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   journal of materials processing technology 201 (2008) 618–622  journal homepage: www.elsevier.com/locate/jmatprotec Planning and analysis of grinding processes for endmills of cemented tungsten carbide  Jenn-Yih Chen a , ∗ , Bean-Yin Lee a , Chi-Hsiang Chen b a Department of Mechanical and Computer-Aided Engineering, National Formosa University, Taiwan b Institute of Mechanical and Electro-Mechanical Engineering, National Formosa University, Taiwan a r t i c l e i n f o Keywords: End millGrinding processApproach and retract sequence a b s t r a c t End mills are widely employed in cutting applications, but there is lack of standard grinding process. Thus the main purpose of this paper is to plan the grinding processes of end mills,including approach and retract sequence of each axis, and grinding parameters, etc. A toolgrinding CAM system was applied to set appropriate parameters in order to find out theoptimal approach and retract distance, and the sequence. In the experiments, a five-axisCNC tool grinder was utilized to grind a square end mill and a ball nose end mill with twoflutes to verify the efficiency for the saving of time. Experimental results show that theefficiency was improved approximately up to 40% by the approach and retract time, andthe overall machining time, 10%. Therefore, the results demonstrated the effectiveness of the proposed processes for the saving of machining time. They also can be applied to grinddifferent shapes of cutting tools.© 2007 Elsevier B.V. All rights reserved. 1. Introduction Due to rapid developments in the modern cutting technology(Schulz and Moriwaki, 1992; Schulz and Hock, 1995; Dewesand Aspinwall, 1997), the demand for all kinds of high pre-cision cutting tools will relatively increase. The grinding forcutting edges of tools is known as the most important andthe final procedure of manufacturing. It is also a critical issuefor determining geometry shapes, cutting performance, wearon the cutting edge, and tool life (Malkin, 1989). In the past, some different accessories must be mounted on conventionaltool grinders to grind the flute, flat gash, and the relief angle,etc. This will lead to undesirable degrading of grinding preci-sion. Most manufacturers acquire precious know-how for toolgrinding. They mainly use trial and error methods which arelaborious and time-consuming, and can not be used as uni-versal applicable standards. Furthermore, geometry shapes of cutting edges are quite complex so that it is difficult to obtain ∗ Corresponding author . Tel.: +886 5 6315322.E-mail addresses: jychen@nfu.edu.tw (J.-Y. Chen), leebyin@nfu.edu.tw (B.-Y. Lee), explorer5270@yahoo.com.tw (C.-H. Chen). the uniform, common, and optimal procedures via conven-tional tool grinders. Therefore, the grinding processes havenotbeenstandardizedyet.Itmakesaretreatforrelativeman-ufacturers for developing this technology. According to thetheoretical analysis or the empirical models, there are vari-oustypesofsimulationforgrindingprocesses(Tonshoffetal.,1992; Badger and Torrance, 2000; Nguyen and Butler, 2005a,b).However,theseareonlythekinematicsimulationofthegrind-ing processes.Currently, more detailed and useful know-how of milling cutters is possessed by some corporations, and it is difficult toacquire. Moreover, the formalization of CAM human machineinterface of the tool grinding software in market conditions isnot completely suitable for end users. Thus, a five-axis CNCtool grinder and CAM system were employed to constructthe grinding processes of end mills and obtain the optimalapproach and retract distance, and the sequence. By meansof 3D simulation, the system can grind the correct tools and 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2007.11.214   journal of materials processing technology 201 (2008) 618–622  619 reach the safety machining. It is very flexible and efficient fortool grinding. 2. Grinding processes planning 2.1. Design procedures of end mills End mills are generally used in mechanical machining. Theparameters of geometry shapes of end mills are the helixangle, rake angle, relief angle, clearance angle, and the land,etc (United State Cutting Tool, 2004). These values are related tomaterialsofmachining,andaffectthecuttingperformanceand tool life. The following are the design procedures of endmills:1. Drawing shapes of cutting tools.2. Accordingtocuttingtools,choosethegrindingprocessandthe sections which would be ground.3. Planning the approach and retract sequence of grinding process.4. Using the mathematical model of space geometry to gen-erate the NC program.5. Executing 3D simulation to confirm the accuracy of tooldesign and programming codes.6. Applyingafive-axisCNCtoolgrindertogrindcuttingtools. 2.2. Grinding processes of a radial rake angle Radial rake angles are varied according to tool functions anddifferent grinding methods. The grinding method of a radialrake angle of milling cutters is divided into the flute and theshearflute.Fig.1showsthedefinitionofeachaxisofafive-axis CNC tool grinder. With the tool grinder to grind a radial helixrake angle, the NC path is generated according to the land,helix angle, rake angle, and the core diameter of an end mill.The grinding sequence is:  X ,  C , and  A  axes start at the sametime first.  