Poster making on major and minor diameter os screw thread

1. Major And Minor Diameters Of Screw Thread
Guided by : Asst. Prof. Brijesh Surati
B.E. MECHANICAL : Shree s’ad vidya mandal institute of technology
B) Bench micrometer:-
 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.
 Then the cylinder is replaced by the threaded work piece and the
new reading is taken.
Introduction:-
 A SCREW THREAD is a ridge of uniform section in
the form of a helix on the external or internal surface
of a cylinder.
 Major diameter:
Diameter of an imaginary co-axial cylinder which
would touch the crests of external or internal thread.
 Minor diameter (Root diameter or Core
diameter):
Diameter of an imaginary co-axial cylinder which
would touch the roots of an external thread.
Method of measuring for external screw thread
1) Major diameter
i. Ordinary micrometer
ii. Bench micrometer
2) Minor diameter
Method of Measuring for Internal screw thread
1) Major diameter
2) Minor diameter
i. Using taper parallels
ii. Using Rollers
1)Major diameter:-
A) Ordinary micrometer:-
For most application a good hand micrometer is quit suitable
for measurement of the major diameter of external thread.
However extreme care is necessary to ensure that only light
presser is applied because the anvils of the micrometer make
contact only at the points of the screw.
Method of measuring for external screw
thread:-
2) Minor diameter:-
 The minor diameter is measured by a comparative method by
using floating carriage diameter measuring machine and small V
pieces which make contact with the root of the thread.
 These V pieces are made in several sizes, having suitable radii at
the edges. V pieces are made of hardened steel.
 The floating carriage diameter-measuring machine is a bench
micrometer mounted on a carriage.
Method of Measuring for Internal screw thread
1) Major diameter:-
 The major diameter of an internal thread is normally measured by
some form of horizontal comparator fitted with ball ended styli of
radius less than the root radius of the thread to be measured.
 One of the styli is attached to a floating head which is kept in contact
with the spindle or dial indicator.
 The major diameter D can be calculated by Knowing the value of x
as,
D= 𝒙 𝟐 −
𝒑
𝟐
𝟐
;where p is pitch of the screw thread.
2) Minor diameter:-
A) Using taper parallels
 For diameters less than
200mm the use of Taper parallels
and micrometer is very common.
 The taper parallels are pairs
of wedges having reduced and
parallel outer edges.
 The diameter across their
outer edges can be changed by
sliding them over each other.
B) Using Rollers:-
 In this method ,precision rollers are
inserted inside the thread and proper
slip gauges inserted the roller as
shown in fig.
 The minor diameter is then the
length of slip gauges plus twice the
diameter of roller.
Dm =X+2d
Where, Dm = minor diameter
X= slip gauges
distance
d= roller diameter.
Conclusion:-
After learning this method one can measured major and minor diameter
of external and internal screw thread.
Reference:-
 BOOKS INDIA PUBLICATION of Mechanical Measurement
and Metrology by J.P. HADIYA & H.G.KATARIYA.

Prepared by:-
Samarth Sojitra
150450119161