Automotive mechanics (volume i)(part 1, chapter4) measuring and checking

Automotive mechanics (volume i)(part 1, chapter4) measuring and checking Measuring and checking 53 Measuring instruments 54 Micrometers 55 Reading micrometers 57 Accuracy and care of micrometers 59 Vernier calipers 59 Dial gauge and its use 60 Depth gauges 61 Marking and checking 61 Tools for marking and checking 62 Other gauges and instruments 65 Electrical test instruments 66 Technical terms 67 Review questions 67

Measuring and checking

Chapter 4

Measuring instruments

Micrometers

Reading micrometers

Accuracy and care of micrometers

Vernier calipers

Dial gauge and its use

Depth gauges

Marking and checking

Tools for marking and checking

Other gauges and instruments

Electrical test instruments

Technical terms

Review questions

Measuring and checking are two important procedures

that are performed in an automotive workshop, and

they must be done accurately In many types

of mechanical service or repair, some type of

measure-ment has to be taken or a check made of the size, fit,

clearance, pressure or other specification With

electrical work, measurements are taken of voltage,

amperage or resistance.

Specifications for components are provided in

service manuals These can include the type, size,

capacity or dimensions of a component, or the

clearance or setting of an adjustment Specifications

can also state the wear limits for parts.

Checking a part against its specifications will

determine whether it is serviceable and suitable for

further use, or whether it is unserviceable and should

be renewed.

Measuring instruments

There are a number of measuring and checking

instruments Some are relatively simple measuring

tools, and these are accurate enough for most general

measurements Others are precision instruments which

are used to take accurate measurements.

Some basic measuring tools are now described.

Steel rule

A steel rule is the basic measuring instrument It is

used for general measurements where great accuracy is

not required Steel rules are usually 150 mm or

300 mm long, and are graduated in millimetres and

half-millimetres When used on edge, a steel rule can

be used as a straightedge to check the flatness of a

surface.

Feeler gauges

Feeler gauges are strips or blades of hardened steel that

are ground or rolled to an accurate thickness They are

usually supplied in sets with a number of blades

(Figure 4.1) Each blade is marked with its thickness in

millimetres They can be used singly, or two or more

blades can be used together to obtain the required

thickness.

Feeler gauges are used to measure small clearances,

such as tappet clearances (see Figure 2.4) With the

correct clearance, the feeler gauge should slide

between the two parts with a slight resistance.

When used with a steel straightedge, feeler gauges

can be used to check the flatness of a part, such as a

cylinder head Feeler gauges can also be used with a surface plate to check flat surfaces (see Figures 4.25 and 4.27).

Wire feeler gauges are similar to flat feeler gauges except that they are round They can be used for checking spark plugs and in other places where a flat gauge would not fit or where a flat gauge would give

an incorrect reading.

Outside calipers

Figure 4.2 illustrates the use of outside calipers to measure the diameter of a shaft The calipers should be adjusted to slip over the shaft with a slight resistance They should not be forced because this would spring the legs and prevent an accurate reading.

When the calipers have been adjusted to size, they are held against a steel rule to read the measurement as shown in Figure 4.3.

Calipers can also be used to compare the sizes of two parts For example, if set to the diameter of one shaft, they can then be used to check whether another shaft has the same diameter If the shafts are the same size, both would have the same resistance to the calipers’ setting.

figure 4.1 A set of metric feeler gauges ranging from

0.05 mm to 0.60 mm

figure 4.2 Using outside calipers to check the diameter

of a shaft

Inside calipers

Inside calipers are used to measure internal

dimen-sions Figure 4.4 shows inside calipers being used to

check the diameter of a large hole.

The calipers are entered into the hole at an angle, as

shown by the dotted lines in Figure 4.4(a), and then

straightened slowly so that they are across the diameter

of the hole The calipers should be adjusted until they

enter the hole in the manner shown, with only a slight

drag The dimension can be read from a steel rule as

shown in Figure 4.4(b).

