A study of the effects of cutter path strategies and orientations in milling .pdf
资源目录
压缩包内文档预览：
编号：122566402
类型：共享资源
大小：2.51MB
格式：ZIP
上传时间：2021-04-20
上传人：221589****qq.com
认证信息
个人认证
李**（实名认证）
湖南
IP属地：湖南
40
积分
- 关 键 词：
-
材料
水合
抛光机
设计
- 资源描述：
-
硬脆材料水合抛光机的设计,材料,水合,抛光机,设计
- 内容简介：
-
Journal of Materials Processing Technology 152 (2004) 346356A study of the effects of cutter path strategiesand orientations in millingC.K. TohSchool of Engineering (Mechanical), University of Birmingham, Edgbaston Park Road, Birmingham B15 2TT, UKReceived 19 February 2003; received in revised form 3 March 2004; accepted 20 April 2004AbstractThe 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; Evaluation1. IntroductionResearch 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 ofthe 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: .sg (C.K. Toh).1.1. Cutter path strategiesMany 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 bullseye 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 XY 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 as0924-0136/$ see front matter 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2004.04.382C.K. Toh/Journal of Materials Processing Technology 152 (2004) 346356347Fig. 1. (a) Offset, (b) raster and (c) single direction raster cutter path strategies.switchbacks 4. When employing this strategy, machiningtime 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 original 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 effectsWang et al. 6, Prabhu et al. 1, Lakkaraju and Raman7 and Jamil 8 conducted analytical studies to identify thebest 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 6work 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 1increments. 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: In offset milling, the selection of a starting point did notsignificantly affect the length cut, although a small varia-tion occurred. The cutting orientation in raster milling had a significantimpact on the length of cut (5100%). There appeared to be no correlation between the optimalcutting orientation and other parameters, such as cutterdiameter and number of cutting edges. The length cut generated by raster milling was shorterthan that generated by offset milling. 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 versuscutting orientation for the triangle and Fig. 3 shows thesample triangle. The shortest cutter path is at an angle of67, which is parallel to the longest edge, AB.Sun and Tsai 9 investigated the effect of offset facemilling on triangular plane surfaces by developing aFig. 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 6.348C.K. Toh/Journal of Materials Processing Technology 152 (2004) 346356mathematical 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 ofabout 918% 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 of5after each simulation, thus changing the orientation ofthe 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. 6and 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 ofstairs 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 Sriramulu11 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 depictsan illustration of raster milling at a cutting orientation angleof 60and 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 0resultedin 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 thatthe lowest length cut can be obtained by moving the cutterC.K. Toh/Journal of Materials Processing Technology 152 (2004) 346356349Fig. 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 ofthe starting point of cut.3. Entrance and exit effectsMost 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 Lakkaraju14 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, lowFig. 