5 Threading Technologies ‘But I grow old always learning many things.’ (640 – 558 BC) [Plutarch: Solon, xxxi] 5.1 Threads An Introduction e originator of the rst thread was Archimedes (287–212 BC), although the rst modern-day thread can be credited to the Engineer and inventor Joseph Whitworth in 1841, where he developed the Stan - dards for today’s screw thread systems. Whitworth’s 55° included angled V-form thread, became widely established enabling thread-locking and unlocking precision parts and of sub-assemblies – paving the way to the build-up of precise and accurate modern- day equipment and instruments. Standardisation of Imperial thread forms in the USA, Canada, UK, and elsewhere, allowed for the interchangeablity of parts to become a reality. Around this time, both in France and Germany metric threads were in use, but it took until 1957 before both the common 60° included an - gled ISO M-thread and Unied thread proles to be - come widely accepted and established (Fig. 95). Along with these and other various V-form threads that have been developed (Fig. 95i), they include quick-release threads such as the Buttress thread: this being a modi - ed form of square thread, along with the 29° included angled truncated Acme form which is a hybrid of a V- form and Square thread. Tapered: gas, pipe and petro - leum-type threads, were developed to give a mechani - cal sealing of the uid, or gas medium, with many other types, including multi-start threads that are now in use throughout the world. V-form screw threads are based upon a triangle (Fig. 95 – top diagram), which has a truncated crest and root, with the root either having a at (as depicted), or a more likely, a radius 1 – depending upon the speci- cation. If screw threads have an identical pitch 2 , but dierent diameters, it follows that they would have dissimilar lead angles. Usually, threads have just one start, where the pitch and the lead are identical – more will be mentioned on multi-start threads later in this 1 ‘Root radius’ , is usually a stronger thread form, as it is less prone to any form of shear-type failure mode in-service. 2 Pitch, refers to the spacing, or distance between any two cor- responding points on adjacent threads, normally taken at the thread’s eective pitch diameter. NB e reciprocal of this pitch, is the threads per inch (i.e. for Imperial units). chapter. Referring to Fig. 95, the angle enclosed by the thread anks is termed the included thread angle (β – as illustrated in Fig. 95 – middle right). is thread form is uniformly spaced along an ‘imaginary cylin - der’ , its nominal size being referred to as the major diameter ( d). e eective pitch diameter (d 2 ) is the diameter of a theoretical co-axial cylinder whose outer surface would pass through a plane where the width of the groove, is half the pitch. erefore, the pitch ( p) is normally associated with this ‘eective’ diameter (i.e. see Fig. 95 – middle right). e minor diameter ( d 1 ), is the diameter of another co-axial cylinder the outer surface of which would touch the smallest diameter. read clearance is normally achieved via truncating the thread at its crest, or root – depending upon where the truncation is applied. ese are the main screw thread factors that con - tribute to a V-form thread, which has similar geom - etry and terminology for its mating nut – for a thread having single-start. 5.2 Hand and Machine Taps Hand Taps Most ‘solid’ 3 taps come in a variety of shapes and sizes (Fig. 94), with hand taps normally found in sets of three: taper, plug and bottoming (Fig. 96). e pro - cess of tapping a hole rstly requires that a specic- sized diameter hole is drilled in the workpiece, this is termed its ‘tapping size’ 4 . e taper tap along with its wrench are employed in producing the tapped thread. 3 ‘Solid taps’ , are as their name implies, but it is possible to use ‘collapsible taps’. ese ‘collapsing taps’ have their cutting ele- ments automatically inwardly collapsing when the thread is completed – allowing withdrawal of the tap – without having to unscrew it, moreover, these ‘collapsible taps’ can be self-set- ting ready for the next hole to be tapped. ey are ‘sized-re- stricted’ by their major diameter. 4 ‘Tapping size’ , refers to the diameter of hole to be drilled that will produce sucient thread depth for the threaded section to be inserted and screwed down, for a particular engineering application. For example, the alpha-numeric notation: M6x1, refers to a metric V-form screw thread of φ6 mm with a pitch of 1mm. It is not necessary to state whether the thread is le-, or right-handed, as the convention is it will be a right-handed single-start thread. In this case, for an M6x1 thread, the tap- ping size can be obtained from the tables, as having a drill size of φ5 mm. Chapter Figure 94. A range of hand and machine taps and a die for the production of precision threads. [Courtesy of Guhring]. Threading Technologies Figure 95. Basic V-form thread nomenclature. [Courtesy of Sandvik Coromant]. Chapter is taper tap has a large length of taper – hence its name, to lead the tap