Metrology of threads

Metrology of Threads

By Prof N D Sadaphal

Assistant Professor

Sanjivani College of Engineering, Kopargaon (Maharashtra State) 423601

Mechanical Engineering

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Metrology of Thread

Prof. N.D.Sadaphal

TE Mechanical

Metrology & Quality Control

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Types of thread External thread: a thread formed on outside of a work piece is

known as external thread. Example: on bolts or screw etc. Internal thread: a thread formed on inside of a work piece is

known as internal thread. Example: nut.

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Screw Thread terminology

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Pitch: The distance from a point on a screw thread to a corresponding point on the next thread measured parallel to the axis.

Lead The distance moved by a nut or a bolt in axial direction in one complete revolutions called lead.

Crest: The outer-most part of the thread is called crest.

Flank: It is the Thread surface that connects crest with root.

Root: The inner most part of the thread is called root.

Major Diameter: Diameter at the crest of the thread measured at right angle to the axis is called as major diameter and is also known as outside diameter.

Minor diameter: The diameter at the core or root of the thread is called minor diameter. It is also called as core or root diameter.

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Effective diameter or Pitch diameter: It is the diameter at the pitch line.

Depth of thread: It is the distance between crest and root measured perpendicular to axis of screw.

Angle of thread: Included angle between sides of thread measured in axial plane.

Helix angle: Angle that the thread makes with plane perpendicular to thread axis at pitch line.

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

Addendum: It is the distance between the crest and the pitch line measured perpendicular to axis of the screw.

Dedendum: It is the distance between the pitch line & the root measured perpendicular to axis of the screw.

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Forms of Thread

Metric thread Unified thread

British Association (B.A.) thread British Standard Whitworth (B.S.W.) thread

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Buttress threads

Knuckle threads

Acme threads

Square threads Sellers thread 7 Prof N D Sadaphal

Errors in Thread • Errors in major & minor diameter may cause interference with

mating parts & will lead to rapid wear & weakening of threads.

• Pitch Error:- The ratio of linear velocity of tool & angular velocity of

work must be correct & this ratio must be maintained constant, otherwise pitch error will occur.

These are classified as,

a. Progressive error

b. Periodic error

c. Drunken error

d. Irregular error 8 Prof N D Sadaphal

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

shorter its nominal value then it is called as progressive error.

This error Caused: •Incorrect ratio of linear velocity of tool & angular velocity of work. •Pitch error in lead screw of lathe machine. •Fault in the saddle guide ways of lathe. 2. Periodic error

These error repeats itself at regular intervals along the thread. If successive portion of thread is short or long than nominal value.

This error Caused: •Incorrect ratio of linear velocity of tool & angular velocity of work. •Teeth error in thread. •Lead screw error.

Sinusoidal form

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3) Drunken error: Drunken errors are repeated once per turn of the 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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4) Irregular errors (Erratic error) • It is vary in irregular manner along the length of

the thread. • Error like variation in major & minor dia. at every

thread. Irregular error causes: 1. Machine fault. 2. Non-uniformity in the material. 3. Cutting action is not correct. 4. Machining disturbances


MEASUREMENT OF VARIOUS ELEMENTS OF THREAD

1) Major diameter

2) Minor diameter

3) Effective or Pitch diameter

4) Pitch


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Ordinary Micrometer •Micrometer is first adjusted on a standard (setting) cylinder having approximately the same diameter as that of major dia. of thread & reading R1 is taken. •Then micrometer is set on the major dia. of the thread & reading R2 is taken.


Major diameter of screw thread = S±(R1-R2) S= dia. of standard cylinder

R1=micrometer reading over setting cylinder

R2=micrometer reading over screw thread

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Measurement by Bench micrometer: C lam p

FiducialIndicator

M easuringA nvils H olding centres

M icrom eter head

Supports

BENCH M ICROM ETER

In order to ensure that all measurement are made at the same pressure, a fiducial indicator is used . It ensures constant pressure for all measurement. 15 Prof N D Sadaphal

Bench micrometer


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•Initially a calibrated setting (standard) cylinder having nearly same diameter as that major diameter of the thread to be measured is used as setting standard. •The setting cylinder is held between anvils & reading is taken. •The cylinder is then replaced by threaded workpiece & again micrometer reading is noted. •Measurement is taken at two or three positions along the thread to detect any taper & Ovality.

Major dia. of screw thread=

D=Dia. of setting cylinder. R1=reading of micrometer on setting cylinder. R2=reading of micrometer on screw thread.