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 Lakkaraju16 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 instantaneous350C.K. Toh/Journal of Materials Processing Technology 152 (2004) 346356Fig. 6. Change of engagement angles when milling at (a) a straight lane, (b) a corner, (c) a convex surface and (d) a concave surface 18.cutting forces were obtained by determining the varying ra-dial width of cut during corner cutting. Based on the cal-culated cutting forces, the estimated deflection errors werethen calculated for the contour accuracy of the pocket andverified using a coordinate measuring machine (CMM).Iwabe et al. 20 developed a simple novel cutter pathstrategy to avoid excessive fluctuation in cutting loads whenmilling in corners. An improved cutter path strategy was de-vised such that instead of cutting at right angle, the cutterpath looped at the inside corner. Fig. 7(a) illustrates the ef-fect of changing the cutter path variation on the radial depthof cut and Fig. 7(b) the maximum chip area. The doublechain line ABC illustrates the original cutter path withouta loop and the improved looped cutter path strategy is de-picted as AA1B1B2B3A1B1C. From Fig. 7(a), theoriginal cutter path exhibited a large radial depth of cut whencutting at the inside corner. By introducing a looped cutterpath, the radial depth of cut reduced to half; hence it effec-tively reduces excessive vibrations that may be encounteredwithout reducing the feed rate. By using a smaller cutter di-ameter, the looped cutter path resulted in a smaller chip areaas compared to the original cutter path; see Fig. 7(b). There-fore, the use of a smaller cutter diameter coupled with theimproved strategy greatly reduces the impact caused whenFig. 7. Influence of the looped and original cutter paths on radial depth of cut and maximum chip area 20.milling corners thus improving the dimensional accuracy ofthe corners.Milling using a worn cutter often introduces edge defectson the workpiece material produced. Such edge defects aremost likely in the form of protrusions or ragged materials,known as burrs 21. Therefore, substantial time may beneeded to spend on manual polishing to remove the burrsformed at the edges. To minimise burr formation, the keyfactor is to prevent the cutter from exiting the workpiecematerial during milling. Based on this concept, Chu andDornfield 21 derived three methodologies to avoid burrformation by altering the cutter exit conditions. The cutterpath strategies deduced had been mathematically provedwithout cutter exits. Fig. 8 presents a modified cutter pathstrategy Ct Cs Ci Camilling around a corner toavoid cutter exit. In this diagram, the exit burrs that existalong PtPacaused by the original cutter path shown as dot-ted arrows will be eliminated. However, a disadvantage isthat the cutter path strategies implemented are not suitablefor milling thin walled sections or ductile materials.Cutter path strategies for machining thin walled sectionshave to be viewed in a different angle. High speed millingof thin webs has been demonstrated successfully by Smithand Dvorak 22 albeit for aluminium workpiece material.C.K. Toh/Journal of Materials Processing Technology 152 (2004) 346356351Fig. 8. A modified cutter path strategy that mills around a corner to avoid cutter exit 21.It was concluded that cutter paths should be chosen suchthat the areas being machined were supported by as muchunmachined material as possible and the direction of cut-ting should proceed from the least supported area to thebest-supported area. When milling thin walled sections, par-ticular attention has to be paid in the correct selection ofcutting speed, feed rate and axial depth of cut in order toavoid distortion to the workpiece structure. Lower cuttingforces, cutting temperatures and tool chatter are particularlyinstrumental to the reduction in distortion 23. With lowcutting forces, cutter deflections can be reduced, which inturn reduces the distortion of the finish part. Lower cuttingtemperatures reduce the thermal strains induced in the work-piece and high cutting speed enables the thin structure to bemachined since chatter is reportedly reduced 23.4. Effects of inclined millingFor finish milling free form moulds and dies, the cutterpath varies along the surface curvature. When finish millingFig. 