removal rate may increase by up to 300% at each suc- cessive infeed. erefore, in order to minimise these induced stresses on the cutting edge and keep them as uniform as possible, the D OC must be reduced with each pass along the workpiece. Thread Finishing and Close Tolerancing In order to obtain a good surface nish/texture, or a close tolerance on the nished thread anks, the CNC machine tool can be programmed to make ei - ther one, or two additional nishing passes. ese ad - ditional passes are termed ‘spring-cuts’ 18 and improve both accuracy and precision in the nal thread form. ‘Spring-cuts’ cannot be utilised on workpiece materi - als that have a tendency to work-harden, for example when thread turning stainless steels and so on, as they cause high tool wear. With work-hardening materials, cuts of <0.03 mm should be avoided, as these materials elastically-deform instead of being cut. e severity of the problem of work-hardening is even greater when thread turning austenitic steels and their equivalents and here, it is recommended that an infeed pass should always be >0.08 mm. ese comments are conned to steels and their alloys and even here, an appropriate number of infeeds by trial-and-error may be neces - sary. When a threading insert breaks, it is normally the result of induced high stresses, so the remedy is usually to increase the number of infeed threading passes. is solution is also true for machining threads in most cast iron grades, the exception here being for austempered ductile irons (ADF). It seems somewhat obvious that the greater the number of passes the threading opera - tion is divided into, the smaller the D OC and the stresses on the threading insert’s tip. Moreover, if this philoso - phy is pursued too far and very many infeed passes are programmed, the tool will simply not be able to cut at all – owing to insucient D OC and this in turn, will result in simply elastic deformation of the workpiece material. Such a ‘timid approach’ to thread cutting will lead to a higher wear-rate, so it becomes necessary to further reduce the number of passes – thereby becom - ing a self-defeating machining objective! 18 ‘Spring-cuts’ , are cuts that create a very light tool pressure on the threaded workpiece, to minimise the elastic deections in the ‘loop’ produced by the tool-machine-workpiece system, thereby improving machined quality. Cutting Forces If a comparison is made between the cutting forces for a threading operation with that of external turn - ing, then the power requirements are higher for thread formation, especially when the chip thicknesses are small. If however, the chip thickness is increased, then the values for plain turning are approached. Hence, one should always attempt to utilise higher chip thick- nesses, as the benets are two-fold: a decrease in the power consumption, combined with an increase in the subsequent production rate. General Comments on Cutting Inserts read cutting demands an insert with a: sharp cut- ting edge, good wear resistance and the ability to with - stand temperature uctuations. A sharp insert cutting edge combined with a favourable geometry are neces - sary, so that a good nal thread surface nish occurs, while simultaneously reducing vibrational tendencies. High wear resistance is crucial, as otherwise the sur - face nish would be impaired and thread tolerance de - viations would occur. Temperature uctuations must be withstood, as the very operation of threading in - troduces uctuations in the insert’s edge temperature: the fast machining pass along the thread causes heat - ing, then the tool is rapidly withdrawn and returned to the start-point – during which time the edge cools. is cyclical process is continuously repeated up to 20 times in quick succession, which could potentially promote fatigue cracks in the insert – termed ‘comb cracks’ (i.e oen previously present aer milling oper - ations – as they had nite ne lines at regular intervals on the tool’s edge, giving them the physical appearance of a ‘comb eect’). is was a real problem some years ago – which has now been overcome with suitable coating technology – and could in the past may poten - tially shorten the insert’s useful life. NB See Appendix 8 for a Trouble-shooting Guide to conventional thread turning problems and remedies. Threading Technologies Figure 107. Threadmilling cutters and typical thread generation operations. [Courtesy of Sandvik Coromant]. Chapter 5.6 