associated ‘mass eect’ 9 meant that the dies could be through-hardened – which gave them an overall ‘bulk hardness’ of greater than HSS. Today, basically dies are either manufactured from micro-grained HSS, or coated cemented carbide. Solid dies (Fig. 99a), do not have any means to com - pensate for die wear, whereas, their split-die nut coun - terparts (Fig. 99b-le), can be manually-adjusted. is adjustment of the die is achieved by turning a centrally mounted grub-screw in the stock body, which along with the xing screws can be made to open, or close on the sha to be threaded. In this manner, achieving the correct thread tolerance, or ‘play’ for the desired tment of its associated mating nut. It is also usual practice, to use a suitable die lubricant, to facilitate in the thread’s production while improving surface nish and prolonging the die’s life – as excessive friction oc - curs during this type of threading process. e major disadvantage of using the solid-type threading dies is that they either have to be unscrewed from the threaded workpiece, or rewound from the thread, using up unproductive time elements, this being particularly important for large batch runs, or in a continuous production environment. Self-open- ing dies 10 (i.e. not depicted) have been utilised for many years on: capstan and turret lathes, single- and multi-spindle automatics and so on, for cutting exter - nal threads. Several types of self-opening die heads are available, ranging from: radial, tangential, or cir - cular arrangement of the multi-point cutting inserts and thread chasers. In most cases, it is usual for the 9 ‘Mass eect’ , is related to the component’s ‘ruling section’. For example, if the part has a large cross-sectional area, when it is quenched from the hardening temperature zone (i.e. this can be found from its associated thermal equilibrium diagram – for the present), it will not exceed the ‘critical cooling velocity’ and only a partial martensitic state occurs. is is because the quench media used could not suciently drastically reduce the part’s temperature with an incomplete atomic transforma- tion occurring and in so doing, the heat-treated component will retain some austenite in the matrix. For this reason, large holes (e.g. designed into in through-hardened Sine-bars) are oen strategically designed in these larger component regions. Moreover, in many cases the larger component cross-sections are reduced, so that the ‘mass eect’ does not occur – apart from the obvious factor of relieving weight, etc. 10 Self-opening dies, are oen termed ‘read chasing die heads’ , whereas in reality a thread is only ‘chased’ once the main thread form has initially been cut. us chasing is employed to give the required t and nish to the nal thread form. die-head cutting elements to be preset to take rstly a roughing cut, followed by nishing cut/chasing of the threads down the bar. At the end of the threaded section, these self-opening dies will automatically open and can then be speedily withdrawn from the threaded portion of the bar. ese self-opening dies can be set to give the correct amount of tolerance, con - trolling the ‘play’ on the thread. Moreover, it is pos - sible to t dierent thread sizes and forms into the die head, for more universal threading applications. Both the radial and tangential threading elements, create less tool ank contact and frictional rubbing on the cut thread. 5.5 Thread Turning – Introduction On conventional engine-/centre-lathes, a single-point thread cutting tool (Fig. 100), has a synchronised and combined linear and rotary kinematic motion for its Figure 100. External and internal threading tool holders and in-serts. [Courtesy of Seco Tools] . Threading Technologies threading insert. is insert is connected to the lead- screw (i.e normally having a very accurately-hardened and ground Acme form) which is precisely synchro - nised to that of the headstock’s rotation. On a CNC turning centre, or similar, this linear motion is reli - ant on the precision and accuracy of the recirculating ballscrew coupled to the programmed cutterpath. In this manner, the threading insert being rigidly held in either the tool post, or turret, generates a spiral groove which when at full depth creates a screwthread of the desired pitch and helix angle. During successive tra - verse feeding passes (i.e. to prescribed depths) along the workpiece the thread is cut. A typical thread is routinely produced on CNC turning centres, using its xed/canned cycles (i.e. ‘bespoke soware’). During these automated threading passes, the tool precisely traverses down the bar’s length, is rapidly withdrawn and moved back to its start point, then fed more deeply beginning another threading pass down the same helical