9. An illustration of four different cutter path orientations 26.Fig. 10. Tool chip contact areas on cutter planes based on different cutter path orientations 27.using a three-axis or five-axis NC machine, the tool axiswith respect to the workpiece surface is crucial in achievingoptimum workpiece surface roughness and accuracy 24.A minimum tool or workpiece inclination angle, known asSturz method or inclined method 25 is defined such thatcutter axis or workpiece material is tilted at a constant anglewith respect to a surface normal. Ball nose cutter is generallyused for finish milling due to the fact that the cutter readilyadapts well to machining free form surfaces. However, finishmilling on a plane surface generally results in poor tool lifesince the effective cutting speed at its tip is zero and theeffective chip space is very small 26. A minimum cutteror workpiece inclination angle is therefore needed to avoidcutting at the tip of the cutter. A large inclination angle onthe other hand can increase surface roughness due to thereason that the cutter deflects more due to higher cuttingforces. In general, four different cutter path orientations areproposed and identified as shown in Fig. 9.When ball nose end milling on inclined surfaces, thetoolchip contact area varies significantly when using dif-ferent cutter path orientations. Fig. 10 shows the toolchip352C.K. Toh/Journal of Materials Processing Technology 152 (2004) 346356Fig. 11. Sculptured core surfaces and cutter path machining strategies 28.contact areas on the cutter planes projected along the cutteraxis. From the figure, the cutter plane is defined as a circularplane, which is perpendicular to the cutter axis. The cutterpath orientation, surface inclination angle and axial depthof cut have a direct influence on the size of the toolchipcontact area. When milling in a vertical upward orientation,the tool chip contact area lies on the upper half of the cut-ter plane. For vertical downward orientation, the toolchipcontact area shifts toward the centre of the cutter plane. Forhorizontal upward or downward orientation, the toolchipcontact area moves outward and simultaneously decreasingits width as the surface inclination angle decreases. The chipgeometry change due to the inclination angle has a signifi-cant effect on cutting forces 27. Kim et al.s work 27 onthe simulation and experimental results of cutting forces oninclined surfaces showed that axial and radial cutting forcesgenerally decreased as the inclination angle increased whenmilling in horizontal or vertical upward orientations. It wasalso observed that cutting forces were in general lower inhorizontal cutter path orientations as compared to millingin vertical cutter path orientations. No particular reason isFig. 12. Effect of cutter path strategies on machining errors 28.mentioned in the paper, however, it is highly believed thattool chip contact area has a significant influence.Further work 28,29 investigated the machining errorscaused by cutter deflection when ball nose end millingsculptured surfaces. The authors took into account fourmain issues such as chip geometry and engagement, cut-ting force, cutter deflection and deflection sensitivity of theworkpiece surface geometry and conducted an experimentto investigate two different cutter path strategies on twoadjacent 2D sculptured core surfaces, see Fig. 11. The re-sults showed that machining errors varied over the surfacesand were different for both surfaces, see Fig. 12. The graphshows that for cutting strategy A, when milling in a horizon-tal downward orientation, larger machining errors resultedas compared to milling in a horizontal upward orientation.In addition, the simulated cutting force magnitude whenmilling in a horizontal downward orientation was higherthan in a horizontal upward orientation. For cutting strat-egy B, vertical downward orientation caused an undercutmachining error. On the other hand, an overcut machiningerror resulted when milling in a vertical upward orientation.C.K. Toh/Journal of Materials Processing Technology 152 (2004) 346356353Fig. 13. Effect of cutter inclination angle and tool overhang on maximum flank wear 31.Lower cutting forces were resulted when milling in a verti-cal downward orientation, causing the cutter to deflect moretowards the machined surface. Conversely for a verticalupward orientation, higher cutting forces resulted and thecutter deflected away from the machined surface resultingin an undercut machining error. The researchers attributedthis phenomenon mainly due to chip load distribution andthe variation of the specific cutting force coefficients at theball end part of the cutter. It was concluded that the size ofmachining errors were also determined by the geometry ofthe sculptured surface, the cutting direction, cutter deflec-tion, machine tool geometric errors, tool wear and thermaleffects 29. Cutter deflection was