Thread Milling Introduction read milling geometry in contrast to that of a basic tap (i.e. having a single spiral shaped tooth Fig. 107ai), has a series of teeth which do not form a spiral, but are congured without pitch (Fig. 107aii). is fun - damental dierence in tool design is attributed to the dierent thread production processes, explained ear - lier. Not only can a thread milling cutter have a similar visual geometry to that of a machine tap (Fig. 107aii), but it can occur with a single radially-mounted blade for milling both external and internal threads (Fig. 108b). Albeit for an internal thread production, there must be sucient working space for the cutter to be able to perform the thread milling task. Tooth Profile and Dimensions e thread milling cutter prole usually conforms to that of the thread to be milled. In certain cases, it may be essential to correct the milled thread’s prole. is being the case, when the diameter of the thread to be milled does not have a denite ratio to the diameter of the thread milling cutter. A major advantage of em - ploying thread milling in the production of threads, is that it can mill a range of threads of diering diam - eters. e one limitation here being that modications of the thread’s pitch is not practicable. If one discounts the tool’s thread pitch, then the de - sign of a thread milling cutter is remarkably similar to that of a machine tap (Fig. 107a). A typical thread milling cutter (Fig. 107aii), is characterised by its cutting section’s size and dimensions. e total tool length and its associated thread length are also part of the cutter’s dimensions. read milling cutter designs can also incorporate either a collar, or not – as the milling situation dictates, together with either a coun - tersinking chamfer, or not. erefore, the thread mill - ing cutter’s cutting section (Fig. 107aii), consists of its: ute length, ute prole, tooth form together with its associated form relief. In a similar fashion to that of a machine tap, the ute length usually incorporates run- out of the utes, although this ute run-out does not have to be as great as that found on machine taps, due to the smaller chips that are produced. read milled chips do not remain in the cutter’s utes during the thread milling process, and as such, will not restrict further chip development. e tooth width is larger than that found on machine taps, with relief grinding creating the necessary clearance angles, required for milling threads. Interference Ratio If the thread milling cutter diameter to that of the nominal thread diameter ratio of 70% is adhered to, then no milled thread prole distortion should take place (i.e. see Fig. 109a), irrespective of the thread’s depth – this fact has been consistently well proven by industrial applications. In Fig. 109a, the illustration depicts the fact that the diameter of the thread milling cutter and its associ - ated prole depth, determine the pressure angle of the thread’s diameter. Helical Interpolation Helical interpolation (Fig. 108a), is the amalgama- tion of two kinematic motions, these being: linear and circular interpolations. erefore, in thread milling, dierent threads can be manufactured by the form of overlaying the pitch direction with that of the direc - tion of rotation of the circular movement. read milling cutters are normally designed for right-hand cutting, with the direction of rotation be - ing generally clockwise. However, by altering a range of kinematic motions, such as: the axial direction of the feed, reverse cutter rotation, or by synchronous milling, all thread combinations can be manufactured – some of which are depicted in Fig. 107c. Depending upon the component features to be thread milled, such as into blind, or through holes and whether horizon - tal, or vertical machining techniques are to be incor - porated, together with the lubrication type and chip removal strategies, these will determine the correct choice of milling procedure to be adopted. Generally, for thread milling production, synchronous milling methods 19 (i.e. Fig. 109b) should be applied whenever possible, as they achieve the following intrinsic ben - ets: lower cutting forces, improved chip formation, longer tool life and improved surface quality. 