unit 3 form measurement

7/28/2019 Unit 3 Form Measurement

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Form Measurement

Unit 3


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Syllabus

Measurement of screw threads-Thread

gauges, floating carriage micrometer-

measurement of gears-tooth thickness-

constant chord and base tangent method-

Gleason gear testing machine - radius

measurements-surface finish, straightness,

flatness and roundness measurements


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Screw thread measurement

Gear measurement

Radius measurement Surface Finish measurement

Straightness measurement

Flatness and roundness measurements.


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Screw thread


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Terminology

Crest

It is top surface joining the two sides of

thread.

Flank

Surface between crest and root.

RootThe bottom of the groove between the two

flanks of the thread


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Contd.,

PitchThe distance measured parallel to the axis from a

point on a thread to the corresponding next point.

Helix angle:

The helix is the angle made by the helix of the

thread at the pitch line with the axis.

Flank angle

Angle made by the flank of a thread with the

perpendicular to the thread axis


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Contd.,

Depth of thread

The distance between the crest and root of the

thread

Major diameter:

Diameter of an imaginary co-axial cylinder

which would touch the crests of external or internal

thread.Minor diameter (Root diameter or Core diameter):

Diameter of an imaginary co-axial cylinder

which would touch the roots of an external thread.


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Contd.,

Addendum Radial distance between the major and pitch

cylinders For external thread.

Radial distance between the minor and pitchcylinder For internal thread.

Dedendum

Radial distance between the pitch and minor

cylinder = For external thread.

Radial distance between the major and pitch

cylinders = For internal thread


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Errors in Screw Thread

Major diameter error

reduction in the flank contact

interference with the matching threads

Minor diameter error

Interference

reduction of flank contact


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Contd.,

Effective diameter error

Pitch error

the total length of thread engaged will be either

too high or too small

Flank angles error

Crest and root error


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Pitch Errors Classification

Progressive error

Periodic error

Drunken error

Irregular error


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Errors in Thread

1) Progressive error:The pitch of the thread is uniform but is longer or shorter

its nominal value and this is called progressive error.


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Contd.,

2. Periodic error

These error repeats itself at regular intervals

along the thread.

Causes of periodic error:

Teeth error in gears.

Lead screw error.Eccentric mounting of the gears.


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Contd.,

Length of thread

Cumulative

pitcherror

Length of thread

Cumulativepitcherror

ERRATIC ERRORSPROGRESSIVE ERROR

Fig (c) Fig (d)


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Contd.,

3) Drunken error:

Drunken errors are repeated once per turn of the thread ina Drunken thread.

In Drunken thread the pitch measured parallel to the thread

axis. If the thread is not cut to the true helix the drunken

thread error will form


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Contd.,

4) Irregular errors:

It is vary irregular manner along the length of

the thread.

Irregular error causes:

1. Machine fault.

2. Non-uniformity in the material.

3. Cutting action is not correct.4. Machining disturbances


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MEASUREMENT OF VARIOUS

ELEMENTS OF THREAD1) Major diameter

2) Minor diameter

3) Effective or Pitch diameter

4) Pitch5) Thread angle and form angle


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Major Diameter

Ordinary micrometer

Bench micrometer


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Measurement by Bench

micrometer:Clamp

FiducialIndicator

MeasuringAnvils Holding centres

Micrometer head

Supports

BENCH MICROMETER


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Measurement by Bench micrometer:

Then the cylinder is replaced by the threaded work piece and the newreading is taken


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Holding centre

Measuring anvil

Holding centre

Measuring anvil

StandardCy

lind

er

Screw

Thread

Measurement of Major diameter


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Measurement of the major diameter of an

Internal thread:

The major diameter is usually measured by thread

comparator fitted with ball-ended styli.

First the Instrument is set for a cylindrical reference

having the same diameter of major diameter of internal

thread and the reading is taken.

Then the floating head is retracted to engage the tips ofthe styli at the root of spring under pressure.

For that the new reading is taken,


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Measurement of the major diameter of an

Internal thread:


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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 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-measuringmachine is a bench micrometer mounted on a

carriage.


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The threaded work piece is mounted between the centers 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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of Internal threads:

The Minor diameter of Internal threads

are measured by1. Using taper parallels

2. Using Rollers.


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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.


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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

One wire

Two Wires

Three wires method


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One wire method


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Two wire method

In this method wires of suitable size are

placed between the standard and the

micrometer anvils


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Three Wire Method


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Pitch measurement

Pitch measuring machine

Tool makers microscope

Screw pitch gauge


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Pitch measuring machine


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Tool makers microscope


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Thread form and flank angle measurement

Thread gauge

Ring gauge

Optical projectors


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Gear Measurement

Types of gears

Spur gear

Spiral gear

Helical gears

Bevel gears

Worm and Worm wheel

Rack and Pinion:


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Gear tooth Terminology


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Tooth profile

It is the shape of any side of gear tooth in its

cross section Pitch circle

It is the imaginary circle of gear that rolls without

slipping over the circle of its mating gear.

Addendum circle

The circle coincides with the crests (or) tops of

teeth.

Dedendum circle (or) Root circle This circle coincides with the roots (or) bottom on

teeth


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Module(m)

It is the ratio of pitch circle diameter to the

total number of teeth Face

Part of the tooth in the axial plane lying

between tip circle and pitch circle. Flank

Part of the tooth lying between pitch circle and

root circle.

Top land

Top surface of a tooth


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Gear Errors

Eccentricity: -

Half the radial run out

Wobble

Run out measured parallel to. the axis of

rotation at a specified distance from the axis

Radial run out

Run out measured along a perpendicular to

the axis of rotation.


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Gear Errors

Eccentricity: -

Half the radial run out

Wobble

Run out measured parallel to. the axis of

rotation at a specified distance from the axis

Radial run out

Run out measured along a perpendicular to

the axis of rotation.


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Gear Errors

Axial run out

Run out measured parallel to the axis of

rotation at a speed

Periodic error

Error occurring at regular intervals


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GEAR MEASUREMENT

1. Run out

2. Pitch

3. Profile

4. Back lash

5. Tooth thickness

6. Alignment


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Run out

In the testing the gears are placed in the

mandrel and the dial indicator of the tester

possesses special tip depending upon the

module of the gear and the tips insertedbetween the tooth spaces and the gears

are rotated tooth by tooth and the variation

is noted from the dial! indicator


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Pitch measurement

Point to point measurement (i.e. One tooth

point to next toot point)

Direct angular measurement


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Tooth to Tooth measurement


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Direct Angular Measurement

Measuring the error by using set dial gaugeagainst a tooth in this method the position of a

suitable point on a tooth is measured after the

gear has been indexed by a suitable angle

If the gear is not indexed through the angular

pitch the reading differs from the original

reading.

The difference between these is the cumulativepitch error


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Profile checking

Optical projection method

Involute measuring machine


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Involute measuring machine


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Backlash

Backlash is the distance through which a

gear can be rotated to bring its nonworking

flank in contact with the teeth of mating

gear The determination of backlash is, first one

of the two gears of the pair is locked, while

other is rotated forward and backward andby the comparator the maximum

displacement its measured


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Tooth thickness measurement

Gear tooth vernier caliper method

(Chordal thickness method)

Base tangent method


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Gear tooth vernier caliper


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Base tangent method

unit 3 form measurement

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