Unit-4�FORM MEASUREMENT�

Need of form measurements - Measurement of Screw Thread - External Thread Measurement - Measurement of Minor Diameter- Measurement of Effective Diameter - Pitch Measurement. Measurement of Gears – Pitch Measurement, Profile Measurement, Tooth Thickness Measurement - Gear Alignment Testing. Radius Measurement – Radius of Circle - Radius of Concave Surface. Surface Finish Measurement – Analysis of Surface Finish - Methods of Measuring Surface Finish. Straightness Measurement - Flatness

Measurement - Roundness Measurement.

FORM MEASUREMENT

• Screw thread measurement
• Gear measurement
• Radius measurement
• Surface Finish measurement
• Straightness measurement
• Flatness and roundness measurements.

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Measurement of Screw Thread

• A screw thread is a helical structure used to convert rotational movement to linear movement and also used to convert rotational torque to linear force.
• 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 core.

• Major diameter or Nominal diameter (d): It is defined as the diameter of an imaginary cylinder that bounds the crest of an external thread or root of an internal thread. It is the largest diameter of the screw thread
• Minor diameter or core diameter or root diameter (dc): It is defined as the diameter of an imaginary cylinder that bounds the root of an external thread or crest of an internal thread. It is the smallest diameter of the screw thread.
• Pitch diameter or Effective diameter (dp) It is defined as an imaginary cylinder diameter, the surface of which would pass through the thread at such points as to make equal width of the thread and width of spaces between the threads.
• Pitch (p): It is an axial distance between two similar points on adjacent threads. It is measured in millimeters.
• Lead: It is defined as the axial distance which a screw thread advances in one rotation of the nut.

For double start threads, lead = 2 × pitch

For multi start threads, lead = n × pitch

Where n = number of starts of the thread

• Root : It is the bottom surface generated by the two adjacent flanks of the thread.
• Depth of thread: It is defined as the perpendicular distance between crest and root.
• Flank: The inclined surface, which joins the crest and root.
• Angle of thread: The included angle between two flank surfaces.
• Slope of the thread: It is defined as the half the pitch of the thread.
• Flank angle (θ): It is the angle between individual flank and the perpendicular to the axis of the thread which passes through the vertex.

• Helix angle: It is the angle made by the helix of the thread at the pitch line with plane perpendicular to the axis.
• Addendum: It is radial distance between the major dia & pitch dia.
• Deddendum: It is the radial distance between the minor dia & pitch dia.

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• Thread angle: The angle of the cross section which is standardized as 60 degree in V - threads but any angle can be used.

Classification of Threads

According to surface:

1.External threads 2. Internal threads

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According to direction or rotation:

1.Right hand thread 2. Left hand thread.

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According to number of starts:

1.Single start thread 2. Multi start thread

Types of Screw Thread

Measurement of screw thread

• It is necessary to measure following parameters of the screw thread to ensure the accuracy of the screw thread.
• Major diameter
• Minor diameter
• Effective diameter
• Pitch

Bench micrometer

• It is a device used to measure the major and minor diameters of the screw thread

Measurement of

major diameter

Measurement of Diameter

Measurement of

major diameter

Measurement of

Minor diameter

Measurement of effective diameter

• One wire method
• Two wire method
• Three wire method
• Thread micrometer

One wire method

Three wire method

Two wire method

Thread micrometer

Pitch measurement

• Tool makers microscope
• Pitch measuring machine
• Pitch gauge

• Worktable is placed on the base of the base of the instrument.
• The optical head is mounted on a vertical column it can be moved up and down.
• Work piece is mounted on a glass plate.
• A light source provides horizontal beam of light which is reflected from a mirror by 900 upwards towards the table.
• Image of the outline of contour of the work piece passes through the objective of the optical head.
• The image is projected by a system of three prisms to a ground glass screen.
• The measurements are made by means of cross lines engraved on the ground glass screen.
• The screen can be rotated through 360°.
• Different types of graduated screens and eyepieces are used.

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Profile Projector -Thread angle and flank measurement

• The optical projections are used to check the thread form and angles in the thread. The projectors equipped with work holding fixtures, lamp, and lenses.
• The light rays from the lens are directed into the cabinet and prisons and mirrors.
• The enlarged image of thread is drawn. The ideal and actual forms are compared for the measurement.

