2. Principles of Measurement
The Principle of linear measurement is essentially a comparison
of the piece under test with a known standard.
Two systems are generally used:
- Metric System
- Imperial or English System
Working standards are calibrated against master standards.
The calibration or testing of measuring tools and instruments
usually requires special equipment.
Ensure instruments are maintained in a serviceable condition:
- Keep instruments clean, avoid misuse.
- Return instruments to cases when not in use.
- Keep the inside of the cases clean.
- Do not attempt to dismantle an instrument.
3. Rules, Calipers & Squares
Steel Rule
Only one possible
reading at edge of
ruler
By using a steel rule properly, parallax errors are avoided.
These are apparent differences in dimensions caused by
changing the point of observation.
4. Rules, Calipers & Squares
Outside Calipers
Calipers are used for measuring or comparing distances and
sizes.
Outside measuring calipers legs are curved and turn inwards.
Outside Spring
Caliper
Outside
Caliper
5. Rules, Calipers & Squares
Inside Calipers
Inside measuring calipers legs are straight and turn outwards.
Inside Spring
Caliper
6. Rules, Calipers & Squares
Engineers Square
Engineers squares are used for checking Squareness and
Flatness of components.
They are also used for marking out (in conjunction with a
scriber).
Engineers
Square
7. Gauges
Feeler Gauges
NB: Feeler gauges should be wiped with a clean cloth before and after use. If
grease or dirt is trapped, a false reading may be given.
Feeler Gauges are manufactured from accurately rolled shims
and are used to measure gap sizes from 0.05mm up to about
1.25mm (0.002 to 0.050 inches). The numbers are marked on
the gauge to give the thickness and thus the gap width.
Feeler
Gauges
8. Gauges
Radius Gauges
Radius gauges are usually made of tool steel and cut with an
inside and outside radius of a stated size. The gauges are
grouped with other sizes to make a set.
An accurately machined component can be easily compared
with the profile of a radius gauge.
Radius
Gauges
9. Gauges
Pitch Gauges
NB: Both Pitch gauges and thread, should be clean and free of burrs. It is
useful to hold the screw up to a light to see small gaps. Use the full length of the
gauge for greater accuracy.
A screw pitch gauge has a set of teeth which are of a known
thread form. The gauge is used to determine the pitch of a
thread on a bolt or screw of any diameter.
10. Gauges
Screw pitches
Metric Screw Thread:
Other screw thread types include:
UNF - Unified Fine
UNC - Unified Coarse
BA - British Association
BS Whitworth (BSF, BSP & Conduit) - British Standard
Where:
p = pitch of the thread
d = depth of the thread
r = radius at the top and
bottom of the threads
11. Gauges
Ring Gauges
A Ring Gauge is a precision-sized gauge used for size control of
external diameters/bars.
It can be fitted over a bar end and moved along the length to
test for roundness and accuracy of diameter.
12. Gauges
Plug Gauges
A Plug Gauge is an precision-sized gauge which is used for size
control of internal diameters/holes.
Go No-Go gauges are double ended. If the Go end is sized to
20mm, the No-Go end is slightly bigger, i.e. 20.020mm. If the
Go end fits in the machined hole and the No-Go end doesn’t
then the hole is the an acceptable size.
The tolerance on the gauges can vary and each gauge is usually
specific to a particular job.
13. Now it’s Your Turn!
Try the Exercises in Section 1 of the Worksheet.
14. The Micrometer is an instrument for measuring and comparing
thickness.
Micrometers
Frame
Locking
Ring
Barrel Thimble
Anvil
Measuring Faces
Spindle
Micrometer
Screw Fixed nut
Slotted to adjust
play in threads
Thimble
Sleeve
Thimble Cap
Ratchet
Stop
15. Micrometers
Using a Micrometer
Hold the frame in your hand, gripping with the 2nd, 3rd & 4th
fingers
Rotate the thimble with the ratchet stop if fitted. Ensure
uniform pressure is applied, so a consistent reading is achieved.
Ratchet Stop
16. Ratchet Stop
Micrometers
Using a Micrometer
If a ratchet stop is not fitted, take care when turning the
thimble.
Do not apply to much pressure, as this can damage the
measuring faces and cause excessive wear of the screw threads.
17. Micrometers
Using a Micrometer
When measuring round objects, roll the object over the anvils to
ensure a reading is taken at the largest diameter.
18. Micrometers
Using a Micrometer
When checking square objects, the micrometer should be
adjusted until there is no angular movement.
Take measurements in two or three places to check parallelism
of material.
