METROLOGY LABORATORY

NAME: VIKASH JHAJHARIA

CLASS : B.M.E. – III; B2

ROLL : 000711201080

EXPERIMENT NO. – 4

TITLE : MEASUREMENT OF EFFECTIVE DIAMETER OF A SCREW THREAD BY ‘TWO WIRE’ METHOD

1. Draw the screw thread micrometer indicating its special feature in the sketch. Which element of thread does it measure?

Ans :>

The special feature of screw thread micrometer is that it has a wedge shaped anvil and a complimentary wedge shaped spindle. The angle of the wedge is the same as the angle of the thread profile to be measured, i.e. 55° for BSW and 60° for metric. The anvil of this type of micrometer can be moved sideways by means of a screw.

The screw thread micrometer measures the effective diameter of a thread.

2. i. In connection with throat measurement practice, what is best size wire?

ii. Why is it so termed? Derive an expression for the best size wire. What will be the corresponding expression for the effective diameter?

Ans :> Best size wire is the wire which when wrapped around the thread touches the flanks of the thread on the effective radius, which is the half of effective diameter. It is s termed because by using this size of wire, we can easily have an accurate measurement of the effective diameter of a thread using simple formula.

Effective diameter is that diameter on which, the width of each thread and the gaps between two threads are equal. If there is no error in the thread, the effective diameter is the mean diameter. α

3. What are the rake correction and compression correction during thread measurement with the help of two wires?

Ans :>

Rake correction :

During thread measurement with the help of wires as the wire remains in contact with the thread flank it makes an angle equal to the screw. For this inclination the section of the wire made by any plane passing through the axis is rather elliptical. But in all our calculation regarding this experiment, as shown before, we consider this section to be circular. To eliminate the error due to this, we need rake correction.

Compression correction :

In the three wire method, the micrometer exerts some force on the wires while measuring the total diameter of the system consisting if the wire and the thread. Due to this force some compression occurs in the wires and to correct the error due to this, we need to add some correction factor called ‘compression correction factor’.

This correction can be formulated as:

Where A = measuring force in Newton

E = Effective diameter in mm.

4. State only the points of difference between three-wire methods of measurement of effective diameter of thread.

Ans :>

Two wire method :

1. The two wire method the job is carried in the floating carriage measuring machine with fiducial indicator.

2. Only a single diameter can be measured.

Three wire method :

1. The outer diameter is measured with an ordinary micrometer.

2. Two diameters are measured and their mean is taken.

: SAMPLE CALCULATION:

For first observation T = M - 2d

= (1.0242 – 2×0.075 ) inch.

Now effective diameter E = T + Pd

So, Pd = 0.9605×P -1.1657×d

= 0.9605×0.1 – 1.1657×0.075 = 0.0091 inch.

E = T + Pd = (0.8742 + 0.0091) = 0.8833 inch.