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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

Screw thread- definition

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Types of Screw thread

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Types of Screw 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.

Multiple-start screw thread: forming two produces this or more helical grooves equally spaced and similarly formed in an axial section on a cylinder. This gives ‘quick traverse’ without sacrificing core length.

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Types of Screw thread

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Screw thread terminology

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Screw thread terminologyPitch: 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’

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Screw thread terminology

Angle of thread: Included angle between sides of thread measured in axial plane.

Helix angle: Angle that thread makes with 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.

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Screw threads are used;To hold parts together (ex: V-threads)To transmit motion & power (Square, Acme threads)

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Errors in screw threadsThere are six important elements in a thread, errors in any one

of which may lead to rejection.

They are Major diameter, Minor diameter, effective diameter, Pitch, Flank angle and the profile at root & crest.

Errors in screw threads may cause interference with mating threads or slackness due to improper flank contact.

Errors in pitch of screw thread may be classified into three types;Periodic errors Progressive errorsErratic errors

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Periodic Errors Periodic errors are those which vary in magnitude along the length of the thread and occurs at regular intervals as shown in fig (a). A ‘drunken thread’ is a particular case of periodic error where the error repeats once per turn.

For a true thread, if the thread is imagined to be unwound from the pitch cylinder, the helix will be a straight line. For a drunken thread, it will be a curve as shown in fig (b).

In a drunken thread, the advance of the helix is irregular in one complete revolution. This is due to thread being not cut to true helix.

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Periodic Errors

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Progressive pitch errorIf the pitch of the thread is uniform but is longer or shorter than its nominal value, then the error is called progressive as shown in fig (c). These errors may be caused by a change in length due to hardening, or by the errors in the pitch of the lead screw, or by the faults in the saddle guide ways.

Erratic errorsThese errors vary in irregular manner along the length of the thread as shown in fig (d). Their causes are difficult to identify. Possible sources are faults in the machine and irregular cutting action resulting from material non uniformity.

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Measurement of major diameter

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Measurement of major diameter

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Measurement of major diameterThe ordinary micrometer is quite suitable for measuring the external major diameter.

It is first adjusted for appropriate cylindrical size (S) having the same diameter (approximately).This process is known as ‘ gauge setting’ . After taking this reading ‘ R the micrometer is set on the major diameter of the thread, and the new reading is ‘R2

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Measurement of major diameter

Excessive pressure may lead to elastic deformation of screw threads leading to errors.

A bench micrometer may be used for greater accuracy which give direct readings of 0.0002 mm.

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Measurement of major diameter

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Measurement of internal thread -major diameter

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Measurement of Minor diameterThe 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.

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Measurement of Minor diameterThe 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.

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Measurement of Minor diameterThe principle of minor diameter is same as that of measuring major diameter except that v -shaped prisms are used.

Prisms of suitable sizes are placed between the standard cylinder and the instrument anvils in order to take a reading first micrometer reading R1.

The standard cylinder is then replaced by the screw thread and a second reading R2 is taken as shown in fig.Then the minor diameter )R~(RSD 212

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Measurement of Minor diameter

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Measurement of flank angle

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Measurement of Minor diameter of internal threads

The Minor diameter of Internal threads are measured by

Using taper parallelsUsing Rollers.

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Measurement of Minor diameter of internal threads

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.

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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.

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Measurement of effective diameter

Effective diameter measurement is carried out by following methods.

One wire, Two wires, or Three wires method.Thread Micrometer method.

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Measurement of effective diameter

Effective diameter measurement is carried out by following methodsTwo wires method

M

M-Dimension over the wire

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Two wires method

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Two wires method

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Two wires methodThen the effective diameter E =T+C

where T =M-2d, & C is a value which depends on diameter of wire, pitch & angle of the screw thread

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Measurement of effective diameter: Three wire method

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Measurement of effective diameter: Three wire method

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Measurement of effective diameter

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Best size wire

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Expression for Best size wire

2sec

2

P

2sec

4

P2D

thread. theofpitch theis P where4

PAB line,pitch on the lies AB since Also .

2secAB2OB2D i.e.

)(D wiresizebest of dia2

1 wireof radiusOBBut

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secAB

2cos

AB

2-90sin

ABOB

OB

AB

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OB

ABAOBSin OAB, triangleIn the

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OB figin shown as words,other In thread.screw theofdiameter effectiveor

linepitch at thecontact makes which one theis wiresizebest The

b

b

b

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Pitch Measurement

The most commonly used methods for measuring the pitch are

Pitch measuring machineTool makers microscopeScrew pitch gauge

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Pitch Measurement

The most commonly used methods for measuring the pitch arePitch measuring machine

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Pitch MeasurementThe most commonly used methods for measuring the pitch arePitch measuring machine

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Pitch MeasurementThe most commonly used methods for measuring the pitch arePitch measuring machine

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Pitch Measurement

Lamp

Hollow base

Collimator lens

Base

Column

Eye pieceOptical head

Mirror

work tablewith carriage

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Pitch Measurement

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GEARSGears are mainly used for transmission of motion & power and must be of accurate profile to obtain exact velocity ratio.