groove, this process being repeated until full thread depth/prole is accomplished. In order to obtain a consistent thread pitch on the workpiece, the feedrate along the threaded portion must exactly co - incide. e thread form is dependent upon the pro - led geometry of the thread cutting insert. In order to achieve the required nal thread prole, the feedrate must be considerably larger than is normally utilised for conventional turning operations. Any V-form thread point angle geometry, is not an ideal edge shape for the production of machined threads if the insert is fed in normal to the workpiece’s axis of rotation (i.e. radial/plunge-fed). Chip control here will be compromised, as each ank of the V-form thread gets successively deeper. is narrowing of the Figure 101. Screwcutting tech- niques on turning centres and suggest- ed methods for improved chip control. [Courtesy of Sandvik Coromant] . Chapter chips from each formed and angled ank face of the V-shaped threading insert (i.e. see Fig. 103ai), creates high localised forces and stresses, which will tend to tear, rather than cut the nal V-form thread prole. In order to minimise these potential high force/shear components when radial/plunge-cutting a thread, the radial infeed passes are progressively reduced with increasing thread depth. e techniques of V-form thread production by radial infeed techniques will be the subject of the next section. .. Radial Infeed Techniques Utilising single-point threading inserts (i.e. see Fig. 104, where a typical sequence of threading passes are depicted – as the V-form thread prole is partially formed), several dierent techniques of thread turning are utilised today, they include: • Radial infeed (Fig. 101 – top-le) – being the most common method, where the threading insert is fed at 90° to the workpiece’s rotational axis. e mate - rial being removed on both sides of the tool’s V- form anks – producing a ‘so’ chip-forming action giving uniform wear to both anks of the insert, NB Here the V-form threading insert geometry forms both anks with lighter cuts as the thread depth progressively increases. • Flank infeed (Fig. 103aii) – is oen known as the ‘half-angle screwcutting technique’ , mainly utilised on a conventional engine-/centre-lathe. Here, the le-hand ank is formed by the tool’s V-form ge- ometry, while the right-hand thread ank is gener- ated by successive passes, as the tool is fed down the face at half the thread’s included angle. Chip control is improved with all ank infeed techniques over ‘plunging’ , enabling the chip to be vectored away from the previously cut surface (i.e see Fig. 101 – middle, where the chips can be steered away from the ank), NB is ‘half-angle technique’ producing the thread’s right ank, is generated by the tool’s right- hand ank – which due to frictional eects, creates here a more pronounced wear rate on the cutting edge resulting in a poor surface nish. • Modied ank infeed (Fig. 101top-middle le) – in this cutting action, the tool is fed to depth in suc - cessive passes at a slightly reduced angle (i.e. nor - mally ranging from 1° to 5°). is screwcutting technique provides an improved ank surface n - ish – compared to the two previous methods, par - ticularly on the either less hard, or for more ductile workpiece materials. Modied ank infeed methods are recommended rather than radial infeeding for larger threads, due to contact on this long ank length which would otherwise result in vibrational eects being superimposed (i.e chatter) onto the nal thread form, NB If the workpiece material’s characteristics include potential machining work-hardening problems, then ank infeed techniques should be avoided, • Incremental feeding 11 (Fig. 101 – top-middle right) – if the thread form is very large, then the incremen - tal thread feeding strategy is normally utilised. ese same radial infeed thread production tech - niques are used for the manufacture of internal threads (Fig. 102a), by either ‘Pull-threading’ – depicted in ‘A’ , where the thread form originates from the inter- nal undercut, as opposed to ‘Push threading’ – shown in ‘ B’ – being toward say, an undercut. In both cases of thread production, the modied ank infeed tech - niques are employed. NB reads manufactured by method ‘A’ allow for excellent evacuation of the chips – being an ideal technique for ‘blind holes’. Conversely, in case ‘ B’ , the swarf would otherwise simply ‘bird’s nest’ 12 in such a hole, unless a through hole is present, as is depicted in ‘ B’. If thread forms are based upon square threads, or their modied trapezoidal forms: Buttress, or Acme (i.e. see Fig. 95i – for examples of these thread proles), it 11 Incremental feeding, is sometimes termed the ‘Alternating ank’ technique, it has the