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Figure 218. Machine tool spindle analysis system. [Courtesy of Lion Precision]. Machining and Monitoring Strategies Figure 219. Machine tool spindle error plots, illustrating spindle condition. [Courtesy of Lion Precision] . Chapter Figure 220. A typical UHSM spindle cardridge listing some of factors aecting such a spindle’s design and its operation. Machining and Monitoring Strategies Figure 221. Attainable cutting parameters – with diering milling spindles, plus HSM is aected by the feedrate and distance to be traversed, prior to the desired velocity being achieved – for conventional slideway motions . Chapter NB Pneumatic spindles can be rotated at excep- tional speeds, by virtue of the ‘ideal condition’ of minimal metal-to-metal contact, although one serious disadvantage being they suer from a low power output, or to be more specic – torque. • Hybrid spindles (Fig. 214-top) – have been de- veloped to answer the major drawback to utilising pneumatic spindles. e hybrid spindle as its name suggests, is a combination of conventional ball- bearing and pneumatic spindles. Here, the spindle design incorporates an aerodynamic thrust bearing with transversal spiral grooves (Fig. 214-top right) thereby creating an intense pressure wave prole, which can withstand up to 300% greater static loads to that of a conventional aerostatic bearing. A typi- cal hybrid aerodynamic spindle bearing allows the assembly to achieve rotational speeds ranging from: 20,000 to 40,000 rev min –1 , with >15.5 kW power at peak speed. NB Hybrid spindle cartridges are signicantly less expensive than the magnetic ‘active’ spindles, but more expensive than pneumatic spindle cartridges. In both of these latter versions, they have a rela- tively long in-service life, as wear-rates are mini- mised, but do not have the stock-removal capability of the former cartridge. In Figs. 220 and 221a, are shown some of the principal factors that aect UHSM spindle performance. In Fig. 220, these factors are represented in an Ishikawa (i.e. ‘Cause-and-eect’) diagram. Here, the many of the in- ter-related eects can be seen, although other factors can also be added, depending upon the local condi- tions of usage: cutting data; workpiece material; wet, dry, or near-dry cutting; together with the machine tool’s overall condition. 9.2 HSM Dynamics – Acceleration and Deceleration If the HSM spindle cartridges – mentioned above – are tted to conventional CNC machine tools, or the more likely low-cost scenario would be to simply t a mechanically-driven speed-increaser. en the result of this HSM spindle tment, will enable high rotational speeds to be produced, but it leaves the CNC-proces- sors somewhat compromised in its ability to produce the desired acceleration and deceleration capabilities. As a practical example of the problems likely to be encountered, the graphs produced in Fig. 221b and c, were drawn from an industrial HSM machining expe- rience at a precision metrology company’s premises, using several of the latest vertical machining centres with the spindle of each machine, being tted with a mechanically-driven speed-increaser (i.e. see Fig. 243a). By utilising the inboard CNC clock – having a reso- lution of 0.0001 seconds, the elapsed times for slide- way motion over varying distances was established. In Fig. 221b, an exponential relationship is depicted for the X-axis, this being a typical situation for the other axes on the machine tool. By a determination of the required motional distance to attain specic veloci- ties, it was possible to illustrate the restrictive nature of both acceleration and deceleration for small slideway motions. In Fig. 221c, this illustrates the eect of the required distances to be executed at various velocities. From Fig. 221c – by way of an example, if a feedrate of 8,000 mm min –1 was utilised, then it would be neces- sary for a minimum movement of the slideway to be 16 mm to momentarily achieve the desired feedrate, which is typical for a machining centre having an ac- celeration of 1.08 m sec 2 . is physical problem in ac- tual positioning to the required component’s dimen- sional feature is not too great a problem for long linear feeding distances – as the slideway velocity could be reached, but acceleration and deceleration becomes exacerbated by the smaller more intricate prismatic features normally found on the more minute, or smaller parts oen produced by HSM milling opera- tions, leading to potential scrappage problems. From the results of inspection procedures con- ducted on the HSM over a range of standardised testpieces, it was concluded that virtually all of the detrimental dimensional eects introduced by the HSM milling operations, could be attributed to severe ‘servo-droop’ – more will