MAGNIFICATION

• Magnification means increasing o/p signal of measuring instrument many times to make it more readable.

• Magnification in measuring instrument may be mechanical, electrical, electronic, optical, pneumatic principle or combination of these.

Methods of magnificationMechanical method of magnification:i) Rack and pinionii) Lever with toothed gearsiii) Cam and gear trainiv) Compound leversv) Twisted stripvi) Lever combined with band wound around

drum

Fig. Rack and pinion

Fig. Cam and gear train

Fig. Compound levers

Fig. Twisted strip

Comparators

• It is a precision instrument employed to compare the dimension of a given component with a working standard (generally slip gauges).

• It does not measure the actual dimension but indicates how much it differs from the basic dimension (working standard).

• NEED• OPERATION

Uses of Comparator:• For calibrating the working gauges • Used as working gauges• Used as final inspection gauges

Essential characteristics of a good Comparator:• Robust design and construction• Linear characteristics of scale• High magnification• Quick in results• Versatility• Minimum wear of contact point• Free from back lash• Quick insertion of work piece• Provision for compensation from temperature effects• Provision for means to prevent damage during use.

• Classification of comparators

• Mechanical comparator• Optical comparator• Mechanical – Optical comparator• Electrical comparator• Pneumatic comparator

Johansson ‘Mikrokator’ comparator

where ,• l is the length of twisted strip, measured along its

neutral axis.• W is the width of twisted strip• n is the number of turns• is the twist of midpoint of strip with respect to

end.

nWionamplificat

d

d2

1.9

Reed type mechanical comparator

Sigma comparator

Total magnification = 1st stage magnification X 2nd stage magnification

= L/x * R/rWhere L = Effective length of armx = Distance from the hinge pivot to the knife edgeR = Length of pointerr = Drum radius

Advantages • Low cost• Do not require external supply• Linear scale• Robust and compact, easy to handle• Suitable for ordinary workshop conditionsDisadvantages • more moving parts, so more friction and less

accuracy• Deformation or slackness reduces accuracy• Range is limited • Parallax error

Optical comparator

(a) Reflection from plane surface

Advantages • High degree of magnification• No contact with part hence no wear • few moving parts, so less friction and high accuracy• No parallax errorDisadvantages • Requires external power supply• Needs dark room • Apparatus is bulky and expensive• Due to lamp heat is generated so changes in setting

Mechanical optical comparator

Mechanical magnification= (l2/l1) Plunger movement causes mirror to tilt by angle ‘α’ then image will by 2αTherefore optyical magnification = 2* (l4/l3)Thus overall magnification = 2*(l2/l1) * (l4/l3)

Pneumatic comparator

Pneumatic comparator

Pneumatic comparator

Advantages • High degree of magnification• No contact with part hence no wear • few moving parts, so less friction and high accuracy• Less pressure of air jet helps to clean dust from part.• Suitable for checking holes, best method to check ovality and

tapers of circular holesDisadvantages • Limited range of measurement• Low response speed• Requirement of auxiliary equipment like pressure regulator• Scale is not uniform • Apparatus is not portable

Electrical magnification

Advantages • High degree of magnification• few moving parts, so less friction and high accuracy• Not sensitive to vibration• Easy to set up and operate• Instrument is small and compact• No need to place indicating device near to measuring unitDisadvantages • Requires external power• Heating of coil• Fluctuation in voltage or frequency of electric supply affect results• Cost is more than mechanical instruments

Electronic comparator