Use of floating carriage type of micrometer - Metrology & Quality Control

Q.  Which of the following statements is true?
- Published on 07 Aug 15

a. In three wire method, each flank of a thread is touched by a wire in axial plane section and this is valid only for a thread having rank angle
b. Compression error is always subtracted from effective diameter value obtained
c. Floating carriage type of micrometer is used for two wire method ----
d. The value of θ is assumed 30o while calculating best wire diameter for Withworth thread

ANSWER: Floating carriage type of micrometer is used for two wire method ----

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    Discussion

  • Sravanthi   -Posted on 26 Oct 15
    Three wire method

    - This method is used to measure the pitch or effective diameter.

    - Generally best wires are used because diameters of these wires have contact with thread flanks at pitch diameter.

    - Accurate results in this method are obtained only if thread angle is correct and wires have contact with straight portion of the flank.


    Compression errors:

    - The force exerted on wires by the micrometers result in compression errors. The diameter measured decreases due to the force applied by micrometer and leads to some degree of compression.

    - Compression corrective = 0.001 (F 2/3 / E 1/3 ) is the formula to calculate compression error. Here, F is the force applied by micrometer in Newton's, E is the effective diameter.


    Floating carriage micrometer

    - This micrometer is used for two wire method.

    - Least count available for floating carriage micrometers are 0.001 mm, 0.002 mm, 0.005 mm, 0.0002 mm.

    - The head stock of this micrometer has precision measuring screw with compensating devices. This is used to ensure error free reading.


    Whitworth thread: The value of θ is assumed as 55o while calculating best wire diameter.

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