be mentioned on this subject shortly. Prior to manufacturing the HSM testpieces, they were designed on a CAD system and their respec- tive tool paths were post-processed by the integrated CAM soware. Hence, with regard to machining cycle times, they were either calculated by the CAD/CAM, or were the actual in-cut times – see Table 15. With re- Machining and Monitoring Strategies gard to these cycle-times for testpiece manufacture, a signicant improvement accrues when utilising HSM milling techniques. Although the increase of actual cy- cle-time from that of the theoretical high-speed CAD/ CAM estimation, can be due to a number of factors as previously noted by Smith and Hanson (1993). Not least of which was found in this case, where the CAM system tended to under-estimate the actual time to machine a component feature. is time-dierence is marginally compounded by the ‘servo-droop’ eects. If the machine tool’s G61 (‘Exact-stop’) mode was employed, the actual cutting time in general showed only a marginal increase, over the normal HSM cut- ting time, although the dynamics of motion tended to be somewhat jerky in action as the command ensured it reached its targeted positions. At normal HSM mill- ing performance, another exacerbating reason for the increase in cutting time over theoretical, was attribut- able to the axis acceleration/deceleration parameters (i.e see Figs. 221b and c). us, these machine tools basically failed to reach the required slideway accel- eration/deceleration then maintain these velocities for about 20% of the total in-cut times, this was for a com- ponent of somewhat moderate dimensional size and pocketing intricacy (i.e. the overall testpiece dimen- sions were approximately: 150 mm in squareness, by 50mm deep). From the testpiece results, various remarks can be made concerning the advantages of employing an HSM strategy over conventional milling practices, these are: • Despite a reduced cut depth, the HSM cycle times are a 66% improvement over conventional milling production techniques, • Using HSM it will signicantly reduce burr forma- tion – although not entirely eliminating it, when compared to that of conventional practice, • Distortion of the thin wall features was minimised by HSM, • When employing a speed-increaser (Fig. 243a), its bearing’s stiness is critically important in order to obtain an acceptable milled surface texture. Finally, by utilising even the most elementary form of HSM approach – using a speed-increaser, highlights the production advantages to be gained from adopting this strategy, albeit for limited periods of continuous cutting time, which normally dictates such ‘increaser’s’ practical usage. .. HSM Dynamics – Servo-Lag Most of today’s CNC machine tools use ‘proportional servo-systems’ , where the axis velocity is proportional to the dierence between the actual position and the command position (Fig. 222a). is ‘error signal’ is utilised by the system to determine any acceleration/ deceleration necessary as well as the steady-state ve- locities. As one can visualise from Fig. 222a, the dis- tance between the actual and commanded positions is commonly termed ‘servo-lag’. is explanation can be taken a stage further in Fig. 222b, where the illustra- tion depicts how a ‘proportional servo-system’ is used to mill a sloping line. In this example, DX and DY are the total programmed changes in position on both the X- and Y-axes, respectively, to go from point ‘A’ to point ‘B’. Conversely, DX L and DY L are the amount of lag on each axis at point ‘C’ along the tool’s path from ‘A’ to ‘B’. Furthermore, in such a system the lag on the X-axis must be proportional to a similar lag in the Y- axis, in order to accurately follow the slope of the line. is aect can be mathematically-represented by the following relationship: DX L DY L = DX DY = Slope of the line . In Fig. 222c, we can gain an appreciation of just what happens when the servo-lag on both axes is not pro- portional. As the machine tool’s axes travels from point ‘A’ to point ‘B’ , the lag on the X-axis is proportionally Table 15. A comparison of the theoretical and actual ma- chining times for the manufacture of testpieces, by various pro- duction routes Machining Method: Theoretical Cad/ Cam Time: Actual In-cut time: Conventional 3.86 3.71 High-speed 1.26 1.50 High-speed (G61)* 1.16 1.55 NB All times in minutes * G61 is the ‘Exact-stop’ mode of machine command and, when ac - tivated, the machine tool will not initialise another movement un- til the previous axis command has been completed (i.e the target- point), thus ensuring an accurate and nal slideway positioning. [Source: Smith and Maxted, 1995] . Chapter Figure 222. The CNC control problem of servo-lag and its aect on the associated HSM motional kinematics. Machining and Monitoring