When you work in the Engineering Industry you will use a range of graduated measuring devices. Metal Machining in particular, requires precise measurements to be made at all times.
In this chapter we’ll cover information that will help you:
A measurement system can be reliable only if it relates to a known standard which never varies. For example, the standard of length in the metric system is the metre which is the distance that light travels in a vacuum in a certain time.
That precise period of time is the basis of length measurement in the International System which is usually referred to as the metric system (or SI units). While it is not necessary for people entering the engineering industry to know that exact length of time, it is helpful to have an idea of the degree of accuracy that is necessary in establishing the international standard.
In round figures, it takes approximately one three-hundred-millionth of a second for light to travel one metre. With modern technology, scientists are able to make the accurate measurements that are necessary to establish this standard.
Officially the standard of length in the metric system is still the metre, but for most purposes in the engineering industry this unit is too large. The millimetre has been adopted as the basic unit of measurement for most engineering applications.
The main advantage of the metric system is that it is a decimal system. This means that conversion from one unit to another is performed in multiples of ten as you can see in the equation below.
1 metre (m) = 10 decimetres (dm)
= 100 centimetres (cm)
= 1000 millimetres (mm)
= 1000 000 micrometres (µm)
Before leaving this page, think of what you've just been reading, and test yourself with these questions:
[[ mc /r /f ][ What is the international (SI) standard unit for the measurment of length? ][ The Metre. ][ * The Centimetre. ][ * The Millimetre. ][ * The Decimetre. ][ * The Kilometre. ][ The Metre is the SI unit of length. ]]
[[ mc /r /f ][ What is the accepted unit for the measurment of length in engineering applications? ][ * The Metre. ][ * The Centimetre. ][ The Millimetre. ][ * The Decimetre. ][ * The Kilometre. ][ The Millmetre is the accepted unit of length in engineering. ]]
The degree of accuracy in measurement is limited to the finest graduation of the measuring device you are using. For example, an engineers rule is usually graduated in millimetres and half millimetres, therefore the most accurate measurement you can take with an engineers rule is to the nearest half millimetre (0.5mm).
If greater accuracy of measurement is required, an alternative measuring device with finer graduations must be used; for example, vernier calipers that can measure to 0.1mm or a micrometer that can measure to 0.01mm.
It is very important to know the capabilities of a range of measuring devices so you can make the correct choice, particularly when working to fine tolerances. For example, if a part was to be machined to 21mm ± 0.2mm, you wouldn’t use an engineer’s rule to check the size because its finest graduation is only 0.5mm.
In this example vernier calipers could be used. A micrometer would be used where finer tolerances are required; for example, for measuring a shaft that is to be turned to 31mm ± 0.02mm.
Four common types of measurement errors are listed below:
Reading errors occur when you misinterpret the position of the graduation which determines the measurement you are taking.
These mistakes can be caused by counting incorrectly from numbered graduations or by misunderstanding how graduations on the device are numbered.
The reverse side of a particular type of steel rule is numbered from 0 in the centre to 150 at each end as illustrated below. These rules are designed for measuring equal distances each side of a centre point.
Using this side of the rule to measure from its end could lead to a reading error. For example, the graduation marked 80 (from the centre) is actually 70 from the end of the rule. As a further example, when you measure from the end of this type of rule the graduation which is 72mm from the end is actually 78mm from 0 (which is at the centre of the rule).
Reading errors of this kind can generally be avoided by using any measuring device only for its intended purpose. In this case, the side of the rule graduated from the centre should be used only for measuring from the centre.
Parallax errors are caused by the incorrect use of a rule. If the rule is placed flat on the work and the line of sight is not at right angles to the surface of the rule a parallax error will occur. The illustrations below show a parallax error. The size or amount of the error is indicated by the letter P.
Parallax errors can be avoided if the rule is used on edge and the line of sight is perpendicular to the edge of the rule. The illustration on the following page shows the correct position for the rule.
Alignment errors will always occur when measuring tools such as steel rules, outside and inside calipers, vernier calipers and micrometers are not aligned at right angles to the work.
The illustration on the right shows a true perpendicular measurement of 50mm between the vertical faces of the machined angle block.
If the rule was not aligned at right angles to these faces the reading taken from the rule would not be a true measurement. The measurement would appear to be greater than 50mm.
Zero or calibration errors can occur with any graduated measuring device. Zero is simply the starting point. If the starting point is not correct it should be properly adjusted (calibrated) where possible or otherwise allowed for when a reading or measurement is being taken.
