Threading Technologies

Chia sẻ: Bành Thị Liễu | Ngày: | Loại File: PDF | Số trang:30

Thêm vào BST

Báo xấu

87
lượt xem 9
download

Download Vui lòng tải xuống để xem tài liệu đầy đủ

dựng từ nhiều trang Internet và World Wide Web, hoặc đơn giản gọi là Web được gọi là tra cứu thông tin toàn cầu. Nó bao gồm hàng triệu các website, mỗi website được xây web. Mỗi trang web được xây dựng trên một ngôn ngữ HTML (Hyper Text Transfer Protocol) ngôn ngữ này có hai đặc trưng cơ bản: 1 Tích hợp hình ảnh âm thanh tạo ra môi trường multimedia 2 Tạo ra các siêu liên kết cho phép có thể nhảy ttừ trang web này sdang trang web khác không cần một trình tự nào. Để ...

Chủ đề:

Bình luận(0) Đăng nhập để gửi bình luận!

Lưu

Nội dung Text: Threading Technologies

5 Threading Technologies ‘But I grow old always learning many things.’ SOLON (640 – 558 BC) [Plutarch: Solon, xxxi]
182 Chapter 5 5 .1 Threads chapter. Referring to Fig. 95, the angle enclosed by the thread flanks is termed the included thread angle (β – as illustrated in Fig. 95 – middle right). This thread form is uniformly spaced along an ‘imaginary cylin- An Introduction der’ , its nominal size being referred to as the major diameter (d). The effective pitch diameter (d) is the The originator of the first thread was Archimedes (287–212 BC), although the first modern-day thread diameter of a theoretical co-axial cylinder whose outer can be credited to the Engineer and inventor Joseph surface would pass through a plane where the width of the groove, is half the pitch. Therefore, the pitch (p) is Whitworth in 1841, where he developed the Stan- dards for today’s screw thread systems. Whitworth’s normally associated with this ‘effective’ diameter (i.e. see Fig. 95 – middle right). The minor diameter (d), 55° included angled V-form thread, became widely established enabling thread-locking and unlocking is the diameter of another co-axial cylinder the outer precision parts and of sub-assemblies – paving the surface of which would touch the smallest diameter. way to the build-up of precise and accurate modern- Thread clearance is normally achieved via truncating day equipment and instruments. Standardisation of the thread at its crest, or root – depending upon where Imperial thread forms in the USA, Canada, UK, and the truncation is applied. These are the main screw thread factors that con- elsewhere, allowed for the interchangeablity of parts tribute to a V-form thread, which has similar geom- to become a reality. Around this time, both in France etry and terminology for its mating nut – for a thread and Germany metric threads were in use, but it took until 1957 before both the common 60° included an- having single-start. gled ISO M-thread and Unified thread profiles to be- come widely accepted and established (Fig. 95). Along 5.2 Hand and Machine Taps 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- fied form of square thread, along with the 29° included Hand Taps angled truncated Acme form which is a hybrid of a V- form and Square thread. Tapered: gas, pipe and petro- Most ‘solid’ taps come in a variety of shapes and sizes leum-type threads, were developed to give a mechani- (Fig. 94), with hand taps normally found in sets of cal sealing of the fluid, or gas medium, with many three: taper, plug and bottoming (Fig. 96). The pro- other types, including multi-start threads that are now cess of tapping a hole firstly requires that a specific- in use throughout the world. sized diameter hole is drilled in the workpiece, this is termed its ‘tapping size’. The taper tap along with its V-form screw threads are based upon a triangle (Fig. 95 – top diagram), which has a truncated crest and wrench are employed in producing the tapped thread. root, with the root either having a flat (as depicted), or a more likely, a radius – depending upon the specifi- cation. If screw threads have an identical pitch, but different diameters, it follows that they would have ‘Solid taps’ , are as their name implies, but it is possible to use dissimilar lead angles. Usually, threads have just one ‘collapsible taps’. These ‘collapsing taps’ have their cutting ele- start, where the pitch and the lead are identical – more ments automatically inwardly collapsing when the thread is completed – allowing withdrawal of the tap – without having will be mentioned on multi-start threads later in this to unscrew it, moreover, these ‘collapsible taps’ can be self-set- ting ready for the next hole to be tapped. They are ‘sized-re- stricted’ by their major diameter. ‘Tapping size’ , refers to the diameter of hole to be drilled that will produce sufficient thread depth for the threaded section ‘Root radius’ , is usually a stronger thread form, as it is less to be inserted and screwed down, for a particular engineering prone to any form of shear-type failure mode in-service. application. For example, the alpha-numeric notation: M6x1, Pitch, refers to the spacing, or distance between any two cor- refers to a metric V-form screw thread of φ6 mm with a pitch responding points on adjacent threads, normally taken at the of 1mm. It is not necessary to state whether the thread is left-, thread’s effective pitch diameter. 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- NB The reciprocal of this pitch, is the threads per inch (i.e. ping size can be obtained from the tables, as having a drill size of φ5 mm. for Imperial units).