Measurement of Major Diameter


Measurement of Minor Diameter

•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.

Two V-piece method


Mainor diameter of screw thread = D±(R2-R1) D= dia. of standard cylinder

R1=micrometer reading over setting cylinder

R2=micrometer reading over screw thread

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Measurement of Effective(Pitch) Diameter By Three wire Method


E M

H

A

B C

D

P

h

EM

Dia 'd'


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This method ensures alignment of micrometer faces parallel to the thread axis.

Here, three wires of exactly known diameters are used, one on one side & the two on the other side. The wires may be held in hand or hung from a stand.

From fig, M=diameter over the wires, E= effective diameter (to be found)

d= diameter of wires, h=height of wire center above the pitch line,

r=radius of wire, H=depth of thread, D=major diameter of the thread.


2

cot22

cosec1dEMOr

2

cot22

cosec1222

cot42

rcosec2EM i.e.

2r2hEM wires,over the Distance

2cot

42cosec

2)(

2cot

42

HCD and

2cot

22cot

2cosec

22cosec ABAD , ABD e triangl

P

PrEr

P

PdCDADhFurther

PPDEH

dthe From


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o Whitworth thread, 55 , of thread 0.64P

E D-0.64P, cosec 2.1657, and cot 1.9212 2

3.1657 1.605 where D is the major diameter of the thread.

For Metric threads, Depth of thread 0.6495P

For depth

M D d P

E

0.6495 , 60 ,cos 2,cot 1.7322 2

3 1.5155

measure the value of M practically & then

compare with the theoretical values using formulae derived above.

After finding the correct value o

oD P ec

M D d P

We can

f M, as d is known, E can be found out.


P

A

P/2

P/4

P itch line

B EST SIZE O F W IR E

B

If the size of the wire is such that, it contacts the flanks at the pitch line, it is called as ‘best size’ of wire which can be determined by geometry of screw thread.

BEST WIRE SIZE


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b

b

In the triangle OAB, , or sin 90-2

sec . 2cossin 90-

22

1 But OB radius of wire of best size wire (D )

2

i.e. D 2 2 sec . Also since AB lies on the pi2

AB ABSin BOA

OB OB

AB ABOB AB

dia

OB AB

P

tch line, AB4

P is the pitch of the thread.

2 sec sec4 2 2 2b

where

P PD



T= Dia. Under the wire, E= Effective/Pitch dia. E=T+P (P=Pitch value) For metric thread P= 0.866p-d (p=pitch of thread, d=dia. of wire used) For whitworth thread P=0.9605p-1.1657d T= (R2-R1)+Ds (R2= Reading over Screw thread R1= Reading over master cylinder Ds= dia. of master cylinder)

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Pitch measuring machine


Pitch measuring machine

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GEAR…..

• Power transmission is the movement of energy from its place of generation to a location where it is applied to performing useful work

• A gear is a component within a transmission device that transmits rotational force to another gear or device


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 gears


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SPUR GEAR

• Teeth is parallel to axis of rotation

• Transmit power from one shaft to another parallel shaft

• Used in Electric screwdriver, clock, washing machine and clothes dryer


External and Internal spur Gear…


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Helical Gear • The teeth on helical gears are cut at an angle

to the face of the gear

• This gradual engagement makes helical gears operate much more smoothly and quietly than spur gears


Helical Gear…


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Rack and pinion

• Rack and pinion gears are used to convert rotation (From the pinion) into linear motion (of the rack)

• A perfect example of this is the steering system on many cars


Straight and Spiral Bevel Gears

Used in differential gear box of bus trucks etc.


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WORM AND WORM GEAR

Used in Lathe machine for movement of saddle along axis of work-piece


NOMENCLATURE OF SPUR GEARS


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Pitch circle- It is an imaginary circle which by pure rolling action would give the same motion as the actual gear. Pitch circle diameter(D) It is the diameter of the pitch circle. The size of the gear is usually specified by the pitch circle diameter. It is also known as pitch diameter. Addendum-It is the radial distance of a tooth from the pitch circle to the top of the tooth. Dedendum- It is the radial distance of a tooth from the pitch circle to the bottom of the tooth. Addendum circle-It is the circle drawn through the top of the teeth and is concentric with the pitch circle. Dedendum circle-It is the circle drawn through the bottom of the teeth. It is also called root circle. Module. It is the ratio of the pitch circle diameter in millimeters to the number of teeth. It is usually denoted by m. Mathematically,


Circular pitch. It is the distance measured on the circumference of the pitch circle from a point of one tooth to the corresponding point on the next tooth. It is usually denoted by Pc ,Mathematically,

Note : If D1 and D2 are the diameters of the two meshing gears having

the teeth T1 and T2 respectively, then for them to mesh correctly,

Clearance. It is the radial distance from the top of the tooth to the bottom of the tooth, in a meshing gear. A circle passing through the top of the meshing gear is known as clearance circle.


