Unit-3- Gauges, Length and Angle Measurement

Gauges types: Slip Gauges - Limit Gauges - Snap Gauges - Plug Gauges - Thread Gauge - Ring Gauge. Length Measurement: Vernier Caliper - Vernier Height Gauge - Vernier Depth Gauge - Micrometer and its Types - Design Aspects in Fixing Least Count of Vernier and Micrometer. Comparators – Mechanical - Pneumatic - Electrical. Angle Measurement- Protractors - Sine bars - Angle Dekkor -

Optical Flats.

Slip gauge

• Slip gauges are the most accurate form of representing size by means of its two parallel surfaces which are very flat.
• They permit a comparison between the working measuring instruments and recognized international standards of measurement.
• Each individual block precisely ground and lapped to a specific thickness to very high standards of accuracy and surface finish.

• The important feature of slip gauges is that they can be joined together by a process called wringing with very little dimensional uncertainty.
• Wringing is defined as the property of the measuring faces of a slip gauge to adhere to the measuring faces of other slip gauges by sliding or pressing without the use of any external means.
• Wringing in slip gauges occurs due to the molecular adhesion between the liquid film and the mating surfaces and partly due to atmospheric pressure.

Classification of Slip Gauges

• Slip gauges are classified into the following types according to their accuracy
• AA – used to calibrate inspection blocks and very high precision gauging. The tolerances are between + 0.10 µ m to - 0.05 µ m.
• A – used as tool room standards for setting other gauging tools. Here tolerances are from + 0.15 µ m to - 0.05 µ m.
• B – used as workshop standards for precision measurement. These have large tolerances from + 0.25 µ m to - 0.15 µ m

Different grade designations are

• Grade 2: This is the workshop grade usually used for setting up machine tools, positioning milling cutters and checking mechanical widths.
• Grade 1: This is used in good tool rooms for precision work like setting up of sine bars and sine tables, setting dial test indicators to zero and for checking gap gauges.
• Grade 0: This is used for setting or calibrating measuring instruments or gauges.
• Grade 00: This is used in standard laboratories as a master set for calibration.
• Calibration grade: In this grade, the actual sizes of the slip gauges are stated or calibrated on a chart provided with the set.

Application of Slip Gauges

• Slip gauges are used as reference standards to set or calibrate the measuring instruments and gauges.
• They are used to set up length dimensions in comparative measurements.
• Used for setting up sine bars for angle measurements.
• Used for setting up tool height at required length in milling cutters.

Advantages of Ceramic Slip Gauges

• These are corrosion resistant.
• These have superior wringability.
• These are resistant to wear and impact

Snap Gauge

• A snap gauge is a limit gauge, having a C-shaped frame with hardened, adjustable anvils at opposite ends, used to check diameters, lengths, and thicknesses.
• The anvils are made up of hard materials such as tungsten carbide for wear resistance.

Rib type snap gauge

• Double ended snap gauge
• Single ended progressive type

Plate Snap Gauges

• Double ended type
• Single ended progressive type

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Double ended snap gauge

Single ended progressive type

Double ended type

Single ended progressive type

PLUG GAUGE

• Plug gauges are of cylindrical form used in the manner of a plug for inspecting internal dimensions.
• Plain plug gauge is used to check the size of a hole. The gauging portion is made of suitable wear resisting steel and hardened, ground and lapped.

Ring Gauge

• Plain ring gauge is a couple of gauges (‘Go’ gauge and ‘No Go’ gauge) having measurement side with the maximum and minimum limit deviation of shaft to assure the interchangeability of the shaft.

Vernier Caliper

• A vernier caliper is a precision measuring instrument for linear dimensions which can measure inside, outside and depth measurements.

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Ty pe B

Vernier Caliper

Type A Vernier

Least Count

• Assume that each division of the main scale is 0.5 mm. Say the vernier scale contains 25 divisions which coincide exactly with 24 divisions of the main scale.so one vernier division is equal to 1/25 of 24 main scale divisions

1 VSD = 1/25× 24 × 0.5 = 0.48 unit

LC = 1 MSD − 1 VSD

= 0.50 − 0.48 = 0.02 units

21 + (12 × 0.02)

i.e 21.24 mm.

Vernier Height Gauge

Construction

• Base
• Beam
• Measuring jaw and scriber
• Graduations
• Slider

Applications of Vernier Height Gauges

• Used as scriber for marking
• Used to check the height of a surface

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Sources of Errors in Vernier Height Gauges

• Errors occur due to
• Slide arm not parallel with the base
• Unclean and damaged base which causes inaccuracy
• Worn-out scriber
• Improper calibration

Precautions While Using a Vernier Height Gauge

• The base should be held firmly to the surface plate to avoid tipping.
• The height gauge and surface plate should be cleaned properly before use.
• They should be kept in their cases when not in use.
• The scriber should be clamped as near as possible to the beam to reduce vibration and increase accuracy.

