metrology1.pdf

Engineering Metrology

Marcian Frank Antony

T.O (CAD/CAM/Design)

Linear Measurement-Comparators

• Necessary measurement of the linear

measurement carried out by the Engineer

by comparing the size of the work piece or

other part with the known size of the end

Gauge.ie comparative measurement

Comparators

• Dimensional comparators are principal

instruments used in Linear Measurement

• Many principles have been used to obtain

suitable degrees of magnification of the

indicating devices relative to the change in

dimension being measured.

Types of Comparators

• Mechanical

• Mechanical-Optical

• Electrical.

• Fluid displacement.

Characteristics of Comparators• The instrument must be of robust design and

construction so as to withstand the effect of ordinary

usage without impairing its measuring accuracy.

• The indicating devices must be such that readings

are obtained in least possible time and for this,

magnification system used should be such that the

readings are dead beat. The system should be free

from backlash, and wear effects and the inertia,

should be minimum possible.

• Provision must be made for maximum compensation

for temperature effects.

Use of Comparators• In mass production, where components are to be

checked at a very fast rate.

• As laboratory standards from which working or

inspection, gauges are set and correlated.

• For inspecting newly purchased gauges.

• Attached with some machines, comparators can be

used as working gauges to prevent work spoilage

and to maintain required tolerances at all stages of

manufacturing.

• In selective assembly of parts, where parts are

graded in three groups depending upon their

tolerance.

Mechanical ComparatorsSystems of Displacement

Amplification used in Mechanical

Comparators

1. Rack and Pinion. Measuring spindle

integral with a rack, engages a pinion

which amplifies the movement of

plunger through a gear train

2. Cam and gear train. In this case the

measuring spindle acts on a cam

which transmits the motion to the

amplifying gear train.

Mechanical Comparators• Lever with toothed sector.

lever with a toothed sector at its

end engages a pinion in the hub

of a crown gear sector which

further meshes with a final

pinion to produce indication

• Compound Levers. Here

levers forming a couple with

compound action are connected

through segments and pinion to

produce final pointer movement.

Mechanical Comparators

• Twisted Taut Strip. The

movement of measuring

spindle tilts the knee

causing straining which

further causes the twisted

taut band to rotate

proportionally. The motion

of strip is displayed by the

Attached pointer.

Dial Indicator

• The linear movement of the

spindle is magnified by means

of a gear and pinion train into

sizable rotation of the pointer

on the dial scale.

• The indicator is set to zero by

the use of slip gauges

representing the basic size of

the part. This is generally used

for inspection of small

precision-machined parts.

Dial Indicator

• This type of comparator

can be used with

various attachments so

that it may be suitable

for large number of

works. With a V-block

attachment it can be

used for checking out-

of-roundness of a

cylindrical component.

Dial Indicator

• Centering of Job in a Four

Jaw Chuck

• Setting the Job Parallel to

With Respect to another

Parallel Surfaces.

• In Conjunction With Vernier

Height Gauges

The Johansson ‘Mikrokator'

• Perhaps the simplest, yet most ingenious

movement used in this type of instrument

is one due to H.Abramson, a Swedish

Engineer, and which is made by C. E.

Johansson L td.

Johansson ‘Mikrokator

Construction

• A thin metal strip carries ,at the center of

its length a very high glass tube pointer.

• From the center the strip is permanently

twisted to form right and left hand helices.

• One end of the strip is fixed to the

adjustable cantilever strip the other being

anchored to the spring strip elbow, one

arm which carries the measuring Plunger.

Working

• As the plunger moves up and down the

elbow acts as bell crank lever and causes

the twisted strip to change its length and

thus further twist or untwist.

• Hence the pointer at the center of the

twisted strip rotates an amount

proportional to the change in length of the

strip.

Working

Mathematical Representation

Working

• The purpose of the cantilever strip other

than anchorage is used to is to allow an

adjustment to be made in amplification. Its

effective length may be varied

• The instrument is surprisingly robust and

is produced commercially in a range of

5000 X magnifications under controlled

laboratory conditions.

The sigma Comparator

• Another mechanical comparator of

ingenious, yet simple design is that

produced in a range of magnifications by

the Sigma Instrument Company.

Construction

• The plunger, mounted on a pair of slit

diaphragms to give a frictionless linear

movement has mounted upon it a knife

edge which bears upon the face of the

moving member of a cross strip hinge.

• This consist of the moving component and

a fixed member connected by thin flexible

strips alternately at right angles at each

other.

Working

• If an external force applied to the to the

moving member it will pivot as would a

hinge about the line of intersection of the

strips.

• If the effective length of this arm is L and

the distance from the hinge pivot to the

knife edge is x then the first stage of the

magnification is L/x

Working

• To the extremities of the 'Y' arm is

attached a phosphor-bronze strip which is

passed around a drum of radius r attached

to the pointer spindle. If the pointer is of

length R then the second stage of the

magnification is R/r and the total

magnification is L/x × R/r

Working

• The magnification can be adjusted by

slackening one and tightening the other

screw, attaching the knife edge to the

plunger and thus adjusting distance x,

while a range of instruments of differing

magnifications can be produced by having

drums of different radii r and suitable

strips.