Micrometers

These are a special type of instrument designed to take

accurate measurements to one-hundredth of a millimetre

(0.01 mm), or in the case of a vernier micrometer, to

one-thousandth of a millimetre (0.001 mm).

There are a number of designs The ones in general

use are outside micrometers, which are used for

external measurements, and inside micrometers, which

are used for internal measurements There are also

depth micrometers and micrometers designed for

special purposes.

■ Micrometers are precision instruments They must

be treated properly to maintain their accuracy and

to prevent them from being damaged.

Outside micrometers

Figure 4.5 shows the construction of an outside micrometer It is a screw-type instrument, consisting of

a frame with an anvil and a threaded sleeve which carries the spindle Turning the knurled part of the thimble screws the spindle in or out in relation to the anvil.

To use the micrometer, the thimble is turned until both the spindle and the anvil are lightly in contact with the object being measured The size is then read from scales which are marked on the sleeve and the thimble (see later section ‘Reading micrometers’).

figure 4.3 Measuring the caliper setting with a steel

rule

figure 4.5 Construction of an outside micrometer

(a)

figure 4.4 Inside calipers

(a) measuring the diameter of a hole (b)

check-ing the measurement against a steel rule

(b)

Small objects that are being measured can be

supported as shown in Figure 4.6(a) For accuracy,

make sure that the anvil and spindle are lightly in

contact with the object and that the micrometer is held

squarely against the surface For larger objects, or parts

to be measured in place, the micrometer is placed over

the part as shown in Figure 4.6(b).

fitted with its shortest anvil, but it also has three other anvils of different lengths It has four setting bars.

■ The setting bars are gauges that have been made to

an accurate length.

Inside micrometers

An inside micrometer is shown in Figure 4.8 It consists of a micrometer head and replaceable spindles It is read in a similar manner to an outside micrometer, using scales on the thimble and on the sleeve of the micrometer head.

While the micrometer head will measure over a small range only, the instrument shown is capable of taking a range of measurements by using spindles

of different lengths The short spindle fitted to the left-hand side of the micrometer head can be removed and the sleeve shown at the top of the illustration can be inserted over it to provide an extension By changing spindles and using the sleeve, the micrometer can be used to measure a wide range of inside dimensions.

To use an inside micrometer, for example to measure a cylinder bore, fit the appropriate spindle (and the sleeve if necessary) to the micrometer head Place the micrometer in the bore and carefully expand

figure 4.6 Measuring with an outside micrometer

(a) small object held in the hand (b) object in

place

figure 4.7 Three sizes of micrometers

(a) 0–25 mm (b) 25–50 mm (c) 25–100 mm

figure 4.8 An inside micrometer and extension spindles

Sizes of outside micrometers

Outside micrometers are available in different sizes.

Three of these are shown in Figure 4.7 They are

identified by their range of measurement: Figure 4.7(a)

is a 0–25 mm micrometer, and Figure 4.7(b) is a

25–50 mm micrometer.

The third micrometer Figure 4.7(c) is an adjustable

micrometer It has replaceable anvils of different

lengths, which gives the instrument a range of

25–100 mm, in steps of 25 mm The end of the anvil is

threaded and fits through a hole in the frame It is held

in place by a knurled nut which allows it to be

removed and replaced.

When an anvil is changed, the micrometer is

checked and, if necessary, adjusted for accuracy This

is done by means of a threaded stop on the anvil and a

fixed-length setting bar between the anvil and the

spindle The micrometer shown in Figure 4.7(c) is

it until the anvil and the spindle are both lightly in

contact with the cylinder walls Move the micrometer

up and down slightly and also from side to side as

shown in Figure 4.9 to obtain an accurate

measure-ment, but do not overtighten it in the bore.