VERNIER DEPTH GAUGE

• Unlike a height gauge, the vernier depth gauge does not have a heavy base.
• The slide is connected with the cross beam of the instrument, which by contacting a reference plane on the object surface, establishes the datum of measurement.
• The end face of the beam (main scale), functions as the movable jaw, in contacting the object element whose distance from the datum is to be measured.

Errors in Vernier Depth Gauge

• the reference surface, on which the vernier depth gauge is rested, is not satisfactorily true, flat and square.
• The base or anvil of the vernier depth gauge not firmly rested on the reference surface.

Advantages

• Amplification is achieved by design and it is not dependent on the parts that can go out of wear or calibration.
• Zero setting adjustment is easy.
• There is no theoretical limit to the scale range.

Disadvantages

• The main disadvantages lie in the instruments on which verniers are used.
• The reliability of reading depends more upon the observer that must instruments.
• No way to adjust for any errors other than zero setting.

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Micrometer Screw Gauge

• The micrometer is an improvement over the measurement of the vernier calliper scale discussed in the past article.
• The accuracy of vernier caliper remains to be 0.02 mm, but most of the engineering precision work demands greater accuracy with sensitivity for which an instrument having both these should be used.
• Micrometer works on the principle of screw and nut. The longitudinal movement of the spindle during one rotation is equal to the pitch of the screw i.e., the distance moved by the nut along the screw is proportional to the number of revolutions made by the nut.
• Therefore by controlling the number of revolution and fractions of a revolution made by the nut, the distance it moves along the screw can be accurately predicted.

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Types of Micrometer Screw Gauge �

The following are the four common types of micrometer screw gauge and 3 special purpose types of micrometers.

• Outside Micrometer
• Inside Micrometer
• Micrometer Depth Gauge
• Bench Micrometer
• Special Purpose Micrometer
• Screw Thread Micrometer
• Vee-Anvil Micrometer
• Thickness Micrometer

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External or Outside Micrometer

• Micrometer or side micrometer is used to measure the dimension of small components for greater accuracy. It provides direct reading and is made in various patterns to suit particular applications

• Each revolution of the thimble moves the spindle face 0.5 mm towards the anvil face. There are 50 graduations on the thimble, one thimble division equals.

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7.5 mm + 0.26 mm = 7.76 mm

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26 * 0.01 = 0.26 mm

Inside Micrometer Calipers

• This types of micrometers, they do not have U-shape frame and spindle. The measuring tips are constituted by the jaws with contact surfaces which are hardened and ground to a radius.
• One jaw is held stationary at the end the second one moves by the movement of the thimble. A locknut is provided to check the movement of the movable jaw. These are used for inspecting of small internal dimensions. Its range is from 5 to 50 mm. It is not so widely used.

Parts of Inside Micrometer

Inside micrometer are used for the measurement of larger internal dimensions. It consists of four parts:

• Measuring head or micrometer unit
• Extension Rods.
• Spacing collars.
• Handle

Micrometer Depth Gauge

• This types of micrometers are used for measuring the depth of holes. Micrometer depth gauge is used for measuring the depth of holes, slots and recessed areas.
• It has got one shoulder which acts as the reference surface and is held firmly and perpendicular to the centre line of the hole.
• For the large range of measurements, extension rods can be used. The screw of the micrometer depth gauge has a range of 20mm or 25mm.
• The length of the micrometer depth gauge caries from 0 to 225mm. The rod is inserted through the top of the micrometer. The rod is marked after every 10mm so that it could be clamped at any position.

Advantages of Micrometer Screw Gauge �

• More accurate than rules.
• Greater readability than rules or vernier.
• No parallax error.
• Small, portable and easy to handle.
• Relatively inexpensive.
• Retains accurately better than verniers.
• Has to wear adjusting facility.
• End measurement.

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Disadvantages Micrometer Screw Gauge

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• Short measuring range
• Single-purpose instrument
• Limited wear area of the anvil and spindle tip.
• End measurements only.