Y  -axis moves forward. Then, the rotation degree of   A -axis is the helix angle which we want.  X  and  Y   axes shift tothe grinding face of a wheel and the cutting face of a milling cutter. Finally,  Z -axis shifts to the approach distance of grind-ing.  X ,  Y  , and  C  axes start simultaneously to grind the flutelength.  Z -axis moves upward in the vertical direction to makethewheelexitthegroove. X , Y  ,and C axesreturntothestarting point simultaneously. Another flute can be ground according to the sequence of the above.Once the tool grinder is utilized to grind a radial rake anglefor shear flutes, the NC path is generated according to theland, rake angle, and the core diameter of an end mill. Thegrinding sequence is:  X  and  C  axes start at the same time Fig. 1 – Definition of each axis of a five-axis CNC toolgrinder. first.  Y  -axis moves forward. Then,  X  and  Y   axes shift to thegrinding face of a wheel and the cutting face of a milling cut-ter. Finally,  Z -axis shifts to the approach distance of grinding. X -axis grinds to the bottom.  Z -axis moves upward in the ver-tical direction to make the wheel exit the groove.  X  and  C  axesreturn to the starting point simultaneously. Another flute canbegroundaccordingtothesequenceoftheabove.Fig.2shows the diagram of grinding the radial rake angle for flutes andshear flutes. 2.3. Grinding processes of an axial rake angle Axial rake angles are varied according to tool functions anddifferent grinding methods. The grinding method of an axialrake angle of milling cutters is divided into the flat gash andthe curve gash. The flat gash is used on square end mills andsmall radius end mills. With the tool grinder to grind an axialflat gash, the NC path is generated according to the rake angleand the gash angle of an end mill. The grinding sequence is: X ,  C , and  A  axes start at the same time first.  Y  -axis moves for-ward. Then,  X  and  Y   axes shift to the grinding face of a wheeland the cutting face of a milling cutter. Finally,  Z -axis shifts tothe approach distance of grinding, and according to the ver-tical displacement, moving angle, moving distance, and thebackout gash angle to grind the axial rake angle.  X ,  Y  , and  Z axesstartsimultaneouslytogrindtothebottom. Z -axismovesupward in the vertical direction to make the wheel exit thegroove. X , Y  ,and C axesreturntothestartingpointsimultane- Fig. 2 – Diagram of grinding the radial rake angle for (a) flutes and (b) shear flutes.  620  journal of materials processing technology 201 (2008) 618–622 Fig. 3 – Diagram of grinding: (a) flat gash and (b) curve gash. ously. Another flute can be ground according to the sequenceof the above.Ballnoseendmillsemploythecurvegashtogrindbladesof theendfaceofcuttingtools.Oncethetoolgrinderisutilizedtogrind an axial curve gash, the NC path is generated according to the rake angle, gash radius, and the gash helix angle of anend mill. In general, the gash helix angle is smaller than thehelix angle of flutes. The grinding sequence is:  X ,  C , and  A axes start at the same time first.  Y  -axis moves forward andshifts to the grinding face of a wheel and the cutting face of amillingcutter.Finally, Z -axisshiftstotheapproachdistanceof grinding,andaccordingtothefrontrakeangle,rearrakeangle,and the depth of the groove to grind the axial rake angle.  X ,  Y  , Z , C ,and  A axesstartsimultaneouslytogrindtothebottom. Z -axismovesupwardintheverticaldirectiontomakethewheelexit the groove.  X ,  Y  ,  A , and  C  axes return to the starting pointsimultaneously. Another flute can be ground according to thesequence of the above. Fig. 3 shows the diagram of grinding  the flat gash and the curve gash. 2.4. Grinding processes of a relief angle 2.4.1. Flat relief angle With the grinder to grind the radial and the axial flat relief angles, the NC path is generated according to the outsidediameter of cutting tools, tilt angle of wheels, relief angle, andthe relief land. The grinding sequence of a radial flat relief angle is:  X ,  Y  ,  C , and  A  axes start at the same time, and shiftto the approach distance of grinding face of a wheel and thecutting face of a milling cutter. There is 1 ◦ difference between  A -axis and the slope of tool shapes. Then, the rotation degreeof   C -axis is the radial relief angle. Finally,  Z -axis moves down-ward to the approach distance.  Y  -axis shifts to the grinding face of a wheel and the cutting face of a milling cutter.  X ,  Y  ,and C axesstartsimultaneouslytogrindthecuttingedgefromthebasetotheendface. Y  -axisreturnsinthehorizontaldirec-tion to make the wheel exit the groove. Then,  X ,  Y  , and  C  axesreturn to the starting point simultaneously. Another flute canbe ground according to the sequence of the above.The grinding sequence of an axial flat relief angle is:  X ,  Y  , C ,and  A axesstartatthesametime,andshifttotheapproachdistance of grinding face of a wheel and the cutting face of amilling cutter.  