19 Synchronous milling methods, can be identied when the thread milling cutting edge emerges with a chip thickness of zero (i.e. h = 0). Threading Technologies Speed Ratio When thread milling, the cutter edge’s speed is cal- culated by the cutting speed (i.e. revolutions) and the feedrate per tooth. With linear movement, the cutting edge’s feedrate is identical to that at the tool’s centre. However, with helical interpolation, it follows a path of a circle in the plane (Fig. 108a). All machine tool CNC controllers will calculate speeds from the tool’s centre, it is necessary to program a command for con - verting the cutting speed (i.e. a contour-related pro - gram). When such a program does not exist, or the central point is programmed, it is necessary to con - vert the feedrate accordingly. It should be mentioned, that the interactive control at the CNC control panel will always indicate the speed at the centre point of the tool and, when running with no load (i.e. usu - ally termed a ‘dry-run’), this speed is simple to check. Furthermore, if this speed is disregarded, the thread milling cutter will run at a speed many times greater than that of the feed, which shortly leads to the cut - ter’s breakage. Figure 108. Thread milling using a single-edged insert for either internal/external threading operations, can be achieved via a complex simultaneous circular interpolation of the ‘x’ and ‘y’ axes and a ‘z’ axis linear motion. [Courtesy of Stellram] . Chapter Figure 109. Threadmilling interference ratio, plus cutter positioning and feeding. [Courtesy of Guhring] . Threading Technologies Internal Thread Milling: Radial Positioning to Nominal Diameter, Via Entry Cycles e thread milling cutter’s radial positioning to the nominal diameter at the start of the thread’s produc - tion, is achieved by so-called ‘entry cycle’ 20 (Fig. 109c), while the movement following the thread’s milling op - eration is achieved by cutter motion from the nomi - nal diameter to the hole’s centre, via a corresponding ‘exit cycle’. read milling cutter approaches to that of the start of the thread, via suitable ‘entry cycles’ can be achieved by several dierent ways, these are: • Linear plunging (Fig. 109ci) – of the thread milling cutter into the workpiece material, creates a very large contact angle at the cutter’s periphery, lead - ing to the undesirable situation of high tool loading and long chips. is problem is particularly acute when the dierences between the thread milling cutter’s diameter to that of the hole’s size is small. Moreover, this radial entry linear plunging tech - nique can leave a small ‘delay mark’ 21 on a portion of the milled thread. NB Linear plunging is not an advisable thread mill- ing technique for the production of accurate and precise small threads. • 90° quarter circle entry cycle (Fig. 109cii) – allows just a small dierence in the diameter between the tool and the thread to remove a large part of the chip volume, during the linear section of the entry cycle. is particular entry cycle strategy, is nor - mally only utilised for relatively large dierences in diameter between the hole size and the cutter’s diameter. NB e 90° quarter circle entry cycle has the advan- tage of a relatively short entry path, together with a simple CNC program. • 180° semi-circle entry cycle (Fig. 109ciii) – the cut- ting force loading of the tool is at its lowest when 20 Entry-cycles, allow the thread milling cutter to be moved in a circular arc to the nominal thread’s diameter. 21 Delay marks, are the result of a slight dwell, prior to the next command line activation in the thread milling program, caus- ing cutting forces to ‘slightly relax’ and then impinge into the machined thread’s surface. the cutter is plunging, due to the contact angle be- ing relatively small during the complete cycle en - try. NB e 180° semi-circle entry cycle necessitates a slightly more sophisticated CNC program, al - though it has been found to be the most cost-e - cient thread milling technique overall. In Fig. 110, is depicted a step-by-step visual interpretation of this actual thread milling process, along with a typ - ical programming example. One distinct advantage that utilising thread milling tooling gives to the quality and tment of matching threads, is that minute variations in the pitch and to a lesser degree its associated depth, can be programmed- in by the operator to modify ‘working clearances’. is has the distinct benet of providing control over the ‘backlash’ between the two mating thread milled parts. 