Metric micrometer

With a metric micrometer (Figure 4.10), the main scale

on the sleeve has 1-millimetre (1.00 mm) and half-millimetre (0.50 mm) divisions The half-millimetre divisions are above the datum line and each fifth division is numbered (0, 5, 10 etc) The half-millimetre divisions are below the datum line and are not numbered.

The scale which is marked around the thimble, has fifty divisions, each representing 0.01 mm Therefore, one full turn of the thimble represents 0.50 mm (50 ¥ 0.01 mm).

The micrometer screw has a pitch of 0.50 mm, so one full turn of the thimble moves its edge 0.50 mm along the main scale, which is one of its half-millimetre divisions.

figure 4.9 Measuring a cylinder bore with a micrometer –

while adjusting to size (a) move it slightly up and down (b) move it from side to side

figure 4.10 The divisions of a standard metric

micro-meter

Reading micrometers

A micrometer has two scales which provide the

measurement – one on the sleeve and the other on the

thimble The sleeve of an outside micrometer is

attached to the frame, it has the main scale and also a

datum line.

The thimble is attached to the spindle and has a

scale marked around its edge The thimble rotates the

spindle as the micrometer is being adjusted to take a

measurement and this also advances the thimble along

the main scale on the sleeve Also, as the thimble is

rotated, its scale moves around the sleeve in relation to

the datum line.

■ When reading a micrometer, the main scale is read

first and then the thimble is added to obtain the

actual measurement.

Reading a metric micrometer

To read a micrometer, the main scale is read to the edge of the thimble and the thimble reading is added The procedure is as follows:

1 From the sleeve, read the number of whole-millimetre divisions which are visible on the main scale above the datum line.

2 Add to this a half-millimetre division if one is visible on the main scale below the datum line.

3 From the thimble, note the division that coincides with the datum line and add this to the previous readings.

Example 1

For the micrometer shown in Figure 4.10, the readings taken in order are:

9 whole millimetres = 9.00 mm

1 half-millimetre = 0.50 mm

48 thimble divisions of 0.01 = 0.48 mm

9.98 mm

Example 2

The scales of the micrometer in Figure 4.11 show the

readings of:

main scale 1.00 mm divisions: 10.00 mm

main scale 0.50 mm division: 0.50 mm

thimble divisions: 0.16 mm

10.66 mm

main scale 1.00 mm divisions: 10.00 mm main scale 0.50 mm division: 0.50 mm

thimble divisions: 0.16 mm vernier divisions: 0.006 mm

10.666 mm

Principle of a vernier

The principle of a vernier is to have two similar scales

of the same overall length, but with one scale having one less division than the other.

With a vernier micrometer, the vernier scale on the sleeve has ten divisions which are equal in length to nine divisions on the thimble scale With this arrange-ment, the vernier scale can read parts of a division of the thimble scale.

The scales operate as a vernier in the following manner:

1 Each thimble division represents 0.01 mm, so that the nine thimble divisions represent a total of 0.09 mm.

2 Because the ten vernier divisions are equal in length to nine thimble divisions (0.09 mm), each vernier division represents one-tenth of 0.09 mm, which is 0.009 mm.

3 The difference between a thimble division (0.010 mm) and a vernier division (0.009 mm) is 0.001.

4 Therefore each vernier division is equivalent to 0.001 mm, and the reading is found by the vernier division that lines up exactly with one of the thimble divisions.

■ In Figure 4.12, only each second vernier division

is marked on the sleeve (numbered 2, 4, 6 etc) Therefore, this micrometer will read in steps of 0.002 mm.

Inch micrometer

The scales of an inch micrometer are shown in Figure 4.13 The main scale on the sleeve of the micrometer has divisions of 0.025 inch The thimble has twenty-five divisions around its circumference, each representing 0.001 inch.

