Wednesday, July 3, 2013

WorkShop Measuring Tools..Definite Read! Part-1

WorkShop Tools! Part-1 

Measuring Tools
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Measuring accurately:

Any measuring tool is liable to damage if it is bumped or dropped. In particular, any instrument that gives readings of 0.1 mm or less:

• Can be damaged or put out of adjustment by unskilled handling.

• Must be checked regularly to ensure it continues to give accurate readings.

They can be checked against working standards of measurement ‑ precisely made steel gauge blocks. Note that steel expands when the temperature rises and contracts when, the temperature fails. Thus measuring accurately is affected by changes in temperature.

When using screw pitch gauges, radius gauges or form type gauges, you should, where possible, hold the work piece and gauge in front of a light background. This allows you to clearly see any differences between the work piece and the gauge.

Basic Measuring Tools:

Using Standard Gauges

Screw Pitch Gauges

A screw pitch gauge is used to find the pitch of a thread. It is a series of thin marked blades which have different pitched teeth. Thread pitch gauges also come in the standard thread forms of metric, Whitworth, BSF, UNF, and UNC which allows both the pitch of the thread to be gauged and the form or shape of the thread, to be checked. Each set of screw pitch gauges has the thread form stamped on it.

Measuring Thread Pitch

Before using a screw pitch gauge, you should measure the approximate pitch of the thread with a rule. To do this for metric threads:
•        put the rule on the thread parallel to the thread axis.
•        line up a major division on the rule with the top or crest of the thread.
•        count the number of crests to another major division, usually 20 - 30 mm.
•        divide the length between the major divisions by the number of crest counted.
•        the answer is the pitch of the thread.
•        then choose the gauge closest to this pitch for the first try.

For imperial threads the method is similar except that the pitch is given as threads per inch (TPI) and so the number of crests in one inch are counted.

  Screw pitch gauge                                                                    Use of screw pitch gauges

Radius Gauges
They are used to check internal and external radii. The gauges are a set of thin blades with a convex (external) and concave (internal) radius of the same size on each blade. The size of the radius is marked on each blade. When the radius on the gauge less than 90 degrees, the gauge is called a fillet gauge.

            Using a radius gauge                                                          Radius gauges

Feeler Gauges
They are used to measure or set clearances between mating parts or for measuring the width of small slots or grooves. In a metric set of feeler gauges the thickness ranges from 0.05 mm to approximately 1 mm in varying steps. The gauges can be built up to produce the thickness required. When using the thinner gauges care should be taken to pull the gauge through a gap rather than push, as by pushing, the gauge will tend to bend and wrinkle or possibly if a sideway movement is used the gauge will tear.

                                                                                              Checking clearances with feeler gauges


Feeler gauge

Thickness Gauges
A thickness gauge is used to measure the thickness of material using a plunger and dial. These gauges are used to measure sheet materials such as paper, plastics, cardboard, leather and sheet metals. They must be handled carefully and kept away from dirt and moisture and returned to their storage box immediately after use.

Form or Profile Gauges
These gauges are used to compare shapes. They can be a fixed shape or profile, or an adjustable type as shown. With the adjustable type, the gauge is set to the master shape as shown, and then compared to the shape being checked.

Adjustable profile gauge

Text Box:
Callipers are used to transfer measurement:

They consist of two legs that are firmly
screwed or fixed together so that they will
maintain the position in which they are set.

Some types of callipers have a spring‑loaded
joint and an adjusting screw to position the legs.

The accurate transfer of measurement when
using callipers, depends upon the feel of
the callipers against the work. This 'feel' is
the light pressure of the callipers as they pass
over the work.

Skill is needed to obtain the correct 'feel' of

Using outside callipers
Text Box:
Outside callipers are used:

•    To measure outside diameters
•    To measure external dimensions
•    To check whether external surfaces are parallel.

Check the diameter of work using outside callipers
 and a rule as follows:

•    Open the jaws of the callipers until they pass
       clearly over the diameter to be measured. The
       work must be stationary when taking readings.

