Chapter No. 05

Screw Thread Measurement

Introduction

• Screw threads are mostly used in all the

components / assemblies / machine tools.

• Function of screw thread is to transmit

power and motion and acting as a

fasteners.

• It is of prime importance to know various

terminology related to screw thread and

their measured process.

Types of Screw Threads

• With worth

• Square

• Acme

Terminology of Screw Threads 1) Pitch - It is the distance measured parallel to

the axis between two consecutive points on adjacent threads.

2) Lead – It is the distance advanced by the thread axially in one revolution.

Lead = Pitch (for single start thread)

Lead = No. of start x Pitch (multistate)

3) Flank Angle – The angle between individual flank and a line perpendicular to the axis of thread, measured in axial plane is called as flank angle.

4) Thread Angle (α) – Angle between two

adjacent flank measured in axial plane is called

as thread angle.

5) Helix Angle – The angle made by helix of a

thread with axis of thread is called as helix

angle.

6) Major Diameter (Dmaj) – It is a diameter of

imaginary cylinder co-axial with screw which

makes a continues helical contact at the crests

of the screw or roots of the nut.

7) Minor Diameter (Dminor) - It is a diameter of

imaginary cylinder co-axial with screw which

makes a continues helical contact at the root of

the screw or crests of the nut.

8) Effective Diameter – Diameter of an

imaginary co-axial cylinder which intersects

the flanks of the threads such that the width of

the thread and width of the space between

threads are equal.

Errors in Threads • In the case of screw threads there are at least five

important elements which require consideration and

error in any one of these can cause rejection of the

thread.

• These five elements are major diameter, minor

diameter, effective diameter, pitch and angle of the

thread.

• Errors on the major and minor diameters will cause

interference with the mating thread. Due to errors in

these elements, the root section and wall thickness will

be less, also the flank contact will be reduced and

ultimately the component will be weak in strength.

• Errors on the effective diameter will also

result in weakening of the assembly due

to interference between the flanks.

• Similarly pitch and angle errors are also

not desirable as they cause a progressive

tightening and interference on assembly.

• These two errors have a special

significance as they can be precisely

related to the effective diameter.

Types of Errors

1) Drunken Thread.

• This is the one having erratic pitch, in which

the advance of the helix is irregular in one

complete revolution of the thread.

• Thread drunkenness is a particular case of a

periodic pitch error recurring at intervals of

one pitch.

• In such a thread, the pitch measured parallel to

the thread axis will always be correct, the only

error being that the thread is not cut to a true

helix.

• If the screw thread be regarded as an inclined

plane wound around a cylinder and if the thread

be unwound from the cylinder, (i.e.

development of the thread be taken) then the

drunkenness can be visualized.

• The helix will be a curve in the case of drunken

thread and not a straight line as shown in Fig.

• It is very difficult to determine such errors and

moreover they do not have any great effect on

the working unless the thread is of very large

size.

2) Progressive Pitch Error.

• This error occurs when the tool work velocity

ratio is incorrect though it may be constant.

• It can also be caused due to pitch errors in the

lead screw of the lathe or other generating

machine.

• The other possibility is by using an incorrect

gear or an approximate gear train between

work and lead screw e.g., while metric threads

are cut with an inch pitch lead screw and a

translatory gear is not available.

• A graph between the cumulative pitch error and the length of thread is generally a straight line in case of progressive pitch error

3) Periodic Pitch Error.

• This repeats itself at regular intervals along the

thread.

• In this case, successive portions of the thread

are either longer or shorter than the mean. This

type of error occurs when the tool work velocity

ratio is not constant.

• This type of error also results when a thread is

cut from a lead screw which lacks squareness in

the abutment causing the lead screw to move

backward and forward once in each revolution.

• Thus the errors due to these cases are cyclic and pitch

increases to a maximum, then reduces through normal

value to a minimum and so on. The graph between

the cumulative pitch error and length of threads for

this error will, therefore, be of sinusoidal form.

4) Irregular Errors.

• These arise from disturbances in the

machining set-up, variations in the

cutting properties of material etc.

• Thus they have no specific causes and

correspondingly no specific

characteristics also.

