@UNIT II ME302 (Thread, Gear and Circularity Measurements)

7/29/2019 @UNIT II ME302 (Thread, Gear and Circularity Measurements)

1/42

SATADRU KASHYAP ME 302(Mechanical Engineering, Tezpur University) (Mechanical Measurements and Instrumentation)

UNIT II: Mechanical Measurements

(Screw thread, Gears, Circularity and Surface finish

Measurements)ME 302: Mechanical Measurements and Instrumentation

Course Instructor: Satadru Kashyap

[email protected]

Tezpur University Intranet: Uploaded E-course Material

Department of Mechanical Engineering

Tezpur University
mailto:[email protected]:[email protected]


2/42


Lead: The axial distance advanced by the screw in one revolution

Pitch: distance between the corresponding points on two adjacent threads

in the same axial plane. pitch = lead / no. of thread starts.

Minor diameterMajor diameter

Pitch/Effective diameter

Helix angle

Flank angle

Height / Depth of thread

External thread

Internal threadAxis of the thread

Screw Threads


3/42


Screw Thread types

Metric Threads

Whitworth thread has an included angle of 55between the flanks and equal radii at crest and

root. (standard nuts, bolts and pipe work.)

Metric Threads: clearance at crest and root so

that contact between mating threads takes

place only on the flanks (included angle of 60).

Buttress Thread (unequal flank angles)

Knuckle Thread


4/42


Measuring Elements of a screw thread: measure the following:

1. Major diameter

2. Minor diameter

3. Effective or pitch diameter

4. Pitch

5. Thread angle and form.

Screw Thread Measurement


5/42


Measurement of Minor Diameter:

Floating carriage micrometer

Applicable for most threads. Vee-pieces - various sizes having suitable radii at the edge.

Standard kept between the micrometer anvils with the help of V- pieces. The fiducial indicator anvil - maintain constant pressure.

The diameter (standard cylinder)known d1 by V-pieces in position r1.

Fix the position of fiducial indicator anvil, the standard cylinder is replaced by screw.

The reading is now taken for screw thread in position r2.

Minor diameter, d = d1+ (r2r1)



6/42


Measurement of Major Diameter:

checked by the use of micrometer or vernier calipers.

The major diameter is measured by bench micrometer (Figure).

Measurements made at constant pressure. Fixed anvil is replaced by fiducial indicator.

Standard cylinder (approx. equal to major diameter) held between anvils. Micrometer

readings noted.

Cylinder is replaced by threaded work pieces and the readings are noted (same process as

minor diameter).

Major diameter = d1 + (r2r1)



7/42


Measurement of Effective/Pitch Diameter:

Two WireMethod:

- place two wires or rods of identical diameter between the flanks of the thread,- measure the distance over the outside of these wires.

The effective diameter E = T + P

T = Dimension under wires = M2d


- wires of hardened steel tosustain the wear and tear.

- They are given a high degree

of accuracy and finish by

lapping.

T can also be determined - place wires over a standard cylinder of diameter (S) greaterthan the diameter under the wires and noting the reading R1 and then taking reading with

over the gauge wires, say R2.

Then T = S - (R1 - R2)


8/42


Measurement of Effective/Pitch Diameter:

Two WireMethod (contd.):

P - depends upon the diameter of wire (d) and pitch of the thread (p).

- constant value (to be added to the diameter under the wires to get the effective diameter).

e.g. P = 0.9605p 1.1657d (Whitworth thread) and P=0.866pd (metric thread).


In Figure, since BC lies on the effective diameter line

=1

2 =

1

2

=

2

2

= =

2

2

2=

2[

2 1]

=

2=

4

2 =

1

2 =

1

2

1

2

= = 4

2

2

[ 2

1]

, =

; Hence = =

[

- 1]

3 wire method: two wires on one side help in aligning the micrometer square to the thread

while the third placed on the other side permits taking of readings.


9/42


Determination of Best Wire: For effective diameter, always best wire to be used.

