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To understand and calculate the thread elements, the following definition relating to screw threads should be known

Major Diameter

It is the largest diameter of the thread. This is the distance between the crests of the thread measured perpendicular to the thread axis.

Pitch/Mean Diameter

The diameter of the thread used to establish the relationship, or fit, between an internal and external thread. The pitch diameter is the distance between the pitch points measured perpendicular to the thread axis. The pitch points are the points on the thread where the thread ridge and the space between the threads are of the same width.

Minor Diameter
It is the smallest diameter of the thread. This is the distance between the roots of the thread measured perpendicular to the thread axis.

This is the included angle of the thread form.

Pitch
It is the distance between the same points on adjacent threads. This is also the linear distance the thread will travel in one revolution.

Root
The surface of the thread that joins the flanks of adjacent threads. The distance between the roots on opposite sides of the thread is called the root, or minor diameter.

Measuring the major diameter

To measure major diameter of the screw, a micrometer, with anvils of a diameter sufficient to span two threads, may be used,.  To eliminate the effect of errors in the micrometer screw and measuring faces, it is advisable first to check the instrument to a cylindrical standard of about the same diameter as the screw. For such purposes a plug gauge or a set of ‘Hoffman’ rollers is useful.

Measuring the minor/core diameter

The diameter over the roots of a thread may be checked by means of a special micrometer adapted with a shaped anvils,or a micrometer may be used in conjunction with a pair of vee pieces ( steel prisms ).  The second method is recommended.  The steel prisms on the micrometer are pressed into the thread groove.  The ends of the prisms are slightly curved and parallel to the root thread.  It is important , when making the test, to ensure that the micrometer is positioned at right angles to the axis of the screw being measured, and when a large amount of such work is to be done, a special ‘floating bench micrometer’ is used.  It is because, it supports the screw and incorporates the micrometer elements correctly located, as well as providing means for suspending the vee prisms.

Checking the core diameter of a thread with an shaped anvil micrometer

Floating Micrometer

The prism values are stated as,
Dm = W – 2T

Note:
Dm       - mean diameter
W        - distance between two prism
T        - prism height (known)
Checking minor diameter by using a micrometer and prisms

Measuring the mean/pitch/effective diameter

The three-wire method is recognized as one of the best methods of checking the pitch diameter because the results are least affected by any error which may be present in the included thread angle.  For threads which require an accuracy of 0.001 in. or 0.02 mm, a micrometer can be used to measure the distance over the wires.  For threads requiring greater accuracy an electronic comparator should be used to measure the distance over the wires.
In the three-wire method, three wires of equal diameter are placed in the thread; two on one side and one on the other side (Fig. 1.6).  The wires used should be hardened and lapped to three times the accuracy of the thread to be
inspected.  A standard micrometer may then be used to measure the distance over the wires.   For greatest accuracy, the best size wire should be used.

The hard round bars (wire) with the same size are positioned opposite to the screw thread groove shown in the diagram above.  The distance is measured between the outside of the round bars.  The most suitable wire size is 0.57735p.  In Fig. 1.7 P is the pitch of the screw thread. The suitable wire size is quite hard to get, usually a size bigger than 0.57735p wire size will be used.

Wires which touch the thread at the pitch diameter are known as "Best Size" Wires.  Such wires are used because the measurements of pitch diameter are least affected by errors that may be present in the angle of the thread.
The above analysis for the distance over wires holds good provided the wire touches the thread somewhere on its right side, and provided the thread angle is correct.  The extremes of wire sizes which touch on the straight sides and which can be measured are shown at (a) and (c), Fig.1.9.  For ISO metric, unified and Whitworth threads these limiting sizes are given in Table 1.1