7 Machinability and Surface Integrity ‘It is common sense to take a method and try it. If it fails, admit it frankly and try another. But above all, try something. ’    (1882 – 1945) [32 nd President: United States of America] 7.1 Machinability Introduction – an Historical Perspective Today, greater emphasis is being placed on a compo- nent’s ‘machinability’ , but this term is an ambiguous one, having a variety of dierent meanings, depending upon the production engineer’s requirements. In fact, the machinability expression does not have an author- itative denition, despite the fact that it has been used for decades. In 1938, Ernst in his book on the ‘Physics of Metal Cutting’ , dened machinability in the follow - ing manner: ‘As a complex physical property of a metal involving: • True machinability, a function of the tensile strength, • Finishability, or ease of obtaining a good nish, • Abrasiveness, or the abrasion undergone by the tool during cutting.’ By 1950, Boulger had summarised these criteria more succinctly in his statement: ‘From any standpoint, the material with the best machinability is the one permit- ting the fastest removal of chips with satisfactory tool life and surface nish.’ is ‘Boulger denition’ leaves some unanswered questions concerning chip-form- ing factors, cutting forces and, has little regard for either the physical and mechanical properties of the material, nor potential sub-surface damage caused by the cutting edge. By 1989, Smith made the point that in fact machinability, had to address these properties and the word ‘metal’ should be substituted by the ex- pression ‘material’ , in a combined general-purpose denition, as follows: e totality of all the properties of a work material which aect the cutting process and, the relative ease of producing satisfactory products by chip-forming methods.’ Even these denitions still lack sucient precision to be of much practical use and by 1999, Gorzkowski, et al., in their powder metallurgy paper concerning ‘secondary machining’ 1 , entitled: 1 ‘Secondary machining’ , is a term used to cover any additional post-machining operations (e.g. drilling, turning and milling, etc.), that has to be undertaken on powder metallurgy (i.e. sintered’) compacts, aer compaction and sintering. Nor- mally, these post-sintering production processes, are only car- ried out to ensure, say: a good turned registered diameter, a precision cross-drilled hole, precise and accurate screwthread, an undercut, or similar* – as this is a last resort, as it adds- value to the overall component’s cost. ‘Machinability’ , stated that: ‘Machinability is a dicult property to quantify.’ Why is this so? It is probably is a combination of many inter-related factors, such as: chemical composition of the workpiece, its micro- structure, heat-treatment, purity, together with many more eects which inuence the overall machining operation. In Fig. 144, this diagram attempts to high- light some of the important factors that aect a com- ponent’s machined state – its ‘machinability’. Although even here, an important factor such as power con- sumption is missing, showing that this is by no means an exhaustive ow-chart of the complex mechanisms that exist when a material is subjected to machining. is is probably why it is virtually impossible to state that one, or another material aer machining, was ei- ther a ‘good’ , or bad’ one to machine. By utilising some ‘impartial and objective testing program’ , it may be pos- sible to ‘rank’ prospective or current materials, or pro- duction tools – in some way, perhaps by way of a ‘De- sign of Experiments’ (DoE), in combination with ‘Value Analysis’ (VA) approach to the production problem. is strategic technique to the problems of ‘machin- ability comparisons’ of diering factors will shortly be mentioned in more detail, aer a brief resumé on just some of the machinability testing techniques favoured today. .. Design of Machinability Tests and Experimental Testing Programmes Over the years, a range of machinability testpieces have been developed – more on this shortly – that are used to assess specic cutting conditions found when machining the actual production part. e assessment of a material’s machinability can be undertaken by two groups of tests, these are machining and non-machin- ing testing programmes. e former machinability group, can be further sub-divided into either ‘ranking’ and ‘absolute’ tests and, it should be mentioned that the latter non-machining tests fall into the ranking category. Oen, ‘ranking’ tests are termed ‘short *Powders when they ll the dies and are compacted, cannot reproduce component features at 90° to the major pressing di- rection – hence, the powders cannot readily move sideways – as such, features, like: screwthreads, transversal features (i.e. undercuts, etc.), must be machined aerward, hence, the term ‘secondary machining’.  