Screw Gauge Experiment.pptx

Experiment of Screw gauge to determine the thickness of a given object.

1. PHYSICS EXPERIMENTS ​ SCREW GAUGE GYANRAJ CHAVAN

2. SCREW GAUGE Aim : To measure the i )Thickness of the Glass Plate ii) Diameter of the metal wire iii) Volume of the given Glass Plate. Apparatus : Screw Gauge , Glass Plate and Metal wire . Description  : Screw Gauge consists of  U shaped metallic frame. To one side of this U frame a long hallow cylindrical tube with a nut inside it, the inner side of cylindrical nut contains a uniform thread cut in it. On the other side of U frame a fixed stud S 1 with a plane face is attached. A screw S 2  is fitted in the cylindrical nut. One side of  the screw S 2  has a plane face similar to that of stud S 1 . The faces of  S 1 and S 2  are plane and parallel to one another. The other end of the screw  carries a milled head ‘H’ attached to a cap ‘C’ with a sloping edge. When the head H is rotated, the screw moves ”to and fro” in the nut.The milled head H is provided with a safety device ‘D’ to rotate the head H. When the object is held between the stud S 1   and  screw S 2  and the head H is rotated using the safety device (D), it produces crackling sound when optimum pressure is applied on the object.

3. SCREW GAUGE

4. SCREW GAUGE The outer surface of long cylindrical nut consists of a thick horizontal line ‘P’ parallel to the axis of cylindrical tube .This line ‘P’ is called Index line. Along the index line a scale is graduated in millimeters. This scale is called Pitch Scale . On the sloping edge of the cap ‘C’ a circular scale is graduated, which consists of 100 equal divisions, this scale is called Head scale. Theory  : The screw gauge works on the principle of screw. When we rotate the head ‘H’ by means of safety device ‘D’ through one complete rotation, the distance moved by the screw for every complete rotation is constant. This constant distance moved by the screw for one complete rotation of head ‘ H ‘ is called Pitch of the screw. If the head scale has 100 equal divisions, then the distance moved by the screw for even 1/100 of a complete rotation can be measured accurately, this is called Least count of screw gauge.

5. SCREW GAUGE    Pitch of the screw No. Of divisions on H scale​ Therefore Least count (L.C) = Procedure  :First we have to determine the least count of the given Screw gauge. To determine the least count of the screw gauge, the head ‘H’ is rotated through certain (say 5) number of complete rotations .The distance moved by the sloped  edge over the pitch scale is measured. Now substitute these values in the formula of pitch of the screw =    Pitch of the screw No. Of divisions on H scale​ Therefore Least count (L.C) =    Distance moved by sloped edge over the pitch scale  No. Of rotations of the screw Pitch of the screw=

6. SCREW GAUGE Now check whether the given screw gauge has any ZERO ERROR or not. To determine the ZERO ERROR, the head H is rotated until the flat end of the screw S2touches the plane surface of the stud s1 (do not apply excess pressure) i.e we have to rotate the head only by means of safety device ‘D’ only.

7. SCREW GAUGE When S 1   and S 2  are in contact ,the zero of the head scale perfectly coincides with the index line as in Fig-(a). In such case there will be no ZERO ERROR and no correction is required. When S 1  and S 2   are in contact ,the zero of the head scale is below the index line as in Fig(b), such ZERO ERROR is called positive ZERO ERROR , and the correction is negative . When S 1  and S 2   are in contact ,the zero of the head scale is above the index line as in Fig(c) , such ZERO ERROR is called  negative ZERO ERROR , and the correction is positive . a) Determine the thickness of glass plate  : The given object glass plate is held between the two parallel surfaces of fix stud S1 and screw tip S2.

8. SCREW GAUGE Note the completed number of divisions on pitch scale, which is called PITCH SCALE READING (P.S.R). The number of the head scale division coinciding with the index line is noted, which is called OBSERVED HEAD SCALE READING  n’. If the given screw gauge has ZERO ERROR (x) the correction is made by adding or subtracting the ZERO ERROR (x) from the  OBSERVED HEAD SCALE READING  n’. The corrected value (n’-x) or (n’+x)  is called the HEAD SCALE READING (H.S.R) n. To calculate the fraction the H.S.R (n) is multiplied by the least count (L.C). Thickness of the Glass plate = Total reading =  P.S.R + n× L.C  – – –  – – (1) Changing the position of glass plate , 5 readings should be taken, and recorded in the table-1. Every time calculate the total thickness of the glass plate using equation (1).

