Precision instruments are required at different stages of manufacturing workshops for good production quality. The level or grade of products is decided and checked on their accuracy. Different measuring instruments are used to check the level of products. The instruments which can measure the smallest and most accurate quantity are called precision measuring instruments. In this chapter, we will discuss different types of precision measuring instruments such as gauges, micrometers, etc their procedure of working, and all related aspects
Vernier Height Gauge
A vernier height gauge is typically used to take precise vertical measurements of various objects. It is possible to measure many different measurements with the gauge, hence, they are most often used in metalworking and other related industries. They often have a scribing tool as the measurement level, allowing the operator to repeatedly mark the vertical distance on pieces of metal that can then be worked on. Heights or vertical distance may also be measured using the bottom of the scribe.
Vernier height gauge can be considered a short of vernier caliper, equipped with a special base block and other attachments which make the instrument suitable for height measurements Along with the sliding jaw assembly, the arrangement is provided to carry a removable clamp. The upper and lower surfaces of the measuring jaws are parallel to the base so that it can be used for measurements over or under the surface. The vernier height gauge is mainly used in the inspection of parts and layout work with a scribing attachment as discussed above..
As an vernier caliper, the vernier height gauge can measure up to 0.02 mm in Metric and 0.001 inch in the British system.
Normally, it is made up of chromium steel or vanadium steel. Usually marking block is used for marking as it has some least count as of offset scriber. In the marking block, the scale can be used for measuring the scale up to 1 / 64 inch or 0.5 mm.
The procedure of Marking Using Vernier Height Gauge
The following are some steps that must be taken while marking with the help of the vernier height gauge.
Step I Check the zero error of the vernier height gauge.
Step II Apply to check media on the job.
Step III Open the sizes on the vernier height gauge and mark them according to the drawing.
Step IV Open the other size on the vernier gauge and mark the line on Job.
Precautions while Using the Vernier Height Gauge
(i) While marking, handle the base in such a manner, so that the main scale remains unaffected.
(ii) Edges of the scriber should be sharpened otherwise while marking extra force will be required.
(ii) Do not punch the marked lines.
(iv) Aware with zero error of scale while measurement.
(v) This device should be kept isolated.
(vi) Oils equipment used after the application.
Vernier Depth Gauge
The vernier depth gauge consists of a graduated scale of either 6 or 12 inches long. It also has a sliding head similar to the one on the vernier caliper. The sliding head is designed to bridge holes and slots. The vernier depth gauge has the of the rule depth gauge. It does not have the accuracy of a micrometer depth gauge. It cannot enter in holes less than 1/4 inch in diameter. However, it will enter a 1/32-inch slot. The vernier scale is adjustable and may be adjusted to compensate for the wear.
The vernier depth gauge differs slightly from the vernier caliper and the vernier height gauge in such a way that the vernier slide assembly remains fixed. At the same time, the steel rule is moved to obtain that is held on the work by one hand while the blade is operated with the other.
The vernier depth gauge has the least count very similar to the vernier calipers ie., 0.001 inches or 0.02 mm.
A base is connected through the head of the depth gauge. For locking purposes of marking, a clamping screw is available at the head. This is a fine adjustment unit that provides accurate data.
Precautions while Using the Vernier Depth Gauge
(i)Clean the measuring faces of the vernier depth gauge with a soft cloth.
(ii) Check the zero setting of the vernier scale by observing the appearance of daylight between the jaws.
(iii) Take the job in your left hand and hold it near the jaws. (iv) Take reading by holding the gauge straight before the eyes.
(v) Do not place the vernier scale in a slanting manner while measuring inside dimensions.
Micrometer
The micrometer is a precision measuring instrument, used for minimum values. Each revolution of the ratchet moves the spindle face 0.5 mm towards the anvil face. The object to be measured is placed between the anvil face and the spindle face.
The ratchet is turned clockwise until the object is trapped between these two surfaces and the ratchet makes a clicking noise. This means that the ratchet cannot be tightened anymore and the measurement can be read.
Micrometer gauges are made in several configurations, but they all rely on the same basic mechanism to take measurements. The measuring tip or spindle, is moved a specific distance by turning a calibrated fine-thread screw. When the spindle contacts with the item being measured, the dimension can be read on a dial or digital readout.
