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)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
Steel rule (reference point) You must consider three factors when using a steel rule 1. Which style of rule do the best job 2. Which measurement divisions (scale) should be used 3. 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 tool ii. Observational error of eye iii. Manipulative error of hand iv. 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.
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 scale see figure 4.6
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
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)
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
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.
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 1. Outside Micrometer 2. Inside micrometer 3. Depth micrometer 4. Others –e.g : i. wire micrometer ii. Screw thread micrometer All micrometers are based on the relation of a screw’s circular movement to its axial movement
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
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) http://www.technologystudent.comhttp://www.technologystudent.com 2)Webster.J, Outdoor Power Equipment, Thomson Delmar Learning, 2000
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
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
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: i. Light source ii. A lens system (to direct the light past the workpiece) iii. A staging table iv. Projection optics (including both lens and table) v. Screen (where the workpiece image is projected)