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Total depth. It is the radial distance between the addendum and the dedendum circles of a gear.

Face of tooth. It is the surface of the gear tooth above the pitch surface.

Flank of tooth. It is the surface of the gear tooth below the pitch surface.

Top land. It is the surface of the top of the tooth. Face width. It is the width of the gear tooth measured parallel to

its axis.


ERRORS in Gear (i) Adjacent pitch error: Actual pitch—design pitch.

(ii) Cumulative pitch error:

Actual length between corresponding flanks—design length.

(iii) Profile error: Deviation of actual profile from Designed profile

(iv)The tooth thickness error:

Actual tooth thickness measured along the surface—design tooth thickness.


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Gear tooth Vernier Measurement of tooth thickness


The tooth thickness is generally measured at pitch circle

•Since the gear tooth thickness varies from the tip to the base circle of the tooth, the instrument must be capable of measuring the tooth thickness at a specified position on the tooth. •The gear tooth Vernier has two Vernier scales. The vertical Vernier scale is used to set the depth (d) along the pitch circle from the top surface of the tooth at which the width (w) has to be measured. While the horizontal Vernier scale is used to measure the width (w) of the teeth.


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• Considering one gear tooth, the theoretical values of w and d can be found out which may be verified by the instrument.

• As shown in the figure , w =chord ADB, but tooth thickness is specified as an = arc distance AEB. Also, the depth d adjusted on the instrument is

slightly greater than the addendum CE“. width w is therefore called chordal thickness and

d is called the chordal addendum.

W=AB=2AD But for gear , θ=360/4N, Where, N= number of teeth.

Pitch circle radius

Chordal thickness Method



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Constant Chord Method


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Base Tangent Method

Base tangent length = one base circular thickness + number of base pitches

Base tangent length is the distance between the two parallel planes which are tangential to the opposing tooth flanks.


David Brown Tangent Comparator

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Base tangent length is,


Gear Rolling Tester

Parkinson Gear Tester 56 Prof N D Sadaphal

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Gear Rolling Tester


Parkinson Gear Tester

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Working Principle

Construction :-

• A standard gear (master gear) is mounted on a fixed vertical spindle & gear to be tested on another spindle mounted on sliding carriage.

• These gears are maintained in mesh by spring pressure. • Two spindles can be adjustable so that their axial distance

is equal to the designed gear center distance. • A scale is attached to one carriage & vernier to other, to

measure center distance. • A recorder is also fitted in form of circular chart.


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• As gears are rotated the movements of sliding carriage are indicated by a dial indicator, and these variation are measure of any irregularities in the gear under test.

• When these gears are in close mesh & rotated, any error in the tooth form, pitch or concentricity of pitch line, will cause a variation in center distance, and movements of carriage are indicated by dial indicator.

• A recorder is used to record or trace the irregularities in the gear under the test.

Working:-


Limitation of Parkinson Gear Tester • Generally Maximum 300mm diameter gear &

150 mm smaller diameter gears are also tested.

• The accuracy is upto .

• Errors are not clearly identified for type profile, pitch, helix angle & tooth thickness.

• Measurements are directly depend upon the master or reference gear.


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Profile Projector • A profile projector is also referred to as an optical comparator, or even known as a shadowgraph, a profile projector is an optical instrument utilized for measurement of screw thread parameter. • The projector magnifies the profile of the specimen, and shows this on the built-in projection on screen. •The X-Y axis of the screen can be aligned correctly using a straight edge of the machined part to analyze or measure by moving work table of Profile Projector.


Tool Maker’s Microscope •It is an ideal measuring instrument for simplifying inspection and precision measurement of diameter, forming tools, gauges as well as template checking of thread and its angles.(it is based on optical system) •Gear tooth flank profile can be tested by projecting on the screen. •Projected profile can be compared with standard profile of tooth which can be seen on projected screen.

STANDARD SPECIFICATIONS: Total Magnification :30x Objective : 2x Eyepiece: 15x Working distance :65mm Field of view : 6mm Prof N D Sadaphal 64

Metrology of threads

Engineering

Transcript of Metrology of threads