identified as the mainfactor. A surface generation model was also developed tostudy cutter deflection errors produced by ball nose endmilling. This predicted the machining errors accurately andthat could provide high product quality and productivitywhen applied in the automotive and aerospace industries.Several researchers have addressed cutter path orienta-tions on inclined workpiece surfaces over the past 15 years.Elbestawi et al. 30 stated that for high speed semi-finishand finish milling of AISI H13 hot work tool steel usingpolycrystalline cubic boron nitride (PCBN) ball nose endmills, a significant increase in tool life was observed whenupward milling at an workpiece inclination angle of 10.The cutter axis was inclined in the feed direction with re-spect to the surface normal. Tool life increased because thetool workpiece contact area was very small and cutting withthe centre of the cutter was avoided.By increasing the cutter inclination angle with respect tothe workpiece, the thickness of cut is reduced. At the sametime, the radial width of cut increases and consequentlyincreases tool chip contact length 31. Fig. 13 shows theresults of the effect of tool overhang and cutter inclinationangle on maximum flank wear of the cutters obtained byTonshoff and Camacho 31. Their results showed that ingeneral, maximum flank wear decreased with increase incutter inclination. It was attributed to the fact that cuttingwork was distributed along the increased cutting edge lengththat reduced the thermal and mechanical loads acting on thecutting edges. Their results also showed that vertical up-ward or downward orientations in general fared better thanhorizontal upward or downward orientations in terms ofmaximum flank wear regardless of tool overhang. Millingin horizontal upward or downward orientations caused toolchatter due to the combination of cutting force direction andcutter position leading to lower system stiffness. It was fur-ther determined that tool overhang had an adverse effect ontool life. By reducing the tool overhang, tool life improveddue to lower tool vibrations coupled with higher rigidity.Schulz and Hock 26 conducted experiments in order toimprove the tool life of ball nose end mills using four dif-ferent cutter path orientations as shown in Fig. 9. It wasconcluded that a vertical upward orientation at an inclina-tion angle of 15was found to be the best for maximumtool life because the cutting forces on the cutting edges andthe cutter vibrations were minimum, see Fig. 14. Tool life interms of length cut for a horizontal upward orientation withthe cutter perpendicular to the feed direction was the low-est. Different inclination angles were tested in order to avoidengagement of the centre tip of the cutter because the zerospeed causing edge chippings would lead to high workpiece354C.K. Toh/Journal of Materials Processing Technology 152 (2004) 346356Fig. 14. Effect of cutter inclination angle and cutter path orientations on length cut 26.surface roughness. It was deduced that thermomechanicalloads on the cutting edge increased with greater cutter incli-nation angles. At the same time, there was a more uniformdistribution of cutting speed along the active part of the cut-ting edge. Another study conducted by Dewes and Aspin-wall 32 showed that a longer length of cut was achieved ona horizontal workpiece rather than the one orientated at 60.It was claimed that a higher average cutting temperature at60caused more rapid tool wear.Chu et al. 33 conducted inclination angle experimentsin order to determine the effects of vibration and feed rateon inclined cutter path orientations. The cutter used was un-coated tungsten carbide ball nose end mill and the work-piece material was a zinc-based alloy. Graphs of fast Fouriertransform (FFT) on magnitude versus frequency for variousinclination angles were plotted with a cutter rotational speedof 3000rpm, together with axial and radial depths of cut of0.05and0.08mm,respectively.Theresultsrevealedthatver-tical upward orientation at low inclination angles gave betterstability than vertical downward orientation as faster cut-ting speeds with the former resulted in lower cutting forces.However, as inclination angle increased, vertical downwardorientation became more favourable because the angle be-tween the tool axis and the resultant cutting force was muchFig. 