Working Principle of Micrometer
The micrometer works on the principle of the screw and nut. The longitudinal movement of the spindle during one revolution is equal to the pitch of the screw. The movement of the spindle to the distance of the ‘pitch can be accurately measured on the barrel and thimble.
Parts of Micrometer
The several parts of the micrometer are discussed below.
(i) Frame
It is a C-shaped structure that maintains the position of the anvil and the barrel thickness of this structure resists the expansion and compression of the job and does not affect the measurement.
(ii) Anvil
It is the stationary part that helps the spindle to grip the object whose dimensions are to be measured.
(iii) Sleeve/Barrel
The measuring scale is available on the sleeve/barrel.
(iv) Lock Nut/Lock Ring/Thimble Lock
It is a knurled part that helps to lock the spindle to take some more measurements accurately.
(v) Screw
It is situated in the barrel and known as the heart of the micrometer.
(vi) Spindle
This is a cylindrical part that moves to an anvil through the thimble to hold the dimensions to be measured.
(vii) Thimble
Symbols are located on it and move with the thumb.
(viii) Ratchet Stamp
It is situated at the end of the device. It pressurizes the items under measurement. It will be free from the influence of the excessive pressure.
Types of Micrometer
Different types of micrometers are discussed here.
1. Outside Micrometer
This device is used to measure the external dimensions of the job. It has a U-shaped frame of steel. An anvil is installed made up of high carbon steel or tungsten carbide tipped. The second end has a spindle, thimble, lock nut ratchet, etc. Threads are present within the spindle and attached to the frame.
Thimble has a cylindrical scale, normally which has 50 or 100. part units. A lock nut is used to fix the position of the spindle.
Least Count of Micrometer
The minimum possible measurement measured or taken by a micrometer is known as the least count of the micrometer.
Least Count of Metric Micrometer
On the sleeve of the metric micrometer, 25 units are present at a distance of 1 mm or so units are present at a distance of 1/2 mm. As the front part of the thimble is equally divided into 50 or 100 parts.
So, least count
Distance traveled by spindle in one revolution (pitch)
= ———————————————————————-
Number of parts on a thimble
1/2
= —— or
50
1
= ——- = 0.01 mm
100
Least Count of British Micrometer
In a British micrometer, 1″ is divided into 10 large divisions. Each 1 is subdivided into 4 sub-divisions.
Hence, 1″ is divided into 40 parts.
So,
Pitch
Least count = ————————————
Total parts on a thimble
1/40
= ———
25
1
= ———- = 0.001″
1000
[ ∵ As thimble is divided into 25 equal parts]
Zero Error of a Micrometer
Move the spindle of the micrometer until it touches the anvil. If the zero mark on the thimble is not aligned with the zero of the datum line of the sleeve, the micrometer is said to have zero error
- If the micrometer reads plus, it has a minus zero error. The error will have to be subtracted from the actual reading
- If the micrometer reads minus, it has a plus-zero error. The error will have to be added to the actual reading.
The procedure of Taking Reading from Outside Micrometer
(i) Get the least count of the given device (normally it should be 0.01 mm or 0.001″).
(ii) Find the range of the micrometer, if the range is 0-25 then no need to add in the measured value.
If ranges are 25-50, 50-75, or 75-100 then 25, 50, or 75 must be added to the measured value.
(iii) Notice the match between the sleeve and the thimble.
For, Let the 23rd line mark be matched to the datum line then
Value of 23rd part of thimble
= Number of thimble × least count
= 23 x 0.01 mm
= 0.23 mm
Handle sc
Measurement of sleeve scale = 6.5 mm
Hence, the final measurement = 6.5+0.23
= 6.73 mm
Similarly, for the British micrometer, let the 17th mark line of the thimble be matched to the datum line.
Then,
Value of the 17th = 17×0.001
Part of the thimble = 0.017″
Value of 3 main divisions in the main scale
=3×0.1=0.3″
value of one sub-division = 1×0.025
= 0.025″
Total measurement 0.017″ +0.3″ +0.025″
=0.342″
Care and Maintenance of Outside Micrometer
(i) Wipe the anvils and, worked perfectly clean before observing the measurement.
(ii) Test the accuracy of the micrometer.
(iii) Do not leave the anvil faces in contact, when the micrometer is not in use.