Threading Technologies 183 . Figure 94. A range of hand and machine taps and a die for the production of precision threads. [Courtesy of Guhring]
184 Chapter 5 . Figure 95. Basic V-form thread nomenclature. [Courtesy of Sandvik Coromant]
Threading Technologies 185 . Figure 96. Hand taps and tapping nomenclature. [Courtesy of TRW-Greenfield Tap and Die] ‘galling’, or tap-breakage problems in-situ could arise. This taper tap has a large length of taper – hence its name, to lead the tap with progressively deeper cuts as Once the taper tap has been through a ‘running hole’ , it is rotated into the workpiece. As the taper tap enters it is often only just necessary to ‘size’ the hole with the the previously tapping sized drilled hole, care should bottoming tap. However, if a ‘blind’/non-through hole, 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, after each ¾ turn, it is counter-ro- tated by about a ¼ turn to break the chips, otherwise ‘Galling’ , is when the tap, or indeed any cutter becomes clogged with the remnants of workpiece material, which will impair its efficiency, or at worse, cause it to break in the par- tially tapped hole.
186 Chapter 5 is to be tapped to depth, then it may be necessary to ting direction, or feed direction and are particularly utilise all three taps in the set, as each successive tap useful for tapping through-holes. Whereas, taps with once rotated to depth, it will have less lead (i.e. taper) straight flutes (Fig. 97bii) in conjunction with a long on the tapped hole, creating a stronger thread – up to chamfer lead, can also give good tapping results. For the thread’s maximum shear strength. blind-holes, right-handed flutes, or straight fluted taps Very large diameter hand taps, require a certain having shorter chamfer lead lengths give acceptable level of skill in ensuring that not only the tapped tapping results. These right-hand fluted taps, allow hole is normal to the surface, but a considerable level chip-flow in the backward direction – up the flutes. of physical strength is necessary to tap such a hole! The chamfer lead length is such, that it allows return movement of chips, but they will not jam and are reli- Curved surfaces are more difficult to tap, particularly ably sheared off. concave ones, as it is often difficult to keep the hand When tapping aluminium, grey cast iron, or certain tap normal to the surface With concave surfaces any brass alloys, the tap should have a short lead length – rotational motion of the tap wrench may be somewhat restricted, without a suitable extension chuck/bar – as- regardless of whether the hole is ‘blind’ , or ‘through- suming workpiece access conditions allow. running’. If, when tapping these workpiece materials, For manual tapping operations, it is often useful to a long chamfer lead length was utilised, the tap would utilise ‘Tapping chucks’. These chucks have a rotational behave like a ‘Core-drill’ with chip-breaker grooves. drive, coupled to a sprung-loaded Z-axis. The tapping This effect would create ‘drilling’ a tapping-sized hole chuck is positioned over the pre-drilled hole and man- to the major diameter – instead of actually cutting the ually-fed down into the hole. Once the tap has engaged required thread. with the hole, it is pulled and simultaneously ‘floated On some machining and turning centres, it is pos- sible to ‘solid tap’ the workpiece, using CNC software down’ the hole being tapped – giving excellent tapped hole accuracy. At ‘bottoming-out’ the tapping chuck developed just for this task. A ‘solid tapping’ operation automatically reverses its direction and ‘drives’ itself requires that the rotation of the spindle and the Z- axis control are fully synchronised, otherwise tapping out of the hole – while the machine’s spindle continues errors would arise. It is possible to calculate the time to rotate in the tapping direction. required for a tapping operation (Degamo, et al. 2003 – modified for metric units), using the following equa- Machine Taps tion: Machine taps (Fig. 97) are utilised across a diverse range of machine tools and