Screw Gauge

Experiment No :2

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.

photo-8-001

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 S_2 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.

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.

Therefore Least count (L.C) = \frac{Pitch of the screw}{Number of divisions on Head scale} .

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 = \frac{Distance moved by sloped edge over the pitch scale}{Number of rotations of the screw}.

Least count L.C =  \frac{Pitch of the screw}{Number of divisions on Head scale} .

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 S-2 touches the plane surface of the stud S_1 (do not apply excess pressure) i.e we have to rotate the head only by means of safety device ‘D’ only.

zero-error

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 S_1 and screw tip S_2. 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\times 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).

Average of the 5 readings  of the glass plate should be calculated, to get the average thickness(t) of the given glass plate.

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 S_1 and screw tip S_2. 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\times L.C

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 =\frac{d}{2} mm.

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’ iv ) Do not apply excess pressure on the object held between the surfaces S_1 and S_2.

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 =

v) Pitch of the screw =\frac{Distance moved by sloped edge over the pitch scale}{Number of rotations of the screw}.

vi) Least count (L.C) =\frac{Pitch of the screw}{Number of divisions on Head scale} .

Table -1 ( Thickness of glass plate ) :

S.No Pitch Scale Reading (P.S.R) amm Observed H.S.R    (n’) Correction   (x) Corrected H.S.R  n=n’-x Fraction  b=n*L.C Total reading  (a+b) mm
1.
2.
3.
4.
5.

Average thickness of the glass plate (t) =        mm

Table – 2 (Diameter of the metal wire):

S.No Pitch Scale Reading (P.S.R) amm Observed H.S.R    (n’) Correction   (x) Corrected  H.S.R n=n’-x Fraction  b=n*L.C Total reading  (a+b) mm
1.
2.
3.
4.
5.

Average diameter  d =       mm

Average radius r = \frac{d}{2} =         mm .

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 )    mm^3

 

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131 thoughts on “Screw Gauge

  1. Hey, I found your blog while searching on Google your post looks very interesting for me. I will add a backlink and bookmark your site. Keep up the good work!

  2. Ex i) Suppose we are dividing 49 MSD in to 50 VSD
    1MSD =1mm
    The principle of vernier is 50 VSD =49 MSD,
    1 VSD = 49/50 MSD
    Least Count = 1 MSD – 1 VSD
    = 1 MSD -(49/50) MSD = (1/50) MSD
    = 0.02 mm
    Ex ii) Ex i) Suppose we are dividing 99 MSD in to 100 VSD
    1MSD =1mm
    The principle of vernier is 100 VSD =99 MSD,
    1 VSD = 99/100 MSD
    Least Count = 1 MSD – 1 VSD
    = 1 MSD -(99/100) MSD = (1/100) MSD
    = 0.01 mm
    Hope this clarifies your doubt.

  3. For Detailed Calculation go to “Question No:3 of practical Exams Question bank”.

  4. Pingback: Contents 1 « A to Z of Physics

  5. its really nice explanation i got all my answers ….really its satisfactory …thanx

  6. Its too good. explanation part is very clear to understand. i feel that their should also be a numerical example of zero error and calculation part such that students can easily understand.

  7. Great work..,Thanks a lot.., Thank u sir for responding to our quests..,Hope that u might continue..,

  8. yea,, nice description abt the micrometer,, and the pictures really helped me a lot,, thankyou

  9. The minimum length which can be measured by the device is called it’s least count

  10. Screw Gauge is used to measure the thickness of thin objects like glass plates,metal sheets and paper .Also used to measure the diameters or radius of thin wires.

  11. it is very good to knw about the screw gauge and its measurement . this video is so owsem and i am able to nkw how to measure the screw gauge and its least count

  12. thnxxx a lot sir it was awesome.I tried a couple of lab manuals but this is simply unmatched. Hats OFF 2 U!!!

  13. its really good… but it can be more fabulous if the author includes an example to explain it.

  14. elaborate, precise and concise to the point a student can understand without any ambiguity.

  15. okay …..but still it didnt clear my doubt about finding the zero error…..it should have been given with an example….

  16. It is too much better for self understanding. I am extremly impressed wid the description and explanation.

  17. Ex i) Suppose we are dividing 49 MSD in to 50 VSD
    1MSD =1mm
    The principle of vernier is 50 VSD =49 MSD,
    1 VSD = 49/50 MSD
    Least Count = 1 MSD – 1 VSD
    = 1 MSD -(49/50) MSD = (1/50) MSD
    = 0.02 mm
    Ex ii) Ex i) Suppose we are dividing 99 MSD in to 100 VSD
    1MSD =1mm
    The principle of vernier is 100 VSD =99 MSD,
    1 VSD = 99/100 MSD
    Least Count = 1 MSD – 1 VSD
    = 1 MSD -(99/100) MSD = (1/100) MSD
    = 0.01 mm
    Hope this clarifies your doubt.

  18. i think this is a wonderful explanation about this concept but still i think there is a need of example how to find the pitch of the screw. please

  19. screw gauge is a very ,,,,,,,,,, easyyyyyyy.
    i will do experiment it is easyyyyyyy.

  20. it is nicely and easy explained with labelled diagram thank you for it…………………

  21. Pingback: How to Check a Screw Gauge | Family - Popular Question & Answer

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