Chapter  Figure 144. The major factors that inuence a machined component’s condition. Machinability and Surface Integrity  tests’ , conversely ‘absolute’ tests are known as ‘long tests’. By their very nature, the ‘short tests’ merely in- dicate the relative machinabilities of two, or more dif- ferent combinations of tool and workpiece. Whereas, the ‘long tests’ can produce a more complete depiction of the anticipated conditions for various combinations of tool and workpiece, but as their name suggests, they are more time-consuming and costly to develop and perform. Some of these test regimes are briey reviewed below, but more information can be obtained from the listed references at the end of this chapter. ‘Ranking’ Machining Tests A series of these ‘ranking’ tests for fast assessment of actual production conditions has been devised over the years and some will be mentioned below, but this is by no means an exhaustive account of all such testing programmes, they merely indicate the relatively well- tried-and-tested techniques, such as: • ‘Rapid facing test’ – this consists of a turning op- eration, requiring facing-o a workpiece, preferably having a large diameter, using an HSS tool 2 . e machinability is assessed by the distance the tool will travel radially-outward, from the bar’s centre, prior to its catastrophic tool failure. is ‘end- point’ as it is known, is compared with a similar trial, where the distance for tool failure by using a reference material 3 was previously determined, NB Although the ‘Rapid facing test’ quickly assesses one particular test criterion that a machinability rating can be based upon, it suers from a number of limitations. Firstly, the material’s diameter may be smaller than that which one would ideally prefer to use for the test. Secondly, if the workpiece mate- rial’s structure is not homogeneous 4 , then this test only indicates properties over the diameter-range 2 ‘HSS tool material’ is utilised, because under these extreme machining conditions, it will rapidly promote catastrophic tool failure as the forces steadily increase together with esca- lating tool interface temperature, as the tool’s edge is fed radi- ally-outward during the subsequent facing operation. 3 ‘Reference materials’ , are normally those workpiece materials that are considered to be ‘easy-to-machine’ , as their name sug- gests they, at the very least, give a ‘base-line’ , or datum, for some form of machinability comparison. 4 ‘Homogeneity of material’ , refers to a uniformity of its micro- structure and having isotropic properties. used. is latter problem of lack of homogeneity of the workpiece material, can be somewhat lessened by boring-out the material at the workpiece’s cen- tre, prior to commencing the test. • ‘Constant-pressure test’ – this is quite a popular testing technique and can be undertaken by a va- riety of methods of machining assessment. For example, in turning, machinability is measured by utilising predetermined geometry in association with a constant feed force. e technique has been used to some eect on the machining of free-cut- ting steels. is test is essentially a measure of the friction between the chip and tool, which is re- lated to the specic cutting temperatures generated whilst machining, together with its eects on the tool’s wear-rate, NB Normally a turning centre has a constant feed force, in order to obtain relevant data. An engine-/ centre-lathe can also be employed to acquire iden- tical data, but a tool-force dynamometer is used to measure this feed force, then plotting a graph of this feed force with its associated frictional ef- fects, but this requires more eort and takes longer. Similar constant pressure tests can be employed for drilling processes. • ‘Degraded tool test’ – consists of workpiece ma- chining with a soened (i.e. degraded) cutting tool. e test’s ‘end-point’ is determined either: when a specied amount of tool ank/crater wear has been reached, or at catastrophic tool failure, NB If machinability testing is carried out on soer and more easy-to-machine materials – typically on various alloys of brass, then just a small variation in soening the tool steel prior to cutting, has a dras- tic eect on the results obtained, but for harder-to- machine materials this eect is signicantly less- ened. • ‘Accelerated cutting-tool wear test’ – as an alterna- tive to deliberately soening the tool (i.e. Degrad- ing tool test), in order to speed-up the machinabil- ity process the cutting speeds are increased. If the cutting speeds are signicantly increased, the tool will not behave according to the predictable tool life  Chapter  equation 5 – due to the articially-elevated cutting temperature generated. NB It is not prudent practice to extrapolate tool- life data beyond that actually obtained during test- ing in order to obtain quantitative information about other ranges and conditions, with diering operations and parameters. As a result, this test is usually classied as a ‘ranking test’. ‘Ranking’ – Non-Machining Tests