Screw Guage.docx

Experiment no 2 Objective: To measure the diameter of the given set of pins by using screw guage.

Apparatus:  

Screw guage. Pins of different diameters.

Theory : The screw gauge is an instrument used for measuring accurately the diameter of a thin wire or the thickness of a sheet of metal. It consists of a U-shaped frame fitted with a screwed spindle which is attached to a thimble. Parallel to the axis of the thimble, a scale graduated in mm is engraved.

WORKING: Micrometers use the principle of a screw to amplify small distances (that are too small to measure directly) into large rotations of the screw that are big enough to read from a scale. The accuracy of a micrometer derives from the accuracy of the thread-forms that are central to the core of its design. In some cases it is a differential screw. The basic operating principles of a micrometer are as follows: 1. The amount of rotation of an accurately made screw can be directly and precisely correlated to a certain amount of axial movement (and vice versa), through the constant known as the screw's lead (/ˈliːd/). A screw's lead is the distance it moves forward axially with one complete turn (360°). (In most threads [that is, in all single-start threads], lead and pitch refer to essentially the same concept.) 2. With an appropriate lead and major diameter of the screw, a given amount of axial movement will be amplified in the resulting circumferential movement.

PARTS: Frame The C-shaped body that holds the anvil and barrel in constant relation to each other. It is thick because it needs to minimize flexion, expansion, and contraction, which would distort the measurement. The frame is heavy and consequently has a high thermal mass, to prevent substantial heating up by the holding hand/fingers. It is often covered by insulating plastic plates which further reduce heat transference. Anvil The shiny part that the spindle moves toward, and that the sample rests against. Sleeve / barrel / stock The stationary round component with the linear scale on it, sometimes with vernier markings. In some instruments the scale is marked on a tight-fitting but movable cylindrical sleeve fitting over the internal fixed barrel. This allows zeroing to be done by slightly altering the position of the sleeve. Lock nut / lock-ring / thimble lock The knurled component (or lever) that one can tighten to hold the spindle stationary, such as when momentarily holding a measurement. Screw The heart of the micrometer, as explained under "Operating principles". It is inside the barrel. This references the fact that the usual name for the device in German is Messschraube, literally "measuring screw". Spindle The shiny cylindrical component that the thimble causes to move toward the anvil. Thimble The component that one's thumb turns. Graduated markings. ZERO ERROR: Least Count of Micrometer Screw gauge = 0.001 cm. Zero Error. It is a defect in a measuring device (VernierCallipers& Screw Gauge). When jaws of a VernierCallipers or Screw Gauge are closed, zero of main scale must coincides with the zero of vernier scale or circular scale in case of screw gauge.

Procedure :         

Find the value of one linear scale division (L.S.D.). Determine the pitch and the least count of the screw gauge and record it step wise. Bring the plane face B in contact with plane face A and find the zero error. Do it three times and record them. If there is no zero error, then record zero error nil. Move the face B away from face A. Place the wire lengthwise over face A and move the face B towards face A using the ratchet head R. Stop when R turns (slips) without moving the screw. Note the number of divisions of the linear scale visible and uncovered by the edge of the cap. The reading (IV) is called linear scale reading (L.S.R.). Note the number (n) of the division of the circular scale lying over reference line. Find total reading and apply zero correction in each case. Take mean of different values of diameter. Measure the length of the wire by stretching it along a half-metre scale. Keeping one end of wire at a known mark, note the position of other end. Difference in position of the two ends of the wire gives the length of the wire. Do it three times and record them.

Observation and Calculation: Sr.no

Material

Main Scale

Circular scale

C.R*L.C (mm)

(mm) 1 2 3

(mm)

Total reading