Theory
Effective Diameter Measurements The effective diameter or the pitch diameter can be measured by . any one of the following methods : (i) The micrometer method (ii) The one wire, two wires, or three wire or rod methods. Theory: Effective diameter of screw thread is the diameter of pitch cylinder which is coaxial with the axis of the screw and intersects the flanges of the thread in such way as to make width of thread and the width of spaces between the threads equal. This is the most important dimension as it decides the quality of the fit between screw thread micrometer and two and three wire method. In Two Wire Method, The effective diameter of a screw thread may be ascertained by placing two wires or rods of identical diameter between the flanks of the thread, as shown in Fig. and measuring the distance over the outside of these wires. The effective diameter E I s then calculated as E=T+P, Where T= Dimension under the wires = M-2d, M=dimension over the wires, d= diameter of each wire The wires used are made of hardened steel to sustain the wear and tear in use. These are given a high degree of accuracy and finish by lapping to suit different pitches. Dimension T can also be determined by placing wires over a standard cylinder of diameter greater than the diameter under the wires and noting the reading R1 and then taking reading with over the gauge, say R2. Then T=dm-(R1-R2). P=It is a value which depends upon the diameter of wire and pitch of the thread. If P= pitch of the thread, then P= 0.866p-d (for metric thread) and Effective Diameter E= T+P.
In Best size wire Method, the wire is of such diameter that it makes contact with the flanks of the thread on the effective diameter or pitch line. The effective diameter can be measured with any diameter wire which makes contact on the true flank of the thread, but the values so obtained will differ from those obtained with best size wires if there is any error in angle or form of thread . t is recommended that for measuring the effective diameter, always the best size wire should be used and for this condition the wire touches the flank at mean diameter line within ±1/5 of flank length