5.7 Thread Rolling – Introduction It is normal to specify thread rolling when substantial quantities of threads need to be manufactured. In es - sence, the production process is one of ‘cold-forming’ , in which the threaded features on the workpiece are formed by rolling a thread blank between hardened dies (Fig. 111). is rolling action, causes the metal to ow radially into the required V-form prole (i.e. see Fig. 111a – inset schematic diagrammatic com - parison between a cut and rolled thread – indicating the ‘directionality of the grain-ow’ 22 ). Due to the fact that no workpiece material is removed in the form of chips, there is no waste material – resulting in substan - 22 ‘Directionality of the grain-ow’ , this anisotropic behaviour of the manipulated grain structure aer rolling is one of plas- tic deformation of the local material (i.e. Fig.111a – inset dia- gram, indicating the V-from rolled thread). is local plastic deformation, raises the material’s: hardness, tensile and fa- tigue strengths, together with its proof stress. However, there is some ‘drop-o ’ in both the thread’s creep strength and its ductility as a result of rolling, but this is tolerated – due to the major benets described. Chapter Figure 110. A typical threadmilling cutter operational sequence, with an illustrated series of cutter motions and a ‘practical’ word-address CNC program. [Courtesy of Guhring] . Threading Technologies Figure 111. Thread rolling techniques – produce a strong thread form. Chapter tial economic savings as a result 23 . e main benets of thread rolling on CNC machine tools, is that due to this cold-working process, rolled threads have high strength, are smoother and more wear resistant then there machined counterparts. e thread rolling pro - duction rates are fast, typically a complete thread can be formed in a second, with the thread quality being consistently high. A principal characteristic of a thread rolling op - eration is that the rolled thread’s diameter is always greater than the original blank diameter. If the pro- spective thread must have an accurate ‘class of t’ , then its blank diameter is marginally increased by 0.05 mm with respect to the thread pitch diameter. When it is desired to have say, the body of a bolt larger than the outside diameter of the rolled thread, then the thread’s blank diameter is produced smaller than the body. .. Thread Rolling Techniques In Fig. 111, can be seen the three basic techniques used to thread roll employed on CNC machine tools, these are: • Two-roll tangential rolling (Fig. 111a), is a similar process to that of ‘knurling’ 24 . As the spindle turns, the workpiece’s pre-rolled diameter is progressively raised to its nal shape, normally over the course of between 20 to 30 revolutions. e tangential thread rolls are fed from the X-axis, at a tangent to the workpiece. When the centreline of the rolls line-up with that of the centreline of the workpiece, the process is complete. Usually rolling a φ20 mm thread at 1200 rpm, takes about 1 second, con - versely, a single-point turned thread would require 23 read rolling, is known as a ‘chipless operation’ and as a re- sult of the ‘cold rolling’ production process , the operation is cleaner and material savings in blank stock weight are of the order of between 15% to 20% – depending upon the size and length of the threaded feature manufactured. 24 Knurling (i.e not illustrated), utilises either two, or three hardened rotating knurls which are pressed into the previ- ously turned outside diameter, thereby giving a ‘gripping’ sur- face pattern – and hence aids in purchase for one’s grip, with normally either a straight-, or diamond-shaped knurl. NB It is possible to utilise tangential sliding knurls to impart the desired ‘imprinted patterned surface’ onto the workpiece’s periphery. 10 times longer to manufacture the same threaded feature, • ree-roll radial rolling (Fig. 111b), is similar in operation to tangential heads, in so far as the work - piece is normally approached from the side, per - pendicular to the major thread’s axis. e radial rolls are sprung-loaded and when they are brought over the workpiece, the tension is released, causing the rolls to rotate and produce a thread. Flats on the rolls allow for work to be inserted and removed. In both the tangential and radial rolling techniques, they are limited to thread lengths that are no greater than the thread roll widths. e principal dierence between these two heads, is that with radial heads the form is completed in just one