•    Gently tap the back of one leg of the callipers
      against a solid part of the work to slightly close
      the jaws.

•    Try the new setting over the work.

Keep the callipers at right angles to the axis of the work.
Continue to adjust the callipers and check the setting
until you feel the jaws just bear against the work.
When the adjustment is correct, the calliper jaws touch
so lightly that the weight of the callipers is sufficient
to make them pass over the diameter of the work.

When you have adjusted the callipers to have the correct
'feet' against the work, proceed as follows:

•     Place a graduated steel rule flat on a machined or
      flat smooth surface.‑

•     Hold the callipers so that one jaw is against the end
       of the rule. Make sure that the calliper jaws lie on a
       line parallel with the edge of the rule.

•    Read off the measurement at the other jaw.

Text Box:  This measurement will be the diameter of the work.

Practise obtaining the correct 'feel' of the calliper jaws against the work by adjusting the callipers over various diameters. Try setting the callipers on flat parallel material.

Using inside callipers

Inside callipers are used:

    To measure internal diameters

    To measure internal dimensions

•     To check whether internal faces are parallel.

Check the inside diameter of a hole using spring inside
callipers and micrometer as follows:

•    Hold the callipers lightly in your right hand with your thumb and first finger on the adjusting nut. Support the weight of the callipers with the middle or third finger.

•    Place one leg of the callipers just inside and at the bottom of the hole.

•    Open the callipers' legs by the adjusting screw until the other leg touches against the top of the hole.

•    Rock the callipers slightly on the lower leg and adjust the screw until you obtain the 'feel' of the callipers in the hole.

Try moving the top leg at right angles to the other
movement. This will ensure that 'feel' is being obtained
directly opposite the bottom leg.

Graduated Measuring Tools

Steel Rules
Steel rules measure lengths to a degree of accuracy of approximately ± 0.5 mm. As a common instrument, it is often misused. The end of the rule must be maintained with its edge square and sharp. A common error is caused by not sighting across the rule at right angles to the graduations. This is called parallax error.

Measuring Tapes
Like any other measuring device care is essential for reliable operation. The blade or tape must be cleaned as it is withdrawn into the housing otherwise it may be difficult to withdraw or if the tape is dirty when it is withdrawn the markings on the tape may be obliterated or damaged causing difficulty in reading.

Depth Gauge
It can be used to measure the depth of holes, slots, or the distance from an edge to another surface.

Vernier Callipers

Standard vernier callipers measure to within 0.05 mm (0.002 in) and 0.02 mm (0.001 in). Digital callipers are available with an accuracy up to 0.01 mm (0.0005 in).

They can be used to measure outside, inside and depth features. They must be stored in a clean, dry place preferably in the pouch or box in which they were originally bought. It is essential that the corners of the inside and outside jaws are protected against damage otherwise inaccurate readings will result.

Hold the vernier so that you are looking at the scale at an angle and in line with the graduated line. Look along rather than at the line. Move into a position where the light strikes from the back of the vernier scale at about the same angle as your line of sight.

Vernier callipers can be read from zero up to the length of the main scale, often 250 millimetres or more. They may also have provision for taking depth readings.

Text Box:  Reading a Vernier:

•  Read the main scale to the left of the zero of the vernier in millimetres.

•  Now look at the vernier scale below. Note which one of the vernier divisions is opposite a line on the main scale.

•   Each of the lines on the vernier scale represents adivision that is 0.02 of a mm shorter than those of the main scale. Multiply the number of the line on the vernier scale by 0.02 and add the result to the reading
of the main scale.

The next sketch shows the reading on a vernier. There are 37 full divisions on the main scale to the left of the zero. This equals 37 millimetres.

The thirty‑third line on the vernier scale is opposite a line on the main scale giving:

33 x 0.02 = 0.66mm

Now add 0.66 mm to the main scale reading of 37 mm to give a total reading of 37.66 mm.

