Unit1 Screw Thread

J3103/1/1 SCREW THREAD

General Objective: To understand the methods of testing andmeasuring elements of ISO and BSW screwthreads.

Specific Objectives: At the end of the unit you will be able to :

Ø Identify the methods of measuring major diameter,minor diameter and mean diameter.

Ø Measure and calculate major diameter, minor

diameter and mean diameter of a screw thread.

Ø To check the thread form by using the opticalcomparator.

UNIT 1 SCREW THREAD

OBJECTIVES

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1.0 INTRODUCTION

1.1 ELEMENTS OF A THREAD

To understand and calculate the thread elements, the followingdefinition relating to screw threads should be known (Fig. 1.1).

rootpitch

All elements of the thread influencethe strength and interchange abilityof screw thread, but the pitch, angleand effective diameter are muchmore important than the otherelements

Figure 1.1 Screw thread terminologythread angle

maj

or d

iam

eter

min

or d

iam

eter

mea

n di

amet

er

INPUT

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1.1.1. Major Diameter

It is the largest diameter of the thread. This is the distance betweenthe crests of the thread measured perpendicular to the thread axis.

1.1.2. Pitch/Mean Diameter

The diameter of the thread used to establish the relationship, or fit,

between an internal and external thread. The pitch diameter is thedistance between the pitch points measured perpendicular to the threadaxis. The pitch points are the points on the thread where the thread ridgeand the space between the threads are of the same width.

1.1.3. Minor Diameter

It is the smallest diameter of the thread. This is the distance

between the roots of the thread measured perpendicular to the threadaxis.

1.1.4. Thread Angle

This is the included angle of the thread form.

1.1.5. Pitch

It is the distance between the same points on adjacent threads. Thisis also the linear distance the thread will travel in one revolution.

1.1.6. Root

The surface of the thread that joins the flanks of adjacent threads.

The distance between the roots on opposite sides of the thread is calledthe root, or minor diameter.

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1.2. MEASURING THE MAJOR DIAMETER

To measure major diameter of the screw, a micrometer, with anvils of a

diameter sufficient to span two threads, may be used,( Fig. 1.2). To eliminatethe effect of errors in the micrometer screw and measuring faces, it is advisablefirst to check the instrument to a cylindrical standard of about the samediameter as the screw. For such purposes a plug gauge or a set of ‘Hoffman’rollers is useful.

anvil

1.3. MEASURING THE MINOR/CORE DIAMETER

The diameter over the roots of a thread may be checked by means of aspecial micrometer adapted with a shaped anvils, (Fig. 1.3) or a micrometer maybe used in conjunction with a pair of vee pieces ( steel prisms ). The secondmethod is recommended ( Fig.1.5). The steel prisms on the micrometer arepressed into the thread groove. The ends of the prisms are slightly curved andparallel to the root thread. It is important , when making the test, to ensure

that the micrometer is positioned at right angles to the axis of the screw beingmeasured, and when a large amount of such work is to be done, a special‘floating bench micrometer’ ( Fig. 1.4 ) is used. It is because, it supports thescrew and incorporates the micrometer elements correctly located, as well asproviding means for suspending the vee prisms.

Figure 1.2 Checking the major diameter with a micrometer

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Fig. 1.4. A Floating Micrometer

Fig. 1.3 Checking the core diameter of a thread with anshaped anvil micrometer

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The prism values are stated as,Dm = W – 2T

Note:Dm - mean diameterW - distance between two prism T - prism height (known)

Tprism

W

1.4. MEASURING THE MEAN/PITCH/EFFECTIVE DIAMETER

The three-wire method is recognized as one of the best methods of checking

the pitch diameter because the results are least affected by any error which maybe present in the included thread angle. For threads which require an accuracyof 0.001 in. or 0.02 mm, a micrometer can be used to measure the distance overthe wires. For threads requiring greater accuracy an electronic comparatorshould be used to measure the distance over the wires.

In the three-wire method, three wires of equal diameter are placed in the

thread; two on one side and one on the other side (Fig. 1.6). The wires usedshould be hardened and lapped to three times the accuracy of the thread to beinspected. A standard micrometer may then be used to measure the distanceover the wires. For greatest accuracy, the best size wire should be used.