Floating Carriage Micrometer

Principle

• It works on principle of Nut and bolt/screw

threads.

• As drum of micrometer rotates by one revolution,

it will move forward by one pitch of internal

threads.

• The movement will be measured using number of

divisions on drum and main scale i.e. Micrometer

principle.

• It is used to measure major diameter, minor

diameter and effective diameter of screw threads.

Construction

• Fig. shows floating carriage or screw thread

measuring machine or bench micrometer.

• It consists of two centers maintained on the pillars

of base.

• The centers are used for fixing the job.

• The floating body (with zero friction) is kept on the

base, which has two pillars on it.

• One pillar consists of a micrometer drum having

least count of 0.0002 mm.

• The other pillar consists of fuducial indicator

which senses the pressure applied on the anvil end.

Reading a Floating Carriage Micrometer

• Floating carriage has three scales i.e. main scale,

circular scale and Vernier scale.

• LC of floating carriage can be calculated by

following process.

Sr.

No.

Scale Smallest

division on

main scale

Number of

division on

next scale

Least count

1 Main scale 0.5 mm ------ 0.5 mm

2 Circular scale 0.5 mm 250 (0.5/250) =

0.002 mm

3 Vernier Scale 0.002 mm 10 (0.002/10) =

0.0002 mm

• Thus least count of floating carriage is 0.0002

mm.

• To take reading,

• TR = MSR + (CSD x LC) + (VSC x LC)

Measurement of Major Diameter

A) Using Simple Micrometer –

Major diameter of screw thread can be measured

by using simple micrometer.

It is most easy and simple method.

It’s least count is 0.01 mm.

By using simple micrometer, it is not possible to

measure up to accuracy of below 0.01 mm

By using simple micrometer, chances of extra

pressure on the job may be there, which

generates errors.

B) Using Floating Carriage Micrometer –

A calibrated master piece is used to avoid errors

at the time of measurement.

1) Note the standard reading of master piece as D1.

2) Measure dimensions of master piece with the

help of floating carriage micrometer as R1.

3) Replace the master piece with screw whose

dimension is to be measured.

4) Measure the dimension of thread of screw as R2.

5) By using following formula, calculate the major

diameter of screw thread.

measuredpiecemasterofadingR

threadscrewofadingR

readingpiecemasterdardSDWhere

RRDDmajor

Re

Re

tan,

)(

1

2

1

121

Measurement of Minor Diameter • Vee piece anvils are to be used for measurement of

minor diameter of screw thread.

• Angle of vee should be less than the angle of

thread to be checked.

1) Note the standard reading of master piece as D1.

2) Measure dimensions of master piece after placing

vee as R1.

3) Replace the master piece with screw whose

dimension is to be measured.

4) Measure the dimension of thread of screw as R2.

5) By using following formula, calculate the minor

diameter of screw thread.

measuredpiecemasterofadingR

threadscrewofadingR

readingpiecemasterdardSDWhere

RRDD or

Re

Re

tan,

)(

1

2

1

121min

Measurement of Pitch

A) Using Profile Projector –

The procedure for major diameter, minor

diameter and pitch measurement is nearly same.

The measurement can be carried out using two

micrometers which are used for measurement of

horizontal and vertical movement of image of

given job on screen.

1) Adjust 0-0 of main scale and Vernier scale of

angle scale on screen.

2) Get the image of thread on screen and adjust the

same at 1-1 as shown in fig. and note the

reading.

3) Change the position of image so that 2-2 should

be the second position of axis and note the

second reading on micrometer.

4) Difference between first and second readings of

micrometer is the required answer i.e. pitch of

screw.

B) Using Screw Pitch Gauge –

Construction of screw pitch gauge is same as

feeler gauge, only difference is that at the end of

the strips V-grooves are given.

It is used to quickly determine the pitch of the

various threads by matching the V-grooves on the

leaves with the V-grooves on the work.

Peter Screw Measuring machine

• The Pitter screw measuring machine employs

various stylus points to suit screw threads that

are to be checked.

• The screw under measurement is held

stationary between centers on the machine.

• The indicator unit, carrying the stylus are

moving on a slides with ball bearings.

• The sliding takes place using micrometer by

which reading can be taken.