A simple way to understand allowing for starting point error is to consider the end of a steel rule such as in the illustration above. Let’s say, as a rather extreme example, that the first millimetre has been worn off the end of the rule. The reading would then appear as 51mm. The true measurement would be calculated in this case by subtracting the zero error (51mm _ 1mm = 50mm). We’ll talk about zero errors in micrometers and verniers in later sections of this chapter.
Before leaving this page, think of what you've just been reading, and test yourself with these questions:
[[ mm /f ][ Match the measurement error with the description of how it can happen: ][ Reading Error ~ By misinterpreting the position of the graduation. ][ Parallax Error ~ By sighting the ruler at an angle other than 90°. ][ Alignment Error ~ By the ruler not being parallel of perpendicular to the work. ][ Zero Error ~ By damaging the end of the ruler. ][ Reading => Misinterpreting; Parallax => Sighting; Alignment => Parallel / Perpendicular; Zero => Damaging. ]]
[[ mc /f ][ What's the best way to allow for a zero error when measuring a length of bar using a steel rule damaged at the end? ][ Measure in the middle of the ruler, and subtract the two end measurements. ][ * Estimate the zero error, and adjust your measurment accordingly. ][ * Cut off the damaged end, and then use the ruler as normal. ][ * Measure the length of bar from both ends, then average the two measurements. ][ You should use a part of the ruler that is not damaged - in the middle of the ruler. ]]
[[ mc /r /f ][ How accurately can you measure with an engineer's rule? ][ * 0.10mm ][ * 0.20mm ][ 0.50mm ][ * 0.75mm ][ * 1.00mm ][ Engineer's rules are usually graduated to 0.50mm. ]]
[[ mc /r /f ][ Which of these devices will give you the finest measurement? ][ * Engineer's Rule. ][ * Calipers. ][ * Vernier Calipers. ][ Micrometer. ][ The Micrometer will give you the finest measurement. ]]
Engineers steel rules are graduated in half millimetres as shown in the illustration below. For general workshop use, rules are available in 150 and 300mm lengths. Accurate steel rules up to two metres in length are available for special purposes. A good quality steel rule is an accurate measuring device which should be used for measuring and marking operations only.
The rule should never be used for any purpose which could bend the rule or damage its edges. In particular, the square end on the rule should never be placed in situations where it could sustain wear. A typical example would be the dangerous practice of measuring a turned shoulder while the job is still rotating in the lathe.
Micrometers are used in the machine shop for taking very accurate measurements. Metric micrometers, such as the one shown below, generally measure in increments of one hundredth of a millimetre (0.01mm) from 0 to 25mm. Micrometers that can measure from 25mm to 50mm, 50mm to 75mm and so on, are also available. The micrometer illustrated is used for taking outside measurements. Other types can be used for taking inside measurements and to measure the depth of holes.
The workpiece is measured between the faces of the fixed anvil and the spindle which is brought into contact with the workpiece by rotating the thimble until a very light pressure is exerted.
Most micrometers of this type have a ratchet device which helps to give a constant pressure. The workpiece and measuring surfaces must be perfectly clean and should be close to normal room temperature.
The spindle screw in a micrometer is precision ground and has a pitch of 0.5mm. This means that each turn of the thimble advances the spindle by exactly 0.5mm. The illustrations on the next page show the micrometer graduations which allow each half millimetre advance of the spindle to be further divided into 50 equal parts.
In the illustration below the graduations are 1.0mm apart. Graduations below the line are offset 0.5mm which means that this scale can measure increments of 0.5mm, the same distance advanced by one turn of the thimble.
Thimble graduations are shown below. The measuring circumference on the bevelled edge of the thimble is divided into fifty equal parts. When the thimble is rotated one of these parts the spindle is advanced by one fiftieth of 0.5mm.
One fiftieth of 0.5mm equals one hundredth of a millimetre (0.01mm). This means that the micrometer is capable of making measurements that are accurate to 0.01mm. From the previous illustrations you can see that only part of the micrometer reading can be taken from the sleeve graduations, while the finer part of the measurement is read from the thimble graduations.
The illustration below shows a total micrometer reading of exactly 7.72 millimetres.
The reading from the sleeve graduations is made up of two parts; seven full millimetres from above the line plus an extra half millimetre below the line. The total reading from the sleeve is 7.50mm.