Certain metric verniers with the vernier scale 49 mm long have each fifth line of the vernier scale numbered from 1 to 10. As each division on the vernierscale represents 0.02 mm, then the fifth line representing 5 x 0.02 which equals 0.1 mm is marked number 1. The tenth line is marked 2, the fifteenth line marked 3, and so on to the end of the scale.

Read this type of scale as follows:
Text Box:  •     Read the main scale as before.

•    Read the numbered divisions of the vernier scale as tenths of a millimetre.

•    Complete the reading by adding the extra 0.02 lines.                                         

Example of a vernier settings:

The main scale reads 60 millimetres. The vernier shows the fifth line which represents 0.5 mm, plus 3 extra divisions which represent:

3 x 0.02 = 0.06 mm.
Total reading is 60
                        + 0.5
                     = 60.56 mm

Some metric verniers have their main scale divided into millimetres and half millimetres, with the vernier scale made 24.5 mm long and divided into 25 equal parts.

The length of each vernier division is therefore one twenty‑fifth of 24.5 mm which equals 0.98 of a mm.

The vernier scale divisions are again 0.02 of a mmshorter than the corresponding main scale millimetredivisions,

The last sketch shows the reading of a vernier reading to 0.02 of a millimetre. It has a vernier scale 24.5 mm long.

There are 37 major divisions on the main scale to the left of the zero, which equals 37 mm. There is also one half‑millimetre division which equals 0.5 mm.

37 + 0.5 = 37.5 mm

The eighth line on the vernier scale is opposite a line on the main scale. Multiply 8 by 0.02 which represents 0.16 and add this to the reading of the main scale.

Main scale                                   37.50
Vernier scale                           +   0.16
Total reading                             =37.66 mm

Exercise – Practice reading the Vernier as shown below:
Text Box:


Text Box:  They enable veryaccurate measurements to be taken. Outside micrometers are used to measure:

• Outside diameters

• Thickness of material

• Lengths of parts.

They are available in various sized frames. All sizes, however, have a measuring range limited to the length of the thread on the spindle.The range is 0 to 25 millimetres.

The principal parts of a micrometer are:



Spindle and Thread

Sleeve or Barrel


A knurled collar or a small lever on the frame can be used to lock the spindle in the barrel.

After the anvils have been set against the work being measured, tighten the spindle lock. This prevents any movement of the spindle while you are reading the micrometer scale.

Remember to loosen the clamp before attempting to take any further readings.

Text Box:  Principles of a micrometer:

The principle of a micrometer that reads to 0.01 of a millimetre is explained below.

Hold a 0‑25 mm outside micrometer by the frame between thumb and first finger of your left hand. Keep the graduations on the sleeve towards you.

Loosen the spindle lock.

Use the finger and thumb of your right hand
on the knurled part of the thimble to screw it
anti‑clockwise. This moves the spindle to the
right and uncovers the graduations on the sleeve.

Look at the gap between the anvils. It is equal to the uncovered 'length of the datum line.

Look at the datum line on the sleeve. It is graduated into millimetres and half millimetres, from zero up to 25 mm, and each fifth millimetre is numbered.

Turn the thimble until zero is level with the datum line. Note the position of the graduation on the sleeve.

Turn the thimble one complete turn. The thimble will
move along one graduation of the sleeve scale. This
is because the pitch of the thread on the spindle is half a millimetre. Two turns of the thimble move the
spindle one millimetre.

Look at the graduations around the thimble.
There are 50 graduations and each fifth graduation
is numbered                                                                                                              

Text Box:
Now wipe the face of the anvils with a piece of clean cloth. Screw the thimble inwards towards the frame until the anvils are touching.

•     Close the anvils gently. Never apply force.

•    Allow your fingers to slip on the knurled part of the thimble.

•  Look at the scales. They should both read zero.

•    Open the anvils by turning the thimble to uncover one division on the thimble scale.