Whenever there seems to be a need to experiment with material cutting using perhaps one of the techniques just mentioned, it is important to establish whether any savings gained will be recouped in the actual pro- duction operation. If a company is unsure of the likely cost benets of such testing, then a strong case can be made not to test the material at all! Fortunately, non- machining tests exist that can be utilised in these doubt- ful situations, rather than ‘working blindly’ – with no relevant cutting data, to base the applied cutting con- ditions upon. Several of these ‘ranking’ non-machining tests can be employed, such as: • Chemical composition test – a variety of tests have been developed by which workpiece materials are ‘ranked’ according to their primary constituents. It is obvious that the results from such tests are only relevant when materials of similar type, having identical processing conditions/thermal history 6 , are to be machined. 5 Taylor’s tool life equation(s), has been utilised for many years, to determine the ‘end-point’ of a cutting insert’s useful life, under steady-state cutting conditions. e basis of the general formula: V c T α = C, has been modied and expanded to obtain an equation for the ‘economical cutting-edge life’ for a speci- ed feed, as follows: T e  = (1/α – 1)(C t /C m  + t c ) Where: T e  = economical tool life, α = slope of the VT-curve (i.e. measured from a plotted graph), C t = cutting-tool cost per cutting edge (i.e. see ‘Machining costs’ – later in the chapter), C m  = machine charge per minute (i.e normally established by the machine shop management), t c  = tool-changing time for the cutting operation – this will vary according to whether the tooling is of the conventional, or quick- change type. 6 ‘ermal history’ , refers to the heat treatment thermal cycle that the component in question was processed, describing the time at temperature, with any modications to the tempera- ture-induced regime on the heat-treated part. NB Given the above limitations, these tests have proved to be quite valid and successful for screen- ing a workpiece material prior to actual machining. Typical examples of this test type, rank materials using a V 60 scale – giving cutting speeds in m min –1 and the machinability index of 100 (i.e. utilised by the ‘Volvo test’ – not shown). A close correlation be- tween the chemical composition test and ‘absolute tests’ has been obtained with accuracies claimed to within 8%. For example, the relationship be- tween chemical composition and cutting speed is: Cutting speed (V 60 ) = 161.5 – 141.4 × %C – 42 – 4 × %Si – 39.2 × %Mn – 179.4 × %P + 121.4 × %S. • Microstructure tests – are principally concerned with the type of microstructure present in say, a steel workpiece, specically: inclusion type, shape and dispersion. e test method gives a good in- dication of the likely machinability, but requires highly-specialised laboratory equipment for such a metallographical investigation although materials can only be ranked, as either: good, bad, or indier- ent. NB Early work here, primarily investigated low- to-medium carbon steel microstructures, notably considering the spacing between pearlite laminae achieved by heat treatment. e pearlite-to-ferrite proportions clearly inuenced the materials hard- ness value (e.g. Brinell). When a cutting speed was selected (e.g. V 80 ), a machinability rating could be obtained for either life at: a constant speed (min- utes), or relative speed for a constant tool life (m min –1 ). It has been observed that when >50% pearlite was present, combined with a relatively high bulk hardness 7 , then good machining characteristics occurred. In recent years, commercially-available steels have trace elements added to aid machinabil- ity, the so-called free-machining steels. Typically, sulphur and manganese additions, create manga- nese sulphide, with their shape, size and distribu- tion within the steel’s matrix, playing a major role in aiding machinability factors. 7 ‘Bulk hardness’ , is a term that is used to state the overall hard- ness of the test specimen, not its micro-hardness – which only establishes localised hardness levels. Machinability and Surface Integrity  • Physical properties test – requires specialist equip- ment in order to perform this test. e physical properties of the workpiece material are utilised in order to determine its machinability ranking. NB Researchers, have produced a general machin- ability equation using a dimensional analysis tech- nique and, by utilising conventional test methods to establish and measure its: thermal conductivity, harness (Brinell), percentage reduction in area, together with the test sample’s length. is ‘Physi- cal properties test’ , gives close agreement with the V 60 cutting speed for a range of ferrous alloys, al- though when brittle materials are assessed, the lack of a yield-point 8 and the much smaller reductions in area – aer tensile testing – may