The wire diameter, such that, - the wire touches the thread on flank portion only.

Effective diameter can be measured with any wire having any diameter, which makescontact on the true flank of the thread; BUT

The best wire size for the given screw thread, is a wire having it's diameter, such that it

would contact the screw thread exactly on the pitch diameter point (shown in the Figure).



10/42


Determination of Best Wire:

At D the wire touching the flank of thread lies on the pitch line or the effective diameter line.


In the Figure, OC and OD perpendicular to CGand DG at point C and D which lies on pitch line.

Metric thread = 60: db= (p/2) Sec(60/2)=0.577p

Whitworth thread = 55; db = (p/2) Sec(55/2) =

0.5636p


11/42


The main parts of the instrument are:1. Rotatable table 2. Swingable head 3. Projection screen

4. Objective lens 5. Measuring stage 6. Ocular

7. Micrometers 8. Prism.

Working: Test piece illuminated by light passing through it.

Illumination - lower side of work piece - objective lens

- prism (deflects the light) - ocular - projection screen.

Objective lens (1X, 1.5X, 3X and 5X)

So, total magnification of 10X, 15X, 30X and 50Xachieved with an ocular of 10X.

The direction of illumination can be tilted with respect

to the work piece by tilting the measuring head and the

whole optical system (screw thread measurement)

Tool Makers Microscope

- optical measuring machine used for external and internal length measurements as

well as measurements on screw threads, profiles, curvatures and angles.


12/42


The scale value of this microscope:

0.01 mm for length measurement. 3' for angle measurement with rotatable table.

1' for angle measurement with angle measuring ocular.

Applications:

(1) Finding the relative position of various points on workby measuring the travel necessary to bring a second point

to the position previously occupied by first.

(2) Measurement of angles by using a protractor eye-

piece.

(3) Comparison of thread forms with master profiles

engraved in the eyepiece and measurement of pitch and

effective diameter.

(4) Comparison of an enlarged, projected image with a

scale tracing fixed to the projection screen.

Tool Makers Microscope


13/42


M/C parts can be out of roundness - clamping distortion, presence of dirt and chips

on clamping surfaces, heat and vibration etc.

Circularity: characteristic form of the entire periphery of a plane (one plane only).Roundness:

condition of a surface of revolution (cylinder, cone or sphere) where all points on surface

intersected by any plane perpendicular to common axis or passing through a common

center are equidistant from the axis.

It is geometric form of a body of revolution in all three dimensions. Distortion may cause breakdown of lubricant film, press fitted parts may collapse under

high load, sealed fits may start leaking.

Lack of roundness in bush bearings - wobbling of the shafts. Roller/ball bearings loose

efficiency and make noise due to roundness errors.

Roundness and Circularity


14/42


Errors of circularity at a cross section

Ovality: This error occurs when there is a difference between the diameters.

Error = Maximum diameter - Minimum diameter = Major axis - Minor axis

Lobbing: In this case, the diameter measured at any two position remains constant. Say

D1but still it is not exactly equal to the original diameter D.

Irregularities of no specific form: In this case, the variation in the diameter occurs due

to irregular surface profile.

Roundness and Circularity


15/42


dial indicator is placed above the work piece (WP) - divided into 12 equal parts.

The WP held in a V-block dial indicator touches it at position 1.

It is rotated from position 212. Variation in surface profile noted by the dial indicator. Procedure repeated 3 times (higher accuracyaverage)

Polar Plot: Circle of dia. = 4 X maximum reading drawn and divided into 12 equal parts.

Inside big circle, draw a small concentric circle of dia. (say 0.5times of work piece).

Values of WP readings plotted & joined (to get actual profile) between small & big circle.

Error = (Error measured from the polar graph / K);where K = constant ; value depends on the shape and work piece and angle of V-blocks;

obtained from a standard chart.

Roundness Measurement by Polar Graph


16/42


Limitations of the method:

V-block angle influences the circularity error (e). It may be possible that indicator shows

no variation ore or 2 e or 3 e for different V block angles. Position of instrument (measurement taken from top or bottom).

