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A Study of the Effects of Cutter Path Strategies and Orientation in Milling(2004)

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The implementation and selection of cutter path strategies and orientations when milling is particularly critical in the aerospace and mould and die industries. Proper selection can lead to substantial savings in machining time, improvement of workpiece surface quality and improvement in tool life, thereby leading to overall cost reduction and higher productivity. The paper identifies and reviews three main areas of literature studies namely analytical analysis on plane milling, entrance and exit effects of the cutter motion and inclined milling effects.
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  Journal of Materials Processing Technology 152 (2004) 346–356 A study of the effects of cutter path strategiesand orientations in milling C.K. Toh ∗ School of Engineering (Mechanical), University of Birmingham, Edgbaston Park Road, Birmingham B15 2TT, UK  Received 19 February 2003; received in revised form 3 March 2004; accepted 20 April 2004 Abstract The implementation and selection of cutter path strategies and orientations when milling is particularly critical in the aerospace andmould and die industries. Proper selection can lead to substantial savings in machining time, improvement of workpiece surface qualityand improvement in tool life, thereby leading to overall cost reduction and higher productivity. The paper identifies and reviews three mainareas of literature studies namely analytical analysis on plane milling, entrance and exit effects of the cutter motion and inclined millingeffects.© 2004 Elsevier B.V. All rights reserved. Keywords:  Cutter path strategies; Orientation; Milling; Evaluation 1. Introduction Research on cutter path generation techniques has beenplentiful over the past decade. Nevertheless, the imple-mentation of the cutter path techniques has been strictlylimited to machining the so-called easy-to-machine work-piece materials. Proper selection of cutter path strat-egy is crucial for achieving desired machined surfaces.Without considering the impact of cutter path selectionwith adequate consideration of the machining outcomesuch as cutting forces, vibration analysis, tool life, cut-ting temperature and workpiece surface integrity, the re-sult can lead to catastrophic cutter failure and thereforelead to unnecessary waste of time, cost and poor surfacequality.This paper aims to give a brief review on the effects of the milling strategies adopted when employing a millingprocess over the past years of research in order to gain abetter understanding on the cutter path effects in millingso as to gear towards the implementation of cutter pathstrategies and orientations when using a high speed millingprocess. ∗ Present address: Singapore Institute of Manufacturing Technology(SIMTech), Machining Technology Group, 71 Nanyang Drive, Singapore638075, Singapore. Tel.:  + 65-67938593; fax:  + 65-67925362.  E-mail address:  cktoh@simtech.a-star.edu.sg (C.K. Toh). 1.1. Cutter path strategies Many forms of cutter path strategies have evolved overthe past 30 years to mill free form surfaces. In general, theycan be classified into three main strategies namely offset,single direction raster and raster strategies. Offset milling,also known as window frame, spiral, meander-type or bull’seye milling, where the cutter usually starts at the peripheryof the face and then proceeds spirally inwards [1]. The cut- ter comes back to the starting point in each cycle and thencuts inwards to the next inner cycle. The cutter then pro-ceeds towards the centre until the entire workpiece surfaceis machined. The cutter path bridges are used to connect thecutter path from the cutter path of outer window frame toinner frame thus achieving a continuous cutter path motion.An illustration of this offset strategy is shown in Fig. 1(a). The cutter path is often used in pocket milling and requiresmore difficult cutter path calculations than raster milling [2].This strategy is commonly used for machining pocket fea-tures. The strategy can also be of an expanded version, i.e.the offset cutter path expands from the inner face graduallyto the peripheral boundaries of the surface to be machined.Raster milling, also known as zigzag, staircase, sweep,hatch or lacing is a strategy where the cutter moves back andforth across the workpiece in the  X  – Y   plane, see Fig. 1(b).This strategy causes the cutter to mill alternatively alongthe spindle direction and then against it, giving up anddown milling, respectively [3]. Such actions are known as 0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2004.04.382  C.K. Toh/Journal of Materials Processing Technology 152 (2004) 346–356   347Fig. 1. (a) Offset, (b) raster and (c) single direction raster cutter path strategies. switchbacks [4]. When employing this strategy, machining time is tremendously reduced and much simpler computa-tionally [5].When using a single direction raster strategy, the cuttermoves in parallel lines scanning across an area to be ma-chined. The cutter mills across the machined surface, stepsover a fixed amount, moves back to the srcinal positionthrough air before milling across another line. Fig. 1(c) illus- trates this cutter path strategy. This results in a down/climbmilling or up/conventional milling direction. 