What is a thread?
A thread is a ridge of uniform
section in the form of a helix on the internal or external surface of a
cylinder (IFI description) or it could be described as a sloping plane curled
around a cylinder.
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External threads are on bolts or screws.
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Internal threads are on nuts.
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There are many forms of threads
but two types are in common use on fasteners.
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Machine Screw Threads - used on bolts, setscrews, machine screws and designed
to mate with preformed threads in nuts or tapped holes.
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Exceptions may be thread forming screws like Taptite or
self-drilling screws like Teksor thread cutters like Type 23's, which form or
cut their own machine screw thread.
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Spaced Threads - used on woodscrews, self-tapping screws, coach screws
and Type 25 thread cutters. Designed to form its own thread, usually in a
pre-drilled hole.
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Exceptions may be self piercing screws such as needle points or
self-drilling screws like Type 17's which create their own hole; some Teksmay
also have spaced threads.
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The major diameter can be measured with a simple calliper rule or slot gauge accurately enough to determine the nominal diameter. A bolt or screw is measured at the crests; a nut is measured at the thread roots.
The effective diameter, minor diameter, flank angle and pitch require specialist measurement equipment for technical accuracy. However, simple measurement at the thread crests will be accurate enough for most practical purposes in measuring pitch and determining thread designation.
For imperial threads, UNC, UNF, BSW and BSF, pitch is expressed in numbers of threads per inch, eg: 1/4 -20 UNC, the 20 being 20 threads per inch or 20 TPI.
For metric and BA threads, the pitch is a single thread measured and expressed in millimetres, eg: M10 x 1.5, the 1.5 being 1.5 mm from the same point on two adjacent threads.
In ordering or referring to these threads, it is not necessary to state the pitch because absence of a thread pitch indicates reference to the standard Australian specification.
Pitch specification would be necessary when referring to metric fine threads which are not covered by Australian Standards and where several different pitches are possible internationally. Also when specifying 1" -14 TPI UNF, which is the common international standard versus Australian standard 1" - 12 TPI UNF.
1" - 14 TPI UNF is also sometimes referred to as 1" - SAE and whilst not absolutely correct, this description may assist in recognition.
Note that in metric and unified, the crests and the roots theoretically should be flat; however, in practice, to aid manufacture and fit, they are rounded inside a maximum outline.
unified and metric (theoretical)
unified and metric (in practice)
Whitworth thread profile is more wave shaped, being a series of radius curves about the pitch line.
withworth
Threads which come to a point at the crest and root, are called complete threads; those that do not are called incomplete threads.
Most fastener machine screws thread forms are incomplete thread types.
Thread angles
Machine screw threads are symmetrical - the angle on both flanks being the same - refer to illustration.
Flank angles for METRIC, UNC and UNF are 30° a total thread angle of 60°.
BSW and BSF are 27.5° a total thread angle of 55°
Because the pitch of some threads is common in the same diameters, it is possible to mate them, eg: BSW and UNC all diameters except 1/2 (where UNC is 13 TPI, BSW is 12 TPI), can be mated together. However, because the thread angles and the profiles differ, the 'fit' will be loose and the mechanical requirements of the fastener will not be achievable. Therefore, mixture of thread forms must be avoided.
The major diameter can be measured with a simple calliper rule or slot gauge accurately enough to determine the nominal diameter. A bolt or screw is measured at the crests; a nut is measured at the thread roots.
The effective diameter, minor diameter, flank angle and pitch require specialist measurement equipment for technical accuracy. However, simple measurement at the thread crests will be accurate enough for most practical purposes in measuring pitch and determining thread designation.
For imperial threads, UNC, UNF, BSW and BSF, pitch is expressed in numbers of threads per inch, eg: 1/4 -20 UNC, the 20 being 20 threads per inch or 20 TPI.
For metric and BA threads, the pitch is a single thread measured and expressed in millimetres, eg: M10 x 1.5, the 1.5 being 1.5 mm from the same point on two adjacent threads.
In ordering or referring to these threads, it is not necessary to state the pitch because absence of a thread pitch indicates reference to the standard Australian specification.
Pitch specification would be necessary when referring to metric fine threads which are not covered by Australian Standards and where several different pitches are possible internationally. Also when specifying 1" -14 TPI UNF, which is the common international standard versus Australian standard 1" - 12 TPI UNF.
1" - 14 TPI UNF is also sometimes referred to as 1" - SAE and whilst not absolutely correct, this description may assist in recognition.
Note that in metric and unified, the crests and the roots theoretically should be flat; however, in practice, to aid manufacture and fit, they are rounded inside a maximum outline.
unified and metric (theoretical)
unified and metric (in practice)
Whitworth thread profile is more wave shaped, being a series of radius curves about the pitch line.
Threads which come to a point at the crest and root, are called complete threads; those that do not are called incomplete threads.
Most fastener machine screws thread forms are incomplete thread types.
Thread angles
Machine screw threads are symmetrical - the angle on both flanks being the same - refer to illustration.
Because the pitch of some threads is common in the same diameters, it is possible to mate them, eg: BSW and UNC all diameters except 1/2 (where UNC is 13 TPI, BSW is 12 TPI), can be mated together. However, because the thread angles and the profiles differ, the 'fit' will be loose and the mechanical requirements of the fastener will not be achievable. Therefore, mixture of thread forms must be avoided.
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