Screw thread measurements

Department of Mechanical Engineering
National Institute of Technology Sikkim
Screw Thread
and
Gear Measurement

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.

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.

Screw Thread - Use
Screw threads are used:
• Tohold parts together-act as fastners (ex: V-threads)
• Totransmit motion &power (Square, Acme threads)

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

 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’

 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.

 Angle of thread: Included angle between sides of thread measured
in axial plane.
 Helix angle: Angle that the thread makeswith 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.

Measurement of Various Elements Of Thread
Tofind 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

Measurement of Major Diameter
Bench micrometer
The instruments
which are used to
find the major
diameter.

Ordinary micrometer
The ordinary micrometer is quite suitable for measuring the
external major diameter.
It is first adjusted for appropriate cylindrical size (S) having
the samediameter (approximately).This process is known as‘ gauge
setting’ .
After taking this reading ‘ Rthe micrometer isset on the major
diameterof the thread,andthe newreadingis‘R2

Measurement by Bench Micrometer:
Clamp
Fiducial
Indicator
M easuring
Anvils Holding centres
Micrometer head
Supports
BENCH MICROMETER

Measurement by 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

Measurement by Bench Micrometer
Then the cylinder is replaced by the threaded work piece and the
new reading is taken

Holding
centre
Measuring
anvil
Holding
centre
Measuring
anvil
Standard
Cylinder
Screw
Thread
Measurement of Major Diameter

Measurement of the Major Diameter of an Internal Thread
An indirect approach of measuring internal dia is obtained by
obtaining the cast of the Thread. The main art thus lies in obtaining a
perfect cast.

Measurement of the Major Diameter of an Internal Thread

Measurement of Minor Diameter
The minor diameter is measured by a comparative method by using
floating carriage diameter measuring machine and small ‘ V pieces which
makecontactwith 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.

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

Measurement of Minor Diameter of Internal Threads
The Minor diameter of Internal threads are measured by
1. Using taper parallels
2. Using Rollers.
1. 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.

For more than 200mm diameter this
method is used. Precision rollers are
inserted inside the thread and proper
slip gauge is inserted between the rollers.
The minor diameter is then the length of
slip gauges plus twice the diameter of
roller.
2. Using rollers:

Pitch Measurement
The most commonly used methods for measuring the
pitch are
1. Pitch measuring machine
2. Tool makers microscope
3. Screw pitch gauge

Hollow base
Lamp
Collimator lens
Base
Column
Eye piece
Optical head
Mirror
work table
with
carriage
Tool Makers Microscope

1.Worktable is placed on the base of the instrument.
2.The optical head is mounted on a vertical column it can be moved up and down.
3.Work piece is mounted on a glass plate.
4.A light source provides horizontal beam of light which is reflected from a mirror
by 90 degree upwards towards the table.
5.Image of the outline of contour of the work piece passes through the objective of
the optical head.
6.The image is projected by a system of three prisms to a ground glass screen.
7.The measurements are made by means of cross lines engraved on the ground
glass screen.
8.The screen can be rotated through 360°.
9.Different types of graduated screens and eyepieces are used.
Tool Makers Microscope

Pitch Measuring Machine
When the pointer is accurately placed in position, the micrometer reading is noted. The
stylus is then moved along into the next thread space, by rotation of the micrometer, and a
second readingtaken.
The difference between the two readings is the pitch of the thread. Readings are taken in
this manner until the whole length of the screw thread has been covered.
Spring loaded head permits the
stylus to move up the flank of the
thread and down into the next space
as it is moved along.
Accurate positioning of the stylus
between the two flanks is obtained
by ensuring that the pointer T is
always opposite to its index mark
when readings are taken.

Measurement of screw thread angle (Flank angle)

Measurement of effective diameter
Effective diameter measurement is carried out by following
methods.
1. Two wires method
2. Three wires method.
3. Micrometer method.

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.

Two Wire Method
M
M-Dimension over the wire

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.

 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.
Three Wire method

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

2 2
2 2
4
H  DEcot
 
P
cot
 and CD 
H

P
cot

2 2 2 2 4 2
Further h  (AD CD)  
d
cosec


 
P
cot



 2 2  4 2
Distance over the wires, M  E  2h  2r
the triangl e ABD , AD  AB cosec
 
d
cosec

2 2 2
From

2


Or M  E d  cosec
 
P
cot

1

 
2

2


   P 
 
P
cot
   2r  E 2r1 cosec  cot
i.e. M  E 2rcosec

2
Three Wire method

2 2
M  D  3.1657d 1.605P where Dis the major diameter of the thread.
For Metric threads, Depth of thread  0.6495P
E  D  0.6495P,  60o
,cosec

 2,cot

1.732
2 2
M  D  3d 1.5155P
We can measure the valueof M practically & then
compare with the theoretical valuesusing formulae derived above.
After finding the correct value of M, as d is known, E can be found out.
E  D -0.64P, cosec

 2.1657, and cot

1.921
For Whitworth thread,   55o
,depth of thread  0.64P
Three Wire method

Best Wire Size



A
P/4
P
Pitch line
P/2
BEST SIZE OF WIRE
B

P
where P is the pitch of the thread.
4
2
ABsec .
2
cos
2
AB

sin90 -
2

The best size wire is the one which makes contact at the pitch line
or effectivediameter of the screw thread.In other words, as shown in fig OB
is perpendicular to flank portion of the thread at the pitch line.
b
D  2 sec


P
sec

b

P
b
But OB  radius of wire 
1
 dia of bestsize wire (D )

i.e.D  2 OB  2ABsec .Alsosince AB lies on the pitch line, AB
2


 
 
OB  


AB  AB

In the triangle OAB, SinBOA  , orsin90 -  
OB 2 OB

AB
  

Best Wire Size

Screw thread measurements

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