Reed Type Comparator

Reed Type Comparator

• The reed mechanism is frictionless device

for magnifying small motions of spindle. It

consists of a fixed block A which is rigidly

fastened to the gauge head case, and

floating block B, which carries the gauging

spindle and is connected horizontally to

the fixed block by reeds C. A vertical reed

is attached to each block with upper ends

joined together. These vertical reeds are

shown in the figure by letter D.

Reed Type Comparator

• Beyond this joint extends a pointer or

target. A linear motion of the spindle

moves the free block vertically causing the

vertical reed on the floating block to

slide past the vertical reed on the fixed

block.

Working• The vertical reeds are joined at the upper

end, instead of slipping, the movement causes both

reeds swing through an arc and as the target is

merely an extension of the vertical reeds, it swings

through a much wider arc.

• The amount of target swing is proportional to the

distance the floating

block has moved but of course very much magnified.

The scale may be calibrated by means of gauge block

(slip gauges) to indicate any deviation

from an initial setting.

Advantages• These are usually cheaper in comparison to other

devices of amplifying.

• These do not require any external supply such as

electricity or air and as such the variations in

outside supplies do not affect the accuracy.

• Usually the mechanical comparators have linear scale

which is easily understood.

• These are usually robust and compact and easy to

handle.

• For ordinary workshop conditions, these are suitable

and being portable can be issued from a store.

Disadvantages

• The mechanical comparators have got more moving parts

than other types. Due to more moving parts, the friction is

more and ultimately the accuracy is less.

• Any slackness in moving parts reduces the accuracy

considerably.

• The mechanism has more inertia and this may cause the

instruments to be sensitive to vibration.

• The range of the instrument is limited as the pointer

moves over a fixed scale.

• Error due to parallax is possible as the moving pointer

moves over a fixed scale.

Limitations• Comparators using this type of linkage

have sensitivities of the order of 0.25

micron per scale division.

• The mechanical amplification is usually

less than 100, but it is multiplied by the

optical lens system. It is available in

amplifications ranging from x 500 to x

1000.

Optical Mechanical Comparators

In this system,

Mechanical amplification= l2/l1and Optical amplification =

l4/l3 * 2.

It is multiplied by 2, because if

mirror is tilted by an angle δθ ,

then image will be tilted by 2 * δθ .

Thus overall magnification of this

system=2*l2/l1 * l4/l3.

Optical Mechanical Comparators

• In this system,

• Mechanical amplification= l2/l1

• and Optical amplification =

• l4/l3 * 2.

• It is multiplied by 2, because if mirror is tilted

by an angle δθ , then image will be tilted by 2

* δθ .

• Thus overall magnification of this

system=2*l2/l1 * l4/l3.

working

• In these instruments small displacements

of the measuring plunger are amplified by

initially a mechanical system consisting

usually pivoted levers followed by further

amplification by a simple optical system

involving the projection of a image.

• The mechanical system causes a plane

reflector to tilt about an axis. The image of

an index is then projected

Zeiss Ultra Optimeter

Advantages• It has small number of moving parts and

hence a higher accuracy.

• In the optical comparators, the scale can

be made to move past a datum line and

thus have high range and no parallax

errors.

• It has very high magnification.

• Optical lever is weightless.

/

Disadvantages

• As the instrument has high magnification,

heat from the lamp, transformer etc. may

cause the setting to drift.

• An electrical supply is necessary.

• The apparatus is usually large and

expensive.

Disadvantages

• When the scale is projected on a screen,

then it is essential to use the instrument in

a dark room in order to take the readings

easily.

• The instruments in which the scale is

viewed through the eyepiece of a

microscope are not convenient for

continuous use.

Pneumatic Comparators

• Industrially, pneumatic comparators, in

which small variations are made in the

dimension being measured with respect to

a reference dimension which are shown by

variation in either

(a)Air Pressure

(b)The Velocity of air flow

Advantages of pneumatic Comparator

over Mechanical Comparator• High amplifications are possible

• No physical contact accuracy is high

• Internal dimensions are readily measured

• Measurement in tolerance and geometry

• Less operation and inspection time or less

cycle time.

Back Pressure Comparators

The air pressure variation system is based on the use of a

two orifice arrangement

Solex Air Gauges

Flow Velocity Comparators

Electrical Comparators

Fluid Displacement Comparators

Slip Gauges

• A gauge block (also known as a gage

block, Johansson gauge, slip gauge, or

Jo block) is a precision ground and

lapped length measuring standard.

Invented in 1896 by Swedish machinist

Carl Edvard Johansson.

• Gauge blocks are the main means of

length standardization used by industry.

Slip Gauges

• Each gauge block consists of a block of

metal or ceramic with two opposing faces

ground precisely flat and parallel, a

precise distance apart.

• Standard grade blocks are made of a

hardened steel alloy, while calibration

grade blocks are often made of tungsten

carbide or chromium carbide because it is

harder and wears less

Wringing

• Wringing is the process of sliding two blocks

together so that their faces lightly bond.