The micrometer screw has 40 threads per inch, so that each turn of the thimble moves it one-fortieth of

an inch (0.025 inch) along the sleeve, which is one division of the main scale Each turn of the thimble also moves it through its twenty-five divisions, this being 0.025 inch (25 ¥ 0.001 inch).

figure 4.11 Metric micrometer scales – the reading

shown is 10.66 mm

figure 4.12 Vernier scale of metric micrometer – the

vernier scale consists of ten divisions, but only each second graduation is shown by the figures 2, 4, 6,

8 and 0

Example 3

The vernier scale of the micrometer shown in

Figure 4.12 has the same setting as Example 2 above,

but with the addition of the vernier scale The ‘6’

division of the vernier is the one that lines up with

a division on the thimble to give a vernier reading of

0.006 mm.

The readings of the scales in the figure are:

Vernier micrometer

A vernier micrometer has an additional scale on the

sleeve, called the vernier scale, with division lines

parallel to the datum line This allows measurements to

an additional decimal place (Figure 4.12).

The vernier scale has five divisions, which start

from zero on the datum line and are marked 2, 4, 6, 8

and 0 (10) Each division represents 0.002 mm and is

read in conjunction with the scale on the thimble The

readings are made to two decimal places, as for a

standard micrometer, and then the vernier reading is

added This is the vernier division which lines up with

a division on the thimble.

To understand the scales, read the main scale first

and then the thimble scale as follows:

1 Read the divisions on the main scale commencing

from zero: the first division below the datum line is

0.025 inch, the second 0.050 inch, the third

0.075 inch and the fourth 0.100 inch This division

is marked with the figure ‘1’.

2 Continue along the scale, the first division beyond

‘1’ is 0.125 inch, the next 0.150 inch, then

0.175 inch and 0.200 inch (which is marked with

the figure ‘2’), and so on to a maximum of

1.000 inch.

3 The thimble has twenty-five equal divisions, each

of 0.001 inch The thimble reading is added to the

main-scale reading.

Example 4

The readings of the scales in Figure 4.13 are:

main scale divisions: 0.200 inch main scale divisions: 0.025 inch thimble scale divisions: 0.024 inch

0.249 inch

Accuracy and care of micrometers

An outside micrometer can be checked for accuracy by

testing for zero error.

Turn the thimble until the end of the spindle is in

light contact with the anvil If the micrometer is

accurate, the zero division on the thimble will line

up exactly with the datum line on the sleeve If these

marks are not in line, then the micrometer can be

carefully adjusted Otherwise, an allowance must

be made by adding or subtracting the zero error from

the reading whenever any measurement is taken.

In the case of larger micrometers, which have

replaceable anvils, or extension bars, the accuracy is

checked against a setting bar The adjustment on the

anvil can be altered to correct any error.

The accuracy of an inside micrometer can be

checked with an outside micrometer Set the inside micrometer to a convenient reading and then check the measurement with an accurate outside micrometer, used in the usual way.

Micrometers should be handled carefully and stored correctly to preserve their accuracy They should be kept clean and should not be overtightened or strained when being used.

■ Outside micrometers should always be left with a gap between the spindle and the anvil when not in use.

Vernier calipers

Vernier calipers are precision instruments which give readings in steps of 0.05 mm or, in some instruments, 0.02 mm They consist of a graduated bar with a fixed jaw and a sliding jaw The bar has the main scale and the sliding jaw has the vernier scale.

The object to be measured is placed between the two jaws, and the sliding jaw is carefully adjusted until both jaws are in contact with the object (Figure 4.14) The measurement can then be read directly from the scales of the instrument.

Both external and internal measurements can be taken For internal measuring, the ends of the jaws are shaped to suit Some calipers have scales for both external and internal measurements.

Reading the scales

Figure 4.15 shows the main scale and the vernier scale

of vernier calipers that read in steps of 0.05 mm The main scale is graduated in millimetres, with each

10 mm (1 centimetre) division numbered 1, 2, 3 etc The vernier scale has twenty divisions, these being numbered (2, 4, 6, 8, 10).

figure 4.13 Sleeve and thimble markings on an inch

micrometer

figure 4.14 Using vernier calipers to measure the length

of a valve spring

The twenty divisions of the vernier scale are

equal in length to nineteen 1.00 mm divisions on

the main scale Each division of the vernier is

there-fore nineteen-twentieth of a millimetre, which is

0.95 mm.

