gear and screw measurement

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MODULE NO – 3

10- 12 Marks

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PREPARED BY,

Afaqahmed M J , AIKTC

Prof. Afaqahmed (c) Copyright

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1. SCREW THREAD MEASUREMENT


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Screw thread-definitionA 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 is a ridge wrapped around a cylinder

or cone in the form of a helix, with the former being

called a straight thread and the latter called a tapered

thread.

The helical groove opened to internal

and external surface


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Types of thread

1. External thread: A thread formed on

outside of a work piece is known as

external thread. Example: on bolts or

studs etc.

2. Internal thread: A thread formed on inside

of a work piece is known as internal

thread. Example: on a nut or female screw

gauge.


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TERMINOLOGY OF SCREW THREAD


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Screw Thread terminology

1. Pitch: The distance from a point on a screw

thread to a corresponding point on the next thread

measured parallel to the axis.

2. Lead :The distance a screw thread advances in

one turn. For a single start threads, lead = pitch,

For double start, lead = 2 x pitch, & so on.

3. Thread Form: The cross –section of thread cut

by a plane containing the axis.

4. Major Diameter: This is the diameter of an

imaginary cylinder, coaxial with the screw, which

just touches the crests of an external thread or

roots o f an internal threads. It is also called as

‘Nominal diameterProf. Afaqahmed (c) Copyright

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5. Minor diameter: This is the diameter of an

imaginary cylinder, co-axial with the screw which

just touches the roots of an external thread or the

crest of an internal thread. This is also referred to as

‘root’ or ‘core diameter’.

6. Effective diameter or Pitch diameter: It is the

diameter of an imaginary cylinder coaxial with the

axis of the thread and intersects the flanks of the

thread such that width of the threads & width of

spaces between threads are equal.

7. Flank: It is the Thread surface that connects crest

with root.

8. Depth of thread: It is the distance between crest

and root measured perpendicular to axis of screw.


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9. Angle of thread: Included angle between

sides of thread measured in axial plane.

10. Helix angle: Angle that the thread makes

with plane perpendicular to thread axis.

11. Flank angle: It is half the included angle of

the thread.

12. Addendum: It is the distance between the

crest and the pitch line measured

perpendicular to axis of the screw.

13. Dedendum: It is the distance between the

pitch line & the root measured perpendicular

to axis of the screw.


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Floating Carriage thread Measurement


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1. It consists of three main units. A base casting carries a pair of

centres, on which the threaded work-piece is mounted. Another

carriage is mounted on it and is exactly at 90° to it.

2. On this is provided another carriage capable of moving towards the

centres. On this carriage one head having a large thimble enabling

reading upto 0.002 mm is provided.

3. Thus the micrometer elements are exactly perpendicular to the axis

of the centres as the two carriages are located perpendicular to each

other.

4. On the fixed carriage the centres are supported in two brackets

fitted on either end. The distance between the two centres can be

adjusted depending upon the length of tie threaded job.

5. job is fitted between the centres the second carriage is adjusted in

correct position to take measurements and is located in position, The

third carriage is then moved till the Fiducial indicator is against the

set point.

Construction & Working


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6. The upper carriage is free to float on balls and enables micrometer

readings to be taken on a diameter without restraint.


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Effective Diameter Measurement1. Two Wire Method


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1. Place two wires / rods of identical diameter between the flanks ofthe thread

2. This method can only be carried out on diameter measuringmachine.

3. Axis of the micrometer is maintained @ 90 degree to the axis ofscrew thread.

4. Micrometer is attached with the wires which shows the readings5. Effective Diameter is calculated as

E = Effective Diameter

E = T + P

Where T = Dimension under the wires = M – 2dM = Dimension over the wiresd = Diameter of each wire

P = Value which depend on Dia & PitchP = 0.9605 p – 1.1657 d (Whitworth Thread)P = 0.866 p – d (Metric thread)


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Effective Diameter Measurement2. Three Wire Method


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Working

1. Most accurate method to measure effective diameter2. Three wires of known diameters are used : one on one side

& two on other side3. This method is precisely aligned with anvil of micrometer4. Wires are either held in hand or hung from a stand to get

adjusted themselves under micrometer pressure.

Calculations to find out effective Dia.1. Whitworth ThreadM = D + 3.1657 d – 1.6 pWhere D = Outside dia2. Metric threadM = D + 3d – 1.5155 p

4. By finding the value of M we can find out E by knowing d.


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VIDEO OF THREE WIRE METHOD


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GEAR MEASUREMENT

a1


Folie 18

a1 aafaq; 18.02.2016


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What is a gear ?

A gear is a wheel with teeth that mesh together with other gears.

Gears change the :

• Speed

• Torque (rot. force)

• Direction of rotating axles.


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Purpose :

� To transmit rotary motion and force.� Gears are used in groups of two or

more.� A group of gears is called a gear train.� The gears in a train are arranged so that

their teeth closely interlock or mesh.� The teeth on meshing gears are the same

size so that they are of equal strength.� The spacing of the teeth is the same on

each gear.Prof. Afaqahmed (c) Copyright

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TYPES OF GEARS

1.According to the position of axes of the shafts.

a. Parallel

1.Spur Gear

2.Helical Gear

3.Rack and Pinion

b. Intersecting

Bevel Gear

c. Non-intersecting and Non-parallel

worm and worm gearsProf. Afaqahmed (c) Copyright

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PARKINSON’S GEAR TESTER


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� This machine, developed by James Parkinson, is popularlyused for testing of spur gears, in pair.