9. SCREW GAUGE Average of the 5 readings  of the glass plate should be calculated, to get the average thickness(t) of the given glass plate. Table -1 ( Thickness of glass plate )  : Average thickness of the glass plate (t) =        mm S.NO. M.S.R a cm  Vernier Coincidence (n) Fraction b = n×LC Total Reading      (a+b) cm 1 2 3 4 5

10. SCREW GAUGE b) Determine the radius(r) of the given metal wire  :The given object metal wire is held between the two parallel surfaces of fix stud S1  and screw tip S2 . Note the completed number of divisions on pitch scale, which is called PITCH SCALE READING (P.S.R). The number of the head scale division coinciding with the index line is noted, which is called OBSERVED HEAD SCALE READING  n’. If the given screw gauge has ZERO ERROR (x) the correction is made by adding or subtracting the ZERO ERROR (x) from the  OBSERVED HEAD SCALE READING  n’. The corrected value (n’-x) or ( n’+x )  is called the HEAD SCALE READING (H.S.R) n. To calculate the fraction the H.S.R (n) is multiplied by the least count (L.C). Diameter of the given wire = Total reading = P.S.R + n  × L.C

11. SCREW GAUGE Changing the position of metal wire, 5 readings should be taken, and recorded in the table-2 . Every time calculate the total diameter (d) of the metal wire using equation (1). Average of the 5 diameter of the metal wire should be calculated, to get the average diameter(d) of the given metal wire. Radius (r) of the metal wire = d/2 mm Table – 2 (Diameter of the metal wire) : S.NO. M.S.R a cm  Vernier Coincidence (n) Fraction b = n×LC Total Reading      (a+b) cm 1 2 3 4 5

12. SCREW GAUGE Precautions  : i ) Pitch scale reading (P.S.R) should be taken carefully without parallax error  ii ) Head scale reading (H.S.R) should be taken carefully without parallax error  iii )Screw must be rotated by holding the safety device ‘D’. v ) The screw is rotated in only one direction either clock wise or anti-clock wise to avoid the back lash error. Observations  : i ) Zero error = ii) Zero correction =                   mm iii ) Distance moved by the head for 5 complete revolutions =           mm iv ) Number of head scale divisions =  iv ) Do not apply excess pressure on the object held between the surfaces S1 and S2.​

13. SCREW GAUGE    Pitch of the screw No. Of divisions on H scale​ Vi)  Least count (L.C) = c ) Volume of Glass plate (v)   : The length (  l  ) , breadth ( b) are determined using vernier calipers  and thickness ( t ) of the glass plate is determined using screw gauge . The values of  l , b and t are substituted in the equation of volume. V = l ×  b × t     Distance moved by sloped edge over the pitch scale  No. Of rotations of the screw  Pitch of the screw= V )

14. SCREW GAUGE What is Pitch of Screw ? It is the distance traversed by  the screw for one complete rotation of head ‘ H ‘ is called Pitch of the screw. What is the formula to calculate Pitch of Screw What is Least count of screw gauge ? What is the formula to calculate Least count of a screw gauge?    Distance moved by sloped edge over the pitch scale  No. Of rotations of the screw  Pitch of the screw= Minimum amount of thickness which can be measured by the screw gauge is called it's Least count.    Pitch of the screw No. Of divisions on H scale​ Least count (L.C) =

15. SCREW GAUGE If Pitch scale is in  of.50mm and number of head scale divisions are 50 calculate L.C of the screw gauge. L.C = Pitch of screw / No of head scale divisions = 0.5/50 =0.01mm What are the differences between Vernier calipers & Screw gauge. I) screw gauge is more accurate instrument when compared to vernier calipers. L.C  of Screw gauge is 0.01mm and L.C of Vernier calipers is 0.1mm . Smaller the L.C higher will be the accuracy. Ii ) We cannot measure  inner diameters & depths with Screw gauge, But, we can measure them with Vernier calipers.

16. SCREW GAUGE