special-purpose tapping Tm = L n � N = + AL + AR π DLn equipment. They can have a variety of flute helices, � V ranging from quick-to-straight flutes (Fig. 97a), de- pending upon the composition of the workpiece ma- Where: Tm = Cutting time (min.), terial to be tapped. When tapping, all machining is L = Depth of tapped hole, or Length of cut (mm), undertaken by the cutting teeth and the chamfer. In n = Feedrate (mm min–), general, the form and length of this chamfer will de- N = Spindle (rpm), pend upon what type of hole is to be tapped. Tapping V = Cutting speed (m min–), ‘through-holes’ is not too difficult, but ‘blind-holes’ AL = Allowance to start the tap (min), can present a problem, associated with the evacu- AR = Allowance to withdraw the tap (min). ation of swarf in the reverse direction to that of the feed. Tap flute spirals that are left-handed and those with spiral points (Fig. 97bi), remove chips in the cut- * To convert to inches, substitute 12 for the 1000 con- stant in the equation and modify the metric units to inches. ‘Tapping depth’ , is an often 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.
Threading Technologies 187 . Figure 97. Machine taps: with and without flutes. [Courtesy of Guhring]
188 Chapter 5 . Figure 98. Fluteles tapping and tool geometry. [Courtesy of Guhring]
Threading Technologies 189 5 .3 Fluteless Taps 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 Fluteless taps (Fig. 98a), do not have cutting edges to be taken. (Fig. 98ai) and produce the desired thread geometry by a ‘rolling action’ of the workpiece material. Threads 5.4 Threading Dies produced by fluteless taps are much stronger than their equivalent machined taps (Fig. 98b). The bulk workpiece material approximately follows the thread’s contour, thereby imparting additional shear strength On shafts, having either straight and tapered external to each thread. Oil grooves are usually incorporated threads these can be manually cut, up to a realistic max- into the taps periphery, to facilitate workpiece mate- imum φ40 mm, with threading dies. In essence, these rial movement and to reduce tap wear rates. Like the threading dies can be considered as analogous to hard- conventional machine taps (Fig. 97), fluteless taps ened threaded nuts with multiple cutting edges (Fig. have a lead to the tap’s edge – termed a ‘forming lead’ 99a). The cutting edges on the front die face are usually (Fig. 98b – left), as opposed to a conventional machine bevelled, or have a spiral lead to assist in starting the tap which has a ‘chamfer lead’ (Fig. 98b-right) which thread on the workpiece. Likewise, it is normal to add forms part of the cutting action. Therefore, the chi- a reasonable chamfer to the bar’s end to be threaded, as pless tap in operation (Fig. 98bi), plastically moves this also helps to gently introduce the thread to depth, workpiece material from the pre-drilled hole into the as the stock and die are manually-rotated down its spaces between the tap’s flanks and in so doing, locally length. As is the case for tapping, it is normal practice work-hardening this material to a limited depth in the to ‘back-off ’ the ‘stock’s’ rotation about every ¾ of a workpiece’s substrate. turn by approximately ¼ of a turn, to facilitate chip- Several factors need to be considered prior to util- breaking. As a result of these ‘leads’ on both the shaft ising fluteless taps on engineering components, these and die, a few threads on the bar’s end will not be to are: full thread depth. Care must be taken when initially • Over-sized diameter of pre-drilled hole – if the starting to cut the thread, as if it is not square to the hole is too large, then insufficient workpiece mate- bar’s axis, then a ‘drunken thread’ will result. Previ- rial will be available to fully form the rolled thread, ously, most dies were manufactured from high carbon • Undersized diameter of pre-drilled hole – too steel and, due to their size, their ‘ruling section’ and its 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 Therefore, 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 ‘Drunken threads’ , are the result of variations in the helix working ability and as a result of this action its lo- angle and its associated pitch differing in uniformity on each cal hardening, are important factors when ‘rolling’ side of the thread’s diameter. Hence, a ‘true’ mating nut, would a thread form. ‘wobble’ somewhat as it is rotated down such poorly manufac- tured threaded shaft – hence, its name: ‘drunken thread’. NB A ‘start-point’ for the size of pre-drilling diam- ‘Ruling section’ , this term relates to the cross-sectional area that can normally be hardened, being significantly influenced eter can be obtained from the tooling suppliers. Of- by the component’s geometry which affects its ‘critical cool- ten some form of experimentation is necessary in ing velocity’ (i.e. usually around 1,000°C sec–) when being order to obtain the optimum diameter, as this pre- quenched. This quenching rate is necessary if the part’s metal- drilled diameter will vary according to the work- lurgical structure is to fully transform into a martensitic state, piece material’s previous processing route. prior to subsequent tempering.
190 Chapter 5 . Figure 99. Die geometries and their nomenclature. [Courtesy of Guhring]
Threading Technologies 191 associated ‘mass effect’ meant that the dies could be die-head cutting elements to be preset to take firstly through-hardened – which gave them an overall ‘bulk a roughing cut, followed by finishing cut/chasing of hardness’ of greater than HSS. Today, basically dies the threads down the bar. At the end of the threaded are either manufactured from micro-grained HSS, or section, these self-opening dies will automatically coated cemented carbide. open and can then be speedily withdrawn from the Solid dies (Fig. 99a), do not have any means to com- threaded portion of the bar. These self-opening dies pensate for die wear, whereas, their split-die nut coun- can be set to give the correct amount of tolerance, con- terparts (Fig. 99b-left), can be manually-adjusted. This trolling the ‘play’ on the thread. Moreover, it is pos- adjustment of the die is achieved by turning a centrally sible to fit different thread sizes and forms into the die mounted grub-screw in the stock body, which along head, for more universal threading applications. Both with the fixing screws can be made to open, or close the radial and tangential threading elements, create on the shaft to be threaded. In this manner, achieving less tool flank contact and frictional rubbing on the the correct thread tolerance, or ‘play’ for the desired cut thread. fitment of its associated mating nut. It is also usual practice, to use a suitable die lubricant, to facilitate in 5.5 Thread Turning – the thread’s production while improving surface finish and prolonging the die’s life – as excessive friction oc- Introduction curs during this type of threading process. The 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 On conventional engine-/centre-lathes, a single-point thread, using up unproductive time elements, this thread cutting tool (Fig. 100), has a synchronised and being particularly important for large batch runs, or combined linear and rotary kinematic motion for its in a continuous production environment. Self-open- ing dies0 (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 ‘Mass effect’ , 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. This is because the quench media used could not sufficiently 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 often strategically designed in these larger component regions. Moreover, in many cases the larger component cross-sections are reduced, so that the ‘mass effect’ does not occur – apart from the obvious factor of relieving weight, etc. 0 Self-opening dies, are often termed ‘Thread chasing die heads’ , whereas in reality a thread is only ‘chased’ once the main . Figure 100. External and internal threading tool holders and thread form has initially been cut. Thus chasing is employed in-serts. [Courtesy of Seco Tools] to give the required fit and finish to the final thread form.