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COMPARATORS

• The comparator is a device, which is generally used to measure the dimensional differences, that means it doesn’t show the exact dimension but only shows the deviation of the dimension from the standard dimension.
• The comparators are kind of linear measurement device. The comparators can be broadly classified as follows:
• Mechanical Comparators
• Optical Comparators
• Pneumatic Comparators
• Electrical Comparators

Essential characteristics of a comparator

• Robust design and construction
• Linear characteristics of scale
• High magnification
• Quick in Results
• Versatility
• Minimum Wear of contact Point
• Free from Oscillations
• Free from backlash
• Quick insertion of work piece
• Adjustable table
• Compensation from temperature effects
• Means to prevent damage

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Mechanical Comparators

• The mechanical comparator are generally used for inspection of components in workshop and tool room industry etc.
• Even small deviations can also be magnified in the mechanical comparator. The mechanical comparator is further classified into four types.
• Dial gauge (or) Dial indicator
• Reed type mechanical comparator

Dial Gauge (or) Dial Indicator

• It consists of a case, circular graduated dial, steam, spindle, pointer, gear train and contact point.
• When the contact point touches against the standard specimen (or) master, the dial scale is set to zero by rotating the case. Now the standard specimen is replaced by the workpiece.
• In case there is no variation in the workpiece dimension as compared to standard specimen, the pointer will show zero reading.

Reed type Mechanical Comparator

• It consists of plunger, contact point, movable member, fixed member, pointer and graduated scale etc., Initially the comparator is set to zero scale with the help of standard specimen like slip gauges.
• Then the work piece is kept at the bottom of plunger. So the contact point will touch the surface of work piece-if any variation occurs, then the plunger will move upward.
• Generally the plunger is attached to movable member. Due to the movement of plunger, the movable member also will move. The reeds are connected between movable member and fixed member.
• Due to movement of reeds, the pointer will move on graduated scale and shows the reading.

Sigma Comparator

• It is used to measure the roughness of the surface by calculating the dimensional difference between the measured value and the standard value of the surface material.

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Advantages of mechanical comparators

• Cheaper than all the other type of comparators
• Does not require any external source of power or air supply
• These comparators use a linear scale that can be easily understood.
• Usually these comparators are robust and compact but are very easy to handle
• These are small in size and can are portable from one place to other very easily without much difficulty

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Disadvantages

• Contains more number of moving parts so there develops friction which in turn reduces the accuracy
• Slackness in the moving parts reduces accuracy very drastically
• These have more inertia so the instrument is prone to vibrational effects
• Limited range of the instrument is another drawback as the pointer moves over a fixed scale
• Parallax error may also arise when proper scale is used

Optical Comparator

• An optical comparator is a measuring device used to analyze and measure the dimensional variations of the workpiece.
• The optical comparator projects the magnified image of the workpiece in the display screen to compare with the standard parameter.
• This device is contactless with a workpiece while projecting its image on the screen for magnification.

Optical Lever / Mechanical Optical Comparator

Zeiss Optical Comparator

Pneumatic Comparator

• The pressurized air is used as a working medium in the pneumatic comparator. In pneumatic comparator, there is no metal to metal contact of gauge and workpiece.
• Hence the accuracy of measurement is increased. The amplification range also increased without reduction in range as compared to mechanical or electrical comparator.
• Flow (or) velocity type pneumatic comparator.
• Back pressure type pneumatic comparator

Flow (or) velocity type pneumatic comparator

Solex pneumatic Comparator

Electrical Comparator

Working Principle

• The armature is placed between two coils. One end of armature, the plunger is supported and other end is connected to flexible strip.
• The amplifier is a device, which is used to amplify the output signal.
• With the help of wheatstone bridge circuit, the meter is set to zero. When the plunger touches the work piece surface, due to variation, the plunger will move up or down.
• Due to plunger movement, the armature also move either up or down. Due to this movement, the change in current or potential will be induced and the meter will show the value of displacement.

Merits

• It consists of less moving parts
• Magnification range is high.
• The instrument is compact.

Disadvantages

• External power supply is needed
• Variation of voltage will affect the output

Angle Measurements

SINE BAR

• Sine bar is a high precision and accurate angle measuring instrument.
• Sine principle: It uses the ratio of two sides of a right angle triangle in deriving a given angle

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• A sine bar consists of hardened, precision ground body made up of high carbon, high chromium, corrosion resistant steel.
• The body is mounted on two precision ground cylinders of equal diameter fixed at its ends.

�Angle Dekkor�

• An optical setup where a collimated beam leaves an optical system and is reflected back into the same system by a plane mirror is called as autocolimation.

Bevel Protractor

• It consists of two arms, one fixed and the other adjustable.
• The body of the instrument is extended to form the fixed arm, known as the stock or base plate. The adjustable arm isattached to an engraved circular scale with a vernier scale.
• It is free to rotate about the centre of the nstrument in a turret mounted on the body and can be locked at any position.
• This protractor is capable of measuring angles from 0 to 360.

Types

• Mechanical bevel protractors
• Optical Bevel Protractor
• Universal Bevel Protractor

Optical Bevel Protractor

Universal Bevel Protractor