revolution, as op- posed to the 20 to 30 revolutions necessary with tangential rolling methods. is fact, makes the ra- dial rolling the fastest of all rolling techniques. For example, if the workpiece spindle is rotating at 1200 rpm, and a φ10 mm thread is to be rolled, it would take just 0.5 seconds to complete, • Two-roll axial rolling (Fig. 111c), these rolls engage the workpiece from its front, along the workpiece’s centreline (i.e. Z-axis). is rolling action is analo - gous to a threading die, or thread-chaser, traversing from one end of the workpiece to the other. Hence, this rolling arrangement is capable of producing very long threads, or threaded portions on the workpiece, moreover, the axial heads support the part during the thread’s manufacture, eliminating the need of a supporting tailstock. In all these thread rolling processes, the operation of thread rolling remains primarily identical in its nal rolled threaded feature on the workpiece and the pro - cess of imparting threads on ductile and to a lesser extent some work-hardening materials, should be en - couraged. ere are other techniques for the produc - tion of rolled threads that have not been shown here, including: reciprocating and at die designs, planetary rolling, etc., they have not been incorporated into this review, because of the diculty of utilising them on CNC machine tools. References Journals and Conference Papers Bolden, A. Tapping Troubles: the Hidden Causes. Cutting Tool Eng’g., 20–25, April 1990. Threading Technologies Burns, S. Keep the Tool Cool during Tapping. Cutting Tool Eng’g., 33–37, April 1990. Hanson, K. Roll your Own [read Rolling]. Cutting Tool Eng’g., 54–58, May 2002. Hazelton, J.L. Tapping the Hard Stu. Cutting Tool Eng’g., 62–68, Mar. 2007. Henderer, T. Solid Synchronicity [Solid Tapping].Cutting Tool Eng’g., 58–63, Feb. 2006. Jonah, A.K. Standard Taps for Exotic Materials. Cutting Tool Eng’g., 26–30, April 1990. Kennedy, B. What’s on Tap? [Tapping Advances]. Cutting Tool Eng’g., 26–35, May 2002. Lewis, B. Challenge on Tap [Tapping Problems]. Cutting Tool Eng’g., 44–48, April 2003. Nelson. D. Swiss reads [Swiss-type, segmented thread- ing]. Cutting Tool Eng’g., 56–62, April 2007. Pontius, K. Low-silicon Lowdown [Tapping Si-Al Parts]. Cutting Tool Eng’g., 58–64, May 2001. Restall, M. e Ins and Outs of read Milling. Cutting Tool Eng’g., 28–33, Aug. 2001. Richter, A. Know your Limits [read Limits and Classes]. Cutting Tool Eng’g., 36–41, Jan. 2005. Rowe, J. e Lowdown on Laydown Inserts [Laydown threading systems]. Cutting Tool Eng’g., 45–48, Oct. 2002. Books, Booklets and Guides Altan, T. Oh, S-I. and Gegel, H.L. Metal Forming: Funda- mentals and Applications. ASM Int. Pub. (Matls. Park, Ohio), 1983. Burrows, L. and Hancox, D. Cra Engineering Data Book. Stanley ornes Pub., 1978. Cottrell, A. An Introduction to Metallurgy. Edward Arnold Pub., 1975. Degamo, E.P., Black, J.T., Kosher, R.A. Materials and Pro- cesses in Manufacturing. John Wiley and Sons Inc., 2003. Precision Cutting Tools. Guhring Pub. 8 th Ed., 2004. Inuence of Metallurgy on Hole Making Operations. ASM Pub. (Ohio), 1978. Kalpakjian, S. Manufacturing Processes for Engineering Ma- terials. Addison Wesley Pub., 1984. Modern Metal Cutting – Part 11: Other Tools. AB Sandvik Coromant Pub., 1981. Modern Metal Cutting. AB Sandvik Coromant Pub., 1994. Reed-Hill, R.E. Physical Metallurgy Principles. Van Nos- trand Reinhold 2 nd Ed., 1973. Rollason, E.C. Metallurgy for Engineers. Edward Arnold Pub. 4 th Ed., 1973. Schey, J.A. Introduction to Manufacturing Processes. Mc- Graw-Hill Book Co. 3 rd Ed., 1999. Wick, C. et al. Tool and Manufacturing Engineers Hand- book – Vol. II: Forming. 4 th Ed., Society of Manuf. Engrs. (Dearborn Mich.), 1984. Chapter . Causes. Cutting Tool Eng’g., 20–25, April 1990. Threading Technologies Burns, S. Keep the Tool Cool during Tapping. Cutting Tool Eng’g., 33 37 , April 1990. Hanson, K. Roll your Own [read Rolling]. Cutting Tool Eng’g., 54–58, May 2002. Hazelton, J.L. Tapping. Milling. Cutting Tool Eng’g., 28 33 , Aug. 2001. Richter, A. Know your Limits [read Limits and Classes]. Cutting Tool Eng’g., 36 –41, Jan. 2005. Rowe, J. e Lowdown on Laydown Inserts [Laydown threading systems]. . Tapping].Cutting Tool Eng’g., 58– 63, Feb. 2006. Jonah, A.K. Standard Taps for Exotic Materials. Cutting Tool Eng’g., 26 30 , April 1990. Kennedy, B. What’s on Tap? [Tapping Advances]. Cutting Tool Eng’g., 26 35 , May 2002. Lewis,