192 Chapter 5 threading insert. This insert is connected to the lead- and moved back to its start point, then fed more screw (i.e normally having a very accurately-hardened deeply beginning another threading pass down the and ground Acme form) which is precisely synchro- same helical groove, this process being repeated until nised to that of the headstock’s rotation. On a CNC full thread depth/profile is accomplished. In order to turning centre, or similar, this linear motion is reli- obtain a consistent thread pitch on the workpiece, the ant on the precision and accuracy of the recirculating feedrate along the threaded portion must exactly co- ballscrew coupled to the programmed cutterpath. In incide. The thread form is dependent upon the pro- this manner, the threading insert being rigidly held in filed geometry of the thread cutting insert. In order to either the tool post, or turret, generates a spiral groove achieve the required final thread profile, the feedrate which when at full depth creates a screwthread of the must be considerably larger than is normally utilised desired pitch and helix angle. During successive tra- for conventional turning operations. verse feeding passes (i.e. to prescribed depths) along Any V-form thread point angle geometry, is not the workpiece the thread is cut. A typical thread is an ideal edge shape for the production of machined routinely produced on CNC turning centres, using its threads if the insert is fed in normal to the workpiece’s fixed/canned cycles (i.e. ‘bespoke software’). During axis of rotation (i.e. radial/plunge-fed). Chip control these automated threading passes, the tool precisely here will be compromised, as each flank of the V-form traverses down the bar’s length, is rapidly withdrawn thread gets successively deeper. This narrowing of the . Figure 101. Screwcutting tech- niques on turning centres and suggest- ed methods for improved chip control. [Courtesy of Sandvik Coromant]
Threading Technologies 193 chips from each formed and angled flank face of the cessive passes at a slightly reduced angle (i.e. nor- V-shaped threading insert (i.e. see Fig. 103ai), creates mally ranging from 1° to 5°). This screwcutting high localised forces and stresses, which will tend to technique provides an improved flank surface fin- tear, rather than cut the final V-form thread profile. ish – compared to the two previous methods, par- In order to minimise these potential high force/shear ticularly on the either less hard, or for more ductile workpiece materials. Modified flank infeed methods components when radial/plunge-cutting a thread, the are recommended rather than radial infeeding for radial infeed passes are progressively reduced with larger threads, due to contact on this long flank increasing thread depth. The techniques of V-form length which would otherwise result in vibrational thread production by radial infeed techniques will be effects being superimposed (i.e chatter) onto the the subject of the next section. fi nal thread form, 5.5.1 R adial Infeed Techniques NB If the workpiece material’s characteristics include potential machining work-hardening Utilising single-point threading inserts (i.e. see Fig. problems, then flank infeed techniques should be 104, where a typical sequence of threading passes are avoided, depicted – as the V-form thread profile is partially • Incremental feeding (Fig. 101 – top-middle right) – formed), several different techniques of thread turning are utilised today, they include: if the thread form is very large, then the incremen- • Radial infeed (Fig. 101 – top-left) – being the most tal thread feeding strategy is normally utilised. common method, where the threading insert is fed at 90° to the workpiece’s rotational axis. The mate- These same radial infeed thread production tech- niques are used for the manufacture of internal threads rial being removed on both sides of the tool’s V- (Fig. 102a), by either ‘Pull-threading’ – depicted in form flanks – producing a ‘soft’ chip-forming action ‘A’ , where the thread form originates from the inter- giving uniform wear to both flanks of the insert, nal undercut, as opposed to ‘Push threading’ – shown NB Here the V-form threading insert geometry in ‘B’ – being toward say, an undercut. In both cases forms both flanks with lighter cuts as the thread of thread production, the modified flank infeed tech- depth progressively increases. niques are employed. • Flank infeed (Fig. 103aii) – is often known as the NB Threads manufactured by method ‘A’ allow for ‘half-angle screwcutting technique’ , mainly utilised excellent evacuation of the chips – being an ideal technique for ‘blind holes’. Conversely, in case ‘B’ , the on a conventional engine-/centre-lathe. Here, the left-hand flank is formed by the tool’s V-form ge- swarf would otherwise simply ‘bird’s nest’ in such a ometry, while the right-hand thread flank is gener- hole, unless a through hole is present, as is depicted in ‘B’. 