with progressively deeper cuts as it is rotated into the workpiece. As the taper tap enters the previously tapping sized drilled hole, care should be taken to ensure it remains normal to the work sur - face, otherwise and angled hole will result. As the ta - per tap is rotated, aer each ¾ turn, it is counter-ro - tated by about a ¼ turn to break the chips, otherwise ‘galling’ 5 , or tap-breakage problems in-situ could arise. Once the taper tap has been through a ‘running hole’ , it is oen only just necessary to ‘size’ the hole with the bottoming tap. However, if a ‘blind’/non-through hole, 5 ‘Galling’ , is when the tap, or indeed any cutter becomes clogged with the remnants of workpiece material, which will impair its eciency, or at worse, cause it to break in the par- tially tapped hole. Figure 96. Hand taps and tapping nomenclature. [Courtesy of TRW-Greeneld Tap and Die]. Threading Technologies is to be tapped to depth 6 , then it may be necessary to utilise all three taps in the set, as each successive tap once rotated to depth, it will have less lead (i.e. taper) on the tapped hole, creating a stronger thread – up to the thread’s maximum shear strength. Very large diameter hand taps, require a certain level of skill in ensuring that not only the tapped hole is normal to the surface, but a considerable level of physical strength is necessary to tap such a hole! Curved surfaces are more dicult to tap, particularly concave ones, as it is oen dicult to keep the hand tap normal to the surface With concave surfaces any rotational motion of the tap wrench may be somewhat restricted, without a suitable extension chuck/bar – as - suming workpiece access conditions allow. For manual tapping operations, it is oen useful to utilise ‘Tapping chucks’. ese chucks have a rotational drive, coupled to a sprung-loaded Z-axis. e tapping chuck is positioned over the pre-drilled hole and man - ually-fed down into the hole. Once the tap has engaged with the hole, it is pulled and simultaneously ‘oated down’ the hole being tapped – giving excellent tapped hole accuracy. At ‘bottoming-out’ the tapping chuck automatically reverses its direction and ‘drives’ itself out of the hole – while the machine’s spindle continues to rotate in the tapping direction. Machine Taps Machine taps (Fig. 97) are utilised across a diverse range of machine tools and special-purpose tapping equipment. ey can have a variety of ute helices, ranging from quick-to-straight utes (Fig. 97a), de - pending upon the composition of the workpiece ma - terial to be tapped. When tapping, all machining is undertaken by the cutting teeth and the chamfer. In general, the form and length of this chamfer will de - pend upon what type of hole is to be tapped. Tapping ‘through-holes’ is not too dicult, but ‘blind-holes’ can present a problem, associated with the evacu - ation of swarf in the reverse direction to that of the feed. Tap ute spirals that are le-handed and those with spiral points (Fig. 97bi), remove chips in the cut - 6 ‘Tapping depth’ , is an oen misleading term, as in many situ- ations holes are tapped too deeply, as its is only necessary to have a full thread form for 1.5D*, as this is where the maxi- mum thread shear strength occurs, which in turn, is related to the shear strength of the workpiece material.*D = thread’s major diameter. ting direction, or feed direction and are particularly useful for tapping through-holes. Whereas, taps with straight utes (Fig. 97bii) in conjunction with a long chamfer lead, can also give good tapping results. For blind-holes, right-handed utes, or straight uted taps having shorter chamfer lead lengths give acceptable tapping results. ese right-hand uted taps, allow chip-ow in the backward direction – up the utes. e chamfer lead length is such, that it allows return movement of chips, but they will not jam and are reli - ably sheared o. When tapping aluminium, grey cast iron, or certain brass alloys, the tap should have a short lead length – regardless of whether the hole is ‘blind’ , or ‘through- running’. If, when tapping these workpiece materials, a long chamfer lead length was utilised, the tap would behave like a ‘Core-drill’ with chip-breaker grooves. is eect would create ‘drilling’ a tapping-sized hole to the major diameter – instead of actually cutting the required thread. On some machining and turning centres, it is pos - sible to ‘solid tap’ the workpiece, using CNC soware developed just for this task. A ‘solid tapping’ operation requires that the rotation of the spindle and the Z- axis control are fully synchronised, otherwise tapping errors would arise. It is possible to calculate the time required for a tapping operation (Degamo, et al. 2003 – modied for metric units), using the following equa - tion: T m = L n �N = π D L n � V + A L + A R Where: T m = Cutting time (min.), L = Depth of tapped hole, or Length of cut (mm), n = Feedrate (mm min –1 ), N = Spindle (rpm), V = Cutting speed (m min –1 ), A L = Allowance to start the tap (min), A R = Allowance to withdraw the tap (min). * To convert to inches, substitute 12 for the 1000 