Number of lobes on WP (elliptical, triangular, quadrilateral etc).

Instrument position (plunger) and point of contact of WP with V-block --- same vertical

plane.

A leaf spring inserted between indicator plunger and WP surface --- else circularity

readings might be affected by surface roughness.

K determination from standard chart

Roundness Measurement by Polar Graph

WP shape

Angle of V-blocks

60 90 108 120

Top Bottom Top Bottom Top Bottom Top Bottom

Elliptical 0 2 1 1 1.38 0.62 1.58 0.42

Quadrilateral 0.1 2 0.415 2.4 1.01 2.01 0.38 1

And so on for different shapes


17/42


Intrinsic Datum System:

simple but they do not give the exact image of the work piece.

Reference is the point on the work piece itself (hence the name intrinsic) and all thevariations in readings are taken with respect to that point.

They do not require expensive tooling for the tests and used for roundness testing.

Roundness Measurement


18/42


Extrinsic Datum System:

Slightly difficult but highly accurate.

They give the true image of the work piece. The reference used is a member of checking/measuring instruments.

Expensive as they require the magnification and recording system and used for roundness

testing where high accuracy is needed.

Roundness Measurement

S f Fi i h


19/42


SF - determines the deviations from the nominal surface w.r.t drawing.

SF obtained by - turning, milling, shaping, planning and grinding, show marked variations

when compared with each other.

SF - judged by degree of smoothness. (visual inspection); - fails to differentiate between

surface produced by same machining operation but under different cutting conditions.

Hence, quantitative evaluation is required.

Surface Roughness (surface texture): They are series of regularly repeated deviations in the

form of a wave, with a ratio of pitch to height.

produced by the trace of an edged cutting tool and plastic flow of the metal. height of the irregularities measured in microns; width is measured in mm.

If surface too rough - wear particles larger - act as abrasives - wear continues at high rate.

If surface too smooth - initial wear slow.

Factors affecting surface roughness are:

Type of coolant usedCutting parameters such as feed, speed and depth of cut

Type of machining

Rigidity of the system (fixtures, cutting tool, WP etc.)

Vibrations Material of tool and work piece.

Surface Finish

S f i i h


20/42


Any material machined by conventional machining process cannot be finished

perfectly. The surface generated will have some irregularities:

First Order: irregularities developed due to the inaccuracies in the machine tool such as lack

of straightness of guide ways, on which tool post is moving.

Second Order: irregularities developed due to the vibrations and rigidity of m/c tools.

Third Order: irregularities due to the cutting parameters (cutting speed, feed, depth of cut).

Fourth Order: irregularities developed due to the rupture of the material during the

separation of the chip from the already finished surface of the work piece.

Surface Finish

S f Fi i h


21/42


C.L.A Index (Ra): Centre line Average Index

To calculate the value of Ra, from a graph it is necessary to have a mean line.

Mean line divide the profile so that, Area above the line = Area below the line. A suitable length L is selected which is called Sampling Length for the given surface.

The average height Ha is calculated as follows:

Ha = (Summation of all area above and below the line / Sampling length) = (A / L)

Ha

= (A / L) = [(A1+A2+A3)+(B1+ B2)] / L

Surface Finish

S f Fi i h


22/42


R.M.S Average (Root Mean Square Number):

Geometrical average of the ordinates of the profile about the mean line.

Mean/Centre line = Sum of areas above the line = Sum of areas below the line. Ifn measurements are made from the mean line above and below the points on the surface

profile denoted by Yi .

Then R.M.S value is

Hrms = ( Yi2 / n)

Surface Finish

S f Fi i h M


23/42


Representation of Surface Roughness:

machining method is milling,

sampling length is 3mm. Direction of lay is perpendicular to the surface,

machining allowance is 1mm and

Representation will be as shown in Figure (b).