19. Micrometers
The Metric Micrometer
1mm
Barrel
1/2mm
Thimble 50 divisions
(1 division =1/100mm)
The pitch of the screw in a metric micrometer is ½mm. 1 rev of
the thimble = ½mm.
The beveled edge of the thimble is divided into 50. Each
graduation = 1/50th of ½mm (1/100mm or 0.01mm).
20. Reading (3)
Thimble
Barrel
Reading (1)
Reading (2)
Micrometers
Reading the Metric Micrometer
Read the number of whole mm divisions visible on the barrel.
Read the number of ½mm divisions visible on the barrel.
Read the line on the thimble which coincides with the axis line.
21. Micrometers
Reading the Metric Micrometer
Reading (3)
= 0.22mm
Thimble
Barrel
Reading (1)
= 10mm
Reading (2)
= 0.50mm
Whole mm divisions visible…… 10 = 10.00mm
½mm divisions visible…………. 1 = 0.50mm
Line on the thimble……………… 22 = 0.22mm
Total Reading…………………….. = 10.72mm
22. Micrometers
The Imperial Micrometer
0.025”
Barrel
0.100”
Thimble 25 divisions
1 division =1/1000
(0.001)
The pitch of the screw in an imperial micrometer is 0.025”. 1
rev of the thimble = 0.025”.
The beveled edge of the thimble is divided into 25. Each
graduation = 1/25th of 0.025 (0.001”).
23. Micrometers
Reading the Imperial Micrometer
Read the number of long vertical divisions visible on the barrel.
Read the number of smaller divisions visible on the barrel.
Read the line on the thimble which coincides with the axis line.
Reading (3)
Thimble
Barrel
Reading (1)
Reading (2)
24. Micrometers
Reading the Imperial Micrometer
Larger divisions visible………… 2 = 0.200”
Smaller divisions visible………. 1 = 0.025”
Line on the thimble…………….. 6 = 0.006”
Total Reading……………………. = 0.231”
Reading (3)
= 0.006”
Thimble
Barrel
Reading (1)
= 0.200”
Reading (2)
= 0.025”
25. The Micrometer Depth Gauge is used for measuring depths of
shoulders and holes.
Micrometer Depth Gauges
Thimble
Barrel
Head
26. Micrometer Depth Gauges
Using the Micrometer Depth Gauge
Screw back the spindle to clear the bottom of the hole and place
Micrometer head over the hole.
Adjust the thimble until the head touches the bottom of the hole.
Read off the hole depth.
27. Micrometer Depth Gauges
Reading the Micrometer Depth Gauge
Read the number of whole mm divisions on the barrel.
Read the number of ½mm divisions on the barrel.
Read the line on the thimble which coincides with the axis line.
NB: Remember the scale is reversed from an ordinary micrometer.
28. Micrometer Depth Gauges
Reading the Micrometer Depth Gauge
Whole mm divisions ……………. 9 = 9.00mm
½mm divisions ……….…………. 1 = 0.50mm
Line on the thimble……………… 29 = 0.29mm
Total Reading…………………….. = 9.79mm
29. Now it’s Your Turn!
Try the Exercises in Section 2 of the Worksheet.
30. The Vernier Caliper is an instrument for measuring sizes of
components.
Vernier Calipers
Nibs
Beam Jaw
Beam
Scale
Top Vernier Scale
(Imperial)
Clamping Screws
Clamping
Block
Bottom Vernier
Scale (Metric)
Fine Adjustment
Screw Wheel
Movable Jaw
(Integral
with Vernier)
31. Vernier Calipers
Using Vernier Calipers
With the clamping screws released, hold the calipers in one
hand, the thumb rested on the fine adjustment screw wheel.
Move the jaw forward with the thumb, until contact is made
with the component.
32. Vernier Calipers
Using Vernier Calipers
Lock the jaws with the other hand and adjust the jaws to just
grip the work piece with the fine adjusting screw.
Remove the caliper and read off the dimension.
33. Vernier Calipers
The Metric Vernier Caliper
The Beam Bottom Scale is 10mm divided into 10 (1mm). Each
1mm is divided into 2 (0.5mm).
On the Bottom Vernier Scale, 12mm divided into 25.
(12/25 =0.48mm) - Each division is 0.48mm.
Difference is 0.5 - 0.48 = 0.02mm.
10mm
0.5mm
0.48mm
Beam Bottom Scale
Bottom Vernier Scale
34. Vernier Calipers
Reading the Metric Vernier Caliper
Read the scale to the left of the 0 on the bottom Vernier.
