Analysis of Shear and Firiction Behaviors in End Milling Process

J. Mater. Sci. Technol, Vol.19 Sppl.1, 2003237Analysis of Shear and Friction Behaviors in End Milling ProcessSeunghan YANG), Seungil CHANG), Youngmoon LEE)t, Heesool KIM2) and Taejo1) School of Mechanical Engineering, Kyungpook National University, 1370, Sankyuk-Dong, Puk-ku, Taegu, 702-701, Korea2) School of Mechanical Engineering, Yeungnam University, Daedong, Gyoungsan, 712-749, Korea[ Manuscript received August 13, 2003)As a new approach to analyze shear behaviors in the shear plane and chip-tool friction behaviors in the chip-toolcontact region during an end milling process, this paper introduces a method to transform an up-end milling processto an equivalent oblique cutting process. In this approach, varying undeformed chip thicknesses and cutting forcesin the up end milling process are replaced with the equivalent of oblique cutting ones. Experimental investigationsfor Inconel 718 were performed to verifty the presented model.KEY WORDS: Up -end milling, Equivalent oblique cutting, Specific cutting energy1. Introduction咖= cos-'(1 -a/r)(3)End milling is an intermittent cutting process performed2.2 Equivalent oblique cutting modelby a rotating tool while workpiece is clamped onto a table andTo establish an equivalent oblique cutting system equal. Foonactionsobtaineded by moving the table.'The undeformed-to an end milling process, cutting and tool geometrical vari-chip thickness and ctting force components vary pridically ables must be matched. Oblique Cuting is the simplet typewith the cutter rotation during the cutting proces.of three-dimensional cutting achieved by a straight cuttingMartellotti1] established the geometric relationships be-edge that is inclined to the coordinate axis perpendicular totween the tool path and the cutting variables in the inter-the cutting velocity vector. Figure 1(C) shows unfolded unde-nmittent cutting process of milling. Tlusty and Macneil2l pre-formed chips along the workpiece movement direction of up-sented a mechanistic model for the prediction of cutting forcesend milling. In the model of spread end milling, the cuttingby multiplying an undeformed chip area by specifc cuttingmeet the z axis with a helix angle (3), which corre-forces, and verified the validity of the model by comparingsponds to the ncination angle () in the equivalent obliquespondscomputed cutting forces with the measured ones. In a metalcutting. The average undeformedi chip thickness, hav is deter-cutting process, however, a chip is produced due to concen-mined on the basis of the same volume of chips produced intrated shear processes that occur at very small intervals inend milling and the equivalent oblique cutting process, as inan extremely limited regionientheshearzone.This chipie,Eq.(4).then experiences severe friction with the tool rake face behav = (aS&Z)/(rd)4)fore being externally discharged. Therefore, the study of acutting process is based on the analysis of the shear processWhereadisti? tool diameter, Z is the pumber of teeth, Se isin the shear zone and the friction process in the chip-toolthe feed per tooth, and a is the radial depth of the cut in thecontact region. A great deal of research on the shear andend milling.the friction processes have been studied, _but mostly in con-The axial depth of cut b and cutting velocity V in endtinuous cutting'Many researchers5~TI have added theirmilling are equal to the width of cut b and cutting velocity Vefforts to a fundamental understanding of intermittent cut-in the equivalent oblique cutting, respectively. Furthermore,ting procsses.s However, no analysis of the shear and friction the radial rake angle ar in end milling equals to the velocityprocesses in intermittent cutting has yet been attempted andrake angle Qv in oblique cutting.Pheraprocesses.there is no general standard to assess thhese processes.Accordingly, in this paper, the shear and friction charac 3. Cutting Experimentsteristics of an up-end milling operation, which is one of themost frequently used intermittent cutting processes, are ana-The milling tests were performed to verify the proposedlyzed based on an equivalent oblique-cutting model.cutting force model. End mills of 8 mm diameter with 30° ,40° and 50° helix aangles were used in the cutting tets. Ta2. Cutting modelble 1 shows the cutting conditions including the geormetricalshape of the end mills and material used in the cutting tests.2.1 Up-end milling modelThe three cutting force components were measured using aFigure 1 shows a three-dimensional model, with a four-piezo- type tool dynamometer and force signals were digitizedtooth cutter, of an up end milling process along with the ge-and stored using a micro-processor-controlled data acquisitionometrical relations between the tool and the workpiece. Thesystem.maximum undeformed chip thickness, hmax, is represented byEq.