PRECISION MEASUREMENT USING MICROMETER SCREW GAUGE

TITLE: PRECISION MEASUREMENT USING MICROMETER SCREW GAUGE

OBJECTIVES

1.              To understand and get familiar with the use of the micrometer screw gauge.

2.              To determine the least count of the micrometer.

3.              To measure the dimensions of slip gauges using the micrometer and calculate the measurement errors by comparing the measured values with standard values.

4.              To analyze the need of micrometer over Vernier caliper.

5.              To discuss the sources of errors and discuss methods to minimize them.

APPARATUS REQUIRED

            1.     Micrometer Screw Gauge

            2.     Set of slip gauges

            3.     Calculator

THEORY

Micrometer Screw Gauge

A micrometer screw gauge is one of the most common and most popular forms of measuring instrument for precise measurement with 0.01mm accuracy.

Principle: It works on the principle of screw and nut, where a small rotary motion is translated into a precise linear movement.

Figure: Micrometer Screw Gauge

Construction: A micrometer screw gauge consists of a U-shaped frame with a fixed anvil on one end and a movable spindle on the other. The spindle is adjusted using a thimble, which is rotated to move the spindle towards or away from the anvil. The thimble includes a circular scale whereas the main scale is marked on the sleeve of the micrometer. A ratchet mechanism at the end of the thimble helps to apply consistent pressure during measurements. A lock nut holds the spindle in place to ensure the accurate reading.

Least Count: It is the value of one division on a thimble, which is connected to the screw.

i.e. L.C. = (Pitch of the spindle screw)/(Number of divisions on the spindle N)

The total reading of the micrometer (TR) = MSR + (CSR * LC)

where, MSR is main scale reading and CSR is circular scale reading.

Zero Error

Before using a micrometer, it is necessary to set zero. To do this, rotate the thimble until the two anvils are touching and the ratchet slips. At this point, reading should be zero. If the zero mark on the sleeve does not coincide with zero on the thimble, we correct the error with slight rotation of the sleeve by spanner.

Slip Gauge

Slip gauges or gauge blocks are universally accepted end standard of length in industry. These were introduced by Johanson and are also called as Johanson Gauges. Slip gauges are rectangular blocks of high-grade steel with exceptionally close tolerances. The cross-sections of these gauges are 9mm x 30 mm for sizes up to 10 mm and 9 mm x 35 mm for larger sizes. Any two slips when perfectly clean may be wrung together. The gap between two wrung slips is only of the order of 0.00635 microns which is negligible.

PROCEDURE

    1.     The measuring flat surface of micrometer was cleaned.

    2.    The micrometer was calibrated by ensuring it was set to zero when the spindle and anvils touch each other.

    3.     We calculated the least count of the micrometer and measured each of the 10 slip gauges by placing in between the anvils.

    4.     We noted the reading on the main scale and reading on the circular scale which exactly coincided with the reference line on the sleeve.

    5.     Total reading was calculated as follows:

T.R.= M.S.R. + (C.S.C. X L.C)

    6.     The error for each slip gauge was calculated by comparing the measured value with the standard value of the slip gauge.

   Error = Measured Value – Standard Value

OBSERVATION

Observation Table 1

CALCULATION

Least Count of Micrometer

Pitch of the spindle screw = 0.5 mm

Number of divisions in circular scale = 50

L.C. = 0.5/50

∴ L.C.  = 0.01 mm

ANALYSIS

The performed measurements suggest that the micrometer screw gauge is very accurate measuring instrument. The least count of 0.01 mm means that any measurement up to the hundredth place can be measured with micrometer. Lack of flatness of slip gauge and parallax error were the major contributing factors for the inaccuracy that arise in the measurement.

Despite the existence of the Vernier caliper, there was need for a micrometer because it could measure small dimensions more precisely. It also overcame the defect in the Vernier caliper's least count limit. Furthermore, it has a ratchet mechanism that ensures consistent measuring pressure and prevents over-tightening during measurement.

DISCUSSION

During the experiment, we found that the circular scale and main scale might not be properly aligned with each other. So, the spindle and anvil were made to touch each other to make sure that they were perfectly aligned in order to make reading easier and more precise. Also, there might be some error such as alignment error, human error, parallax error, environment error, etc which can be reduced by careful handling of the slip gauges. During measurement of slip gauge, the ratchet should be carefully spun with no extra force to ensure accurate reading and minimum error.

CONCLUSION

In conclusion, the micrometer proved to be a highly reliable and precise instrument for measuring small dimensions. While various sources of error were identified, proper calibration and careful handling can reduce the errors. The experiment shows that the significance of micrometer precision measurement in the field that requires high accuracy.

PRECAUTIONS

    1.     Clean the micrometer by wiping of oil, dirt and dust.

    2.     Clean the measuring faces of the anvil and spindle with a clean piece of cloth.

    3.     Set the zero reading of the instrument before measuring.

   4.   While measuring fine adjustment should be made by ratchet so that uniform measuring pressure is applied.