Figure 1.5 Checking minor diameter by using a micrometer and prisms

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Figure 1.6 Three wire method

The hard round bars (wire) with the same size are positioned opposite tothe screw thread groove shown in the diagram above. The distance is measuredbetween the outside of the round bars. The most suitable wire size is 0.57735p.

In Fig. 1.7 P is the pitch of the screw thread. The suitable wire size is quite hardto get, usually a size bigger than 0.57735p wire size will be used.

Fig. 1.7. Conditions when measuring with wires

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1.4.1. Best Size Wires.

Wires which touch the thread at the pitch diameter are known as"Best Size" Wires. Such wires are used because the measurements of

pitch diameter are least affected by errors that may be present in theangle of the thread.

The above analysis for the distance over wires holds good providedthe wire touches the thread somewhere on its right side, and provided thethread angle is correct. The extremes of wire sizes which touch on thestraight sides and which can be measured are shown at (a) and (c),

Fig.1.9. For ISO metric, unified and Whitworth threads these limitingsizes are given in Table 1.1

Thread

Form

Max.

Wire

Min.

Wire

‘Best

Wire’

Size range for

Best wire

ISO metric andUnified

1.01p 0.505p 0.557p 0.534p0.620p

Whitworth 0.853p 0.506p 0.564p 0.535p0.593p

Table 1.1. Wire sizes for thread measurement ( p = pitch of thread)

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Note:W = Distance over wiresDE = Pitch/ Effective DiameterDw = Wire diametera = 600

From the Fig. 1.8, mean/pitch diameter can be calculated by applying thefollowing formula;

AD = AB cosec2a = r cosec

2a

H = DE cot2a =

2P cot

2a

CD = 0.5H =4P cot

2a

h = AD – CD = r cosec2a –

4P cot

2a

B

C

D

W

DE

P/2

r

A

h

E

60o

Pitch (P)

a

2a

H

Figure 1.8. Three-wire measurement

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and distance over wires (W)

= DE + 2h + 2r

= DE + 2 {r cosec2a –

4P cot

2a } + 2r

= DE + 2r cosec2a -

2Pcot

2a + 2r

= DE +2r ( 1 + cosec2a) –

2Pcot

2a

and, since 2r = d (the diameter of the wire),

W = DE + d ( 1 + cosec2a ) –

2Pcot

2a (1)

From this general formula we may apply the special adaptation for

common threads.

Figure 1.9. a) ISO metric and unified b) Whitworth

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(a) ISO metric and unified Fig. 1.9 (a)

The effective diameter lies 0.3248p inside the crest of the thread,

Hence DE = D – 0.6496p

a = 60° and cosec2a = 2

cot2a = 1.732

W (over wires) = DE + d (1 + cosec2a ) –

2Pcot

2a

=D – 0.6496p + d(3) –2P (1.732)

= D +3d- 1.5156p (2)

(b) Whitworth Fig. 1.9(b)

Depth of thread = 0.64p, so that DE = D – 0.64p

a = 55° and cosec2a = 2.1657 cot

2a = 1.921

Hence W ( over wires) = DE + d { 1 + cosec2a } -

2P cot

2a

= D -0.64p + d 3.1657) -2P (1.921)

= D + 3.165d - 1.6 p (3)

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1.5. OPTICAL COMPARATOR

An optical comparator or shadowgraph (Fig. 1.10a and 1.10b) projects anenlarge shadow onto a screen where it may be compared to lines or to a masterfrom which indicates the limits of the dimensions or the contour of the part beingchecked. The optical comparator is a fast, accurate means of measuring orcomparing the work piece with a master. It is often used when the work piece isdifficult to check by other method. Optical comparators are particularly suited

for checking extremely small or odd-shaped parts, which would be difficult toinspect without the use of expensive gauges.

Optical comparators are available in bench and floor models, which areidentical in principle and operation. Light from a lamp passes through acondenser lens and is projected against the work piece. The shadow caused bythe work piece is transmitted through a projecting lens system, which magnifies

the image and casts it onto a mirror. The image is then reflected to the viewingscreen and is further magnified in this process.

The extent of the image magnification depends on the lens used.Interchangeable lenses for optical comparators are available in the followingmagnifications: 5 x, 10 x, 31.25 x, 50 x, 62.5 x, 90 x, 100 x, and 125 x.