advantage of imparting a uniform wear to both of the cutting insert’s V-form anks, thereby sig- nicantly increasing the tool life. 12 ‘Bird nesting’ , is a term that refers to the rotational entangle- ment and build-up of work-hardened swarf at the bottom of a ‘blind hole’ , which can create some problems in internal thread production. Threading Technologies Figure 102. External and internal threading operations and the eect that the helix has as the diameter changes – for a given pitch. [Courtesy of Sandvik Coromant] . Chapter is advisable to pre-machine the thread with a groov- ing tool – with the tool’s width being the equivalent of the thread’s root spacing dimension. Not only does this pre-machining strategy of employing a grooving tool reduce the number of threading passes to just ank nishing, the tool can have a chip-breaker pres - ent during the rough machining stage to eectively re - move the bulk stock and its associated swarf. .. Thread Helix Angles, for Single-/Multi-Start Threads e fundamental basis underpinning any thread form is the helix angle, which in this case is denoted by the Greek symbol ‘ ϕ’ – as schematically illustrated in Fig. 102b. One way of describing how the helix angle’s geometry is created, is to imagine that a right-angle triangle is formed by a thin wire which has been unwound from a parallel cylindrical sha, whose diameter equates to its ‘eective diameter’. en, this unwound wire length (i.e. πD) would be its circumfer - ence, acting as a base for the triangle (Fig. 102b). e perpendicular height of right-angled triangle is equal to the pitch 13 ‘p’ , or the lead 14 – in the case of a single- start thread. e angle that the hypotenuse makes with the base is its helix angle ‘ϕ’. From the schematic dia- gram in Fig. 101c, if the pitch ‘p’ remains constant and the diameter ‘ D 1 ’ is decreased (i.e. ‘D 1 ’ → ‘D 3 ’), then the helix angle proportionally increases (i.e. ‘ ϕ 1 ’ → ‘ϕ 3 ’). In the case of multi-start threads, the pitch and the lead dier, as shown in Fig. 106c. In this illustration for the cutting the triple-start thread, the usual approach to its manufacture is for the three successive starts to be individually completed to form the ‘triple-start’ , with each start being angularly displaced 120° with respect to each other. Alternatively, if one start is be - gun with the rst threading pass, then the second start is similarly machined and so on – for the number of starts required, then the threading insert is advanced 13 ‘Pitch’ – can be dened as the distance between correspond- ing points on adjacent threads, normally expressed in metric units as ‘mm’ , or in Imperial units as threads per inch. 14 ‘Lead’ – being dened as the axial distance through which a point on the thread advances during one revolution of the thread ×. is helix angle ‘ϕ’ is also known as its ‘lead angle’ NB Both the pitch and the lead are identical for single-start threads. to a deeper thread depth and the process is repeated until the full thread form has been completed. As pre - viously mentioned, the pitch is not the same as the lead for multi-start threads and the lead can be easily calculated as follows: Lead = np Where: n = number of starts, p = pitch (mm). For example, in the case of the triple-start thread illus - trated in Fig. 106c, for say, a V-form metric thread of 6 mm pitch, then the lead will be: 3 × 6 mm = 18 mm. NB is means that if a mating nut was rotated down this triple-start thread, it would be linearly displaced by 18 mm in one revolution – allowing the nut to be rotated in, or out quickly (i.e. because of its larger he - lix angle), but to the detriment of an increased axial loading. Although this load is distributed across the contact between all the multi-start threads. .. Threading Insert Inclination e threading insert is carefully ground by the tool- ing manufacturer to provide the correct thread prole. is insert must operate with a radial cutting rake of 0°, if the correct thread form is to actually imparted to the formed thread (Fig. 103). e lead angle of the ank surface varies at dierent points between the crest and the root of the thread, increasing toward the root – the opposite is true on an internal thread. Due to this eect, the actual cutting rake varies along the insert’s cutting edge, becoming more positive on the leading edge and more negative on the trailing edge – the closer it gets to the thread’s root. In order to minimise such threading insert rake angle variation the insert is inclined 15 , so that its top face is perpendicular to a 15 reading shimming – the tool holder is delivered tted with a shim that gives an eective side inclination angle of 1° – be- ing the most common type. Although shims can be changed in degree increments from: –2° to 4°, by simply tting a dier- ent shim angle. Likewise, internal threading tool holder incli- nations can be changed, by tting such shims. Threading Technologies line indicating the mean lead angle ‘λ’ 16 – measured at the pitch diameter (Fig. 103b). is insert inclination 16 reading insert top face geometry – instead of a at/straight top face to the insert, today, it is oen angled (i.e. shown in Fig. 103bi – bottom le), which enables improved control of the developing chip. produces a symmetrical side clearance (i.e. depicted in Fig. 103bi – bottom le diagram) and is important in ensuring a uniform edge wear on both anks, re - sulting in increased insert useful life. e fact that this small threading insert inclination, causes one ank to cut slightly below, while the other cutting marginally above the centre-line of the workpiece – for a at top faced insert, is of no practical signicance at normal lead angles for either the function of cutting, or the Figure 103. Thread for- mation by radial and ank infeeds, to-gether with threading insert inclina- tion angles. [Courtesy of Sandvik Coromant] . Chapter thread’s prole. Further, a small deviation from the exact symmetry required in the insert’s inclination is also acceptable, without too obvious a disadvantage. us, the inclined insert can be utilised to cut threads of between 0° and 2° with an inclination of 1° and, still produce a satisfactory thread. is thread production technique is only true for the normal, symmetrical threads (i.e. ‘V-forms’); in the case of ‘saw-toothed’ Figure 104. External threading with an indexable insert – chip formation in a partially-formed/generated thread for a single pass along the bar. [Courtesy of Sandvik Coromant] . Threading Technologies threads (e.g. Buttress), it should be borne in mind that the ‘straight-anked’ ones – those with angles between 0° to 7° – in particular, oer side clearances which may be adequate. In Fig. 103c a graph depicting the thread - ing insert inclination angles is given for diering helix angles, with the helix angle calculation derived as fol - lows: tan λ = P / D × π Where: λ = Helix angle (°), P = Pitch (mm, or threads per inch), D = Eective pitch diameter (mm, or inches). Metric threading inserts are characterised by their thread prole and the associated pitch, being ex - pressed in millimetres. e shape and size of the in - sert will determine the completed thread form. One threading insert can be utilised to cut all threads of this prole and size, irrespective of their thread diam - eter, or whether they are: right- or, le-hand, single- or, multi-start (i.e. see Figs. 105b and 106a, for internal and external le- and right-hand threading congura - tions, respectively). In the case of internal thread inclination angles, the tool must be ‘canted-over’ at the angle ‘ λ’ (i.e. see Fig. 103bii), so that the cutting edge is situated normal to the centreline. Oen, the toolholder shank has to be ground-away to avoid fouling on the internal hole’s diameter as shown in Fig. 105a. Here (Fig. 105a), the distance from the tool tip to the rear of the toolholder shank – denoted by the dimension ‘ D’ , is relieved to ‘D mod ’ to avoid fouling on the curvature of the hole, as the tool is fed-out of the thread depth at the end of its successive ‘threading passes’. .. Thread Profile Generation e prole of a thread can be cut by several dierent techniques and diering types of inserts – depending whether ‘topping’ the is required. For example, if a V- form proled threading insert is utilised (Fig. 106bi), no actual machining is undertaken of the thread’s top. In this situation, it is necessary to ensure that the Figure 105. Internal threading operations for right- and left-hand threads. [Courtesy of Sandvik Coromant] . Chapter Figure 106. External threading operations and insert forms. [Courtesy of Sandvik Coromant]. Threading Technologies pre-machined workpiece – when producing external threads – has the exact size for the major diameter re - quired, conversely, for internal threads this must be the minor diameter that is pre-machined. Due to the sharpness of the thread produced by this technique, it is oen necessary to ‘chase the thread’ 17 aerward. In the case of proled threading inserts, the com- plete thread prole is cut from a slightly oversized blank. Usually, three distinct proling inserts could be used in thread production, these are: • V-form (Fig. 106bi) – has the ability to machine a range of thread proles, with the nose radius pre - cisely and accurately ground for the smallest pitch to be cut. As a result of this tightly ground nose radius, the insert’s life is shorter than with other proling insert versions, as its size has not been optimised for individual thread geometries. From an