
Screw Thread
 Sr.no Name
En.No
 1. Shaikh Mohsin R.
140550119091
 2. Patel Rohan G.
150553119023

2. 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.

4. Classification of thread
 1) According to the surface on which the threads
are cut
1] External thread : These thread are cut on
the surface of a cylindrical bar.

5.  2] Internal thread : These thread are cut into the
surface of cylindrical hole of bar or cone.

6. 2)According to the direction of rotation of the
threaded cylinder with respect to engagement with the
other part :
1] Right handed thread : A right handed thread
is one in which the nut must be turned in a
right handed direction to screw it on.

7.  2] Left handed thread : A left handed thread is one
in which the nut would be screwed on by turning it
to the left .

8.  Single start threaded : A start threaded screw one
in which there is a movement of one thread for
one complete turn round the screw or bolt.
 Pitch and lead are same in this case.

9. Multi start thread :
There is a movement of more than one thread .
For double start thread lead is twice the pitch.
For triple start thread lead is three times the pitch.
Lead =pitch x Number of start

11.  1] Vee threads :
 British Association (B.A.) threads are exclusively used
for small screw like in clocks etc. thread is rounded V-
form.
 British standard whitworth (B.S.W.) threads are used in
general machine construction.
 2] transmission threads :
 Square threads are most commonly used for
transmission of power as in vices, clamps and in jacks

12.  Acme thread is stronger than square thread and
easy to make it . It is used in lead screw of lathe
machine for transmission of power.
 Buttress thread resist heavy axial loads, loads
acting in the direction of the axis Example of uses
of buttress thread occur in quick acting vices and
in breech mechanism of guns.

13. To find out the accuracy of a screw thread it will
be necessary to measure the following:
1) Major diameter.
2) Minor diameter.
3) Effective or Pitch diameter.
4) Pitch
5) Thread angle and form

14. The instruments which are used to find the major
diameter are by
 Bench micrometer
Bench micrometer

15. For getting the greater accuracy the bench micrometer is used for measuring
the major diameter.
In this process the variation in measuring Pressure, pitch errors are being
neglected.
 The fiducial indicator is used to ensure all the measurements are made at
same pressure.
The instrument has a micrometer head with a vernier scale to read the
accuracy of 0.002mm. Calibrated setting cylinder having the same diameter as
the major diameter of the thread to be measured is used as setting standard.
After setting the standard, the setting cylinder is held between the anvils and
the reading is taken

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

17.  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

18. M
M-Dimension over the wire

19. 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.

20. Three Wire method

21. 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.

22. Errors in thread:
 A rise during the manufacturing or storage of threads
 Error may cause due to the following elements in thread
1.Major diameter error : Cause reduction in flank contact
and interference with the matching threads
2.Minor diameter error : Cause interference, reduction of
flank contact
3.Effective diameter error : If it is small, the threads will be
thin on the external screw and thick on the internal screw

23. 4.Pitch error : Defined as total length of thread engaged is
either too high or too small
Types:
I .Progressive error
Ii .Periodic error
Iii .Drunken error
Iv .Irregular error

24.  I . Progressive error:
 Pitch of the thread is uniform but it is
longer or shorter to its nominal value
 Causes
 Incorrect linear and angular velocity ratio
 Incorrect gear train and lead screw
 Saddle fault
 Variation in length due to hardening

25.  ii. Periodic error:
 Repeated itself at regular intervals
along the thread
 causes
 Non uniform tool work velocity
ratio
 Teeth error in gears
 Lead screw error
 Eccentric mounting of the gears

26.  Iii . Drunken error:
 Repeated once per turn of the thread
in drunken thread
 In drunken thread, pitch is measured
parallel to thread axis
 If the thread is not cut to true helix,
drunken thread error will be formed

27. Iv . Irregular error : Varies in irregular manner along the
length of thread
 Causes :
 Machine fault
 Non uniformity in the material
 Cutting action is not correct
 Machining disturbances