The movement of the anvil  =       of a complete turn.                            50
        50         of   0.5 mm     =   0.01 mm

•    Continue turning the thimble until the tenth line of the thimble is level with the datum line.

•    Hold the micrometer up to the light. By carefully looking at the anvils you should be able to see a small gap. It is 0.1 of a millimetre.

•    Continue turning until the fiftieth line of the thimble is level with the datum line.

The anvils will now be 0.5 of a millimetre apart.
The first graduation on the sleeve will now be visible.

Turn the thimble one more complete turn to open the anvils to 1 millimetre.

If you find that the micrometer does not read zero when the anvils are touching and you are sure that they are clean, the micrometer needs adjusting.

Text Box:
Reading a metric micrometer:

•    Read on the barrel scale the number of
      millimetres that are completely visible.

•    Add any half millimetres that are completely  visible.

•    Note the number of the graduation on the
     thimble scale that is level with the datum line.

•    Add the thimble reading to the other reading.

The sketch shows a micrometer set to a reading.

There are 5 millimetres between the zero and the thimble. There is also one graduation of 0.5 of a millimetre. The twelfth fine of the thimble scale is level with the datum line.

The reading of the micrometer would be:

5.0 mm
+ 0.5 mm
+ 0.12 mm
            = 5.62 mm

Text Box:  Using outside micrometers:

Skill is needed to obtain accurate measure­ments when using a micrometer.

Excessive pressure during adjustment will:

•     Give inaccurate readings

•     Cause strain on the thread

•     Distort the frame.

As you adjust the micrometer anvils against the work, you should feel a light pressure or resistance against the surface. Develop this 'feel' by constant practice, measuring articles of accurately known size.

Some micrometers have a spring‑loaded ratchet which will ensure constant adjusting pressure.

Accurate measurements can be made with the
assistance of the ratchet, provided the micrometer is kept square to the work.

Measure with an outside micrometer as follows:
Hold the outside micrometer in your right hand,
with the graduations on the main scale towards you.

Support the frame on the lower centre of your
palm. Use the little or third finger to hold the frame
to the palm.

Place the middle finger behind and sup­porting the

Keep the first finger and thumb free to adjust the
knurled thimble.

Text Box:  Close the anvils until you feel them just touching the work.

Allow your finger and thumb to slip on the knurled thimble to obtain the correct pressure.

Move the work slightly between the anvils or pass the micrometer over the work by moving your wrist.

Make any further adjustment of the thimble until you obtain the right 'feel'.

When you are satisfied with the feel of the anvils against the
 work, proceed as follows:

•     Remove your fingers from the thimble.

•     Turn the micrometer towards you.

•     Read the measurement.

Sometimes‑ it may be more convenient to hold the
micrometer with both hands by:

•    Supporting the frame between the fingers and thumb
of your left hand

•    Using the thumb and finger of your right hand to adjust
 the thimble.

Exercsie - Read the Micrometers 1. to 10. shown below:
Text Box:

Care During Use

When using any measuring instrument, whether it be a gauge or a graduated instrument, the points listed should be carefully followed.

1       Never drop the instrument
2       When not in use leave the instrument in its case or on a clean rag, never on a hard steel bench
3       Never allow dirt, filings, cutting oils or any other foreign substances to come in contact with the instrument
4       Do not put the instrument on top of or under other instruments or tools
5       Never measure moving objects.
6       Ensure that the instrument is correctly set to zero before use

Storage Procedures
Correct storage procedures will lead to long and reliable service from any gauge or graduated measuring device. The following points should be observed when storing these instruments.

1       Clean the instrument thoroughly during and after use
2       Lightly oil or wrap the instrument in oiled paper
3       Store the instrument in is own case or in a box where it is protected from outside damage
4       Store the instrument in a dry place away from corrosive chemicals or solvents


  1. It's interesting to see the techniques used and the different types of optical measurement being used to accomplish these tasks!


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