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. External threads are on bolts or screws. Internal threads are on nuts. There are many forms of threads but two types are in common use on fasteners. Machine Screw Threads - used on bolts, setscrews, machine screws and designed to mate with preformed threads in nuts or tapped holes. 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. 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. 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. Basic Features Major (nominal) diameter   Effective (pitch) diameter   Minor (root) diameter   Pitch   Flank   Crest 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. 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. 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. These screws have a tapered tri-lobular thread which roll forms its own mating thread whilst being driven into a prepared hole. Because of the resultant snug fit of the threads, the screw is vibration resistant. It can also be replaced by a conventional screw. Suitable for steel, die casting, aluminium. Which have a slot milled along the shank point. This will cut a thread in soft metals and hard plastics. Also used to remove paint from threads of captive nuts on painted panels, eg: automotive. All machine screw threaded products, bolts or screws have common technical terms when referring to the thread. Lead or start of thread Threaded portion Thread run out Lead... is the point at which the thread groove is visible on the point of the screw. Threaded portion... is the total section of the screw on which there is a thread. Thread run out... is the point at which the thread and the plain shank meet. Basic Features Major (nominal) diameter Minor (root) diameter Pitch The major diameter can be measured with a simple calliper rule or slot gauge accurately enough to determine nominal diameter. The measurement is taken on the crests. The minor diameter and the pitch require specialist measuring equipment for technical accuracy. However, simple measurement at the crests will be accurate enough for most practical purposes in measuring pitch and determining thread designation. The diameter of imperial spaced threads is expressed as - gauge or 'number' #. The pitch of imperial spaced threads is expressed as - threads per inch (TPI). eg: a standard AB self-tapping screw, therefore, would be:- 6-20 where 6 is the gauge number and 20 is the TPI or 10-16 where 10 is the gauge number and 16 is the TPI. For metric spaced threads which, apart from coach screws are a soft conversion from imperial, the diameter and pitch are expressed in mm. The pitch being the distance between the same point on two adjacent threads, again the crests will suffice as the measuring point. eg: the equivalent to a 6-20 imperial would be 3.5-1.27 mm where 3.5 mm is the equivalent for .138 (the major diameter of 6 gauge) and 1.27 mm is the equivalent pitch for a thread of 20 TPI. To maintain simplicity, most spaced thread products continue to be referred to in their imperial designations and the use of pitch is not necessary for standard self tappers or for woodscrews. eg: No. 6 STS or 6 gauge STS No. 8 WS or 8 gauge WS is acceptable and sufficient The proliferation of pitch availabilities in self-drilling type products, particularly the building fastener ranges, demands the use of pitch designations to ensure accurate description as many of these products can be available in two versions of spaced threads and a version of machine screw thread. Designed to form their own thread in pre-drilled holes in timber, they incorporate a woodscrew type rolled spaced thread which is dimensionally a soft conversion from imperial. However, designation of the size or nominal diameter is in millimetres, eg: M6, M8, M10 and the hexagon dimensions are the same as for hexagon metric commercial bolts. As with woodscrews, there is no necessity to designate pitch in the description. Designed to form a matching thread in the materials being joined. Usually into pre-drilled or pre-punched holes in sheet metals (needle point or S point versions self pierce or self drill). They are heat treated and hardened, are often used into spring steel clips or speed nuts and can also be used in aluminium castings, plywoods, soft and high impact plastics, zinc die castings. Hi - Lo designed for plastic materials it combines two thin walled threads, one higher, one lower. This gives a high pull out strength coupled with reduced incidence of plastic cracking. Designed for tamper proof fixing in plastic and metal castings. It features multiple start, very coarse spiral threads, is driven with a hammer and usually has a round or button shaped head. Type 17 Designed for fixing sheet metal, fibro cement sheet, aluminium sheet or timber panelling to timber supports. No drilling of either the sheets or the supports is necessary; the gimlet point will pierce and self drill the sheet and the milled slot will cut a pilot hole and thread whilst drilling. Type 25 Designed for use in die castings and or hard plastics. The blunt point assists square location into a prepared hole and the milled slot will cut the threads and clear the chips whilst driving. Wormed or cut thread woodscrews - have sharply defined threads on a tapering shank to a gimlet point. Rolled thread woodscrews - employ a type 'A' spaced thread and a rolled taper point (this will look similar to a gimlet point). Both of these products are designed for quality cabinet making, furniture and joinery. Longthread woodscrews - have the same type 'A' spaced thread and gimlet style point as the above; however, the thread extends the full length of the screw to the underside of the head. These are designed for use in composite timbers such as particleboard and craftwood, giving longer thread engagement and higher pull out strength. Note: each of these woodscrews requires a pre-drilled hole and in high quality work, the wormed woodscrew also requires a counter bored and countersunk hole for quality results. The are also some double threaded products called twin start or twin fast which usually have needle type points and two extra coarse threads running inside each other. This gives the same total number of threads engaging, so maintains pull out strength, but halves drive time.     Click here to download the complete Training Manual. (588kb)   You will need Adobe Acrobat to view this file. 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