Strategies less than the lag on the Y-axis. is error might be the result of the ‘servo-gains’ 12 between the X- and Y-axes not being properly synchronised. Normally, ‘servo- gain’ can be expressed in units of: mm min –1 [i.e. velo- city / mm (i.e. distance in 0.001)] of lag. us, lag can be determined using the following relationship: Lag L (mm) = Feedrate F Gain G = , �. = . mm . For example: If a machine tool’s moving axis is travelling along its slideway at 2,500 mm min –1 and the servo has a gain of 2, the lag will be 1.25 mm, as indicated in the fol - lowing calculation: L (mm) = F G = , �. = .mm . .. Effect of Servo-lag and Gain on Corner Milling If two axes with correctly matched servo-lags can move in a straight line from point ‘A’ to point ‘B’ , then to comprehend the eect of gain, let us consider what occurs when milling a right-angled corner at a con- stant feedrate without stopping (Fig. 222d). Whilst milling the corner from ‘A’ to ‘B’ and the onward to ‘D’ , the servo develops a steady lag (DX L ), until sucient command signals have been generated to reach point ‘B’. It is at this position that the con- trol begins to generate commands toward point ‘D’ , although the actual slideway has not yet reached point ‘B’ , owing to the servo-lag (DX L ). At this point the X- axis will begin to decelerate and, simultaneously, the Y-axis begins to accelerate, that is the velocity is pro- portional to the distance between the command signal and the actual position. Acceleration factors aect the slideway motions producing the result that the dis- tance from ‘B’ to ‘C’ is always greater than DX L . Fur- thermore, this curved path is not a circular arc, but an exponential curve, with the amount of variance from the sharp right-angled corner being dependent on the 12 ‘Gain’ or to be more specic: ‘servo-gain’ , in this instance, is a measure of the servo’s responsiveness. us, the higher the gain, the lower the lag. magnitude of servo-lag 13 , which itself depends upon the aect of feedrate and gain – according to the previ- ous formula. .. Effect of Servo-Lag and Gain Whilst Generating Circular Paths For one to fully understand just what happens when milling complex contours, it will be helpful to consider the simple case of a milled path where two straight lines are joined by a semi-circle (Fig. 222e). In this situation, the milling operation occurs at a constant feedrate moving from point ‘A’ in a straight line until the command dimension reaches point ‘B’ . However, at this point, because of the eect of servo-lag, the axis motion will have only reached point ‘B L ’. erefore, as the control command is moving forward at a constant rate, it begins to generate commands toward point ‘C’. is action results in the axis motion beginning to move away from the desired path at point ‘B L ’. e dot- ted line depicted in Fig. 222e, shows the actual path taken by the cutter and as one can visually observe, from points ‘B L ’ to ‘C L ’ , the deviation from the desired path is shown as ‘e’. In this example, the magnitude of ‘e’ is determined as a function of the: feedrate; gain; plus the desired radius. When the radius error approaches the pro- grammed radius, the resulting machined prole ap- pears distorted and is hence, impracticable. Speci- cally, if one needed to mill a 25 mm radius at a feedrate of 2,500 mm min –1 with a machine tool gain being: 25 mm/min/0.001, then the error generated would be approximately 0.125 mm, equally, if the gain was increased to 100 mm/min/0.001, the maximum er - ror ‘e’ will be considerably reduced to approximately 0.008 mm. A machined curve is an approximation on CNC machine tools, in that the prole is constructed from a series of short connected segments, or chords. e controlling factor on the length of such segments is the deviation between the centrepoint of any chord 13 ‘Servo-lag’ , is sometimes referred to in the literature as: ‘Servo-droop’ – due to its ‘rounding-eect’ at the corners, this being particularly prevalent when fast feedrates are selected, creating fast tool path velocities, particularly when normally undertaking high-speed milling operations. Chapter and a point at right angles on the programmed curve. e linear distance between these two points is usually termed the ‘maximum allowable chordal deviation’ and is a function of the CNC controller’s executive soware. So, the resultant machined curve is a combination of the chordal deviation and the servo-lag for a particu- lar machine tool. To illustrate this condition, Fig. 220f shows the culmination of servo-lag when following a contour, with the curve ‘C1’ being the desired con- tour, ‘C2’ a linear approximation (i.e. the programmed path), ‘C3’ is the actual generated path resulting from servo-lag utilising a high gain servo and nally, ‘C4’ being the path