The process of wringing

involves four steps• Wiping a clean gauge block across an oiled pad

• Wiping any extra oil off the gauge block using a

dry pad

• The block is then slid perpendicularly across the

other block while applying moderate pressure

until they form a cruciform.

• Finally, the block is rotated until it is inline with

the other block.

Slip Gauges

Slip Gauges

Available in a set of 81, 49, 41, 35 & 25

pieces (inch unit)

Available in a set of 112, 105, 87, 50, 33

& 27 pieces (metric unit)

Range (mm) Steps (mm) No. of Pieces

1.001 – 1.009 0.001 09

1.010 – 1.490 0.010 49

0.50 – 24.50 0.050 49

25, 50, 75, 100

25 04

1.0005 --- 01

TOTAL 112

Typical Gage Block Set

Grades of Slip gauges• Reference (AAA):

Small tolerance(±0.05 μm or ±0.000002 in) used to

establish standards

• Calibration (AA):

(Tolerance +0.10 μm to −0.05 μm) used to calibrate

inspection blocks and very high precision gauging

• Inspection (A):

(Tolerance +0.15 μm to −0.05 μm) used as toolroom

standards for setting other gauging tools

• Workshop (B):

large tolerance (tolerance +0.25 μm to −0.15 μm) used

as shop standards for precision measurement

Handling of Slip Gauges

• In use, the blocks are removed from the

set, cleaned of their protective coating

(petroleum jelly or oil) and wrung together

to form a stack of the required dimension,

with the minimum number of blocks.

Handling of Slip Gauges• Gauge blocks are calibrated to be

accurate at 68 °F (20 °C) and should be

kept at this temperature when taking

measurements. This mitigates the effects

of thermal expansion.

• The wear blocks, made of a harder

substance like tungsten carbide, are

included at each end of the stack,

whenever possible, to protect the gauge

blocks from being damaged in use.

Sine Bar

• Used when accuracy of angle must be

checked to less than 5 minutes

• Consists of steel bar with two cylinders of

equal diameter fastened near ends

– Centers of cylinders exactly 90º to edge

– Distance between centers usually 5 or 10

inches and 100 or 200 millimeters.

• Made of stabilized tool hardened steel

Characteristics Permissible

Tolerance• Flatness of upper and lower surfaces

0.001 mm

• Parallelism of upper and lower surfaces

w.r.t. datum surface when resting on it

0.001 mm

• Flatness of side faces 0.005 mm

• Squareness of side faces to upper surface

0.003/25 mm

Parallelism of side faces to the axes of

rollers 0.01/25 mm

Flatness of end faces 0.003 mm

Characteristics Permissible

Tolerance

• Parallelism of side faces to the axes of

rollers 0.01/25 mm

• Flatness of end faces 0.003 mm

• Mean diameter of rollers 0.002 mm

• Distance between the roller axes ± 0.003

mm

Sine Bar

Features of Sine Bar

• The two rollers must have equal diameter

and be true cylinders.

• The rollers must be set parallel to each

other and to the upper face.

• The precise centre distance between the

rollers must be known.

• The upper face must have a high degree

of flatness.

Use of Sine Bar

• Measuring known angles or locating any

work to a given angle.

Use of Sine Bar

• Checking of unknown angles

Use of Sine Bar

Assingment

• Limitations

• Precautions in use of Sine Bar

• Sine Table

• Sine Center

Auto collimator

Principle

1.The distance between the reflector and the lens has no effect on

the Separation distance between source and image.

2. For high sensitivity, i.e. a large value of x for it small angular

deviation,δ,a long focal length is required.

Auto collimator

Limit Gauges

Gage is a device / equipment used for

investigating the dimensional fitness of a part

for a specified function

Gaging is defined as a process of measuring

manufactured materials to assure the

specified uniformity of size and contour

required by the industries

Gaging accomplishes two functions: Controls the dimensions of a product within the prescribed

limitations

Segregates or rejects products that are outside these

limitations

System of Limits and Fits

Why Limits Required?

• It is impossible to make a part to a

specified definite size except by remote

chance.

• If by chance a part is made exactly to the

size required, it is impossible to measure it

accurately enough to prove it.

Fits

• Interference fit. The minimum permitted

diameter of the shaft is larger than the

maximum allowable diameter of The hole.

Fit

• Transition fit. diameter of the largest

allowable hole is greater than that of the

smallest shaft, but the smallest hole is

smaller than the largest shaft.

• Clearance fit. The largest permitted shaft

diameter is smaller than the diameter of

the smallest hole.

Definitions

• Limits of size. These are the maximum

and minimum sizes allowed in a given

component.

• Tolerance is the maximum size variation

which will be tolerated on a given

component.

• Allowance is the size difference between

the limiting conditions of, size on the two

components.

Representation of Fits• Hole basis system. For a given nominal

size the limits on the hole are kept

constant and a series of fits are obtained

by varying the limits on the shafts.

• Shaft basis system. A series of fits can

be arranged for a given nominal size using

a standard shaft and varying the limits on

the hoIe.

Queries - Discussion