15. Effect of cutter path orientations on length cut and cutting forces 35.smaller than with vertical upward orientation. Cutting forcesare important in determining workpiece tolerances. Low cut-ting forces, ideally in the direction of the cutter axis are es-sential for high workpiece dimensional accuracy. Forces alsorelate to spindle power and torque requirements. Kruth andKlewais 34 proposed an algorithm, which evaluated thecutter inclination dynamically when milling free form sur-faces. They determined that the best surface roughness wasalways achieved at low inclination angles 10. The best re-sult was always achieved in a horizontal upward/downwarddirection using a corner radius end mill.Ng et al. 35,36 carried out experiments based solely on45workpiece inclination on ball nose end milling Inconel718 nickel based super alloys. Fig. 15 depicts the effectof different cutter path orientations on tool life in terms oflength cut using a two-flute ball nose end mill. Their resultsshowed that milling in a horizontal downward direction gavethe lowest tool wear and longest length cut regardless of toolcoatings used, see Fig. 15(a). The effect of tool coatings andcutter path orientation on resultant cutting forces is shownin Fig. 15(b).Here, higher cutting forces were observed when millingin downward orientations than in upward orientations. Thisis in line with the results obtained by Kang et al. 37. TheyC.K. Toh/Journal of Materials Processing Technology 152 (2004) 346356355Fig. 16. Effect of different cutter path orientations on length cut 37.suggested that this was due to lower cutting speed and hencelower cutting temperature that would favour increased work-piece/tool adhesion and BUE. That in fact caused the lowesttool life when milling in a vertical downward orientation.FFT vibration analysis showed that vibrations were presentwhen milling in horizontal upward and vertical upward ori-entations. They deduced that when milling in downward ori-entations, the resultant forces acted at an angle of 1645from the cutter Z-axis. This meant that the majority of theforces were transmitted to the Z-axis that caused a stablemachining process. Whereas with upward orientations, thecutter milled the workpiece at an angle between 45 and 74from the cutter Z-axis. This created a higher tendency forchatter because the majority of the cutting forces acted topush the cutter away from the machined surface.Gaida et al. 38 conducted milling tests using two incli-nation angles of 15 and 60on P20 cold work tool steel.The cutter path orientation adopted was horizontal upwardorientation. Despite the large scatter, tool life was optimalat 60m/min for 15and 120m/min for 60. A general trendwas observed such that tool life decreased with increase incutting speed. It was also observed that tool life was lowerfor 60inclination angle compared to 15inclination angle.They deduced that this could be due to the difference in cut-ting speed distribution along the contact zone and the chipformation process at different inclination angles.Kang et al. 37 carried out comparative studies of millingcharacteristics on different inclined planes using the fourcutter path orientations as shown in Fig. 9. Fig. 16(a) showsthat when milling in a horizontal downward orientation atan inclined angle of 15, tool life was the lowest. It was be-cause up and down milling occurred simultaneously aroundthe tool axis promoting excessive vibrations that resultedin excessive tool chipping and flank wear. Here, the resultsgenerally suggested that milling in vertical or horizontal up-ward orientations gave better results than in vertical or hor-izontal downward orientations in terms of length cut. Fromtheir analysis of results, it was deduced that up and downvibrations occurred along the feed direction when milling ina vertical downward orientation. This in turn resulted in adetrimental effect on tool life. This was in stark contrast withNg et al.s results 35,36. Analysis of the chip formationfor horizontal upward orientation showed that long-wedgedshapechipswereproduced,whichshowedthatstablemillingwas achieved by dispersing the stresses acting on the cuttingedge.5. ConclusionsThere are three main cutter path strategies that are com-monly employed in industries namely, offset, raster and