The difference between a main scale division of

1.00 mm and a vernier scale division of 0.95 mm is

0.05 mm, which is the reading of each vernier division.

Each second graduation is therefore 0.10 mm and these

are shown as longer lines on the scale with each

second long line marked: 2, 4, 6, 8 and 10.

To read the scales, first read the main scale to the

left of the ‘0’ mark on the vernier and then add the

vernier reading, which is the division line that

coincides with a line on the main scale.

Example

For Figure 4.15, this is:

main-scale divisions: 13.00 mm vernier divisions: 0.40 mm

13.40 mm

It can be seen that the ‘4’ division line of the vernier

is the only one that aligns with a division on the main

scale If the instrument is gradually opened a little

wider, the next graduation will align to give a vernier

reading of 0.45 mm, and so on in steps of 0.05 mm.

Vernier calipers capable of reading in steps of

0.02 mm have a vernier scale divided into fifty

divisions against a main scale of forty-nine divisions.

The difference between a division on the main scale

and one on the vernier scale is therefore one-fiftieth of

a millimetre, which is 0.002 mm.

Dial gauge and its use

This is an instrument which, as its name suggests, has a

face or dial The dial is marked with divisions of

0.01 mm, and a pointer, operated by a plunger, is moved

around the dial to indicate the reading A smaller pointer

on the face of the dial gauge counts the number of full rotations of the large pointer in 1 mm divisions.

The instrument is clamped or supported so that the plunger can be set against the part being checked The bezel (ring) on the edge of the dial is then turned to set

it to zero, that is, the ‘0’ on the dial is set in line with the pointer.

A dial gauge does not take direct measurements, but shows variations from the zero setting These variations are transferred from the plunger to the pointer.

■ The pointer will show a plus reading on one side of zero and a minus reading on the other.

A dial gauge has many applications It can be mounted on a housing to check the end-play of a shaft,

or against a gear to check its clearance Mounted against the face of a flywheel as shown in Figure 4.16, the dial gauge will check runout Mounted on a base, it can be used with a surface plate to check the flatness of a surface, or it can be used to check the straightness of

a shaft resting in vee blocks or mounted between centres Figure 4.17(a) and (b) show a dial gauge and acces-sories which enable it to be supported in various ways while readings are taken When used with a magnetic base, it can be easily attached to iron and steel parts The dial gauge Figure 4.17(c) has a long extension and is used for checking cylinder bores Movement of the plunger at the lower end of the gauge is transferred

up through the extension to the dial gauge at the top Plungers of different lengths can be fitted to suit different-sized cylinder bores These are in a box Figure 4.17(d) that contains the gauge and accessories.

figure 4.15 Scale of vernier

A – main scale, each division is 1 mm;

B – vernier scale, each division is 0.05 mm;

the reading shown is 13.40 mm

figure 4.16 A dial gauge being used to check flywheel

runout

dial gauge

engine flywheel

Depth gauges

Two types of depth gauges are shown in Figure 4.18.

These are used to measure the depth of holes or

recesses, and also the height of parts.

The depth gauge Figure 4.18(a) is used by placing

it on the surface from which the measurement is to be

taken and then sliding the thin steel scale to the depth

of the hole being measured The depth is then taken

directly from the scale.

For more accurate readings, a micrometer depth

gauge Figure 4.18(b) can be used This is fitted with a

micrometer head which is read in the same way as an outside or inside micrometer.

Figure 4.19 shows one type of application for a micrometer depth gauge This is being used to check the dimension between the surface of a cylinder head and the head of a valve Comparing this to the manufacturer’s specifications will help to determine the type of reconditioning that will be carried out.

