Unit 13: Mechanical Operations
LO5: Be able to quality assure components
Planned quality assurance checks
Instructions and answers for teachers
These instructions should accompany the OCR resource ‘Planned quality assurance checks’ activity which
supports Cambridge Technicals in Engineering Level 3.
The Activity:
This activity offers an
This activity offers an
opportunity for English
opportunity for maths
skills development.
skills development.
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Suggested
timings:
1-2 hours
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Activity 1: General dimensional metrology knowledge
Typical answers to the problems are given below:
a) What is meant by the terms ‘accuracy’ and ‘precision’ when making measurements?
Accuracy is how close a measured value is to the true value.
Precision is how close a series of measurements are to each other (if a measurements if repeated
several times)
b) What is meant by the term “resolution” of a measuring instrument?
Resolution is the smallest quantity that a measuring instrument can read.
c) If a dimension contains a tolerance of 100 microns, what does the capability of the chosen measuring
instrument need to be to provide a reliable result?
Resolution of at least 0.1mm
d) Illustrate a vernier scale and how to read it.
An enlarged view of the above calliper shows it has a resolution of 0.02 mm. The reading shown is
3.58 mm. The 3 mm is read off from the upper (fixed) data scale. The 0.58 mm is obtained from the
lower (sliding) indicating scale at the point of closest alignment between the two scales. The
superimposed red markings show where the readings are taken.
e) Illustrate a micrometer, label the various parts; discuss the principle of its operation and
how to read it.
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The diagram below shows a labelled micrometer:
Anvil
Spindle
Locking Lever
Thimble
Frame
Sleeve
The spindle of a micrometer has 2 threads per millimetre, and thus one complete revolution moves
the spindle through a distance of 0.5 millimeter. The longitudinal line on the frame is graduated with
1 millimetre divisions and 0.5 millimetre subdivisions. The thimble has 50 graduations, each being
0.01 millimetre (one-hundredth of a millimetre). Thus, the reading is given by the number of millimetre
divisions visible on the scale of the sleeve plus the particular division on the thimble which coincides with
the axial line on the sleeve.
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Activity 1: Appropriate usage of measuring equipment in dimensional
metrology
Learners will need to be provided with suitable objects to measure, as well as measuring equipment
such as a rule, analogue vernier calliper, digital calliper, micrometer as well as a vernier protractor.
The appropriate type of equipment to measure the various dimensions highlighted is as follows :
a) Overall length – A rule can be used but for better accuracy, the outer jaw of an analogue vernier
calliper should be used. Accuracy of analogue and digital calliper are the same unless both have
different resolution. Measurements should be taken more than once to ensure accuracy and precision
of the result.
b) Overall height – A rule can be used but for better accuracy, the outer jaw of an analogue vernier
calliper should be used. Accuracy of analogue and digital calliper are the same unless both have
different resolution. Measurements should be taken more than once to ensure accuracy and precision
of the result.
c) Hole measurement – A rule or calliper can be used to measure the position of the hole centre. The
inner jaw of an analogue or digital calliper should be used to measure the bore of the hole.
d) Stepped face – A rule and a vernier calliper can be used but for better accuracy a micrometer should
be used.
e) Angle measurement – A vernier protractor (or similar) should be used to measure any angled faces.
The main purpose of the activity is for the learners to familiarise themselves with measurement
equipment. Any object with an adequate number of geometrical features can be used.
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