(iv) Do not use the cotton to clean the micrometer.
(v) Oil the thread properly.
(vi) Avoid frequent dismantling and assembling.
2. Large Inside Micrometer
The inside micrometer is used to measure the internal dimensions of the job. It is not possible to measure less than 2″ or 50 mm sleeves with has 1/2″ or 10 mm marking scale. An extension rod is used in order to measure the long-size jobs. There are three sets of extension rods available in A, B, and C respectively. It is very similar to the outside micrometer but does not possess a U frame and ratchet. The thimble is situated above the sleeve. A fixed anvil is available to clamp the extension bar and spindle at the other end.
The dimensions of 2″ to 2.5″ are directly measured with a micrometer. A spacing collar of 1/2″ is attached to it for measuring the dimension above 2.5″. An extension bar is fitted for the measurement exceeding that one. The match specifications of three types of bars are discussed below.
(i) Six extension bars with a 1/2″ spacing collar exist in this set A. It is used to measure 2″ to 8″.
(ii) B set has 10 extension bars with a 1/2″ spacing collar. It is used to measure 2″ to 12″.
(iii) C set has 9 extension bars with 1″ and 2″ extension collars. It is used to measure 2″ to 32″.
This micrometer has a thread handle at one end which is used to measure the depth of the job.
3. Small Inside Micrometer
The small inside micrometer is obtained by slight variations in the outside micrometer. It has a spindle similar to the vernier one is a static jaw while the other is a moving jaw, fixed. at sleeve.
It is moveable with the spindle through the thimble. Both jaws have a similar thickness of 5 mm. It has the least count of 5 mm.
It has a marking mete opposite to the thimble. It is available in many ranges, there are 5-25 mm, 25-50 mm, 50-75 mm, and 75-100 mm. Similarly, the British micrometer is available in 0.2-1″,1-2″, 2-3″ and 3-4″.
Precautions While Using the Large and Small Micrometers
(i) Wipe the anvils and the work must be perfectly cleaned before taking measurements.
(ii) Do not leave the anvil faces in contact, during the operation.
(iii) Do not apply excessive pressure while taking the measurement.
(iv) Oiling is to be done in the thread properly.
(v) Do not use Cottone waste to clean the micrometer.
(vi) Test the accuracy of the micrometer before use.
4. Depth Micrometer
The depth micrometer consists of a stock on which, a graduated sleeve is fitted. The other end of the sleeve is the thread with a 0.5 mm pitch, having a V thread. It has a thimble, which is internally threaded to the same pitch and form, mates with the threaded sleeve, and slides over it.
The other end of the thimble has an external step machined and threaded to accommodate a thimble cap. A set of extension rods is generally inserted inside the thimble and the sleeve.
A depth micrometer is widely used to measure the depth of the holes, slots, projections, recesses, keyways, etc. The working principle of this micrometer is the same as that of the outside micrometer. The measuring faces of the stock and rods are hardened, tempered, and ground. The measuring face of the stock is perfectly machined flat. The extension rods may be removed and replaced according to the size of the depth to be measured.
5. Screw Thread Micrometer
It is similar to the outside micrometer in the body structure and is used to measure the pitch diameter of screw threads. It has a moveable spindle and an end is fixed in the shape of a V-grooved Le, fixed anvil.
In measuring screw threads, the angle of the spindle point and the sides of the anvil come in contact with the surface of the thread. So, the reading of the instrument indicates the pitch diameter of the thread.
For measuring the diameter of threads, the anvil has V and the spindle has an angle of 60° while for the Whitworth thread, it is 55°. The set of interchangeable, anvil is also made to suit particular threads.
It is measured to an accuracy of 0.2 mm. Each major division on the main scale is 1 mm and the minor division is 0.5 mm. There are 25 divisions on the vernier scale which are equal to the 24 divisions of the main scale. Therefore, the length of each vernier scale is 0.5 of 24/25, and since the main scale divisions are 0.5 mm, the difference between them is 0.02 mm.
Precautions While Using the Screw Thread Micrometer
(i) Clean the vernier and main scale before use.
(ii) The position of the vernier is zero mark.
(iii) Never take the measurement on the moving job.
(iv) Do not place the micrometer in a slanting manner while measuring the inside dimensions.
(v) Check the micrometer for zero error.