Surface Finish Measurements

S f Fi i h M t


24/42



Indication of Surface Roughness

The value orvalues defining the principal criterion ofroughness areadded to the symbols

a- surface roughness value

If it is necessary to impose maximum

and minimum limits of the principal criterion ofsurface roughness, both values shall be shown

maximum limit (a1) ;minimum limit (a2).

Roughness a obtainedby any production

process

Roughness a obtained byremoval of material by

machining

Roughness a shall beobtained without removal

of any material

S f Fi i h M t


25/42



S f Fi i h M t


26/42


Mechanical Methods using Stylus:

A stylus (fine to follow the tiny surface deviations) is lightly pressed on the specimen and

is moved along the surface.

Simultaneously, recordings made - vertical and horizontal displacements.

In some instrumentsrecord stylus movement on a chart - surface contour (2D / 3D).

Datum for the up and down movement - provided by stylus skids.

Vertical magnifications - very high to record small values (microns),

Horizontal magnifications - low (mm).

Stylus - light beam, a jet of air, or a condenser plate; most commonly, diamond having a

cone and a spherical tip of 0.0005 in or 0.0001 inch is used.

Diamond can be formed into above shape and stays that way with uniformity (hard

material).

Difficult to confine the light beam, air jet, or condenser plate to a small enough size to beable to follow the surface irregularities.


S f Fi i h M t


27/42


Tomlinson Surface Recorder:

Construction and working:

The tip of the diamond probe has aradius of about 2 microns.

lever is attached to a vertical

cylinder on one end and a diamond

needle supporting a smoked glass.

Surface irregularities detected by theprobe magnified by vertical cylinder

on to the needle/glass combination.

Magnification = lever length /dia. of

vertical cylinder.

The record obtained on the small

glass is further magnified by optical

projection.


S f Fi i h M t


28/42


Tomlinson Surface Recorder:

Advantages:

simple. low cost.

gives reliable results.

Disadvantages:

delicate and requires great care.

slow in operation.

not suitable for rapid and continues

use on the shop floor.


S rface Finish Meas rements


29/42


The Talysurf Surface Roughness Tester:

Construction and working: (operates on electrical principles).

The measuring head is fitted with the probe and the skid.

Motion of the measuring head - given by Gear Box.

The unit can be moved vertically over guide ways (hand wheel & lead screw).

The diamond probe (radius ~ 2 m) and gear box (give max travel of 12 mm) to it.

The averaging meter & pen recorder - provides graphical record.




30/42


The Talysurf Surface Roughness tester: Construction and working:

The arm carrying the stylus forms an armature (pivoted - center of E-shaped arm).

Two legs of E-shaped armtwo induction coils carrying an A.C. current.

A small air gap is left between the armature and outer limbs of E-shaped arm. movement of stylus changes air gap of primary and secondary coils. This modulates the

carrier wave. After amplification, modulated carrier wave demodulated and separated out to

get the information on surface roughness.


Gear


31/42


Gear is a mechanical drive which transmits power through toothed wheel.

Accuracyprime importance in manufacturingtest and measure the gears precisely.

Proper inspection concentrate on raw materials, check the machining of blanks, heat

treatment and the finishing of teeth.

Gear blank - tested for dimensional accuracy (face width, bore, outside diameter), and

eccentricity, tooth thickness. Precise gears - tip radius, shape of root provided and surface

finish are also measured.

Types of gears

1. Spur gear

2. Spiral gear

3. Helical gears

4. Bevel gears

5. Worm and Worm wheel

6. Rack and Pinion etc.

Gear

Gear Terminology


32/42


Pitch circle diameter(PCD): diameter of a circle

which will produce the same motion as the

toothed gear wheel.

Pitch circle: imaginary circle of gear that rolls

without slipping over the circle of its mating gear.

Addendum (A) = (1/Pd) = m.

Dedendum circle (or) Root circle

Pressure angle (a): angle made by the line of

action with the common tangent to the pitch

circles of mating gears.

