Sistema Operativo

Sellmeier showed that if the chamfer width b_fand feed per tooth f_z are increased, the specific edge coefficients K.e increase significantly as well [Sel12a, p. 84]. A comparatively minor influ-ence on the specific cutting coefficients K.cwas also observed. Fig. 7.1 shows investigations for a larger feed per tooth range and including the new tool concept.

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mean forces F

feed per tooth f_z feed per tooth f_z feed per tooth f_z sharp cutting edges

variation range (6 measurements)

chamfered cutting edges prototype (∆R = 10 µm) hydraulic expansion chuck:

Schunk Tendo E machine tool:

Heller MCi16

three-component dynamometer:

Kistler 9257B

workpiece:

Al 7075 T651

end mills:

- sharp tool - chamfered tool - prototype ap = 5 mm

n = 2,400 min–1 v_c = 150.8 m/min

–FfN

Ff

800 1,200 1,600

0 400 600 1,000 N

200

0.08 0.16 mm 0.32 0.08 0.16 mm 0.32 0.08 0.16 mm 0.32 vf

a_e = 20 mm

z y x

Fig. 7.1: Influence of new tool concept on the mean values of the feed and feed normal force for full immersion milling tests in dependence of f_z.

The chamfer width b_fand the radial offsetΔR were determined using a contour measuring device (Perthometer Concept Contour PCV200) and an optical measuring machine (Walter Helicheck), respectively. Forces were measured with a three-component dynamometer. On the left diagram the arithmetic mean values of the feed and feed normal force_F¯

fand_F¯

fN, respectively, for the tool with sharp cutting edges are shown. Six experiments were conducted at each feed per tooth.

The feed distance was l_f = 100 mm for each experiment and corresponds to the length of the workpiece. The dashed lines with round markers correspond to the mean values of_F¯

fand_F¯

fN

from the six experiments. The gray area corresponds to the deviation range. As expected, the

forces increase linearly with f_zand with only small deviations.

A comparison of the process forces between the tool with sharp cutting edges (dashed lines) and chamfered cutting edges (solid lines) is given in the middle diagram. The distance from the ordinate and, in particular, the slope of¯_F

f are significantly higher in case of chamfered cutting edges. For full immersion milling tests, the following convenient relationship between the mean process forces and the coefficients applies [Alt00, p. 46]:

¯F_f= –N_ta_p

4 K_rcf_z– N_ta_p π ^Kre F¯_fN = +N_ta_p

4 K_tcf_z– N_ta_p π ^Kte

(7.1)

For the sake of simplicity, influences such as non uniform tooth pitches are not considered at this point. In the case of high feed rates, the average value of the feed force is almost doubled.

Furthermore, the chamfered cutting edges cause a higher variation range of the mean forces. In the event that the process is stable in the case of both cutting edge geometries, the use of tools with chamfered cutting edges results in an increased power consumption and thus in a reduced efficiency. The mean forces for a hybrid tool with a radial offset ofΔR = 10µm are shown in the right diagram of Fig. 7.1. The values of the mean forces range between the values of the tool with sharp and chamfered cutting edges. In particular,_F¯

f is significantly lower than for the tool that has only chamfered edges. Consequently, the required power consumption is lower.

On the basis of these results, it is clear that the increased forces are due to a contact between the chamfer and the workpiece. No regenerative chatter or large vibrations occurred in all tests that were carried out. This makes clear that a contact between the chamfer and the workpiece also occurs when no large vibrations arise during the machining process. This can also be proven by similar simulations as already carried out in Chapter 6.1.5. Fig. 7.2 a) shows the indentation area A^∗_pdin case of no vibrations.

A contact occurs between the rear part of the chamfer and the workpiece along the whole immer-sion range, similar to the detailed view in Fig. 6.7 atφ = 161^◦. If f_zincreases, A^∗_pd obviously increases significantly. Fig. 7.2 b) shows for this case how the damping forces contribute to the forces in feed and feed normal direction. For f_z = 0.32 mm, the mean value of the damping force in feed direction F_pd,fis four times higher than the damping force in feed normal direction F_pd,fN. This higher increase in feed direction corresponds to the experimental results for the tool with chamfered cutting edges in Fig. 7.1. The increased damping at high feed rates also coincides with the experience that an unstable process often becomes stable if feed rate is increased. It must be noted that the simulation was carried out for a tool with a diameter of D = 10 mm.

In case of D = 20 mm no immersion between the chamfered cutting edge and the workpiece occurs in the simulation. It can be assumed that the heat-induced expansion of the material as well as vibrations with small amplitudes are already sufficient for a contact. However, this is not taken into account during the simulation. The influence of the chamfer on heat development in the workpiece will be discussed in the experimental results.

In addition to the high forces, a further negative effect of the tool with chamfered cutting edges appeared at the highest feed rate, as shown in Fig. 7.3.

In two out of six experiments at f_z = 0.32 mm, severe smearing occurred. For such high feed

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a) b) damping forces in feed and

feed normal direction

feed

per tooth f_z feed per tooth f_z influence of fz on indentation

mean forces Fpd,.

immersion angle f(t)

indentation area Apd(t)*

process:

cutting speed:

radial depth of cut:

axial depth of cut:

process damping coeff:

Fig. 7.2: Simulation of the influence of feed per tooth f_zon damping forces in case of no vibrations. radial depth of cut:

axial depth of cut:

spindle speed:

Fig. 7.3: Smearing caused by the end mill with chamfered cutting edges at high feed per tooth.

rates, the effective clearance angleα_effcan become negative in the area of the chamfer. Thus, the heat induced into the workpiece due to its contact with the chamfer increases noticeably. In such a case, the chamfer angleα_fmust be increased. In addition to strong smearing traces at the machined surface, the workpiece material adhered on the tool. As shown in the diagram, forces increased abruptly, similar to chatter occurrence, at the beginning of the smearing after a feed

distance of approximately 37 mm.

In summary, these investigations already show the first advantages of the new tool concept. The process forces are lower compared to the tool with chamfered cutting edges. As a result, the heat generation as well as the required power is also lower. This in turn can reduce the incidence of smearing and increase productivity, respectively.