The thimble graduation that coincides with the line on the micrometer sleeve is 22. This represents an additional measurement of 0.22mm because each graduation on the thimble represents 0.01mm.
Calibrating or zeroing a micrometer involves checking and adjusting the graduations to accurately register a zero reading when the spindle is in contact with the fixed anvil at normal pressure and at normal room temperature. Basic micrometers are usually designed with an adjustment screw for the anvil which allows these fine corrections to be made.
Micrometers are delicate precision instruments which must be treated with a great deal of care and respect. Satisfactory results can only be achieved in the workshop if the accuracy of the micrometer is maintained. Good work practice should include the following:
The Vernier Scale
The vernier scale is used on a range of measuring devices such as calipers, height gauges and depth gauges. The vernier scale allows much finer measurements to be taken than would be possible with a basic or plain scale.
A measuring device that uses the vernier principle has a fixed or main scale and a movable scale which is called the vernier scale. The graduations on the vernier scale are smaller than the graduations on the main scale.
The difference in length between a main scale graduation and the corresponding vernier scale graduation is the finest measurement that can be taken with that particular measuring device.
Basic Vernier Calipers
The illustration below shows a basic vernier caliper which is accurate to 0.1mm. (Imperial scales have been omitted for clarity of illustration and explanation.)
The illustration above shows that the ten graduations on the vernier scale correspond to nine millimetres on the main scale. This means that each vernier scale graduation is 0.9mm in length. The difference in length between a main scale graduation and a vernier scale graduation is 0.1mm which means the vernier caliper is capable of measuring to one tenth of a millimetre.
A measurement is taken by pressing the vernier scale release button and sliding the moving jaw up to the work with a very light pressure. When the spring loaded button is released the vernier scale is locked in position.
The illustration below shows the vernier caliper set to a measurement of 14.7m. The enlarged inset shows the position of the vernier scale in relation to the main scale.
Reading The Vernier
The zero on the vernier scale is between the fourteen and fifteen millimetre graduations on the main scale.
This means the measurement is over 14mm but less than 15mm.
The part of the measurement over 14mm is read from the vernier scale. The small triangle indicates that the seventh graduation on the vernier scale is aligned with a graduation on the main scale, while the others are not. As each graduation on the vernier scale represents a difference of 0.1mm, seven graduations represent a difference of 0.7mm.
The part of the measurement over 14mm is equal to 0.7mm.
Vernier Calipers Accurate To 0.02mm
The illustration below shows a more accurate vernier caliper that is capable of measuring to one fiftieth of a millimetre.
Fifty graduations on the vernier scale correspond to 49mm on the main scale. This means that each vernier scale graduation is 0.98mm in length (49mm divided by 50 equals 0.98mm). The difference in length between a main scale graduation (1mm) and a vernier scale graduation (0.98mm) is 0.02mm. This is the finest measurement that can be taken with the vernier caliper. The illustration below shows the vernier caliper set to a measurement of 37.74mm.
Each set of five vernier graduations represents 0.1mm (5 x 0.02mm). For example, number 4 on the vernier scale represents 0.4mm.
Zero on the vernier scale is between 37mm and 38mm on the main scale. The part of the measurement over 37mm is indicated by the triangle which points to the vernier scale graduation that is aligned with a graduation on the main scale. The number on the left of the aligned graduation is 7, which represents 0.7mm. The two graduations between the 7 and the arrow represent 0.04mm (2 x 0.02mm).
Using The Vernier Caliper
In practice, a vernier scale graduation may not be exactly aligned with a main scale graduation. In this case, the vernier graduation selected should be the one that is closest to a main scale graduation.
To measure with the type of vernier caliper illustrated on the previous page, loosen the locking screws and bring the moving jaw up close to the work. Lightly tighten the fine adjustment locking screw and turn the fine adjustment nut to bring the jaws into contact with the work using a very light pressure. Then lightly tighten the moving jaw locking screw. Always make sure the vernier caliper is positioned at right angles to the work to avoid alignment errors.
Vernier Depth Gauge
Vernier calipers have a depth gauge which is built into the back of the beam and connected to the moving jaw. When the jaws are closed, the vernier reading is zero and the end of the depth gauge is perfectly flush with the end of the beam. When the jaws are opened, the distance that the depth gauge protrudes from the end of the beam is equal to the jaw opening and the vernier reading as shown in the illustration below.