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 flank infeed techniques If thread forms are based upon square threads, or their over ‘plunging’ , enabling the chip to be vectored modified trapezoidal forms: Buttress, or Acme (i.e. away from the previously cut surface (i.e see Fig. see Fig. 95i – for examples of these thread profiles), it 101 – middle, where the chips can be steered away from the flank), NB This ‘half-angle technique’ producing the thread’s right flank, is generated by the tool’s right- Incremental feeding, is sometimes termed the ‘Alternating flank’ technique, it has the advantage of imparting a uniform hand flank – which due to frictional effects, creates wear to both of the cutting insert’s V-form flanks, thereby sig- here a more pronounced wear rate on the cutting nificantly increasing the tool life. edge resulting in a poor surface finish. ‘Bird nesting’ , is a term that refers to the rotational entangle- ment and build-up of work-hardened swarf at the bottom of • Modified flank infeed (Fig. 101top-middle left) – in a ‘blind hole’ , which can create some problems in internal this cutting action, the tool is fed to depth in suc- thread production.
194 Chapter 5 . Figure 102. External and internal threading operations and the effect that the helix has as the diameter changes – for a given pitch. [Courtesy of Sandvik Coromant]
Threading Technologies 195 is advisable to pre-machine the thread with a groov- to a deeper thread depth and the process is repeated ing tool – with the tool’s width being the equivalent until the full thread form has been completed. As pre- of the thread’s root spacing dimension. Not only does viously mentioned, the pitch is not the same as the this pre-machining strategy of employing a grooving lead for multi-start threads and the lead can be easily tool reduce the number of threading passes to just calculated as follows: flank finishing, the tool can have a chip-breaker pres- Lead = np ent during the rough machining stage to effectively re- move the bulk stock and its associated swarf. Where: n = number of starts, 5.5.2 T hread Helix Angles, p = pitch (mm). f or Single-/Multi-Start Threads For example, in the case of the triple-start thread illus- The fundamental basis underpinning any thread form trated in Fig. 106c, for say, a V-form metric thread of is the helix angle, which in this case is denoted by the 6 mm pitch, then the lead will be: 3 × 6 mm = 18 mm. Greek symbol ‘ϕ’ – as schematically illustrated in Fig. NB This means that if a mating nut was rotated down 102b. One way of describing how the helix angle’s geometry is created, is to imagine that a right-angle this triple-start thread, it would be linearly displaced triangle is formed by a thin wire which has been by 18 mm in one revolution – allowing the nut to be unwound from a parallel cylindrical shaft, whose rotated in, or out quickly (i.e. because of its larger he- d iameter equates to its ‘effective diameter’. Then, this lix angle), but to the detriment of an increased axial unwound wire length (i.e. πD) would be its circumfer- loading. Although this load is distributed across the ence, acting as a base for the triangle (Fig. 102b). The contact between all the multi-start threads. perpendicular height of right-angled triangle is equal to the pitch ‘p’ , or the lead – in the case of a single- start thread. The angle that the hypotenuse makes with 5.5.3 T hreading Insert Inclination 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’ is decreased (i.e. ‘D’ → ‘D’), then the The threading insert is carefully ground by the tool- helix angle proportionally increases (i.e. ‘ϕ’ → ‘ϕ’). ing manufacturer to provide the correct thread profile. In the case of multi-start threads, the pitch and the This insert must operate with a radial cutting rake of lead differ, as shown in Fig. 106c. In this illustration for 0°, if the correct thread form is to actually imparted the cutting the triple-start thread, the usual approach to the formed thread (Fig. 103). The lead angle of the to its manufacture is for the three successive starts to flank surface varies at different points between the be individually completed to form the ‘triple-start’ , crest and the root of the thread, increasing toward the with each start being angularly displaced 120° with root – the opposite is true on an internal thread. Due to this effect, the actual cutting rake varies along the respect to each other. Alternatively, if one start is be- insert’s cutting edge, becoming more positive on the gun with the first threading pass, then the second start leading edge and more negative on the trailing edge – the is similarly machined and so on – for the number of starts required, then the threading insert is advanced closer it gets to the thread’s root. In order to minimise such threading insert rake angle variation the insert is inclined, so that its top face is perpendicular to a ‘Pitch’ – can be defined 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. ‘Lead’ – being defined as the axial distance through which Threading shimming – the tool holder is delivered fitted with a shim that gives an effective side inclination angle of 1° – be- a point on the thread advances during one revolution of the thread ×. This helix angle ‘ϕ’ is also known as its ‘lead angle’ ing the most common type. Although shims can be changed in degree increments from: –2° to 4°, by simply fitting a differ- NB Both the pitch and the lead are identical for single-start ent shim angle. Likewise, internal threading tool holder incli- threads. nations can be changed, by fitting such shims.
