# CHAPTER 4 MEASUREMENT INSTRUMENTS

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CHAPTER 4 MEASUREMENT INSTRUMENTS
DPT312 METROLOGY 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 Which style of rule do the best job Which measurement divisions (scale) should be used 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
Inherent instrument error or tool Observational error of eye Manipulative error of hand 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 which an 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: Vernier caliper height gages, depth gages, gear tooth instrument protractor

Read the number of the whole divisions on the main scale that appear to the left of zero (0) on the vernier Read the largest numbered graduation on the main scale that lies to the right of the index (0) on the vernier scale Read the largest whole mirror division to the right index Find the vernier graduation that mosy exactly coincide with any graduation on the main scale see 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
Outside jaws: used to measure external lengths Inside jaws: used to measure internal lengths Depth probe: used to measure depths Main scale (cm) Main scale (inch) Vernier (cm) Vernier (inch) 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
Scriber (the most important use of the scriber is for layout rather than measurement) See figure 4.10 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 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

MICROMETER INSTRUMENT
A micrometer , sometimes known as a micrometer screw gauge, is a device used widely in mechanical engineering and machining for precisely measuring, along with other metrological instruments such as dial calipers and vernier caliper. (wikipedia)

MICROMETER INSTRUMENT
Types of micrometer Outside Micrometer Inside micrometer Depth micrometer Others –e.g : wire micrometer Screw thread micrometer All micrometers are based on the relation of a screw’s circular movement to its axial movement

Figure 4.13: Outside micrometer caliper

Micrometer parts frame: The C-shaped body that holds the anvil and barrel in constant relation to each other. anvil: The shiny part that the spindle moves toward, and that the thing to be measured rests against. barrel/sleeve: Also called the stock. The stationary round part with the linear scale on it. Sometimes vernier markings. lock-ring/lock nut/thimble lock/Clamp ring: The knurled part (or lever) that one can tighten to hold the spindle stationary, such as when momentarily holding a measurement. screw (not seen): The heart of the micrometer. It is inside the barrel. spindle: The shiny cylindrical part that the thimble causes to move toward the anvil. thimble: The part that one's thumb turns. Graduated markings. ratchet stop: Device on end of handle that limits applied pressure by slipping at a calibrated torque

Proper way to use micrometer

Depth micrometer

Depth micrometer The depth gauge micrometer is a precision measuring instrument, used by engineers to measure depths. Each revolution of the rachet moves the spindle face 0.5 mm towards the bottom of the blind hole. They come in sets with different length depth rods of different ranges measurement The ratchet is turned clockwise until the spindle face touches the bottom of the blind hole. The scales are read in exactly the same way as the scales of a normal micrometer Ref:1) 2)Webster.J, Outdoor Power Equipment, Thomson Delmar Learning, 2000

Wire micrometer To measure diameter of wire

Feeler gage A feeler gage is a simple tool used to measure gap-widths
Feeler gauges are mostly used in engineering to measure the clearance between two parts. They consist of a number of small lengths of steel of different thicknesses with measurements marked on each piece. They are flexible enough that, even if they are all on the same hinge, several can be stacked together to gauge intermediate values

Feeler gage

Screw pitch gage The use of screw pitch gage provides quick and accurate method of checking the thread pitch of a fastener The leaves of this measuring tool are marked with the various pitch To check the pitch, simply match the teeth of the gage with the threads of the fastener Then read the pitch from the leaf Ref : J. Erjavec, Automatic Transmission, Thomson Delmar Learning, 2004

Screw pitch gage

Profile projector Profile projector is also commonly called as optical projector or optical comparator It is an instrument which projects the large shadow of the profile of the workpiece on a glass screen From this projection of the workpiece, measurement can be made directly or indirectly The profile projector has a table that can be moved laterally and from front to back The workpiece is placed on the table and moved into position so that its enlarged shadow comes on the screen. Ref: Serving Ohio, Kentucky & Indiana, Manufacturing technology, New Age Publishe, 2005

Profile projector Profile projector consist of: Light source
A lens system (to direct the light past the workpiece) A staging table Projection optics (including both lens and table) Screen (where the workpiece image is projected)

Profile projector

References Dotson C.L., Fundamentals of Dimensional Metrology, Thomson Delmar Learning, 2006 Webster.J, Outdoor Power Equipment, Thomson Delmar Learning, 2000 J. Erjavec, Automatic Transmission, Thomson Delmar Learning, 2004 Serving Ohio, Kentucky & Indiana, Manufacturing technology, New Age Publisher, 2005