Gandhinagar Institute of Technology

Mechanical Measurement and Metrology

Mechanical Engineering

4th : B - 2

Prepared By :Bhautik Malviya (130120119094)

Pavan Narkhede (130120119111)

Darshit Panchal (130120119114) Guided By:

Prof. Manish Meghani

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.

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.

2] Internal thread : These thread are cut into the

surface of cylindrical hole of bar or cone.

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.

2] Left handed thread : A left handed thread is one

in which the nut would be screwed on by turning it

to the left .

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.

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

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

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.

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) Pitch5) Thread angle and form

The instruments which are used to find the major

diameter are by

Bench micrometer

Bench micrometer

Clamp

FiducialIndicator

MeasuringAnvils Holding centres

Micrometer head

Supports

BENCH MICROMETER

The ordinary micrometer is quite suitable for measuring theexternal major diameter.

It is first adjusted for appropriate cylindrical size (S) havingthe same diameter (approximately).This process is known as ‘gauge setting’ .

After taking this reading ‘ R the micrometer is set on themajor diameter of the thread, and the new reading is ‘R2

For getting the greater accuracy the bench micrometer is used for measuringthe major diameter.

In this process the variation in measuring Pressure, pitch errors are beingneglected.

The fiducial indicator is used to ensure all the measurements are made atsame pressure.

The instrument has a micrometer head with a vernier scale to read theaccuracy of 0.002mm. Calibrated setting cylinder having the same diameter asthe 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 andthe reading is taken

The Minor diameter of Internal threads are measured by

1. Using taper parallels

2. Using Rollers.

Measurement of Minor diameter of

Internal threads: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.

Measurement of Minor diameter of

Internal threads:Using rollers:

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.

Measurement of screw thread angle

(Flank angle)

Measurement of effective

diameter

Effective diameter measurement is carried out by following methods.

1.One wire method2. two wires method3. three wires method.4. Micrometer method.

The effective diameter can not be measureddirectly but can be calculated from themeasurements made.

Wires of exactly known diameters are chosen suchthat they contact the flanks at their straightportions.

If the size of the wire is such it contacts the flanksat the pitch line, it is called the ‘best size’ of wirewhich can be determined by geometry of screwthread.

The screw thread is mounted between the centers& wires are placed in the grooves and reading M istaken.

M

M-Dimension over the wire

P

AP=OP-OA

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.

Three Wire method

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.

Three Wire method

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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

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

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

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

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

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