Zero Error
Zero error in vernier calipers is usually caused by wear on the measuring faces of the jaws. Damage such as a bent beam, which can be caused by incorrect use or poor work practice, may also be a source of error. Calibration errors can generally be avoided if the instrument is used and stored correctly.
Care Of A Vernier Caliper
Vernier calipers are precision measuring devices which must be maintained properly and treated with a great deal of care and respect. Good work practice should include the following:
Before leaving this page, think of what you've just been reading, and test yourself with these questions:
[[ mc /r /f ][ One complete turn of the thimble on a micrometer will result in what change of jaw gap? ][ * 0.01mm ][ * 0.10mm ][ * 0.05mm ][ 0.50mm ][ 1.00mm ][ One turn of the thimble corresponds with 0.50mm of gap change. ]]
[[ mc /r /f ][ The bevelled edge of the thimble is divided into how many parts? ][ * 200 parts ][ * 100 parts ][ 50 parts ][ * 20 parts ][ * 10 parts ][ The edge has 50 graduations around its circumference. ]]
[[ mr /f ][ Which of the following practices regarding use of a micrometer seem reasonable? ][ * It must be kept at room temperature. ][ * The spindle should be stored firmly against the anvil. ][ The anvil and spindle should be cleaned before use. ][ It should be cleaned before being put away. ][ The workpiece must be clean and dry. ][ The micrometer doesn't have to be kept at room temperature; the spindle should not be stored against the anvil. ]]
[[ mc /f ][ How accurately can you measure with a standard vernier caliper? ][ To 0.1mm ][ * To 0.2mm ][ * To 0.5mm ][ * To 0.05mm ][ Standard vernier calipers can measure accurately down to 0.1mm. ]]
[[ mc /r /f ][ "The vernier scale graduation nearest to the main scale graduation determines the measurement." - true or false? ][ True. ][ * False. ][ This is true - the measurement is read where the graduations meet. ]]
Digital instruments are usually much easier to read than other types of measuring devices. Measurements taken with the vernier calipers and micrometer require the addition of two or more separate readings from graduated scales. When a digital device is being used the measurement is simply read straight from the digital display.
Digital measuring devices are usually powered by a small battery. In good quality instruments these batteries can last from one to three years with normal use. Battery life should be maximised by making sure that digital measuring devices are switched on only while they are being used.
Some digital displays show measurements to hundredths of a millimetre. However, you should not assume that the device is accurate to one hundredth of a millimetre. The manufacturer’s specifications generally state the degree of accuracy of a device and what you need to do to maintain that accuracy.
Digital measuring devices should never be cleaned with petrol, acetone or other organic solutions and care must be taken to ensure that an external electric charge is never applied to the device.
Digital measuring devices are generally supplied in foam padded protective cases. When a device is not in use it should be stored in its protective case, preferably with a desiccant such as a sachet of silica gel.
The desiccant absorbs and displaces moisture from the air reducing the corrosive effects of humidity on metal surfaces and electronic circuitry. (Warning: Desiccants should be handled with care and may be harmful if ingested.) Good work practice and care requirements for other types of graduated measuring devices should also be followed when you are using digital devices.
The illustration below shows a basic electronic digital caliper, sometimes called a digital vernier even though it doesn’t have a vernier scale.
Example Specifications
The following is a typical manufacturer’s specification for a basic digital measuring device such as the caliper illustrated above:
These specifications affect the way you can use a digital measuring device in the workshop. You need to be sure that the accuracy of the measurement is within the specified tolerance of ± 0.03mm. The accuracy of the instrument cannot be guaranteed if measuring speed is more than 1.5m per second, if ambient temperature is not between 5°C and 40°C or if relative humidity is 80% or more.
In many workshops you can’t control the temperature and humidity but measuring speed can be controlled easily. For example, when using a digital vernier the thumb wheel should be rotated to move the slider slowly and at a constant speed.
Setting Zero
Clean the jaws of the caliper thoroughly before you switch on the power, then bring the jaws together using a light and constant pressure on the thumb wheel. Press the zero setting button to set zero then open and close the jaws again to verify zero. Basic digital devices usually need to be zeroed every time you switch the power on.
Special Purpose Digital Calipers
Some digital calipers are designed with special attributes, such as water resistance, for use in workshop conditions where measuring devices may regularly come into contact with water based substances such as coolant. These devices are specially designed with very high dust and water protection levels.
The model shown below requires to be zeroed only once for the life of the battery. From then on, each time the display is switched on it will indicate the actual slider position regardless of where the jaws were when the power was switched off.