2. Analytical analysis on plane milling effects Wang et al. [6], Prabhu et al. [1], Lakkaraju and Raman [7] and Jamil [8] conducted analytical studies to identify the best cutter path strategies and the optimum angle orientationofacutterpathwithrespecttoaplaneworkpiece.Thestudieswereallcarriedoutonplanesurfaceswithoutinternalislandsof material.Early examples of evaluation studies for milling werepublished [1,6] in terms of orientation of cutter paths withrespect to a reference point on a flat plane and selectionof a starting point on a convex polygon. Wang et al.’s [6]work involved a systematic study to identify the optimumcutting angle orientation, which affected the total length cutwhen face milling a surface. The work concentrated on basicpolygons from triangles to heptagons. Two milling strate-gies were employed: (a) offset milling; (b) raster milling. Inoffset milling, each vertex was chosen as the starting point,whereas in staircase milling, the cutting orientation was ex-aminedbyvaryingtheorientationanglebetweencutterpathsand workpiece polygon in 1 ◦ increments. By altering thestarting point and cutting orientation, a calculation was per-formed for the length of cut and cutting time (assuming thatthe latter was proportional to the former). The process plan-ningproceduresforoffsetandrastermillingweredeveloped.The conclusions of the work were: ã  In offset milling, the selection of a starting point did notsignificantly affect the length cut, although a small varia-tion occurred. ã  The cutting orientation in raster milling had a significantimpact on the length of cut (5–100%). ã  There appeared to be no correlation between the optimalcutting orientation and other parameters, such as cutterdiameter and number of cutting edges. ã  The length cut generated by raster milling was shorterthan that generated by offset milling. ã  For raster face milling of plane surfaces, the optimumcutting orientation was generally parallel to the longestedge of the polygon. Fig. 2 is a plot of length cut versus cutting orientation for the triangle and Fig. 3 shows the sample triangle. The shortest cutter path is at an angle of 67 ◦ , which is parallel to the longest edge, AB.Sun and Tsai [9] investigated the effect of offset facemilling on triangular plane surfaces by developing a Fig. 2. Effect of cutting angle orientation on length cut on face millingof an irregular triangle shown in Fig. 2 [6]. Fig. 3. A sample triangle for optimisation in terms of the cutting angleorientation of cutter path [6].  348  C.K. Toh/Journal of Materials Processing Technology 152 (2004) 346–356  mathematical model to determine the effects of varyingstarting points and short cuts on total length cut. They de-duced that varying the starting point on different positionin each vertex angle resulted in a variation of about 10%in length cut. The inclusion of short cut had a variation of about 9–18% where the starting point was located at thesame vertex angle. When compared to raster milling [10],Sun and Tsai proved that cutter path length cut required foroffset milling was shorter. This conclusion was in contrastwith Wang et al. [6] because from the evaluation results,they suggested that the variation of a starting point did notsignificantly alter the length cut. However, Sun and Tsaidid realise a major impact on the shortening of length cutby varying the starting point.Lakkaraju and Raman [7] claimed that although analyticalmodelling is an easy way to determine the optimum cutterpath for a face milling operation, it ignores several physicalparameters and in many cases this makes the modelling un-realistic. In order to make things more realistic, factors suchas cutter diameter and cutter path overlap were consideredin addition to cutter path orientation. Only the raster millingstrategy was used and the radial depth of cut was taken as80% of the tool diameter. Experiments were carried out onthree-, four- and five-sided convex shapes. Sets of cutter pathfor each geometry were generated by applying rotations of 5 ◦ after each simulation, thus changing the orientation of the object relative to the cutter path. At each orientation, thedistance travelled by the cutter was measured. Graphs of dis-tance travelled against cutter path orientation with respect tothe part were developed. These indicated a cyclic relation-ship with maxima and minima occurring at regular intervals.It was also found that the minimum values occurred at dif-ferent orientation angles for different shapes. In other words,there exists an optimum path for every shape at a specificorientation. In their later work  [10], they developed an ana-lytical model to relate the total length cut to orientation asan arithmetic progression of trigonometric functions basedsolely on object geometry and cutter diameter. Their ana-lytical modelling results are consistent with Wang et al. [6]and Prabhu et al. [1] such that the lowest length cut can beobtained by moving the cutter parallel to the longest edge.Jamil [8] introduced a modified raster method for evaluat- ing cutter path for face milling three-sided convex surfaces.Unlike previously discussed methods, this did not adopt aniterativeapproachbutinsteadasemi-analyticalapproachandit was claimed that this produced better results as comparedto previous models. Findings indicated that optimal cutterpaths were most likely to be obtained when the number of ‘stairs’ was minimised and corresponded to the parallel sideof the largest edge, particularly when the triangle had an ob-tuse angle. However, this was not confirmed when the trian-gle had no obtuse angle. In this case, the path length shouldbe evaluated for each side of the triangle to determine theoptimum solution. The analytical models developed by theabove mentioned researchers are far too complex for simplepolygonal shapes. On the other hand, Arantes and Sriramulu[11] derived much simpler equations and deduced that theoptimal length cut could be obtained by limiting the calcu-lations to the directions parallel to the edges of the polygon.Sarma [4] suggested that the number of switchbacks inraster milling, rather than length cut is a major