(1).4. Results and Discussionhmx=r-√(r-a)2+{√2-(r-a)2-s}2 (1)4.1 Cutting forcesUsing Eq.(5) tangential and radial forces (Fr and F), withwhere, φ1 is the rotation angle when the undeformed chip30°，40°，and 50° helix angle were calculated from the exper-thickness, h reaches its maximum, and中is the rotation an-imentally obtained cutting force data F: and Fy. Figures 2gle when a cutting edge escapes from the workpiece中and (a) to (c) show the results.2 can be expressed by Eqs.(2) and (3), respectively.中国煤化工0φ = cos~'{(r -a)(r - hamax)}(25)MYHCNMHG1][幻↑Prof, to whom correspondence should be addressed,where, φ is the rotation angle of the tool.E- mail: ymlee@knu.ac.kr.238J. Mater. Sci. Technol, Vol.19 Suppl.1, 2003_W(a) schematic(b) cross section(c) spread up end millingFig.1 Schenatics of up-end milling procs, (向) schenatic, (b) cross secion, (e) spread up end mlling00[()Upend miling MA30Upend mir 0HA407]upems milin CHASO)wwwa wrw mum100 1804 .0 6.08 610 612604 600 .O .10 82604-600 6.06 6.10 612Tme/sTme1sTene/SFig.2 Tangential and radial cutting forces in up end miling, (a) 30°, (b) 40°, (c) 50°Table 1 Cutting conditionsWorkRadial depth Axial depth Cutting Radial rakeHelixNumber Feed permaterialof cut, aof cut, tvelocity, Vangle, rangle, βof teeth, Z tooth, St/mm/(m/min)/deg.Inconel 7181230_830, 40, 500.06Table 2 Shear and friction characteristicsThe shear process consumes 55%~64% of the specific cut-30~4050°ting energy and the chip-tool friction process consumes thbalance. In addition, the specific cutting energy decreasedFriction characteristicswith an increase in the undeformed thickness. As such, theseFriction force, Fc/N269.0 223.9 263.7} theseSpecifc friction energy, u/MPa 748.3 657.1870.8results coincided well with those of previous studiesShear characteristicsShear force, F。/N145.1148.9155.95. CnclusionsShear strain, .3.373.38 3.19Shear stress, τ/MPa349.3339.6335.6.(1) A method to transform an end milling into an equivaSpecifc shear energy, un/MPa 1177.1 1147.8 1070.6lent oblique cutting system was established for analyzing theCutting characteristicsshear and friction characteristics of an up-end milling process.Specifie cutting energy, u/MPa 1925.4 1804.9 1941.4(2) When the helix angle increases from 30° to 40°, theu/u0.39 0.36 0.45 specific cutting energy decreases. However, as helix angleug/u0.610.640.55 turns from 40° to 50° the specific cutting energy increasesand this is mainly due to the increase of frictional energy.(3) The experimental results demonstrated that the pro-The main-, feed-, and thrust-cutting forces (Fw, Fa, andposed model is suitable for analyzing the cutting characteris-FI) in the equivalent oblique cutting model correspond retics of intermittent-cutting procCesses.spectively to the tangential, radial-, axial-cutting forces (Fr,F, and F) of the intermittent cutting model.REFERENCES4.2 Friction and shear characteristics1 ] M.E.Mrellotti: Trans. ASME, 1941, 63, 677.Table 2 shows the friction and shear characteristic factorsJ.Tlusty and P.Macneil: CIRP, 1975, 24, 21.M.E.Merchant: Trans. ASME, 1944, 66, 168.of the equivalent oblique cutting processes corresponding to[4] E.Usui, A.Hirota and M.Masuko: Trans. ASME, 1978, 100,the up-end millings presented in Table 1. The factors were222.calculated by substituting the replaced cutting forces into the[5] L.Zheng, S.Y Liang and S.N.Melkote: Trans. ASME, 1988,oblique cutting model8l.As shown in Table 2, as the helix angle was increased from中国煤化工LsME, 1999 121, 173.30° to 40°, the specific cutting energy u decreased. However,C 000 122, 3.MHCNMHG.IKSPE,20001(1),when the helix angle was increased from 40° to 50°，the spe-cific cutting energy, u increased. The latter result appears to[9] M.C.Shaw, N.H.Cook and P.A.Smith: Trans, ASME, 1952,be due to the increase of frictional energy.74, 1055.