� The principle of this device is to mount a standard gear on afixed vertical spindle and the gear to be tested on anothersimilar spindle mounted on a similar spindle mounted on asliding carriage, maintaining the gears in mesh by springpressure.

� Movements of the sliding carriage as the gears are rotatedand indicated by a dial indicator, and these variations are ameasure of any irregularities in the gear under test ,alternatively a recorder can be fitted ,in the form of a waxedcircular chart and records made of the gear variation inaccuracy of mesh.


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When the waxed paper recorder is fitted, the chart makes arevolution for each one of the gears mounted on the slidingcarriage. As the chart moves and rotates, the line tracedrecords the movements of floating carriage. A circle is drawnat the same time as the record as shown in figure.

METHOD OF MEASUREMENT


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LOGOTOOL MAKER’S MICROSCOPE


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� A ray of light from a light source fig. b is reflected by a

mirror through 90’ It then passes through a

transparent glass plate (on which flat parts may be

placed ).

� A shadow image of the outline or counter of the

workspaces passes through the objective of the optical

head and is projected by a system of three prisms to a

ground glass screen.

� Observations are made through an eyepiece.

Measurements are made by means of cross lines

engraved on the ground glass screen.

� The screen can be rotated through 360’; the angle of

rotation is read through an auxiliary eyepiece.

WORKING PRINCIPLE


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LOGOPROFILE PROJECTOR


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• The idea of using profile projector, which is created by James Hartness and

Russell W. Porter, came from mixing optics and measurement in a device.

• The use of the term comparator for metrological equipment, had existed in

other forms prior to Hartness's work; but they had remained in realms of

pure science (such as telescopy and microscopy) and highly specialized

applied science (such as comparing master measuring standards).


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(i) A single setting of the specimen provides observation, comparison andinspection of several dimensions and form characteristics in a projector.(ii) Several people can observe the projected image simultaneously. Thus,projectors are handy tools when images are to be inspected by a group ofpeople.(iii) The image can be magnified according to requirement. Thus, dimensions tobe inspected individually or their interrelation with other dimensions of thesame part can be observed without any additional instruments.(iv) Projector provides direct measurements of various lengths and angles.Lengths are measured by graduated rulers and angles by drafting protractors.(v) Precise comparison is possible in projectors. Standard comparator charts,specially for repetitive forms such as circular arcs with different radii, angles,thread forms, gear contours can be used on optical projector screen. Suchstandard charts are made of glass plates.(vi) There is no physical contact between the specimen and the measuringinstruments in projectors. Thus, specimen to be inspected is free frommechanical distortion or defects. This increases the accuracy in measurement.(vii) Unlike the mechanical gauges, which undergo wear and tear due to prolonguses, measurements by optical projectors are free from wear.(viii) Optically obstructed surface elements can be traced by means ofprojectors.

PROS


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Coordinate Measuring Machine


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Coordinate Measuring Machine (Schematic)


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A coordinate measuring machine (CMM) is a devicefor measuring the physical geometricalcharacteristics of an object. This machine may bemanually controlled by an operator or it may becomputer controlled. Measurements are defined bya probe attached to the third moving axis of thismachine. Probes may be mechanical, optical, laser,or white light, amongst others. A machine whichtakes readings in six degrees of freedom anddisplays these readings in mathematical form isknown as a CMM.

Definition : �


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INTRODUCTION

• Ogden developed an inspection machine in

1956,which is considered to be the first coordinate

measuring machine (CMM).

• A Coordinating measuring machine is an

electromechanical system design to perform

coordinate metrology.

• CMMs are also considered as transducers that can

convert physical measurements into electrical

signals.

• They are versatile in their capability to record

measurement of complex profiles with high

sensitivity (0.0025 mm) and speed.


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COMPONENTS OF CMM

To accomplish measurements in 3-D, a basic CMM is

composed of the following components:

• Probe head and probe to contact the workpart surfaces

• Mechanical structure that provides motion of the probe

in three Cartesian axes and displacement transducers to

measure the coordinate values of each axis

In addition, many CMMs have the following components:

• Drive system and control unit to move each of the three

axes

• Digital computer system with application software


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TYPES OF CMM

As per the mechanical structure the CMM are classified

into six types , which are as follows:

� CANTILEVER

� MOVING BRIDGE

� FIXED BRIDGE

� HORIZONTAL ARM

� GANTRY

� COLUMN


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MECHANICAL STRUCTURES OF CMM


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CMM WORKING

• After placing a workpiece on themachine table, a probe is used tomeasure different points on it bymapping the x, y, z coordinates.

• The probe operates either manuallyvia an operator or automatically viaa control system.

• These points are then uploaded to acomputer interface where they canbe analyzed using modelingsoftware (e.g. CAD) and regressionalgorithms for further development.


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PROS OF CMM

� Dimensional measurement

� Profile measurement

� Angularity or orientation

� Depth mapping

� Digitizing or imaging

� Shaft measurement


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gear and screw measurement

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