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Thread gauges

• Classification of thread gauges:

Plug screw gauges (used to measure Internal threads)

Ring screw gauges (used to measure external threads)

Caliper gauges (used to measure external threads)

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GEARS MEASUREMENT

• Positive drive (i.e without slipping) is required for some precision machines, and if the distance between the driver and follower is very small, the gears (or) toothed wheels are used.
• Gears are used for transmitting motion and power from one shaft to another with constant velocity ratio.

Gear terminology

• Tooth profile It is the shape of any side of gear tooth in its cross section.
• Base circle It is the circle of gear from which the involute profile is derived. Base circle diameter Pitch circle diameter x Cosine of pressure angle of gear
•  Pitch circle diameter (PCD) The diameter of a circle which will produce the same motion as the toothed gear wheel.
• Pitch circle It is the imaginary circle of gear that rolls without slipping over the circle of its matiug 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.
• Pressure angle (a) It is the angle making by the line of action with the common tangent to the pitch circles of mating gears.
• Module(m) It is the ratio of pitch circle diameter to the total number of teeth. Where, d = Pitch circle diameter. n = Number f teeth.
• Circular pitch It is the distance along the pitch circle between corresponding points of adjacent teeth.

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• Addendum Radial distance between tip circle and pitch circle. Addendum value = 1 module.
• Dedendum Radial distance between itch circle and root circle, Dedendum value = 1 .25module.
• Clearance (C) Amount of distance made by the tip of one gear with the root of mating gear. Clearance = Difference between Dedendum and addendum values.
• Blank diameter: The diameter of the blank from which gear is out. Blank diameter = PCD + 2m
• 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.
• Lead angle The angle between the tangent to the helix and plane perpendicular to the axis of cylinder.
• Backlash: The difference between the tooth thickness and the space into which it meshes. If we assume the tooth thickness as t and width ‘ t then. Backlash = t2 – t1

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ERRORS IN SPUR GEAR

• Gear Blank Run-out Errors
• Gear Tooth Profile Errors
• Gear Tooth Errors
• Pitch Errors
• Runout Error of Gear Teeth
• Lead Error
• Composite Error
• Assembly Errors

• Profile error: - The maximum distance of any point on the tooth profile form to the design profile.
• Pitch error: - Difference between actual and design pitch
• Cyclic error: - Error occurs in each revolution of gear
• Run out: - Total range of reading of a fixed indicator with the contact points applied to a surface rotated, without axial movement, about a fixed axis.
• 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.
• Undulation: - Periodical departure of the actual tooth surface from the design surface.
• 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 Teeth Pitch Measurement

• Circular pitch is defined as the distance from a point on one tooth to the corresponding point on the next tooth, measured around the pitch circle.

It can be measured by

• Tooth to tooth pitch measurement
• Direct angular measurement

Tooth to tooth pitch measurement

Direct angular measurement

Profile measurement

• Optical projection technique
• Involute measuring machine
• Tooth displacement method
• Computer-controlled probe scanning method

Optical projection technique

Involute measuring machine

Computer-controlled probe scanning method

Tooth displacement method

Tooth thickness measurement

• Gear tooth vernier caliper method
• Base tangent method
• Constant chord method
• Rolling gear test method

Gear tooth vernier caliper method

Constant chord method

• Constant chord is defined as the chord joining two points on opposite faces of the tooth.
• The constant chord method avoids the difficulties occuring in tooth vernier method in which the chordal thickness and chordal addendum depend upon the number of teeth.

Base tangent method

• The base tangent method measures the span of any number of teeth between the two parallel planes, which are tangential to the opposite tooth flanks.
• The span length is a tangent to the base circle. This distance is known as base tangent length.

Rolling Gear Test

• The variations of centre distance can be measured when the gear under test is made to rotate with a master gear with high quality.
• Characteristics during rotation and test for the general accuracy of gear by checking its composite error. This test is generally known as rolling gear test.

Single contact (or) fixed centre distance method

• The gear is mated with a master gear on a fixed centre distance and set in such a way that only one tooth side makes contact.
• Angular transmission error of the driven gear is measured by means of the measuring carriage that transmits the centre to centre distance deviation to a dial indicator.

Dual contact (or) variable centre distance method

• Two tooth flanks are in mesh, the measurement result represents the sum of the variation of both tooth flanks.