cause potential ranking problems. ‘Absolute’ Machining Tests As their name implies, the ‘absolute tests’ are utilised in order to obtain a comprehensive data-gathering machining-based activity, on particular types of work- piece and cutting tool combinations. Many of these ‘absolute testing’ techniques have been devised, with several of them listed below, including the: • Taper-turning test – being undertaken by turning a tapered workpiece. As a result of turning along the taper, the cutting speed will proportionally increase with increasing taper diameter – this also being in proportion to the cutting time. By originally estab- lishing the cutting speed, the changing-rate of the 8 ‘Yield-point’ , refers to the strain* at which deformation be- comes permanent, when the material is subjected to some form of mechanical-working. e yield-point strain for fer- rous and many ductile materials is well-dened, illustrating a ‘sharp’ transition from elastic-to-plastic deformation – where a permanent ‘set’ occurs. However, this is not the case for many brittle materials, here when say, a tensile test is con- ducted, an articial ‘proof-stress’ value is used to intersect the stress/strain curve plotted, to establish its safe-working level of operation – see the relevant References for more in-depth details. *‘Strain’ , is a measure of the change in the size, or shape of a body – referring to its original size, or shape. For ex- ample, linear strain is the change per unit length of a linear dimension – aer some form of mechanical working. For a tensile test specimen that has been subjected to a tensile test, it refers to its linear dimensional change from its original gauge length. cutting speed in conjunction with the amount of tool ank wear – for two separate tests – allows the values of the constants (i.e.‘α’ and ‘C’) in Taylor’s equation for tool wear – see Footnote 5 – to be de- rived and, the tool life established for a range of fu- ture cutting tests. As the D OC must be consistently maintained throughout the test, either a CNC pro- gram must be written – using one of the standard ‘canned-routines’ available, or a taper-turning at- tachment is necessary on an engine-/centre-lathe, NB Some major advantages accrue from this com- prehensive testing technique, not least of which is that results are valid for a range of pre-selected cut- ting speeds and, the test is of relatively short du- ration, but closely agree with many thorough and longer test methods. Although, the results obtained may not be representative of actual cutting condi- tions, owing to the fact that the cutting tool, ma- chines at diering diameters throughout the taper turning test. • Variable-rate machining test – achieves similar results to the previously described ‘Taper-turn- ing test’. In this case, the increase in cutting speed is obtained by turning a parallel testpiece axially, whilst simultaneously increasing the cutting speed as the tool traverses longitudinally along the work- piece. Once again, the constants are derived for the ‘Taylor equation’ aer a minimum of two tests have been completed, NB e main advantages of this method over the ‘Taper-turning test’ , are that a standard testpiece can be used and the results probably reect truer actual turning conditions – in that consistent diam- eters are being turned, although this argument is somewhat debased, if the turning of complex free- from component geometry is demanded for the production part. • Step-turning test – was developed to overcome some of the problems associated with the two pre- viously described testing techniques. In the ‘Step- turning test’ method, a range of discrete diameters and speeds are utilised to determine the ‘Taylor’s constants’. is test, shows close agreement with re- sults obtained from the two previously-mentioned ‘absolute test’ methods, • HSS tool wear-rate test – this test assesses machin- ability by measurement of the tool’s ank wear, pro-  Chapter  duced when machining free-cutting steels, with the major parameters being the elemental additions to the metallurgical composition of these steel grades. NB ese tests are undertaken in a similar manner to the: ISO 3685:1977 Standard, for a long ‘absolute test’ , but it was withdrawn in mid-1984. All of the above ‘absolute testing’ programmes, relate to turning operations, principally due to the fact that the tool is engaged in the workpiece test sample for a reasonably lengthy period of time. is tool/workpiece engagement, allows for ‘steady-state’ conditions to be developed, having the additional benet of producing relatively consistent ‘Taylor constants’. From a more practical viewpoint, the author has developed some other testpieces, which have proved somewhat useful in actual industrial machining applications, where a more representative machinability situation was de- manded. Just some of these testpieces, along with a discussion of their relative merits, will now proceed. Practical