THREAD.docx

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THREAD

ELEMENTS OF A THREADTo understand and calculate the thread elements, the following definitionrelating to screw threads should be known

Major DiameterIt 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 DiameterThe 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 DiameterIt is the smallest diameter of the thread. This is the distance between the

roots of the thread measured perpendicular to the thread axis.

Thread AngleThis is the included angle of the thread form.

PitchIt is the distance between the same points on adjacent threads. This is

also the linear distance the thread will travel in one revolution.
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RootThe 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 measuremajor diameter of the screw, a micrometer, with anvils of adiameter 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 DIAMETERThe 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. Thesteelprisms on the micrometer are pressed into the threadgroove. 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 micrometerelements correctly located, as well as providing means for suspending the vee

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

Floating MicrometerThe prism values are stated as,

Dm = W2T

Note:

Dm - mean diameterW - distance between two prism

T - prism height (known)

Checking minor diameter by using a m icrometer and prisms
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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 bepresent 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 wireshould 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.
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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

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