contributorto machining time. It is believed that the ratio of maximumcutting velocity to maximum acceleration is huge especiallyin the context of HSM. Therefore, switchbacks contribute toa majority of the total machining time. To reduce the numberof switchbacks, the author developed a concept known ascrossing function, which is a measure of how many times theradial depth of cut at some angle intersects with the contourof a polygon. It is further proved that the reduction of thecrossing function, i.e. the number of switchbacks, alwayscorrespond to the minimum width of a convex polygon byorienting the cutter path across it.Raster and offset cutter path strategies have their advan-tages and disadvantages. Although raster milling has gen-erally been found to produce a shorter cutter path, scallopmarks that are left on the walls of a machined pocket can-not be completely removed. With the offset strategy, scal-lop marks left can be removed creating a smooth surface.A hybrid machining strategy developed by Gay and Veera-mami [12] combined the benefits of both cutter path strate- gies such that scallops could be eliminated at the same timeas achieving a low cutter path length. Their analytical resultsshowed that the hybrid machining strategy was better thanoffset strategy in terms of length cut and the results more sig-nificant with larger pocket size and smaller internal angles.This was because the radial depth of cut required to avoidmaterial overlap decreased as the inner angle increased andsubsequently resulted in a shorter length cut, see Fig. 4 forillustrations.The analytical models developed by the researchers men-tioned above do not take into account the state of tool wearon a cutter that is influencing the length cut. The ignoranceof taking tool wear into consideration can result in poor toollife and workpiece surface quality. This in effect will resultin an increase in cost and waste of time. Based on thesefacts, Fry et al. [13] investigated the effect of varying cut-ting angle orientation on tool wear when raster face millinga rectangular hot rolled medium carbon steel. Fig. 5 depicts an illustration of raster milling at a cutting orientation angleof 60 ◦ and the graph detailing the effect of length cut andtool wear area per length on the cutting angle orientation.In general, tool wear and length cut increased with increas-ing orientation angles. The results suggest that the cuttingangle orientation and length cut have a significant effect onthe tool wear. The cutting angle orientation of 0 ◦ resultedin a length cut of about 4800mm. Fry et al. [13] provedthat by raster milling in  Y   instead of   X   direction parallel tothe longest edge, the length effectively could be reduced byapproximately 914mm making it the shortest cutter path.Therefore, the study confirms the findings of Wang et al. [6],Prabhu et al. [1] and Lakkaraju and coworkers [7,10] that the lowest length cut can be obtained by moving the cutter  C.K. Toh/Journal of Materials Processing Technology 152 (2004) 346–356   349Fig. 4. Illustrations of cutter paths when manoeuvring around a smaller inner angle and a larger inner angle to reflect the unmachined area [12]. parallel to the longest edge, subject to optimal selection of the starting point of cut. 3. Entrance and exit effects Most of the papers mentioned above suggest that shorterlength cut results in lower machining time and higher toollife. This conclusion may be misleading, as they did notconsider other process parameters. Raman and Lakkaraju[14] developed a software program to incorporate the effectof entrance and exit angles of the cutter and cutter geome-try with reference to the raster cutter path employed. Theirsimulation results showed that cutter geometry and entranceand exit conditions had a detrimental effect on tool life.Ng and Raman [15] concluded that by increasing the radialdepth of cut, shorter length cut resulted since more mate-rial was removed. However, this was coupled with high cut-ting forces and surface error that could eventually cause toolfracture and consequently low tool life. When finish millingwhere the workpiece surface quality is crucial, low radialdepth of cut is preferable such that low cutting forces canbe maintained to avoid undesirable vibrations. Hence, low Fig. 5. Effect of cutting angle orientation on length cut and tool wear area per length cut [13]. workpiece surface roughness and surface accuracy can beachieved. On the other hand, length cut is longer that canhave a detrimental effect on tool wear formed on the cutter.Every time the cutter enters and leaves a machined sur-face, it is subjected to rapid cutting load changes. Such con-ditions arise when milling and are characterised as entranceand exit conditions [16]. When high speed milling, the con- stant material removal rate resulted along the cutter pathcreates a uniform cutting load. When milling in a corneror a concave surface, the material to be removed increasesdue to a higher engagement angle, see Fig. 6. This increasesthe radial depth of cut and chip area rapidly creating a fluc-tuation in cutting forces that can result in excessive cuttervibrations. Consequently, the fluctuating cutting forces cre-ate undercutting of the corner [17]. Raman and Lakkaraju[16] analysed the impact of the locus of cut, entrance andexit on face milling through extensive literature. They inte-grated these tool life process variables into their program toenable simulation of machining strategies to be more real-istic. Law and Geddam [19] developed analytical equationsfor estimating the cutting forces and tool deflection errorsfor straight and corner slot cutting as well as milling in-side corners with small radial immersion. The instantaneous
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