196 Chapter 5 . Figure 103. Thread for- mation by radial and flank infeeds, to-gether with threading insert inclina- tion angles. [Courtesy of Sandvik Coromant] line indicating the mean lead angle ‘λ’ – measured at produces a symmetrical side clearance (i.e. depicted the pitch diameter (Fig. 103b). This insert inclination in Fig. 103bi – bottom left diagram) and is important in ensuring a uniform edge wear on both flanks, re- sulting in increased insert useful life. The fact that this small threading insert inclination, causes one flank to cut slightly below, while the other cutting marginally Threading insert top face geometry – instead of a flat/straight top face to the insert, today, it is often angled (i.e. shown in above the centre-line of the workpiece – for a flat top Fig. 103bi – bottom left), which enables improved control of faced insert, is of no practical significance at normal lead angles for either the function of cutting, or the the developing chip.
Threading Technologies 197 . 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] thread’s profile. Further, a small deviation from the of between 0° and 2° with an inclination of 1° and, still exact symmetry required in the insert’s inclination is produce a satisfactory thread. This thread production also acceptable, without too obvious a disadvantage. technique is only true for the normal, symmetrical Thus, the inclined insert can be utilised to cut threads threads (i.e. ‘V-forms’); in the case of ‘saw-toothed’
198 Chapter 5 . Figure 105. Internal threading operations for right- and left-hand threads. [Courtesy of Sandvik Coromant] threads (e.g. Buttress), it should be borne in mind that and external left- and right-hand threading configura- the ‘straight-flanked’ ones – those with angles between tions, respectively). 0° to 7° – in particular, offer side clearances which may In the case of internal thread inclination angles, the tool must be ‘canted-over’ at the angle ‘λ’ (i.e. see Fig. be adequate. In Fig. 103c a graph depicting the thread- ing insert inclination angles is given for differing helix 103bii), so that the cutting edge is situated normal to angles, with the helix angle calculation derived as fol- the centreline. Often, the toolholder shank has to be lows: ground-away to avoid fouling on the internal hole’s diameter as shown in Fig. 105a. Here (Fig. 105a), the tan λ = P / D × π distance from the tool tip to the rear of the toolholder shank – denoted by the dimension ‘D’ , is relieved to ‘Dmod’ to avoid fouling on the curvature of the hole, as Where: λ = Helix angle (°), the tool is fed-out of the thread depth at the end of its P = Pitch (mm, or threads per inch), successive ‘threading passes’. D = Effective pitch diameter (mm, or inches). 5.5.4 T hread Profile Generation Metric threading inserts are characterised by their thread profile and the associated pitch, being ex- pressed in millimetres. The shape and size of the in- The profile of a thread can be cut by several different sert will determine the completed thread form. One techniques and differing types of inserts – depending threading insert can be utilised to cut all threads of whether ‘topping’ the is required. For example, if a V- this profile and size, irrespective of their thread diam- form profiled threading insert is utilised (Fig. 106bi), eter, or whether they are: right- or, left-hand, single- no actual machining is undertaken of the thread’s or, multi-start (i.e. see Figs. 105b and 106a, for internal top. In this situation, it is necessary to ensure that the
Threading Technologies 199 . Figure 106. External threading operations and insert forms. [Courtesy of Sandvik Coromant]