Marking and checking

When carrying out repairs to motor vehicles, it is sometimes necessary to use hand tools to fit one part to another, to mark off a dimension to locate and then drill a hole, or to carry out some similar modification

to a component At times, it may be necessary to make

a small part such as a plate, bracket, tool or holding fixture.

Making such articles requires the use of hand tools, first to mark out the shape wanted, then to cut and form the metal to shape Tools are also needed to check and measure for accuracy as the work proceeds,

so that the finished article is true to shape and size.

Marking out

This is the process of placing lines on the surface of the metal to show how it will be worked The location

of these is normally obtained from a sketch or drawing

of the article to be made or repaired If the surface of the work is flat, it can be coated with a marking material so that the lines will be seen For a piece of black mild steel, chalk can be rubbed over the surface For bright steel, copper sulphate solution or blue marking dye can be used.

Measurements are made with a steel rule and marks

figure 4.17 Types of dial gauges

(a) and (b) Universal dial gauge (c) and (d) cylinder-bore gauge DIS

figure 4.18 Depth gauges

(a) Depth gauge (b) micrometer depth gauge

figure 4.19 Using a micrometer depth gauge on a

cylinder head and valve DAIHATSU

micrometer depth gauge cylinder head

are made with a steel scriber A centre punch is used to

mark the location of holes.

Tools for marking and checking

Various tools are used to mark out and transfer

measurements to the surface of the work Following

are the tools that are commonly used.

Rules and tapes

Steel rules are usually 150 mm or 300 mm long, and

are graduated in millimetres and half-millimetres.

They are used for all normal workshop measurements.

For longer measurements, such as the track or

wheelbase of a vehicle, a steel tape is used Steel tapes,

being flexible, can also be used to measure curved

surfaces or larger round objects.

Scriber and prick punch

A scriber is a piece of round hard steel about 3 mm

diameter with a long sharp point It is used to draw

lines or marks on the surface of the work It is used

along the edge of a steel rule or a try square.

A prick punch is similar to a centre punch, but it is

ground to a much sharper point Prick-punch marks

are small punch marks which are spaced along a

scribed line They are permanent marks, compared

to scribed lines which are liable to rub off When

marking, light prick-punch marks should accurately

split the scribed line.

Dividers and jenny calipers

Dividers are used for marking dimensions, stepping

out distances, transferring measurements and scribing

circles (Figure 4.20) A small punch mark at the centre

will locate the leg of the dividers when scribing circles.

Jennie calipers (often called jennies) are a

combination of calipers and dividers They can be

used to scribe lines parallel to the edge of the work.

They are set to size by placing the caliper leg against

the end of a steel rule and setting the divider leg to the

required graduation on the rule The caliper leg is

then placed against the edge of the work and the

divider leg is drawn across the work to scribe a line

parallel to the edge (Figure 4.21(a)).

Jennie calipers can be used to find the centre of a

round bar Apply a coating of chalk or marking

compound to the end of the bar and set the jennies a

little shorter than the bar radius Using the caliper leg

against the edge, scribe four lines about 90º apart, as

shown in Figure 4.21(b) The centre of the space between the scribed lines is the centre of the bar.

■ Jennie calipers are sometimes referred to as odd leg calipers because they have one caliper leg and one divider leg.

Steel try square

A steel try square has a stock with a blade at 90º It is used when a line is to be scribed at right angles to the edge of the work It is also used for checking internal and external right angles (Figure 4.22).

To check an external angle, the inside of the stock

of the try square is held firmly against one finished surface, with the blade slightly clear of the work The work is held up to the light, and the blade of the square brought slowly down to contact the surface being checked as shown in Figure 4.22(a) An internal angle

is checked in a similar manner with the outside of the square as shown in illustration Figure 4.22(b).

figure 4.20 Dividers being used to mark a circle

figure 4.21 Using jennie calipers

(a) scribing a line parallel to the edge of the work (b) locating the centre of a round bar