6. Vernier Micrometer
Ordinary metric micrometers can measure only an E accuracy of ±0.01 mm. For taking more accurate measurements, an vernier micrometer is used. Vernier micrometers can measure to an accuracy of ±0.001 mm. It is similar to the ordinary micrometer except that it has an additional vernier scale graduated from the sleeve. It has ten vernier graduation lines, marked parallel about the datum lines. The space between these t lines is equal to 9 (nine) divisions at the thimble.
We know that,
the value of a single division of thimble = 0.01 mm
So, that the value of 9 division = 0.01 × 9 = 0.09 mm
It is equal to the 10 division of the vernier scale.
∴ The value of one division of the vernier scale
0.09
= ——–
10
= 0.009 mm
Least count = One division of thimble – one division of vernier
0.01-0.009
= 0.001 mm
The procedure of Taking Reading from a Vernier Micrometer
For the measurement of any job through the vernier micrometer, the following procedure must be followed.
(i) Firstly, read the scale of the micrometer carefully i.e., read the divisions of thimble that lie down the datum line.
(ii) Now, read the vernier scale i.e., face at thimble.
(iii) For the value for the vernier scale, multiply the sleeve division and least count.
(iv) For the accurate measurement of the vernier micrometer, add the division of the micrometer and the division of the vernier scale.
Example 1 A cylinder is measured by a micrometer, its range is 1″-12″. Thimble is ahead at 3 divisions to the main scale, 7 divisions of thimble in below of datum line and 8 division of vernier scale is right on the any division of thimble.
find out the value of vernier measurement.
Solution The range of micrometers is 1 inch to 12 inches. From figure,
Value of range = 1.00 inch
Main division = division of 3 = 3 x 0.01 = 0.3000 inch
Subdivision = division of 2 <= 2 x 0.025 = 0.0500 inch
Thimble division = division of 7 = 7 x 0.001 = 0.0070 inch
Vernier division = division of 8 = 8 x 0.0001 = 0.0008 inch
∴ Total division = 0.3+0.05 +0.007 + 0.0008 = 0.3578 inch
Example 2 A micrometer, which has a range of 0-25 mm, is used to measure a job. Its thimble is below the datum line and shows the 5 main divisions of the sleeve. The 8th division of vernier is the right face of any division of thimble. Find out the value of the vernier micrometer.
Solution
Range of micrometer = 0 mm to 25 mm
Main division = 5 x 1.000 = 5.000 mm
Sub-division = 0x0.500 00.000 mm
Thimble division = 0 x 0.010 = 00.000 mm
Vernier division = 8 x 0.001=00.008 mm
Total division 5.0+00+0.0+ 0.0085.008 mm
Dial Test Indicator
It is an instrument of high precision, used for comparing and determining the variation in the sizes of a component. It can not give a direct reading of the sizes like micrometer and vernier calipers. A dial test indicator magnifies small variations in sizes by means of a pointer on a graduated dial. It consists of a plunger, which slides through a circular body and moves through causing the pointer to turn and record the magnitude of the plunger movement.
The plunger slides through two bosses in the instrument case and is held in contact with the work by a small tension spring. The magnification of the small movement of the plunger is converted into a rotary motion of the pointer on a circular scale.
An important feature of the dial test indicator is that the scale can be rotated by a ring Berel, enabling it to be set readily to zero.
The dial test indicator is used in conjunction with the stand i.e.,
(i) General purpose holder with cast iron base.
(ii) Magnetic stand with universal clamp.
(iii) Magnetic stand with flexible post.
Procedure to Use a Dial Test Indicator
(i) Attach the dial test indicator with a suitable holder.
(ii) Bring the plunger tip into contact with the workpiece surface so that the pointer makes one or more revolutions.
(iii) Move the indicator relative to the workpiece or job..
(iv) Read the dial test indicator for plus or minus variations.
Precautions While Using the Dial Test Indicator
(i) Place the workpiece to be checked on the surface plate.
(ii) Applying checking media on the workpiece.
(iii) Take the job in left hand always and measure carefully.
(iv) Do not leave the anvil face in contact with the machine during the operation.