generated by a low gain servo. rough servo-lag, a smoothing of any contour occurs owing to the lagged cutter path, this causes severe contour prob- lems with respect to part accuracy and precision for the simple arc geometry depicted in Fig. 222e. Clearly then, servo-lag and gain promote a variety of eects on complex shapes, depending upon their geometry and tolerance, with these errors becoming still more complicated when one considers three-dimensional milling contouring. In many circumstances the cutting of three-dimensional proles may necessitate utilising four, or more axes with either one, or two of them be- ing rotary axes being necessary to create the required tool paths to produce the component. e servo-lag and gain on all axes must be considered when manu- facturing complex part geometries. Regardless of the workpiece’s geometry, or the number of axes utilised, there is one factor that should be emphasised concern- ing potential errors created by servo-lag. If servo-lag is extremely large, then this ‘lag’ can easily exceed the positioning errors in the machine tool’s basic speci- cations. .. CNC Processing Speed Possibly the main factor limiting contouring speed is the CNC’s inherent processing speed, with each pro- grammed-block 14 generated for every axis having to 14 ‘Programmed blocks’ , these are basically the ‘G-’ and ‘M-’ and ‘Auxiliary-codes’ which make up each individual block’s line, with successive blocks in a logical sequence containing the whole CNC program. Generally speaking, the smaller the number of blocks – for the successful production machining of the part, the more ecient and rened has been the pro- gramming. (Smith et al., 1993) be read, interpreted, the activated to obtain dynamic slideway motions. is CNC exercise is usually re- ferred to as the ‘block processing time’. e maximum allocated time for block processing of information is dependent on the length of slideway stroke (i.e. chord length) and its associated feedrate. It is possible to cal- culate the maximum block processing time (T b ), as follows: T b = Maximum stroke length Feedrate For example: if we require a prole’s chord length (i.e. stroke length) of 0.50 mm, in order to maintain con - touring accuracy whist milling at 3,000 mm min –1 , or 50 mm s –1 , with a maximum block processing time, then this ‘time’ should be less than: T b = . , � = . = . s, or ms Many CNC’s have block processing times typically within the range of 30 to 60 ms, as can be seen from the above example, the CNC program would suer from ‘data starvation’ , whilst the controller attempts to catch up on its data processing. Such ‘starvation’ would cause hesitation in the slide motions, slowing down the cutting time and leaving ‘dwell marks’ 15 on the ma- chined workpiece’s surface. Since this ‘data starvation’ eect is unacceptable, a lower feedrate must now be programmed to overcome the problem and as a re- sult, the cycle-time increases. In the above example, if an older CNC was tted to the machine tool with the controller’s block processing time being 60 ms, the cut would have taken six times longer to generate the prole, than a more modern CNC controller having a processor capable of 10 ms. So that we can fully-comprehend the CNC process- ing speed problem, let us now consider two widely dif- fering machining applications: 1. Complex three-dimensional milling of a hob – to manufacture a die utilised in the production of in- tricate and expensive military metal buttons. Such 15 ‘Dwell marks’ , here are the result of an ‘untimed delay’ in the program’s execution, created in this instance, by data starva- tion (i.e. block processing speed was simply not fast enough). ese untimely delays in the activation of programming blocks cause the rotating cutting tool to rest and press against machined surface and thus, generate minute ‘gouging-eects’ in the surface. (Source: Seames, 1990; Smith, 1993) Machining and Monitoring Strategies a hob will more than likely have very ne detailed work on its surface, perhaps with radii as small as 0.25 mm, requiring a tool tip radius of 0.025 mm. In order to machine the button’s elaborate features with such a small milling cutter, the spindle speeds might need to reach 40,000 rev min –1 , utilising a feed per revolution of 0.008 mm, giving a feedrate of 320 mm min –1 . Many production engineers would not consider this as an example of high-speed mill- ing, but let us look more closely at this particular machining problem. If the controller has a servo- gain of 4, with a feedrate of 320 mm min –1 , this means that the servo-lag would be 0.75 mm min –1 , which is consistent with milling radii of 0.25 mm. However, if the servo-gain was 1, this would cause a servo-lag of 0.320 mm min –1 and in this case, it obviously could not machine that button’s