Read the mark where the Vernier Scale is Exactly the same as
the Beam Bottom Scale.
Beam Bottom Scale
Bottom Vernier Scale
Reading (1)
Reading (2)
35. Vernier Calipers
Reading the Metric Vernier Caliper
Bottom Vernier Scale reading…. = 30.50mm
Mark where scales are aligned... 14 (x 0.02) = 0.28mm
Total Reading……………………… = 30.78mm
Beam Bottom Scale
Bottom Vernier Scale
Reading (1)
30.5mm
Reading (2)
14
36. Vernier Calipers
The Imperial Vernier Caliper
0.024 inch
0.025 inch
1 inch
Beam Top Scale
Top Vernier Scale
The Beam Top Scale is 1 inch, divided into 10. Each of the 10 is
divided into 4 (0.025 inch).
On the Top Vernier Scale, 6/10 is divided into 25.
(1/25 of 6/10 = 0.024 inch)- Each division is 0.024 inch.
Difference is 0.025 - 0.024 = 0.001 inch.
37. Vernier Calipers
Reading the Imperial Vernier Caliper
Read the scale to the left of the 0 on the Top Vernier.
Read the mark where the Vernier Scale is Exactly the same as
the Beam Top Scale.
Beam Top
Scale
Top Vernier Scale
Reading (1) Reading (2)
38. Vernier Calipers
Reading the Imperial Vernier Caliper
Top Vernier Scale reading……… = 3.225 inch
Mark where scales are aligned... 16 (x 0.001) = 0.016 inch
Total Reading……………………… = 3.241 inch
Beam Top
Scale
Top Vernier Scale
Reading (1)
3.225 inch
Reading (2)
16
39. Vernier Calipers
Using Digital Calipers
Digital Calipers are operated in the same way as Vernier
Calipers.
Just read off the digital display (can be switched between Metric
and Imperial).
40. The Vernier Height Gauge is
mostly used to measure
surface heights and for
making precise markings on
components.
The gauge has a sharp finger
attached which can be used
for both purposes.
Vernier Height Gauge
Metric Scale
Imperial
Scale
Fine
Adjustment
Clamp
Clamping
Screw
Base
Finger
Clamp
Fine
Adjustment
Screw
Finger
41. Vernier Height Gauge
Using the Vernier Height Gauge
The Height Gauge can be used to measure the height of a
component.
In this case, the height of the component is the same as the
Vernier Gauge reading.
42. Vernier Height Gauge
Using the Vernier Height Gauge
The Height Gauge can be used to measure the Underside of a
component.
In this case, the height of the component is the Vernier Gauge
reading plus the height of the Finger (7.95mm).
43. The Vernier Protractor is used for precisely measuring angles on
components.
Vernier Protractor
Blade Clamp Screw
Blade
Dial
Clamp
Main Scale
Vernier Scale
44. Vernier Calipers
Using a Vernier Protractor
Most protractors are graduated 0 to 90 degrees reading in two
directions.
The Blade can be slid round to any position and clamped. One
side is flat to allow it to lie flat on a workpiece.
45. Vernier Protractors
The Vernier Protractor
The 12 divisions on the Vernier equal 23 degrees on the main
scale.
Each of the 12 divisions is 11/12th of a degree, 1 Vernier div is
1/12th degree = 5 mins
12 Vernier
Divisions
46. Vernier Protractor
Reading the Vernier Protractor
Read the number of whole Degrees visible on the main scale.
Read the number of mins where the Vernier Scale is Exactly the
same as the main scale.
Reading (1)
Reading (2)
47. Vernier Protractor
Reading the Vernier Protractor
Whole degrees visible……………... = 52
Mark where scales are aligned….. = 40
Total Reading………………………... = 52 degs 40 mins
Reading Left to Right
Reading (1)
52
Reading (2)
40
48. Vernier Protractor
Reading the Vernier Protractor
Whole degrees visible……………… = 127
Mark where scales are aligned….. = 20
Total Reading………………………... = 127 degs 20 mins
Reading Right to Left
Reading (1)
127
Reading (2)
20
49. Now it’s Your Turn!
Try the Exercises in Section 3 of the Worksheet.
50. Slip blocks are used as standards for precision length
measurement in the Engineering Industry.
Slip Blocks
51. Slip Blocks
Four Grades of Accuracy
Workshop Grade
Inspection Grade
Calibration Grade
Reference Grade
Care of Block Gauges
When not used, Slips should be kept in their Case.
Avoid Excessive Handling - Don’t touch lapped faces.
Wipe slips clean before and after they are used.
