Thread measurement
The screw - thread nomenclature is shown in Figure. Threads are normally specified by the major diameter Though there are a large variety of threads used in engineering, the most common thread encountered is the metric V-thread shown in Figure.
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| External screw thread |
The para meter that are normally measured are-
● Major diameter
◆ Micrometer
● Pitch diameter
◆ Screen thread micrometer
◆ Wire method
● Pitch
◆ Screw pitch gaye
◆ Pitch measuring machine
● Thread form
◆ Optical projector
The major diameter can be measured using an external micrometer with the anvil and spindle touching the thread surface. Care has to be taken to see that the thread surface is touched with minimum pressure. Since the crest surface that will be in contact with the micrometer is very small, any excess pressure is likely to deform the thread and cause error in measurement. To measure the pitch diameter, screw - thread micrometers have a specially designed spindle and anvil as shown in Figure. The end of the spindle of this type of micrometer is pointed to form a 60 ° cone, and the anvil has the form of a 'V' to fit over the thread. The sharp tip of the spindle is ground off to make sure that only the pitch diameter is measured rather than the root or minor diameter. The swivel of the V-shaped anvil compensates for the thread lead. A screw - thread micrometer is generally designed to measure threads within a narrow range of pitches and it is marked on the micrometer.
Measurement Over Wire
Another method used to measure precision threads is using the wires. Small hardened steel wires are or precision size and accurately ground. Two wires are placed over the thread grooves in the opposing directions of the thread as shown in Figure Then the size is measured using a micrometer with the spindle and anvil touching the wire surface. Then the pitch diameter is calculated as
Pitch diameter, D = M + P
where P = 0.866p - G for metric threads,
p = Pitch of the thread, and
G = Diameter of the wire used
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| Measuring screw thread using a micrometer |
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| Nomenclature of external screw thread |
Three - Wire System
In the three - wire method, two of the wires are placed in the thread groove on one side and one wire is diametrically opposed as shown in Figure. The measurement is done using a micrometer or preferably a diameter - measuring machine for more accuracy. The accuracy of the pitch - diameter measurement depends on the measuring instrument, the wire diameter and the contact force applied.
Pitch diameter,
D = M - [ G(1 + cosecA) - pcotA/2]
where P = 0.866p - G for metric threads,
p = Pitch of the thread, and
G = Diameter of the wire used
A = Thread angle
Ideally, the wire diameter. G is to be selected such that it makes contact with the flank of the thread on the pitch line. As a result for each pitch, there is a best size.
Best wire diameter, G = p/2 sec(A/2)
The method normally used for quick checking threads, particularly on production lines is the use of thread - ring gauge for external threads and thread - plug gauge for internal threads. When using a thread ring or plug gauge, a "GO" and "NO GO" gauge is used. This type of gauge requires no extra training or skill to use.