Digital micrometers are similar in basic construction and operation to other micrometers but they are much easier to read. The model shown below has an optional ratchet stop to ensure that the correct pressure is applied to the spindle when a measurement is taken.
Zero is set by pressing the zero button when the spindle is in the zero position.
Digital micrometers should always be used within the limits set by the manufacturer’s specifications. Work practice and care requirements for other types of micrometers should also be followed when you are using a digital micrometer.
The micrometer shown below is a dual mode instrument that has both mechanical and digital operation.
The mechanical mode is accurate to 0.01mm and the digital mode can be accurate to 0.001mm on some models. As well as the zero setting function, some types can have the origin set to any position of the spindle. This is very useful where incremental measurements need to be taken from a point other than zero.
Before leaving this page, think of what you've just been reading, and test yourself with these questions:
[[ mr /f ][ Which of the following statements regarding digital measuring devices are reasonable? ][ These are easier to read than analogue types. ][ * Two readings are needed to generate a measurement. ][ They are usually powered by a small battery. ][ Only turn them on when being used. ][ Digital devices give a single, direct reading. ]]
[[ mr /f ][ How should digital measuring devices be cared for? ][ They should be stored with dessicant to keep them dry. ][ They should be kept in their caes until needed. ][ * They should be cleaned with petrol acetone or other organic solutions. ][ keep them away from stray electric fields and charges. ][ They should never be cleaned with any solvent. ]]
[[ so /f ][ Place these steps in order for setting up a digital caliper for use: ][ Jaws should be cleaned. ][ Power is switched on. ][ Bring jaws together with light, constant pressure. ][ Press the Zero setting button. ][ Open and reclose jaws to verify zero setting. ][ Jaws cleaned => Power on => Jaws together => Zero setting => Open and reclose jaws. ]]
Several different types of analog dial indicators, as well as digital indicators, are used in the workshop for a wide variety of testing applications which could include:
The illustration below shows a digital indicator set up to test a workpiece held in a four jaw lathe chuck. The indicator is usually supported by a magnetic stand from the lathe bed or attached to the lathe tool post.
The indicator is brought carefully into contact with the work until the spring loaded contact point is lightly depressed.
The origin button is then pressed to zero the indicator and the work is rotated carefully by hand to test the setting of the workpiece.
Any fluctuation of the reading shown on the digital display will indicate that the workpiece is not running truly in the four jaw chuck.
Caution: The lathe should not be running while the workpiece is being tested and adjustments are being made.
The photograph below shows an analog dial indicator which could be used to perform the same tests as the digital indicator. It operates mechanically through a system of gears and levers so that movement of the contact point can be indicated by the pointer on the dial.
When the indicator has been positioned correctly at 90° to the axis of the workpiece, the zero point can be set. On most dial indicators this is carried out by rotating the dial until the zero is aligned with the pointer and then lightly tightening the locking screw.
Like other measuring and testing devices used in engineering, dial indicators are delicate precision instruments which should always be treated with a great deal of care.
The following could be used as a general guide:
Before leaving this page, think of what you've just been reading, and test yourself with these questions:
[[ mr /f ][ What are dial indicators useful for? ][ Checking out-of-round or out-of-centre on a lathe. ][ Setting work accurately in machines. ][ Testing the accuracy of machine slides. ][ * Measuring the inside diameter of small pipes. ][ Dial gauges won't fit inside small pipes. ]]
Various kinds of special purpose hand operated gauges are used in the engineering workshop. Two of the most common ones are illustrated below.
Feeler gauges, or thickness gauges, are used to measure small spaces such as the clearance required between two parts when you are setting up work or adjusting machines. The photograph below shows a common set of feeler gauges ranging from 0.05mm to 1mm in increments of 0.05mm. Using feeler gauges requires a certain ‘feel’. The gauge should fit the space with a very light pressure.
The photograph below shows a set of thread pitch gauges. The gauge leaves fit a range of pitches, generally available in metric, whitworth and unified threads. A common range is 0.35mm to 6mm with 21 or 30 leaves.
Before leaving this page, think of what you've just been reading, and test yourself with these questions:
[[ mr /r /s /f ][ Which of the following feeler gauges would you combine to set a thickness of 1.35mm? ][ 0.05mm ][ 0.10mm ][ * 0.20mm ][ 0.40mm ][ 0.80mm ][ * 1.6mm ][ 1.35 = 0.80 + 0.40 + 0.10 + 0.05 ]]