economic viewpoint, due to the V-form prol - ing insert’s ability to cut a wide variety of thread pitches, less inserts need to be stocked, • Full-form (Fig. 106bii) – has the ability to prole the thread’s crest and is therefore manufactured to exactly the specication of the required thread pro - le. Such full-prole inserts simplify thread pro - duction, as no prole is deeper than its specica - tion, allowing them to be a stronger insert thereby resulting in improved tool life, • Multi-point form (Fig. 106biii) – with this multiple- pointed proling insert, the rst tooth roughs-out the thread and is therefore slightly set back in com - parison to the second tooth on the insert, which acts almost like a ‘chaser’ which fully-forms and blends the various thread proling elements upon the nal threading pass. Cutting conditions need to be rigid and stable for this type of insert to operate correctly – due to its longer cutting edge length. It is essential to ensure that the recommended in-feed values are used, to ensure that cutting forces are balanced for both of the cutting teeth. One advantage of utilising these multi-point threading inserts is that the num - ber of threading passes can be reduced by almost 50% – as it cuts deeper than its counterparts, when compared to the single-proling insert forms. 17 ‘Chasing a thread’ , refers to using a chasing tool with the ex- act thread prole which is utilised to follow the thread along, thereby deburring and forming the desired prole simultane- ously. .. Threading Turning – Cutting Data and Other Important Factors Whatever type of thread to be cut, whether it is a: V-form, Multi-start, Trapezoidal, or Tapered, it is gen - erally quite dicult to vary such factors as the: cutting speed, feed and, to a lesser extent the D OC , indepen- dently of one another, without certain consideration of some limiting factors. e typical limitations imposed when cutting a thread, will now be discussed. Cutting Speed Typical limitations imposed by the action of cut- ting a thread include, reducing the cutting speed by 25% – compared to ordinary turning, as the insert’s shape limits heat dissipation. If a high a chip load occurs due too great a cutting speed selected, then the cutting temperature can approach that of say, a cemented carbide’s original sintering temperature. As a result of this elevated temperature, the binder phase may soen, causing potential cutting edge plastic deformation. e remedy here seems quite easy, simply reduce the cutting speed, but this may in - crease the risk of BUE. is BUE may cause the chips to become welded onto the cutting edge from which they are shortly fragmented and continuously carried away – taking a minute portions of the insert’s edge along with them. e problem can be minimised by specifying a tougher grade of carbide for the threading insert, or choosing a multi-coated insert. Normally, the cutting speeds for any threading operation should not be less than 40 m min –1 , when machining with any cemented carbides. Feed and D OC e feed in millimetres per revolution, must coincide with the desired pitch, or lead – when cutting multi- start threads. Hence, if the cutting speed is modied, the feedrate will also have to be increased, or de - creased, so that the feed per revolution is constantly maintained. So, the critical factor here is to achieve some form of control over the D OC when threading. Each threading pass along the workpiece causes an in - creasingly larger portion of the insert’s cutting edge to become in contact in the threading operation, accord - ingly, tool forces will proportionally increase. If the D OC is kept constant during several passes, the chip- Chapter . tech - niques are used for the manufacture of internal threads (Fig. 102a), by either ‘Pull-threading’ – depicted in ‘A’ , where the thread form originates from the inter- nal undercut, as opposed to ‘Push threading’ – shown in ‘ B’ – being toward say, an undercut. In both cases of thread production, the modied ank infeed tech - niques are employed. NB . – the tool holder is delivered tted with a shim that gives an eective side inclination angle of 1° – be- ing the most common type. Although shims can be changed in degree increments from: 2 to 4°, by simply tting a dier- ent shim angle. Likewise, internal threading tool holder incli- nations can be changed, by tting such shims. Threading Technologies line indicating the mean lead angle ‘λ’ 16 – measured at the pitch diameter (Fig. 103b). is insert inclination 16 . the contact between all the multi-start threads. .. Threading Insert Inclination e threading insert is carefully ground by the tool- ing manufacturer to provide the correct thread prole. is insert must operate with a radial cutting rake of 0°,