Machining an AISI 4340 using different rake angles

The second application concerns the cutting of a rectangular block of high strength AISI 4340 steel of length 7mm and width 3.6mm, at a velocity of 3.33 m/s, a cutting depth of 0.1 mm and a rake angle of 0º and 6º. Material behavior is given by a Johnson Cook law that takes into account thermal softening and strain rate hardening (Table 11). Conductivity and specific heat does not depend on temperature, we consider them constant. The following assumptions are made: First, the tool is supposed to be rigid and friction is neglected. Furthermore, the thermal exchange between the part and the tool are also neglected. The inertia of the part is neglected. Implicit dynamics was used. Time steps were of _{1.25 10} 8 _{which necessitates of 2.5 10}4 _{steps for a tool travel}

of 1.1 mm. Only insertion of particles was used in this example.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 20 40 60 80 100 120 140 tool displacement(mm) fo rc e( N /m m ) cutting force thrust force

134 (a)

(b)

(c)

Figure 42. Continuous chip formation using the rate dependent Johnson Cook hardening law and a rake angle of 6º : (a) von Mises (MPa) ; (B) strain rate(1/s); (c)

temperature(K)

For a tool rake angle of 6º, deformation is largely confined to the primary shear zone and to the boundary layer adjacent to the tool (Figure 42). No shear localization occurs and a continuous chip formation is predicted. A typical distribution of temperature field within the workmaterial is shown inFigure 42. Highest temperatures are observed on the outside surface of the chip currently in contact with the rake face. Temperature in the direction of the shear plane is found to vary from high of about 780 K near the cutting edge to about 500K near the unmachined free surface. Also temperature along the rake face changes

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from 780K near the cutting edge to 750K at the point where the contact between the tool and the chip come to an end.

Figure 43. Cutting and thrust force vs. tool displacement for a rate dependent Johnson Cook hardening law and a rake angle of 6º

The largest accumulated plastic strains occur within the boundary layer adjacent to the tool. In this region, the flow of the material is facilitated by thermal softening and the plastic strains attain values up to 4. Strains in the chip interior remains within 1-1.5 range upon exit from the primary shear zone. Figure 43 shows the horizontal and vertical predicted cutting forces. It is found that horizontal cutting force rise quickly to a value of 170N per mm of width of cut within a short distance of 0.05mm. Then as the chip thickness and cutting temperatures in the deforming zone stabilize, the horizontal cutting force holds to a constant value of 170N/mm. The steady state vertical force component, also known as thrust force was found to average around 42 N per mm width of cut. The contact length between the tool and the workpiece, the deformed chip thickness and the shear angle are 0.15 mm, 0.16 mm and 30º.

For a tool rake angle of 0º, deformation is largely confined to the primary shear zone and to the boundary layer adjacent to the tool (Figure 44). No shear localization occurs and a continuous chip formation is predicted. The maximum number of particles is near 3325. A typical distribution of temperature field within the workmaterial is shown in Figure 44. Highest temperatures are observed on the outside surface of the chip currently in contact with the rake face. Temperature in the direction of the shear plane is found to vary from high

0 0.2 0.4 0.6 0.8 1 0 50 100 150 200 tool displacement(mm) fo rc e( N /m m ) cutting force thrust force

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of about 790 K near the cutting edge to about 510K near the unmachined free surface. Also temperature along the rake face changes from 790K near the cutting edge to 760K at the point where the contact between the tool and the chip come to an end.

(a)

(b)

(c)

Figure 44. Continuous chip formation using the rate dependent Johnson Cook hardening law and a rake angle of 0º : (a) von Mises (MPa) ; (B) strain rate(1/s); (c) temperature(K) The largest accumulated plastic strains occur within the boundary layer adjacent to the tool. In this region, the flow of the material is facilitated by thermal softening and the plastic strains attain values up to 3,5 . Strains in the chip interior remains within 1-2 range upon exit from the primary shear zone. Figure 45 shows the horizontal and vertical predicted cutting forces. It is found that horizontal cutting force rise quickly to a value of 180N per mm of width of cut

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within a short distance of 0.05mm. Then as the chip thickness and cutting temperatures in the deforming zone stabilize, the horizontal cutting force held to a constant value of 180N/mm. The steady state vertical force component, also known as thrust force was found to average around 50 N per mm width of cut. The contact length between the tool and the workpiece, the deformed chip thickness and the shear angle are 0.16 mm, 0.167 mm and 28º.

Figure 45. Cutting and thrust force vs. cutting tool displacement for a rate dependent Johnson Cook hardening law and a rake angle of 0º

A comparison of the predicted cutting and thrust forces for two different rake angles (0º and 6º) shows that an increase in rake angle implies a decrease in forces, due to thermal softening phenomena is more localized when the rake angle is increased. Furthermore, with a rake angle of the 6º the contact length is reduced and the shear angle is increased due to the faster curling of the chip. Finally, the deformed chip thickness is reduced due to an increase in the rake angle. 0 0.2 0.4 0.6 0.8 1 0 50 100 150 200 tool displacement(mm) fo rc e( N /m m ) cutting force thrust force

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3.6 Implicit, IMPLEX or explicit time integration