con - stant in the equation and modify the metric units to inches. Chapter Figure 97. Machine taps: with and without utes. [Courtesy of Guhring]. Threading Technologies Figure 98. Fluteles tapping and tool geometry. [Courtesy of Guhring]. Chapter 5.3 Fluteless Taps Fluteless taps (Fig. 98a), do not have cutting edges (Fig. 98ai) and produce the desired thread geometry by a ‘rolling action’ of the workpiece material. reads produced by uteless taps are much stronger than their equivalent machined taps (Fig. 98b). e bulk workpiece material approximately follows the thread’s contour, thereby imparting additional shear strength to each thread. Oil grooves are usually incorporated into the taps periphery, to facilitate workpiece mate - rial movement and to reduce tap wear rates. Like the conventional machine taps (Fig. 97), uteless taps have a lead to the tap’s edge – termed a ‘forming lead’ (Fig. 98b – le), as opposed to a conventional machine tap which has a ‘chamfer lead’ (Fig. 98b-right) which forms part of the cutting action. erefore, the chi - pless tap in operation (Fig. 98bi), plastically moves workpiece material from the pre-drilled hole into the spaces between the tap’s anks and in so doing, locally work-hardening this material to a limited depth in the workpiece’s substrate. Several factors need to be considered prior to util - ising uteless taps on engineering components, these are: • Over-sized diameter of pre-drilled hole – if the hole is too large, then insucient workpiece mate - rial will be available to fully form the rolled thread, • Undersized diameter of pre-drilled hole – too small a hole will be likely to cause the chipless tap to jam – as it attempts to roll the thread, possibly leading to tap breakage, NB erefore, precise control over the diameter of the pre-drilled hole is imperative. • Workpiece material’s characteristics – both the bulk hardness and more importantly, its mechanical working ability and as a result of this action its lo - cal hardening, are important factors when ‘rolling’ a thread form. NB A ‘start-point’ for the size of pre-drilling diam- eter can be obtained from the tooling suppliers. Of - ten some form of experimentation is necessary in order to obtain the optimum diameter, as this pre- drilled diameter will vary according to the work - piece material’s previous processing route. In Appendix 7, some tapping problems are given, with possible causes and solutions that may be of use in identifying any potential remedial machining action to be taken. 5.4 Threading Dies On shas, having either straight and tapered external threads these can be manually cut, up to a realistic max - imum φ40 mm, with threading dies. In essence, these threading dies can be considered as analogous to hard - ened threaded nuts with multiple cutting edges (Fig. 99a). e cutting edges on the front die face are usually bevelled, or have a spiral lead to assist in starting the thread on the workpiece. Likewise, it is normal to add a reasonable chamfer to the bar’s end to be threaded, as this also helps to gently introduce the thread to depth, as the stock and die are manually-rotated down its length. As is the case for tapping, it is normal practice to ‘back-o ’ the ‘stock’s’ rotation about every ¾ of a turn by approximately ¼ of a turn, to facilitate chip- breaking. As a result of these ‘leads’ on both the sha and die, a few threads on the bar’s end will not be to full thread depth. Care must be taken when initially starting to cut the thread, as if it is not square to the bar’s axis, then a ‘drunken thread’ 7 will result. Previ- ously, most dies were manufactured from high carbon steel and, due to their size, their ‘ruling section’ 8 and its 7 ‘Drunken threads’ , are the result of variations in the helix angle and its associated pitch diering in uniformity on each side of the thread’s diameter. Hence, a ‘true’ mating nut, would ‘wobble’ somewhat as it is rotated down such poorly manufac- tured threaded sha – hence, its name: ‘drunken thread’. 8 ‘Ruling section’ , this term relates to the cross-sectional area that can normally be hardened, being signicantly inuenced by the component’s geometry which aects its ‘critical cool- ing velocity’ (i.e. usually around 1,000°C sec –1 ) when being quenched. is quenching rate is necessary if the part’s metal- lurgical structure is to fully transform into a martensitic state, prior to subsequent tempering. Threading Technologies Figure 99. Die geometries and their nomenclature. [Courtesy of Guhring]. Chapter . 5 Threading Technologies ‘But I grow old always learning many things.’ (640 – 558 BC) [Plutarch: Solon, xxxi] 5 .1 Threads An Introduction e originator of the rst thread was. Archimedes (287– 212 BC), although the rst modern-day thread can be credited to the Engineer and inventor Joseph Whitworth in 18 41, where he developed the Stan - dards for today’s screw thread systems until 19 57 before both the common 60° included an - gled ISO M -thread and Unied thread proles to be - come widely accepted and established (Fig. 95). Along with these and other various V-form threads