Module (m): ratio of PCD to no. of teeth (D/T)

Circular pitch: distance measured along

circumference of pitch circle from a point on one

tooth to the corresponding point on the next tooth(Pc = D/T = m)

Diametral Pitch: number of tooth of the gear per

mm of pitch circle diameter. (Pd= T/D)

Gear Terminology

Gear Measurements


33/42


Measurement of gear tooth thickness:

Gear tooth Vernier:

Tooth thickness varies from the tip to basecircle of the tooth

Instrument must be capable of measuring the

tooth thickness at a specified position.

Only possible fixed position available for

measurement. Tooth thickness - generally measured at pitch

circle (pitch line thickness) of tooth.

The gear tooth in the vernier has two vernier

scales

set for the width w of the tooth & the height h from the top (at which w

occurs).

Gear Measurements

Gear Measurements


34/42


Gear Measurements

Gear Measurements


35/42


Gear Measurements

Gear Measurements


36/42


Gear Measurements

Constant chord method for measuring tooth thickness:

Gear Measurements


37/42


Gear Measurements

Constant chord method for measuring tooth thickness:

Gear Measurements


38/42


Gear Measurements

Base Tangent Method:

David brown tangent

comparator:

consists of a fixed and

movable anvil.

limited movement

micrometer on the moving

anvil side.

base tangent is adjusted by

setting the fixed anvil at a

desired place with a locking

ring.

The number of teeth overwhich measurement is to be

made is obtained from a

gear handbook.

of vernier

calliper

Gear Measurements


39/42


Gear Measurements

Gear Testing Machine:

Composite gear checkingtooth-tooth variation and total composite variation.

Parkinsons Gear Tester: principle - standard gear (fixed vertical spindle) and gear to betested (spindle on a sliding carriage).

Both gears maintained in mesh by spring pressure.

As gears are rotated, movement of the sliding carriage is indicated by a dial indicator.

It is used to detect (i) poor tooth form (worn or inaccurate cutting tool) (ii) pitch circle

eccentricity (inaccurate centering of the gear blank prior to tooth cutting).

Gear Measurements


40/42


Gear Measurements

Gear Tooth Profile Measurement:

Toolmaker`s microscope:

Gear tooth profile can be inspected on the screen of toolmaker`s microscope connected to a

profile projector. Measuring gear tooth profile is projected on the projector`s screen, and then standard involute

profile is also projected on the same screen.

Comparemeasured and standard profile - deviations of the measured tooth profile .

Method is good for gears with module < 1.

When the width of the gear blank is large, it is necessary to give proper illumination so thatthe projected tooth should be free of shadow.

In order to conduct test on successive teeth the gear blank must be properly clamped on the

rotary table of toolmaker`s microscope.

Advantages:

This method can be used for small gears (module up to 1 mm).

Method can also be used for measurement of tooth thickness, base pitch, space width etc.

Limitation:

Gear`s size limits this technique.

Gear Measurements


41/42


Gear Measurements

The Klingeluberg involute tester: gear tooth profile (an involute curve)

Disc dia. = base circle dia. of gear under test.

Gear mounted on same spindle as disc.

Straight-edge contact the disc (move tangentially) - disc will be rotated without slip.

Magnification lever fitted with a diamond stylus that contacts the tooth flank at the base circle

is hinged at A.

Movt. of stylus magnified (250X) through a lever and gear mechanism and then recorded.

Gear Measurements


42/42

SATADRU KASHYAP ME 302

Gear Measurements

The Klingeluberg involute tester: gear tooth profile (an involute curve)

Due to the linear motion of the straight-edge and the rotational motion of the gear tested, the

stylus, which will trace an involute curve, will slide over the tooth flank.

If profile of flank - exactly involute, the lever will remain stationary.

Then, recorded curve - straight line parallel to the direction of the motion of the flat surface.

Irregularities in straightness of recorded line - errors in the gear tooth profile.

Inclination of the recorded line to the direction of motion of the recording paper - the errors of

the base circle.

Documents

@UNIT II ME302 (Thread, Gear and Circularity Measurements)