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GEARSBase circle: It is the circle from which gear teeth profiles are generated.

Pitch circle: It is an imaginary circle which by pure rolling action, would produce the same motion as the toothed wheel. The size of the gear is usually specified by the pitch circle diameter.

Pitch point: It is a common point of contact between two pitch circles of two meshing gear wheels.

Pressure angle: It is the angle between the common normal to two gear teeth at the point of contact and the common tangent at the pitch point.

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GEARS

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GEARSStandard Tooth Specifications

Standard pressure angles, 14.5o (old), 20o, and 25o

Two mating gears must have the same diametral pitch, P, and pressure angle, φ.

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GEARS

Generation of the involute curve

rG / rP = constant (constant speed ratio)

All common normal have to intersect at the same point P

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GEARSAddendum: It is the radial distance from the pitch circle to the tip of the tooth.Dedendum: It is the radial distance from the pitch circle to the root of the tooth.Face : It is the part of the tooth surface which is above the pitch surface.Flank : It is the part of the tooth surface which is below the pitch surface.Circular pitch : It is the distance measured on the circumference of the pitch circle from a point on one tooth to the corresponding point on the adjacent tooth. Module: It is the ratio of the pitch circle diameter in millimeters to the number of teeth.Face width: It is the width of the gear tooth measured parallel to its axis.

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GEARS

P = N / d

P = diametral pitch, teeth per inchN = number of teethd = pitch diameter (gear diameter)

m (module, mm) = d / N

Metric system

p (circular pitch) = πd / N Pp = π

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GEARSFollowing Elements are to be checked while carrying out gear measurement

Geometrical Parameters Tooth Thickness Addendum Depth Tooth Spacing Over ‘X’ Number of teethFunctional Parameters Pitch Variation Involute profile

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GEARSFollowing Elements are to be checked while carrying out gear measurement Functional Parameters while meshing Runout Backlash Contact Area Noise

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GEARS measurement elements Runout.Pitch Profile Lead Back lashTooth thicknessConcentricityAlignment

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RUNOUT TESTINGIt means eccentricity in the pitch circle.It will give periodic vibration during each revolution of the gear. This will give the tooth failure in gears.

The run out is measured by means of eccentricity testers.In this testing process, the gears are placed in the mandrel and

the dial indicator of the tester posses special tip depending upon the module of the gear and the tips inserted between the tooth spaces and the gears are rotated tooth by tooth and the variation is noted from the dial indicator.

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RUNOUT TESTING

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PITCH MEASUREMENT

There are two ways for measuring the pitch variation . Point to point measurement (i.e. One tooth point to next toot

point) Direct angular measurement

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PITCH MEASUREMENT- POINT TO POINT

The instrument has three tips.One is fixed measuring tip and the second is sensitive tip, whose position can be adjusted by a screw and the third tip is adjustable or guide stop.

The distance between the fixed and sensitive tip is equivalent to base pitch of the gear.

All the three tips are contact the tooth by setting the instrument and the reading on the dial indicator is the error in the base pitch.

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PITCH MEASUREMENT- POINT TO POINT

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BACKLASH MEASUREMENT Backlash is the distance through which a gear can be rotated

to bring its nonworking flank in contact with the teeth of mating gear.

Numerical values of backlash are measured at the tightest point of mesh on the pitch circle.

There are two types of backlash Circumferential backlash Normal backlash

.

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BACKLASH MEASUREMENT

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GEARSMeasurement of tooth thickness Measurement of tooth thickness by

Gear tooth vernier caliper.Constant chord method.Base tangent method.Measurement by dimension over pins

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GEARS

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GEARS

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GEARS

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GEARS TOOTH VERNIER CALIPER

Considering one tooth, the theoretical values of w & d can be found which may be verified by the instrument.The fig shows the chord ADB which is width w but tooth thickness is arc distance AEB.Also the depth d adjusted on the instrument is slightly greater than the addendum CE, & hence the width w is called chordal thickness & d is called chordal addendum.

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GEARS TOOTH VERNIER CALIPER

NN

Nmd

N

Nmm

Nmd

N

NmAlso

mNm

mRCE

NmNw

mNR

N

R

whereFrom

90cos

21

2

90cos

22

90cos

2RcosOD

2)(OEOCmodule, one toequal is Addendum

OD)-OC(d fig thefrom Also .90

sin

2

2

teethof No

dia circlePitch m Module

radius. circlepitch R where

4N

3602RsinAOsin22AD wADO, triangleIn the

teethofnumber theis N 4N

360 Also 2AD.AB wfig, the

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The Constant Chord MethodVernier method like the chordal thickness and chorale addendum are dependent upon the number of teeth.

Due to this for measuring large number of gears different calculations are to be made for each gear. So these difficulties are avoided by this constant chord method

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The Constant Chord MethodEfficient for measuring large no of gears, each having different no of teeth but same module

Property : If an involute tooth is considered symmetrically in close mesh with basic rack form , then it is observed that when gear rotates and all teethes come in mesh with rack , for the given size of tooth (same module), the contact is always occur at point two points

A & F as shown in fig. i.e the distance AF remains constant and known as constant chord.

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The Tangent Method

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The Tangent Method