gear and screw measurement
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MODULE NO – 3
10- 12 Marks
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PREPARED BY,
Afaqahmed M J , AIKTC
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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
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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
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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
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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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