A comparator chart or master form mounted on the viewing screen is used

to compare the accuracy of the enlarged image of the work piece being inspected.Charts are usually made of translucent material, such as cellulose acetate orfrosted glass. Many different charts are available for special jobs, but the mostcommonly used are linear-measuring, radius, and angular charts. A vernierprotractor screen is also available for checking angles. Since charts areavailable in several magnifications, care must be taken to use the chart of thesame magnification as the lens mounted on the comparator.

Many accessories are available for the comparator, increasing theversatility of the machine. Some of the most common ones are tilting work

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centres, which permit the work piece to be tilted to the required helix angle forchecking threads; a micrometer work stage, with permit quick and accuratemeasuring of dimensions in both direction; and gauge blocks, measuring rods,and dial indicators used on comparators for checking measurement. The surfaceof the work piece may be checked by a surface illuminator, which lights up theface of work piece adjacent to the projecting lens system and permits this imageto be projected onto the screen.

1.5.1. To check the angle of a 60o thread using an optical comparator

1. Mount the correct lens in the comparator.2. Mount the tilting work centres on the micrometer cross-slide

stage.3. Set the tilting work centres to the helix angle of the thread.4. Set the work piece between centres.

5. Mount the vernier protractor chart and align it horizontallyon the screen.

6. Turn on the light switch.7. Focus the lens so that a clear image appears on the screen.8. Move the micrometer cross-slide stage until the thread image

is centralized on the screen.

9. Remove the vernier protractor chart to show a reading of 30o.10. Adjust the cross-slides until the image coincides with the

protector line.11. Check the other side of the thread in the same manner.

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Note: If the threaded angle is not correct or square with the centre line,adjust the vernier protractor chart to measure the angle of the threadimage. Other dimensions of the threads, and width of flats, may bemeasured with micrometer measuring stages or devices such as rods,

gauge blocks and indicators.

helix angle

Figure 1.10 (a). Checking a thread form on an optical comparator

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Figure 1.10 (b) Principle of the optical projector

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TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITHTHE NEXT INPUT…!

1.1. Draw and label a schematic drawing of how you would check thecore diameter of an external V-thread.

1.2. Using ‘best’ wire sizes determine the distance of the wire for M 20 x2.5 ISO metric thread.

1.3. Why is the three-wire method is one of the best method ofmeasuring the pitch diameter of a V thread?

1.4. With the aid of a labelled diagram, briefly explain how you woulduse an optical comparator to check the thread angle of 60o

ACTIVITY 1

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1.1

Tprism

W

Dm = W – 2T ; T = prism height (known)

1.2. 20 mm x 2.5 mm pitchBest wire diameter = 2.5 x 0.577

= 1.443 mm.

From formula W = D + 3d – 1.5156P

= 20 + 3 (1.443) – 1.5756 (2.5)

= 20.54 mm

1.3. The results are least affected by any error which may present in theincluded thread angle.

FEEDBACK ON ACTIVITY 1

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1.4. Using an optical comparator to check the thread angle of 60o.

To check the angle of a 60o thread using an optical comparator

1. Mount the correct lens in the comparator.2. Mount the tilting work centres on the micrometer cross-slide stage.3. Set the tilting work centres to the helix angle of the thread.4. Set the work piece between centres.

5. Mount the vernier protractor chart and align it horizontally on thescreen.

6. Turn on the light switch.7. Focus the lens so that a clear image appears on the screen.8. Move the micrometer cross-slide stage until the thread image is

centralized on the screen.

9. Remove the vernier protractor chart to show a reading of 30o.10. Adjust the cross-slides until the image coincides with the protector

line.11. Check the other side of the thread in the same manner.

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Note: If the threaded angle is not correct or square with the centre line,

adjust the vernier protractor chart to measure the angle of the threadimage. Other dimensions of the threads, and width of flats, may bemeasured with micrometer measuring stages or devices such as rods,gauge blocks and indicators.

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1. Calculate the effective diameter of M35 x 5.5 threads by using three wiremethod. The distance between wires is 35.60 mm.

Used formula E = M – 3d + 0.866P ; when d = 0.577P and P = pitch.Sketch the measurement setup.

2. Using the ‘best’ wire sizes, determine the distance over wires for (a)43 in

Whitworth, (b) M 20 x 2.5 ISO metric threads.

SELF-ASSESSMENT 1

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1. E = 23.263 mm

2. (a) 0.0564 in, (b) 1.4425 mm

FEEDBACK OF SELF-ASSESSMENT 1

Three wire method

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Education

Unit1 Screw Thread