We all know that Vernier callipers are one of the precision instruments that can actually measure with an accuracy of up to 0.1 mm. At the same time, higher accuracy can be attained using Screw Gauge. A Screw Gauge can take measurements up to 0.01 mm to even 0.005 mm. The sole idea behind this experiment is to find how to measure the thickness of a sheet of paper using a screw gauge.
Aim of the Experiment
To measure the thickness of a given sheet using a screw gauge.
Sheet of Paper
Screw gauge is one of the precision instruments that can be used to accurately measure the thickness of a paper, or even the diameter of a thin wire. The structure of a screw gauge consists of a U-shaped frame along with a screw spindle that is attached to the thimble. In a screw gauge, mm scales are engraved, running parallel to that of the thimble.
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The head section of the screw gauge consists of a ratchet, that restricts the over-tightening of the screw. Within the thimble section, it consists of a circular scale, that is divided into 50 or 100 equal parts. It is also termed as a head screw, which moves over the pitch scale while operating.
The anvil, which is the stud with a plane end surface, forms the U shaped frame. It can be found on the opposite side to the tip of the screw. You will find the zero of the head scale coinciding with that of the pitch scale. This occurs when the tip of the screw comes in contact with that of an anvil.
The pitch of the screw gauge can be defined as the distance travelled by the spindle per rotation. The pitch of the screw gauge can be determined by the distances travelled by the screw, divided by the total number of rotations.
The formula for pitch is given by:
Pitch of the Screw = (Distance travelled by the screw) / (No. of Full rotation taken)
Least Count of the Screw Gauge
When the tip of the screw gauge is turned by one division of the head scale, the least count (LC) is taken.
The formula for calculating least count is given by:
Least Count = (Pitch) / (Total number of divisions in the circular scale)
Zero Error and Zero Correction
By looking at the screw gauge image, one needs to consider the zero error in the calculation. The zero error can be calculated by completely rotating the screw until it touches the anvil. Make sure that the edge of the cap is located at the zero marking. The screw gauge needs to be kept vertical so that its zero is facing downward.
By attaining the same position, you can come across three circumstances:
The zero marks from the circular scale align with that of the reference scale. Here, no scope of zero error or zero correction can be found.
The zero marks from the circular scale actually remain above that of the reference scale. Here, the zero error is considered positive, while the zero correction is negative.
The zero marks from the circular scale are actually below the reference line. Here, the zero error is considered negative, while the zero correction is positive.
In order to properly conduct the experiment, make sure to go through the following procedure carefully:
Make sure to insert the sheets in between the studs of the screw gauge. Take the calculation of thickness from five different positions.
Make sure to calculate the average thickness, while determining the correct thickness by zero error.
Least count of the screw gauge: _____ mm
Zero Error of screw gauge: _____ mm
Mean thickness of the given number of papers: _____ mm
Mean corrected thickness of the paper
Thickness observed through screw gauge – Zero error: _____ mm
So, the thickness of the given sheet of paper is _____ mm.
1. What does it mean by backlash error?
In order to make room from the play of the screw, there is a little space from the nut. With gradual usage over time, this space starts to increase. So, when you turn the screw in one direction, the stud tends to move the same. As for the opposite direction, the stud is expected to move in the same manner, but it suddenly stops for a while. This particular error is commonly termed as a backlash error. In other words, backlash error can be defined as the one that is introduced on moving the nut in another direction.
2. What are the physical attributes that contribute to error in screw gauge?
By looking at the screw gauge diagram, one can find there are three physical attributes that can cause errors in screw gauge. The first and foremost one is temperature. Since, screw gauge is made out of metal, upon the increase in temperature, its diameter tends to increase proportionately. The second source of error is manufacturing defects. In fact, no two screw gauges can attain the same precision due to the error. Lastly, human error is worth considering, too, as it can also create hindrance with the outcome of the experiment.