Digital Dial Indicator
A digital dial indicator is also called a dial indicator gauge. It shows the small movement by displaying them with a needle on a graduated dial face. A dial indicator looks and works like a car’s speedometer. The faces of dial indicators come in two designs Le, balanced and continuous The digital dial indicator is faster and easier to read than the traditional dial indicator. It has an accuracy of 0.001 mm. It is very similar to ordinary indicators in construction and differs in graduation. It is used for comparing and determining the variation in the sizes of a component. It magnifies small variations in sizes. The direct reading of the deviations gives accurate conditions of the parts being tested.
The magnification of the small movement of the connecter or connecting pin or stylus is converted into a rotary motion of the analog meter.
Precautions While Using the Digital Dial Indicator
(i) Wipe the connector or anvil and clean the work before measurement.
(ii) Used the dial indicator carefully and read the correct measurement.
(iii) Do not apply excessive force while taking measurements.
(iv) Take reading by holding the dial indicator straight before the eyes.
Compactors
Compactors are instruments calibrated by means of end standards to measure unknown dimensions. The purpose of a compactor is the detect and display the small differences between the unknown linear dimensions and the length of the standard.
The difference in lengths is detected as a displacement of a sensing probe. The important and essential function of the instruments is to magnify or amplify the small input displacement so that it is displayed on an analog scale. Compactors are classified on the basis of the type of the amplification method used and they are of the following type.
(i) Mechanical Compactor
(ii) Optical Compactor
(iii) Phlegmatic Compactor
(iv) Electrical Compactor
From the above types of computer, Mechanical computer is widely used. From this point of view, have to study only about the mechanical compactor.
Mechanical Compactor
Conventional mechanical methods of magnification are not suitable in constructions of the obtain yer mechanical compactors as they cause backlash and friction The functioning of a mechanical compactor may be understood by studying a type of mechanical compactor Let us read about a read compactor which is strictly a mechanical compactor.
A spindle is attached to the moveable member which is in contact with the component to be measured. A movable member moves through a distance x, in response to displacement with respect to a fixed member. The movable member is constrained by bluster strips or reed R₁ to move relative to the fixed member and the pointer is attached to reed R₂. A small input displacement produces a large angular movement (x) of the pointer. the scale is Calibrated by means of a gauge block and indicates the difference in displacement of the fixed and movable elements. There are many other systems that are used for mechanical compactor but there is a limit to magnifications that can be achieved with purely a mechanical compactor.
Calibration of Measuring Instruments
Calibration is a comparison between the standard measurement and measurement that uses your instrument. The calibration of the measuring instruments has two objectives. It checks out the accuracy of your word and it determines the traceability of the measurement. The accuracy of all measuring devices degrades over time.
It has the same points of measurement, as follows.
(i) Testing
It is determined, if the instrument’s demand is fulfilled, i.e., ratchet alignment, accuracy of measurement, etc.
(ii) Adjustment
To eliminate the measuring deviation. Such as, if there is stackness between the anvil and spindle then tight it removes the error and eliminates the zero error by adjusting the spindle.
(iii) Certification
For a testing process of a measurement system, to be certified as required by measurement needs such as maintenance of the precision instrument.
(iv) Standards
A measuring instrument needs to be regularly checked, using a calibration process. The calibration process measures an object with known proportions. An object with proportions is called a standard.
British System of Measurement
measurement is most widely used for industrial measurements. But in certain industries, the used.The British system of measurement is still being In this system of measurement the inch, its multiples, and sub-divisions are used to represent length measurements. For e.g., 36 inches or 3 feet make 1 yard. 5280 feet or 1760 yards make 1 mile.
Conversions from the inch to metric and vice-versa
1″ = 25.4mm or 2.54mm
1 yard = 36″ or 0.9144m
1 mm = 0.03937″
1 metre = 1000mm or 39.37″
Example 3 Conversion of metric into inch.
(i) 0.05 mm and (ii) 1.25 mm
Solution (i)1 mm = 0.03937″
∴ 0.05 mm = 0.05 x 0.03937″ =0.00196″
(ii) 1.25 mm = 1.25 x 0.03937″ = 0.0492125″
Example 4 Conversion of inch into metric.
(i) 3/4″ and (ii) 1/1000″
Solution (i) 3/4″ = .75″=0.75x 25.4 = 19.05 mm
(ii) 1/1000″ 0.001″ = .001 x 25.4 = 0.0254 mm