Testpieces – for CNC Applications e premise behind the development of the testpiece depicted in Fig. 145, was to attempt to ‘mirror’ the ac- tual production operations and to a lesser extent, the physical geometry of a particular component part. Here, the component geometry was devised to be ma- chined on either a machining centre, or a turning cen- tre with the facility of driven tooling and at the very least, having an indexing workholding spindle/chuck. With this testpiece, the part is preferably a thick-walled tube that can be bored out, OD turned, circular inter- polated (i.e. milled), drilled and tapped – as the drill- ing size, is also an M6x1 tapping size. is allows the component’s geometric features to be inspected ‘On- machine’ – using metrological inspection routines in association with touch-trigger probes and, ‘O-ma- chine’ employing a CNC Co-ordinate Measuring Ma- chine (CMM). ese identical parts were from a series of exhaustive tests undertaken on both ferrous metals and aerospace-grade aluminium stock. Of particular note, was that when a milled circular interpolated fea- ture – the boss, was assessed on the machining centre, it gave more accurate readings than its equivalent in- spection routine on the CMM. is perceived dier- ence in accuracy and precision, was the result of part changes caused by both relaxation of the clamping forces – upon release – and the greater temperature dierential between these workpieces when inspected on the CMM. However of note, was the fact that in general for the inspection of part features, the CMM showed a four times improvement in repeatability, to that of the touch-trigger probing undertaken on the machine tool, as the following Table 9 indicates: e above type of practical ‘testing regimes’ are gen- erally termed: ‘Production Performance Tests’ (PPT). Typically, these PPT’s can be utilised to determine the maximum production rate – in parts per hour. Al- though it must be said, that with shis normally con- sisting of between 6 to 8 hours duration of potential ‘in-cut time’ , this to a certain extent, limit’s the achiev - able machined surface nish requirement, particularly if a ‘Sister tooling strategy’ is not operated. One of the main problems connected with PPT’s, is that invari- ably free-cutting metals are usually selected for long- term testing, meaning that any wear-related data takes awhile to accrue. Despite this slight reservation, actual cutting data can be employed, which represents almost optimum machining conditions, leading the way to Table 9. A comparison of the machined component tes- tpiece accuracies by either: ‘On-’ , or ‘O-machine’ inspection procedures PARAMETERS: MACHINES*: - equipped with Renishaw touch- trigger probes: Machining Centre (Vertical) CMM (LK CNC Micro4) Scope Full range of: X-, Y- and Z-axes Direction of test Uni-directional Positional Accuracy ±13 µm X-axis ±8 µm Y-axis ±5 µm Z-axis ±6 µm Repeatability ±10 µm ±2.5 µm * Machine tools here, are part of a fully-industrial Flexible Manufac - turing Cell (FMC), comprising of Cincinnati Milacron equipment: 200/15 Turning Centre, 5VC Vertical Machining Centre, T 3 776 Ro- bot- equipped with twin back-to-back grippers – for component loading/unloading, LK Micro4-CMM, DeVlieg Tool Presetting Ma- chine, Component workstation, Cell Controller, all equipped with Sandvik Coromant quick-change tooling (Block Tools and Varilock Tooling), plus DNC-link to a CAD/CAM workstation – being desi- gned and developed by Cincinnati Milacron and the Author, when acting as an Industrial Engineering Professor at the Southampton Solent University. . Machinability and Surface Integrity  Figure 145. General machinability test piece for CNC machine tools. NB Holes marked ‘A, B and C’ are machined at dierent cutting speeds, as are the turned, bored and milled dimensions. .  Chapter  ‘full’ production operational machining, meaning that with some degree of condence, manufacturing dic- tates and objectives will be met. In Fig. 146, a commercial (PPT) testpiece has been developed showing typical machining data employed, based upon the secondary machining operations de- manded by many companies on Powder Metallurgy (P/M) components – where light nishing cuts, or ac- curate and precise screwthreads are demanded. Here, the cutting insert can turn three dierent diameters – usually in some form of arithmetic progression 9 , so that feedrate longitudinally can be metrologically as- sessed. Moreover, the insert’s passage over the surface can be metallographically-inspected and a micro- hardness ‘footprint’ across a tapered section can be undertaken, to see if any surface/sub-surface modica- tions have occurred. More will be said on this subject later in the chapter, when discussing the eects of ‘ma- chined surface integrity’. is design of using a thick- walled tube (Fig. 146), that can be produced from ei- ther wrought stock, or P/M compact processing – the latter, giving a controlled ‘density’ 10 