intri- cate detailing. In such circumstances, it would be necessary to appreciably reduce the feedrate to say, 75 mm min –1 to generate the button’s contours, leading to the cycle-time increasing by 400%. Let us also now consider the impact of block process- ing time under these conditions. To mill a radius as small as 0.25 mm, we would need to produce linear stroke lengths of just 0.075 mm – to reproduce ac - ceptable button detailing. is intricate contouring work requires a block processing time of 15 ms. If the CNC controller has a block processing time of just 60 ms, then the feedrate must be limited to 75 mm run –1 which again, increases milling time by a factor of four. 2. ECM pattern electrode for a Turbine fan (i.e large aluminium casting) – here, the electrode’s geom- etry has very gentle three-dimensional curves. In this situation the chosen CNC machine tool’s mill- ing spindle has a 250,000 mm min –1 capability, cou- pled to adequate power to cut at a feed of 0.25 mm rev –1 . is production requirement produces a feed- rate of 62,500 mm min –1 (i.e. being the product of: 250,000 x 0.25) would be possible. For accuracy and precision, a chordal deviation (C d ) of 0.005 mm would indicate a stroke length of 0.75 mm – if the minimum radius of curvature for the Turbine fan’s geometry was 25 mm. Assuming that the servo- gain of 1 was available, then we would obtain errors as large as 0.125 mm and with such errors, the ma - chine tool would not produce an acceptable part. Further, at 62,500 mm min –1 , if the block process- ing time (T b ) was 60 ms, this ‘timing’ would require stroke lengths of 2.5 mm instead of the 0.75 mm we needed for the required accuracy and precision. erefore, in order to eliminate the eects of low gain, or slow processing time, it is necessary to de- press the feedrate, resulting in the cutting time be- ing increased up to 400%. When considering these two practical examples from a metaphorical sense, the former method can be com- pared to that of racing a go-kart on a small tight track, while the latter method is similar to a highly tuned sports car racing on a longer and smoother track. e go-kart may only reach speed a of 30 km h –1 , whereas the sports car may hit speeds of >200 km h –1 . e corner forces and reaction times are similar for both methods, even though the speeds are vastly dierent. Looked at from yet another viewpoint, we can say that the frequency of response of both the drive and car, that is their servo-gain and processing time, are very similar in both examples even though the speeds (feed- rates) are radically dierent. In the day-to-day production environment, the du- plication of specic and precise contours is the end re- sult of a combination of many inter-related factors. As the number of machine tool axes required to produce sculptured part surfaces increases, the diculty of ob- taining the desired prole also becomes proportion- ally problematic. So, machine tools that would nor- mally produce excellent general-purpose machining work, may not be either accurate, nor ecient enough to manufacture complex part contouring geometries. at is, unless their CNC processors can achieve block processing speeds of <10 ms, with servo-system gains of up to 4, having sucient ‘look-ahead’ 16 capabilities (i.e. see Fig. 222g) that are required, for any realistic and practical HSM applications. 16 ‘Look-ahead’ facilities, are when the controller has the abil- ity to look-ahead through the following sequenced program- ming blocks* to determine successive motions and actions – an important feature for any HSM applications. Many of today’s sophisticated CNC controllers can look-ahead through a considerable number of these blocks, thereby prompting the controller’s response, prior to undertaking any command exe- cutions. *A ‘Block’ can be dened as: A set of words, characters, digits or other elements handled as a unit – hence, the term ‘block’ – which creates a sequence of lines of a computer pro- gramming language, that can then be activated upon by the machine tool‘s CNC controller, producing the necessary pro- grammed-actions. (Source: Smith et al., 1993) Chapter . Chapter Figure 22 0. A typical UHSM spindle cardridge listing some of factors aecting such a spindle’s design and its operation. Machining and Monitoring Strategies Figure 22 1. Attainable. cartridge. In Figs. 22 0 and 22 1a, are shown some of the principal factors that aect UHSM spindle performance. In Fig. 22 0, these factors are represented in an Ishikawa (i.e. ‘Cause -and- eect’) diagram visualise from Fig. 22 2a, the dis- tance between the actual and commanded positions is commonly termed ‘servo-lag’. is explanation can be taken a stage further in Fig. 22 2b, where the illustra- tion
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