• The stylus which is mounted on a leaf spring,

falls in and out of each thread ; the pointer of the

indicator reads zero (it is adjusted to read zero in

the first groove) when this stylus is in a central

position in each successive thread.

• The micrometer reading is taken each time the

indicator reads zero.

• The instrument is more accurate for the

measurement of pitch.

Measurement of Angle of Screw Thread

A) Using Profile Projector –

Adjust 0-0 to main scale and Vernier scale.

Adjust the image of the thread so that one flank

coincide the vertical axis.

Rotate the screen scale assembly so that the

vertical axis should coincide with another flank

of the same thread.

Note reading of main scale and Vernier scale (for

angle measurement) and get the required answer.

Measurement of Effective Diameter

• It is like a simple micrometer, but ends of anvils

are of Vee shape.

• Care to be taken about flanks of thread should

come in contact of vees.

• The value should be equal to (for With worth

thread)

• Pitch diameter = D – 0.6403p

• Where 0.6403p = depth of thread

• D = Outside diameter of thread

• p = pitch of thread

• For matching helix angle, for every thread new

micrometer is to be used and initial reading of

micrometer is to be zero.

• But thread micrometer shows variation in

drunken thread can be best noted point.

Best size of wire

• Best size of wire is that size which makes contact

with the thread flank.

sec2

sec4

2

2

)&4

(sec4

seccos)90sin(

)90sin(

sin,

PPd

rdwireofsizeBest

rOBP

ABP

r

ABABAB

OB

OB

AB

OB

ABBOAAOBIn

Measurement of Effective Diameter

• Effective diameter can be measured by using

floating carriage micrometer which is more

accurate.

• For measurement of effective diameter following

methods can be used

a) Two wire method

b) Three wire method

c) Direct micrometer method.

Two Wire Method

• Two wire method consists of use of two

identical best size of wires.

• Fig. shows the measurement of thread with

two wire method.

• The wires are to be inserted in such way that,

they should be inserted in the same thread and

the flank surfaces are tangent to the wire

• The dimension of thread + wire is indicated by

‘R’.

• Various terms are defined as

• Diameter over wire = R

• Diameter under wire = T

• Pitch value = P. It is the difference between

effective diameter and diameter under wire.

• From fig. it is clear that, effective diameter (E) is

addition of diameter under wire and pitch value.

diameterwiredWhere

dRTPdR

PTE

}2{)2(

Three Wire Method of Thread Measurement

• Three wire method of thread measurement

Pitch Value

• Pitch value is defined while measurement of

effective diameter of screw thread.

• It is the difference of effective diameter and

diameter under wire.

• Pitch value depends upon type of thread, pitch of

thread and diameter of wire.

• For example – for metric threads

• P = 0.86602 x (pitch) – (diameter of wire) P = 0.86602 x (pitch) – (diameter of wire)

Profile Projector

• Construction –

• It consists of a light source which incident light

on the mirror to create the profile of the object.

• Condensers are used to reduce intensity of light

for avoiding fatigue to the operator.

• Table is made up of glass on which job can be

mounted.

• Magnifying lenses are used to magnify the image.

• The magnification up to 10X, 20X, 50X, 100X,

200X, 500X, 1000X can be possible.

• Two micrometers are used to measure travelling

of table in X and Y direction with least count of

0.01 mm.

• The screen is made up of greenish colour glass

which shows the magnified image.

• The screen consists of two perpendicular axis

used for reference and a circular scale with

markings of angles in degrees.

• Vernier scale can be used to increase the accuracy

of measurement of angle.

• Working –

• When the light rays incident light on mirror ‘M1’

through condenser and lens, it will be reflected in

upward direction and passes through the table

made up of glass.

• The profile of the object is created at this point.

• This will be magnified by using magnification

lens.

• The magnifying image would be reflected on

mirror ‘M2’ then to mirror ‘M3’ and finally

visible on screen.

Pitch Measurement by Profile projector

Measurement of Major diameter by profile

projector

Measurement of Major diameter by profile

projector

Angle Measurement by profile projector

Advantages of Profile projector

• No wear and tear.

• Magnifying devices can be used hence accuracy

is more.

• Magnification of small components can be done

effectively.

•End Of This Unit