A -axis rotates to the grinding angle that is 90 ◦ minusthereliefangle,andthentherotationdegreeof  C -axisis90 ◦ .Finally, Z -axismovesdownwardtotheapproachdistance. Y   and  Z  axes start simultaneously to grind from the outsidediameter to the center of the end face of tools.  Y  -axis returnsin the horizontal direction to make the wheel exit the groove.Then,  Y  ,  Z , and  C  axes return to the starting point simultane-ously. Another flute can be ground according to the sequenceof the above. Fig. 4 shows the diagram of grinding the radialflat relief angle and the axial flat relief angle. 2.4.2. Concave relief angle Once the tool grinder is used to grind the radial and the axialconcave relief angles, the NC path is generated according totheoutsidediameterofcuttingtools,tiltangleofwheels,relief angle, and the relief land. The grinding sequence of a radialconcave relief angle is:  X ,  Y  ,  C , and  A  axes start at the sametime, and shift to the approach distance of grinding face of awheel and the cutting face of a milling cutter.  A -axis rotatesto orthogonalize with tool shapes.  Z -axis moves downwardto the approach distance.  X -axis shifts to the grinding faceof a wheel and the cutting face of a milling cutter. Finally, X ,  Y  ,  C , and  A  axes start simultaneously to grind the cutting edge from the base to the end face.  X -axis returns in the hor-izontal direction to make the wheel exit the groove. Then,  X , Y  ,  C , and  A  axes return to the starting point simultaneously.Another flute can be ground according to the sequence of theabove.The grinding sequence of an axial concave relief angle is: X ,  Y  ,  C , and  A  axes start at the same time, and shift to theapproach distance of grinding face of a wheel and the cutting face of a milling cutter.  A -axis rotates to orthogonalize with Fig. 4 – Diagram of grinding: (a) radial flat relief angle and (b) axial flat relief angle.   journal of materials processing technology 201 (2008) 618–622  621 Fig. 5 – Diagram of grinding: (a) radial concave relief angle and (b) axial concave relief angle. toolshapes. Z -axismovesdownwardtotheapproachdistance. X -axis shifts to the grinding face of a wheel and the cutting face of a milling cutter. Finally,  X ,  Y  ,  C , and  A  axes start simul-taneously to grind from blades to the center of the end faceof tools.  X -axis returns in the horizontal direction to makethe wheel exit the groove. Then,  X ,  Y  ,  C , and A axes return tothestartingpointsimultaneously.Anotherflutecanbegroundaccording to the sequence of the above. Fig. 5 shows the dia- gram of grinding the radial concave relief angle and the axialconcave relief angle. 2.4.3. Eccentric relief angle With the tool grinder to grind an eccentric relief angle, theNC path is generated according to the outside diameter of cutting tools, tilt angle and shape of wheels, and the relief angle. The grinding sequence is:  X ,  Y  ,  C , and A axes start atthe same time, and shift to the approach distance of grind-ing face of a wheel and the cutting face of a milling cutter.Accordingtovariouswheels,therotationdegreesof   A -axisaredifferent.  Z -axis moves downward to the approach distance.Finally,  X ,  Y  , and  C  axes start simultaneously to grind the cut-ting edge from the base to the end face.  X -axis returns in thehorizontal direction to make the wheel exit the groove. Then, X ,  Y  , and  C  axes return to the starting point simultaneously.Another flute can be ground according to the sequence of theabove.Fig.6showsthediagramofgrindingtheeccentricrelief  angle. Fig. 6 – Diagram of grinding the eccentric relief angle. 3. Experimental results and analysis In the experiments, the tested cutting tools were the squareend mill and the ball nose end mill with two flutes and 8mmin outside diameter. In the practical grinding, the parame-ters of grinding processes were fixed, only the parametersof the approach and retract sequence are changed. The sizeof cutting tools shapes maybe need to compensate in grind-ing. Therefore, the shifting sequence and the distance of eachaxis should be taken into consideration to avoid collision. Inindustry,eachaxiswillreturntooriginalpointofmachineandshift to the grinding point when the grinding processes arechanged.Thedifferenceofgrindingefficiencyoftheindustrialapplication and the approach and retract sequence are com- Table 1 – Efficiency analysis of approach and retract procedures Items Cutting tools for testing 8mm square end millwith two flutes8mm ball nose end millwith two flutesProposed grinding processGrinding processin industryProposed grinding processGrinding processin industry Machining time ( a ) 354s 396s 424s 471sPractical grinding time ( b ) 291s 291s 352s 352Approach and retract time ( c = a − b ) 63s 105s 72s 119sSaving time of approach and retract ( d ) 105 − 63=42s 119 − 72=47sEfficiency improved of approach andretract ( d  / c )42/105=40% 47/119=39.5%Overall efficiency improved of machining ( d  / a )42/396=10.6% 47/471=10%

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