200 Chapter 5 5.5.5 T hreading Turning – pre-machined workpiece – when producing external C utting Data and Other threads – has the exact size for the major diameter re- I mportant Factors 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 Whatever type of thread to be cut, whether it is a: is often necessary to ‘chase the thread’ afterward. V-form, Multi-start, Trapezoidal, or Tapered, it is gen- In the case of profiled threading inserts, the com- erally quite difficult to vary such factors as the: cutting plete thread profile is cut from a slightly oversized speed, feed and, to a lesser extent the DOC, indepen- blank. Usually, three distinct profiling inserts could be dently of one another, without certain consideration of used in thread production, these are: some limiting factors. The typical limitations imposed • V-form (Fig. 106bi) – has the ability to machine a when cutting a thread, will now be discussed. range of thread profiles, with the nose radius pre- cisely and accurately ground for the smallest pitch Cutting Speed to be cut. As a result of this tightly ground nose radius, the insert’s life is shorter than with other Typical limitations imposed by the action of cut- profiling insert versions, as its size has not been ting a thread include, reducing the cutting speed by optimised for individual thread geometries. From 25% – compared to ordinary turning, as the insert’s an economic viewpoint, due to the V-form profil- shape limits heat dissipation. If a high a chip load ing insert’s ability to cut a wide variety of thread occurs due too great a cutting speed selected, then pitches, less inserts need to be stocked, the cutting temperature can approach that of say, a • Full-form (Fig. 106bii) – has the ability to profile cemented carbide’s original sintering temperature. the thread’s crest and is therefore manufactured to As a result of this elevated temperature, the binder exactly the specification of the required thread pro- phase may soften, causing potential cutting edge file. Such full-profile inserts simplify thread pro- plastic deformation. The remedy here seems quite duction, as no profile is deeper than its specifica- easy, simply reduce the cutting speed, but this may in- tion, allowing them to be a stronger insert thereby crease the risk of BUE. This BUE may cause the chips resulting in improved tool life, to become welded onto the cutting edge from which • Multi-point form (Fig. 106biii) – with this multiple- they are shortly fragmented and continuously carried pointed profiling insert, the first tooth roughs-out away – taking a minute portions of the insert’s edge the thread and is therefore slightly set back in com- along with them. The problem can be minimised by parison to the second tooth on the insert, which acts specifying a tougher grade of carbide for the threading almost like a ‘chaser’ which fully-forms and blends insert, or choosing a multi-coated insert. Normally, the various thread profiling elements upon the final the cutting speeds for any threading operation should threading pass. Cutting conditions need to be rigid not be less than 40 m min–, when machining with any and stable for this type of insert to operate correctly cemented carbides. – due to its longer cutting edge length. It is essential to ensure that the recommended in-feed values are Feed and D OC used, to ensure that cutting forces are balanced for The feed in millimetres per revolution, must coincide both of the cutting teeth. One advantage of utilising these multi-point threading inserts is that the num- with the desired pitch, or lead – when cutting multi- ber of threading passes can be reduced by almost start threads. Hence, if the cutting speed is modified, 50% – as it cuts deeper than its counterparts, when the feedrate will also have to be increased, or de- compared to the single-profiling insert forms. creased, so that the feed per revolution is constantly maintained. So, the critical factor here is to achieve some form of control over the DOC when threading. Each threading pass along the workpiece causes an in- creasingly larger portion of the insert’s cutting edge to ‘Chasing a thread’ , refers to using a chasing tool with the ex- act thread profile which is utilised to follow the thread along, become in contact in the threading operation, accord- thereby deburring and forming the desired profile simultane- ingly, tool forces will proportionally increase. If the DOC is kept constant during several passes, the chip- ously.