3STEEL RULESteel rules are all narrow steel strips with one set or more of graduated marksThese marks are referred to as a scaleThe 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 timeMeasuring with a graduated rule is commonly called direct measurement
5Steel rule (reference point) You must consider three factors when using a steel ruleWhich style of rule do the best jobWhich measurement divisions (scale) should be usedWhich method of holding both rule and part allows us to obtain the most precise measurementThese factors help us to the best relationship among the reference point on the part and the graduations of the rule
6Figure 4.2: The right way to use a rule is usually easiest, fastest and reliable
7The role of error Measurement errors with steel rules come from Inherent instrument error or toolObservational error of eyeManipulative error of handbias
8i. Inherent instrumentCan be eliminated by choosing a quality steel rulerQuality rules are engine engraved; a machine called ruling engine cuts each graduationLow cost rules are stamped or printed, whereas engine engraved are sharp.
9ii. Observational error Parallax is important form of observational error, in whichan object appears to shift when observer changes his of her position.See figure 4.3To combat parallax; always place the scale edge of the rule as close as possible
104.3: Parallax errorThe observer B would correctly measure x as 16 division,While A would measure 15 and C would measure 17.
11iii. Manipulative error See figure 4.4Many common manipulative errors are caused by ‘cramping ‘- the use of excessive forceWhen you squeeze rule or other instrument tightly, you may forcing it against the partFor reliable measurement, always use a light touch.
13iv. The problem of biasBias means that we unconsciously influence each measurement we makeSee figure 4.5
14Figure 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.
15VERNIER INSTRUMENTVernier instrument are used most tool-rooms, die-making and laboratory work but rarely for modern production inspectionThey are referred to as one of the nonprecision intruments, not for their lack of precision, but because they lack the amplification of other instrumentVernier instrument today includes:Vernier caliperheight gages,depth gages,gear tooth instrumentprotractor
16How to read vernier instrument? Read the number of the whole divisions on the main scale that appear to the left of zero (0) on the vernierRead the largest numbered graduation on the main scale that lies to the right of the index (0) on the vernier scaleRead the largest whole mirror division to the right indexFind the vernier graduation that mosy exactly coincide with any graduation on the main scalesee figure 4.6
181. 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 economicalVernier calipers are slide calipers with a vernier scale attached
19Figure 4.7: Parts of a vernier caliper Outside jaws: used to measure external lengthsInside jaws: used to measure internal lengthsDepth probe: used to measure depthsMain scale (cm)Main scale (inch)Vernier (cm)Vernier (inch)Retainer: used to block movable part to allow the easy transferring a measurement
20Vernier caliper (cont’) Vernier calipers can measure internal dimensions using inside jawsexternal dimensions using outside jawsdepth measurements by the use of a probe that is attached to the movable head and slides along the centre of theClamping screw that locks the moveable jaw to the beam can minimize error
222. VERNIER HEIGHT GAGEThe height gage is essentially a vernier caliper with an entire surface plate as its fixed jawThe 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)
24Height gage attachment Scriber(the most important use of the scriber is for layout rather than measurement) See figure 4.10Depth 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 4.11Indicator holder
28The problem with height gage A- instability, wooble (tall, thin column sways freely- can destroy the reliability of measurementB- instabilityC- magnification of setup errors- dirt, surface plate error, burr on height gage baseD-magnification of instrument error- wear and abuse can disturb the squareness of the column to the base of the moveable jaw to the column.
30MICROMETER 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)
31MICROMETER INSTRUMENT Types of micrometerOutside MicrometerInside micrometerDepth micrometerOthers –e.g :wire micrometerScrew thread micrometerAll micrometers are based on the relation of a screw’s circular movement to its axial movement
33Micrometer partsframe: 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
38Depth micrometerThe 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 measurementThe 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 micrometerRef:1)2)Webster.J, Outdoor PowerEquipment, Thomson DelmarLearning, 2000
40Feeler 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
42Screw pitch gageThe use of screw pitch gage provides quick and accurate method of checking the thread pitch of a fastenerThe leaves of this measuring tool are marked with the various pitchTo check the pitch, simply match the teeth of the gage with the threads of the fastenerThen read the pitch from the leafRef : J. Erjavec, Automatic Transmission, Thomson Delmar Learning, 2004
44Profile projectorProfile projector is also commonly called as optical projector or optical comparatorIt is an instrument which projects the large shadow of the profile of the workpiece on a glass screenFrom this projection of the workpiece, measurement can be made directly or indirectlyThe profile projector has a table that can be moved laterally and from front to backThe 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
45Profile projector Profile projector consist of: Light source A lens system (to direct the light past the workpiece)A staging tableProjection optics (including both lens and table)Screen (where the workpiece image is projected)