sin-gle direction raster. The analytical analysis on the cutter pathstrategies has been mainly on the evaluation and determina-tion of the best cutting angle orientation on a plane surface.Furthermore, a substantial amount of literature study focuseson the entrance and exit effects when the cutter enters or ex-its a corner. The survey also suggests that inclined machin-ing has been carried out in relation to tool life, cutting forceand workpiece surface quality. It can be concluded that toollife is optimum when machining in a vertical upward ori-entation at an inclined workpiece angle of 15. When ma-chining at a workpiece inclination angle of 45or above,the general consensus is that downward orientation in par-ticular the horizontal downward orientation is preferable interms of longer tool life.Finish milling are the subject of attention for most of thework mentioned above. In comparison, rough milling ap-pears to receive little attention. Furthermore, there is littleor no data on the effects of cutter path strategies and ori-entations on workpiece surface integrity. Future work willfocus on these two main areas when adopting a high speedmilling methodology.AcknowledgementsThe author would like to extend his gratitude to Profes-sor Alan Ball, former head of the School of Manufacturingand Mechanical Engineering, Professor Mike Loretto, for-merheadoftheInterdisciplinaryResearchCentre,Mr.DavidAspinwall, Head of the Machining Research Group for theprovision of facilities and Universities UK for funding viathe award of an Overseas Research Scholarship. Thanks also356C.K. Toh/Journal of Materials Processing Technology 152 (2004) 346356extend to Mr. Steve Hobbs, Delcam International Plc. andMr. Alan Pearce, Miracle Engineering Europe for their in-volvement in this project.References1 P.V. Prabhu, A.K. Gramopadhye, H.P. Wang, A general mathematicalmodel for optimising NC tool path for face milling of flat convexpolygonal surfaces, Int. J. Prod. Res. 28 (1) (1990) 30101.2 Y.S. Suh, K.W. Lee, NC milling tool path generation for arbitrarypockets defined by sculptured surfaces, Comput.-Aided Des. 22 (5)(1990) 273284.3 A. Hatna, R.J. Grieve, P. Broomhead, Automatic CNC milling ofpockets: geometric and technological issue, Comput. Integr. Manu-fact. Syst. 11 (4) (1998) 309330.4 S.E. Sarma, The crossing function and its application to zigzag toolpaths, Comput.-Aided Des. 31 (9) (1999) 881890.5 K. Tang, S.Y. Chou, L.L. Chen, An algorithm for reducing toolretractions in zigzag pocket machining, Comput.-Aided Des. 30 (2)(1998) 123129.6 H. Wang, H. Chang, R.A. Wysk, A. Chandawarkar, On the efficiencyof NC tool path planning for face milling operations, J. Eng. Ind.,Trans. ASME 109 (4) (1987) 370376.7 R. Lakkaraju, S. Raman, Optimal NC path planning: is it reallypossible? Comput. Ind. Eng. 19 (14) (1990) 462464.8 A.T.M. Jamil, A semi-analytical method of finding an optimum cutterpath for face milling 3-sided convex surfaces, Int. J. Prod. Res. 36 (2)(1998) 343355.9 R.H. Sun, Y.C. Tsai, An analytical model for optimisation of NCtool path, J. Chin. Soc. Mech. Eng. 14 (5) (1993) 483491.10 R.K. Lakkaraju, S. Raman, S.A. Irani, An analytical model foroptimisation of NC tool cutting path, Int. J. Prod. Res. 30 (1) (1992)109127.11 J.C. Arantes, P. Sriramulu, Optimisation of tool path in staircasemilling operation, in: Proceedings of the second Industrial Engineer-ing Research Conference, 1993, pp. 345349.12 Y.S. Gay, D. Veeramami, Hybrid machining strategy for 2.5D pocketmachining, in: Proceedings of the 1996 Fifth Industrial EngineeringResearch Conference, Minneapolis, MI, USA, 1996, pp. 187192.13 C.G. Fry, T.L. Fry, S. Raman, Experimental verification of tool weareffects in alternate path traversal in milling, in: Proceedings of the199
- 温馨提示:
1: 本站所有资源如无特殊说明，都需要本地电脑安装OFFICE2007和PDF阅读器。图纸软件为CAD,CAXA,PROE,UG,SolidWorks等.压缩文件请下载最新的WinRAR软件解压。
2: 本站的文档不包含任何第三方提供的附件图纸等，如果需要附件，请联系上传者。文件的所有权益归上传用户所有。
3.本站RAR压缩包中若带图纸，网页内容里面会有图纸预览，若没有图纸预览就没有图纸。
4. 未经权益所有人同意不得将文件中的内容挪作商业或盈利用途。
5. 人人文库网仅提供信息存储空间，仅对用户上传内容的表现方式做保护处理，对用户上传分享的文档内容本身不做任何修改或编辑，并不能对任何下载内容负责。
6. 下载文件中如有侵权或不适当内容，请与我们联系，我们立即纠正。
7. 本站不保证下载资源的准确性、安全性和完整性, 同时也不承担用户因使用这些下载资源对自己和他人造成任何形式的伤害或损失。
人人文库网所有资源均是用户自行上传分享，仅供网友学习交流，未经上传用户书面授权，请勿作他用。
|
2:不支持迅雷下载,请使用浏览器下载
3:不支持QQ浏览器下载,请用其他浏览器
4:下载后的文档和图纸-无水印
5:文档经过压缩，下载后原文更清晰
|
川公网安备: 51019002004831号