PARKINSON GEAR ROLLER TESTER

• The gears are mounted on the two mandrels, so that they are free to rotate without measurable clearance.
• The left spindle can be moved along the table and clamped in any desired position.

• Good circle shows the good gear.
• Moderate circle shows the moderate gear.
• Unsatisfactory circle shows the faulty gear.

MEASUREMENT OVER PINS OR BALLS

• Lead checking
• Lead is the axial advancement of helix for one complete turn, as in the threads of cylindrical and teeth of helical gears.
• Backlash checking
• One of the two gears is locked, while the other is rotated backward and forward as far as possible.
• Circumferential backlash
• Normal backlash

Concentricity measurement

• The concentricity of the gear is checked by mounting the gear between centres and measuring the variation in height of a roller placed between successive teeth.

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Alignment checking

• The alignment of the gear teeth with respect to the axis of mounting is determined using a parallel bar placed between the gear teeth.

SURFACE FINISH MEASUREMENT

• The surface texture is defined as the regular (or) irregular surface spacings which tend to form a pattern on the surface.
• Types of Irregularities
• Primary texture (or) Roughness
• Secondary texture (or) Waviness

• Primary texture (Roughness) - The primary texture is caused, due to the short wavelength irregularities in the surface.
• Secondary texture (Waviness) - The secondary texture is also called as waviness. Waviness is the widely spaced component of the surface texture.

• Lay- Lay indicates the direction of predominant surface pattern produced and it reflects the machining operation used to produce it.

Production method

Surface Roughness values

Reasons for measuring the surface texture

• To predict the performance of the work material.
• To control the manufacturing process.

Factors Affecting the surface finish

• Machining variables
• Cutting speed
• Feed
• Depth of cut
• Geometry also influences the surface finish.
• Nose radius
• Rake angle
• Side cutting-edge angle
• Cutting edge

Analysis of Surface Finish

• Peak to valley height method
• The average Roughness
• Form factor or bearing curve.

Peak to valley height:

• This method measures the maximum depth (height) of the surface irregularities over a given sample length, and largest value of the depth (height) is accepted as a measure of roughness.

The Average Roughness:

• C.L.A. Method - Centre Line Average or Arithmetic Average (AA) is defined as the average values of the ordinates from the mean line, omitting the signs of the ordinates.

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• R.M.S. Method - It is defined as the square root of the arithmetic mean of the value of the squares of co-ordinates of the surface measured from a mean line.

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• Ten point height Method - average difference between the five highest peaks and five lowest valleys of Surface Roughness is taken for a given sample length measured from a line parallel to the mean and not crossing the profile.

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Form factor- The form factor is defined as the ratio between the area of material above the baseline to the area of the enveloping rectangle.

MEASUREMENT OF SURFACE FINISH

• Surface Inspection by comparison method
• Touch Inspection
• Visual Inspection
• Scratch Inspection
• Surface Photographs
• Reflected light Intensity
• Micro Interferometer
• Microscopic Inspection and
• Wallace surface dynamometer
• Direct Instrument method
• Stylus probe Instrument
• Profilometer
• Tomlinson surface meter and
• Taylor - Hobson Talysurf

Stylus probe Instrument

Profilometer

(a) Contact profilometer

(b) Non-contact profilometer

(c) Stylus profilometer (Mechanical profilometer)

Tomlinson surface meter

Taylor - Hobson Talysurf

ROUNDNESS (OR) CIRCULARITY

• Roundness is the measure of how closely the shape of an object approaches that of a circle.

Types of Irregularities of a Circular Part

• Ovality
• Lobing
• Irregularities of No specific form

Causes of out-of roundness

• Poor bearings in the spindle
• Poor alignment of shafts between centers.
• Stress caused due to compression of the part at the points of contact when held in a chuck.

Methods of Roundness Measurement

• Diametral Method
• Circumferential confining gauge
• Rotating on centers
• V-Block Method
• Three point Probe
• Roundness measuring spindle
• Reference circles
• Roundness measuring machines

Diametral Method

Circumferential Confining Gauge

Rotating on Centres

V-block Method

Three Point Probe Method

Roundness measuring Spindle

a) Overhead Spindle

b) Rotating table

Reference Circles

• Least Squares Reference Circle (LSCI)
• Minimum Circumscribed Circle (MCCI)
• Minimum Zone Reference Circles (MZCI)
• Maximum Inscribed Circle (MICI)

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