Metrology and Measurements MCQs | Page - 5

Explanation: factors affecting the accuracy of straightness measurement by optics are air turbulence, mechanical vibrations, optic errors, slope errors, optics not fixed in the correct position, optics not fixed rigidly, localised heat sources etc.

Explanation: straight edges are extremely useful for setting up machines such as planers. precision straight edges are hardened and lapped on the edges by a small radius, which makes a blunt knife edge straight to a few thousands of an mm.

Explanation: an internal thread is that which formed on the inside of a workpiece e.g. on female screw gauge or a nut. an external thread is that which formed on the outside of a workpiece e.g on bolts or studs.

Explanation: a thread is truncated sometimes at the crest or at the root or at both root and crest and root. the radial distance from the crest to the nearest apex of the fundamental triangle is truncation at the crest is and at the root it is the radial distance from the root to the nearest apex.

Explanation: addendum for an external thread is defined as the radial distance between the major cylinder and pitch cylinder and for internal addendum is the radial distance between the minor cylinder and pitch cylinder. dedendum for external thread is the radial distance between the pitch cylinder and minor cylinder and this is the radial distance between the major and pitch cylinders for internal thread.

Explanation: for a straight thread, major diameter is the diameter of the major cylinder. it is also known to as the outside diameter or full diameter for external threads. sometimes it is also called crest diameter.

Explanation: in a screw thread axial thickness is the distance between the opposite faces of the same thread. it is measured on the pitch cylinder and in a direction parallel to the thread axis.

Explanation: functional diameter is also known as virtual diameter. for an internal or external thread, this is the pitch diameter of the enveloping thread, lead and flank angles having the full depth of engagement but not at roots and crests.

Explanation: thread per inch is the reciprocal of the pitch(inches). the angle made by the thread helix at the pitch line with plane perpendicular to the axis is known as lead angle. lead angle is measured in an axial plane.

Explanation: in case of straight thread, effective diameter is the diameter of the pitch cylinder and is also called as pitch diameter. effective diameter is the most important dimension at it decides the quality of the fit between the nut and the screw.