CHAPTER 4MEASUREMENT INSTRUMENTS

DPT312METROLOGY

ROSHALIZA HAMIDON

STEEL RULE

VERNIER INSTRUMENTS

MICROMETER INSTRUMENTS

OTHER INSTRUMENTS

STEEL RULE

Steel rules are all narrow steel strips with one set or more of graduated marks

These marks are referred to as a scale

The number of subdivisions of unit of length on a rule is called discrimination. (see figure 4.1)

When you measure with a rule, you use the interchange method of measurement because you observe both ends of the part feature at the same time

Measuring with a graduated rule is commonly called direct measurement

Figure 4.1: Discrimination

Steel rule (reference point)

You must consider three factors when using a steel rule

1. Which style of rule do the best job2. Which measurement divisions (scale) should

be used3. Which method of holding both rule and part

allows us to obtain the most precise measurement

These factors help us to the best relationship among the reference point on the part and the graduations of the rule

Figure 4.2: The right way to use a rule is usually easiest, fastest and reliable

The role of error

Measurement errors with steel rules come from

i. Inherent instrument error or toolii. Observational error of eyeiii. Manipulative error of handiv. bias

i. Inherent instrument

Can be eliminated by choosing a quality steel ruler

Quality rules are engine engraved; a machine called ruling engine cuts each graduation

Low cost rules are stamped or printed, whereas engine engraved are sharp.

ii. Observational error

Parallax is important form of observational error, in whichan object appears to shift when observer changes his of her position.

See figure 4.3 To combat parallax; always place

the scale edge of the rule as close as possible

4.3: Parallax error

The observer B would correctly measure x as 16 division,While A would measure 15 and C would measure 17.

iii. Manipulative error

See figure 4.4 Many common manipulative errors

are caused by ‘cramping ‘- the use of excessive force

When you squeeze rule or other instrument tightly, you may forcing it against the part

For reliable measurement, always use a light touch.

Figure 4.4: Manipulative Error

iv. The problem of bias

Bias means that we unconsciously influence each measurement we make

See figure 4.5

Figure 4.5: The problem of bias

- Assume that the dimension need to be 14 cm (5 ½ in).- Because that graduation is easier to read than 13.5 cm (5 31/64 in) or 14.5 (5 33/64 in) cm, you might biased to read 14 instead of the accurate measurement.

VERNIER INSTRUMENT

Vernier instrument are used most tool-rooms, die-making and laboratory work but rarely for modern production inspection

They are referred to as one of the nonprecision intruments, not for their lack of precision, but because they lack the amplification of other instrument

Vernier instrument today includes:1. Vernier caliper 2. height gages,3. depth gages,4. gear tooth instrument 5. protractor

How to read vernier instrument?

1. Read the number of the whole divisions on the main scale that appear to the left of zero (0) on the vernier

2. Read the largest numbered graduation on the main scale that lies to the right of the index (0) on the vernier scale

3. Read the largest whole mirror division to the right index

4. Find the vernier graduation that mosy exactly coincide with any graduation on the main scalesee figure 4.6

Figure 4.6: Reading a vernier in inches

1. VERNIER CALIPER

The simplest of vernier instrument Important in tool-room, die making,

model making etc. They provide long measurement

range (6 to 80in) and are economical

Vernier calipers are slide calipers with a vernier scale attached

Figure 4.7: Parts of a vernier caliper

1.Outside jaws: used to measure external lengths 2.Inside jaws: used to measure internal lengths 3.Depth probe: used to measure depths 4.Main scale (cm) 5.Main scale (inch) 6.Vernier (cm) 7.Vernier (inch) 8.Retainer: used to block movable part to allow the easy transferring a measurement

Vernier caliper (cont’)

Vernier calipers can measure internal dimensions using inside jaws

external dimensions using outside jaws depth measurements by the use of a

probe that is attached to the movable head and slides along the centre of the

Clamping screw that locks the moveable jaw to the beam can minimize error

Figure 4.8: Alignment consideration

2. VERNIER HEIGHT GAGE

The height gage is essentially a vernier caliper with an entire surface plate as its fixed jaw

The surface plate is not part of height gage, but the height make efficient use of the surface plate, because the gage sits right on the plate.

They are available in size from 250 to 1500 mm (10 to 60 in)

Figure 4.9: Vernier height gage

Height gage attachment

1. Scriber(the most important use of the scriber is for layout rather than measurement) See figure 4.10

2. Depth gage attachment(we can convert the instrument to a depth gage with very large range, which allow us to measure relative height differences in inaccessible places)See figure 4.11

3. Indicator holder

Figure 4.10: Scriber

Figure 4.11: Depth gage attachment

The depth gage attachment gets into otherwise inaccessible places

The problem with height gage

A- instability, wooble (tall, thin column sways freely- can destroy the reliability of measurement

B- instability C- magnification of setup errors- dirt,

surface plate error, burr on height gage base

D-magnification of instrument error- wear and abuse can disturb the squareness of the column to the base of the moveable jaw to the column.

Figure 4.12: Height gage error