across and along the part, makes it particularly ‘ideal’ for any secondary machining machinability trials. Boring operations can also be conducted on such a testpiece geometry, al- lowing roundness parameters and its associated ‘har- monic prole’ to be metrologically assessed, in conjuc- tion with any ‘eccentricity’ with respect to the OD and 9 ‘Arithmetic progressions’ , are normally utilised for many ap- plied machining (PPT) trials as they give a ‘base-line’ for the research work and increase at a controlled amount. For ex- ample, a feedrate, could begin and increase as follows: 0.1, 0.4, 0.7, 1.0, 1.3, … mm rev –1 – with the ‘common dierence’ being 3. As a mathematical expression, this simple arithmetic pro- gression, can be written as follows: a, a+d, a+2d, a+3d, a+4d, a+5d, … where the ‘common dier- ence’ is ‘d’ , giving the: n th term as: a+(n–1)d. 10 ‘P/M Density’ , refers to either the uncompacted, or free-par- ticulates and is termed its ‘Apparent density’ (AD). is term AD, is used to refer to the loose material particulates prior to PM compacting, to describe the density of a powder mass ex- pressed in grammes per cubic centimetre of a standard volume of powder. is AD diers from that of its ‘compacted density’ – which will vary depending upon the consolidation (i.e. com- pacting) technique utilised. For example, double-compaction – pressing the powder in the dieset from both ends, or us- ing ‘oating diesets’ – to simulate double compaction, in this latter case, pressing from one end only, will produce a more uniform bulk density throughout the ‘green compact’ as it is known – prior to its subsequent sintering process. ID – these machined surfaces both being produced in a ‘one-hit machining’ operation – then inspected by a suitable roundness testing machine. e main advantage of using industrial-based (PPT) testpieces similar to that shown in Fig. 146, is that ‘canned-cycles’ 11 , can be used to produce the un- dercuts, turning passes, or screwcutting operations on each part. Moreover, optional ‘programmed-stops’ can be written, allowing the research-worker/operator, to have the facility to stop machining at a convenient point as desired, at the press of a button – giving a measure of control to the automated CNC machining processes. If a series of testpieces are to be machined, it is important that all of the parts machining sequences are known and that they are laid-out in a consequtive logical fashion. is allows one to measure the dete- rioration with machining time for the sequence of tes- tpieces produced. To this end, not only should some unique and logical part numbering system be used, but in the case of P/M testpieces, the top and bottom for each compact should be established. As when each one was initially compacted, its local density have var- ied and, for consistency for all machining undertaken with each test piece, it needs to be held in the same orientation. Oen it is possible to amalgamate two previous ranking machining test regimes into one, this is the case with ‘Accelerated Wear Test’ (AWT) illustrated in Fig. 147, this test being a combination of both the: ‘Rapid Facing’ and ‘Degraded Tool’ tests – previously described. In the case of the AWT technique, this hy- brid test’s aim is to assess the relative machinability of either wrought, or secondary machined P/M compacts 11 ‘Canned-cycles’ , this is a preset sequence of events that is ex- ecuted by issuing a single command, which may remain active throughout the program, or in this case will not, for a par- ticular ‘canned-cycle’ *. For example, once the preset values/ dimensions together with the required tool osets have been established, then a preparatory function entitled a ‘G-code’ can be used, such as a G81 code, which would initiate a sim- ple drilling cycle, in association with the following G84 code which would then specify a tapping cycle on this drilled hole, or alternatively, a G32 code commences a threading cycle and so on. – which considerably reduces both the complesaty and overall length of a CNC program. *G-codes fall into two categories, they are either ‘modal’ , or ‘non-modal’. A ‘modal’ G-code, remains ‘active’ for all subse- quent programmed blocks, unless replaced by another ‘modal’ G-code. Conversely, a ‘non-modal’ G-code will only aect the programmed block in which it appears. Machinability and Surface Integrity  . sucient precision to be of much practical use and by 19 99, Gorzkowski, et al., in their powder metallurgy paper concerning ‘secondary machining 1 , entitled: 1 ‘Secondary machining , is a term. cutting conditions found when machining the actual production part. e assessment of a material’s machinability can be undertaken by two groups of tests, these are machining and non-machin- ing. hence, the term ‘secondary machining .  Chapter  Figure 14 4. The major factors that inuence a machined component’s condition. Machinability and Surface Integrity  tests’ , conversely ‘absolute’