Use soft linen or cambric cloth to clean slips.
Least Expensive
Most Expensive
52. Slip Blocks
Wringing Slips
Select the blocks needed to make up the pack and clean them.
Start with the biggest pair and twist them at 90° under slight
pressure.
53. Slip Blocks
Wringing Slips
For slips less than 1.27mm (.05 inch) wring by sliding together.
To prevent scratches on slips, protector blocks can be used on
the ends on stacks (2mm) which are cheaper to replace.
54. Slip Blocks
Calibration Charts
Blocks are calibrated by comparing them to master gauges
The deviation of each slip is recorded and included in the block
set stating the error.
For High accuracy the calibration chart is essential.
55. Slip Blocks
The Effect of Cumulative Errors
Error due to deviation from true size.
Errors due to grease or dirt between ringing faces.
Errors due to expansion caused by exposing gauges to strong
light or excessive handling.
56. Choosing a Measuring instrument
Basic Rules
Choose instrument in keeping with the tolerance and
dimensions to be measured.
Use an instrument that gives a direct reading wherever
possible.
Don’t use damaged or worn instruments.
Instrument resolution is not always the accuracy to which it
can be used.
Inbuilt accuracy's often exceed resolution.
NB: The larger the size being measured, the more difficult it will be to attain a
high standard of accuracy.
57. Choosing a Measuring instrument
Instrument
Or Measurement
Type of
Measurement Range
Value of smallest
Graduation
(Resolution)
Suggested
Reliability
Steel Rule Direct 150 – 600mm
6 - 24 in
.5mm
.02in
±.5mm
±.02in
Depth Gauge Direct 150mm
6in
.5mm
.02in
±.5mm
±.02in
Calipers Transfer 150mm
6in
none ±.5mm
±.02in
Vernier Calipers Direct 600mm
24in
.01mm
.001in
±.05mm
±.002in
Vernier Depth Gauge Direct 300mm
12in
.01mm
.001in
±.05mm
±.002in
Vernier Height Gauge Direct 600mm
24in
.01mm
.001in
±.05mm
±.002in
Micrometers Direct 0-25mm
0-1in
.01mm
.001in
±.01mm
±.001in
Direct 25-50mm
1-2in
.01mm
.001in
±.01mm
±.001in
Inside Micrometers Direct 2.5-130mm
0.1-5.2in
.01mm
.001in
±.01mm
±.001in
Depth Micrometers Direct 0-300mm
0-12in
.01mm
.001in
±.01mm
±.001in
Slip Gauges End Standard Up to 100mm
Up to 4in
.001mm
.0001in
.0005mm
.00001in
58. Checking Surface Texture
Roughness average index
The Roughness average (Ra) index is an accepted method of
measuring surface finish.
The table shows a series of surface textures which can be
produced by various manufacturing processes.
59. Checking Surface Texture
Using Comparison Plates
Comparison plates are a range of blocks machined to various
calibrated surface textures, each texture measured in microns
(µm) and micro-inches (µin).
60. Checking Surface Texture
Visually ...
Clean work piece and appropriate comparison plate.
Compare the workpiece and plate from many different angles.
The workpiece finish should be at least as good as, if not better.
61. Checking Surface Texture
... or by Touch
Draw your fingernail lightly over the comparison plate.
Repeat the operation on the workpiece and note any differences.
The workpiece finish should be usually better than the plate.
62. Checking Surface Texture
With an Instrument
Pickup
Stylus
Pickup
Mount
Traverse Unit
and Electronics
Controls and
Display
Pickup
Lead
63. Checking Surface Texture
With an Instrument
Pickup
Stylus
Measurement
Length
Surface to be
measured
The Pickup is pulled across the surface for a pre set distance.
A diamond tip stylus measures the roughness of the surface.
The result in Ra (Roughness average) is shown on the display.
65. Checking Surface Texture
What is Ra?
Mathematically, Ra is the arithmetic average value of the
profile departure from the mean line, within a sampling
length.
A method of visualising how Ra is derived is as follows:
Graph A: A mean line X-X is fitted to the measurement data.
66. Checking Surface Texture
What is Ra?
Graph B: The portions of the profile within the sampling length
“l” and below the mean line are then inverted and placed above
the line.
Graph C: Ra is the mean height of the profile above the original
mean line.
Ra
67. Checking Surface Texture
Ra has its Limitations
All the above profiles have different form, but the Ra value is
the same. Other parameters are used to calculate these
differences.
Ra
Ra
Ra
Ra
68. Now it’s Your Turn!
Try the Exercises in Section 4 of the Worksheet.
