Cutting speed and feed based analysis of chip arrangement in the dry horizontal turning of UNS A92024 alloy.

Batista, Moises ; Sanchez-Carrilero, Manuel ; Rubio, Eva Maria 等

1. INTRODUCTION

The UNS A92024 is an Aluminium alloy commonly used in the aircraft industry. This alloy can be processed by machining depending on its final application. Aerospace requirements involve high quality levels and, if possible, an online monitoring in order to preserve the workpiece design conditions.

The machining process can be monitored by different methods. One of the most single methods involves the analysis of the chip arrangement (Rubio et al., 2006).

In the case of wrought aluminium alloys, standard ISO 3685 gives a classification of aluminium chip based on its 3D external morphology (ISO, 1993), which, in some studies, has been considered a first approach to its machinability (Carrilero & Marcos, 1996).

However, chip arrangement studies have not considered the cross section of the chip. Lateral compression of the chip can also be analysed in order to improve the conclusions extracted of the single morphology analysis.

In this paper, a first approach to the combined study of the chip morphology, the cross section and cutting parameters has been developed.

2. EXPERIMENTAL

UNS A92024 cylindrical bars (150-200 mm long, 80-120 mm diameter) were horizontally turned with a depth of cut of 1 mm in a CNC Lathe avoiding the use of cutting fluids in order to have a reference of the most severe conditions.

Tests were performed by combining the cutting parameters included in Tab. 1.

The chips obtained were collected, identified and ISO 3685 classified after being observed by Stereoscopic Optical Microscopy (SOM) techniques.

After that, the cross section of chip was obtained using a vision measuring system and defining a CAD model for it.

3. RESULTS AND DISCUSSION

Table 2 shows images acquired by SOM after different dry turning tests. For each image, the corresponding ISO 3685 associated form has been included.

On the other hand, an increase of cutting does not mark a clear tendency. In effect, a dependence of feed can be distinguished. So, when small values of feed are applied an increase of the cutting speed provokes an approximation of chip morphology to a chip nest form. However, when higher feed values are used, smaller and closer to spiral chip can be found, Tab. 3. Nevertheless, this does not suppose a better finish quality of the workpiece as it was demonstrated in previous works (Rubio et al., 2006; Sebastian et al., 2002).

[TABLE 2 OMITTED]

[TABLE 3 OMITTED]

Fig. 1. shows images of the chips cross section for the different cutting speeds and feeds applied

[FIGURE 1 OMITTED]

The cross section images have been pre-processed by using SOM/Video/CAD utilities. From these CAD models a first evaluation of the cross section can be made.

As it can be appreciated in those curves, and according to images recorded in Figure 1, the cross sections values are mainly affected by the feed applied, not having a very soft (or even inappreciable) variation with the cutting speed applied.

As regards as all above commented, feed provokes a cross section increasing, as it can be expected from single models such those which consider the initial section as the product of depth by feed.

However, cutting speed seems to have a weak influence in the cross section values. Marginal parametric models as

S = a x [v.sup.n] (1)

were tested for analysing the cutting speed influence. In all of cases n values were

[absolute value of n][less than or equal to] 0.05 (2)

This results confirm the small influence of cutting speeds on section.

On the other hand, marginal parametric models as S = b x [f.sup.m] (3)

were tested for analysing the feed influence. In all of cases m values were

0.80 [less than or equal to] m [less than or equal to] 0.95 (4)

This result indicates that this feed dependence is close to that marked theoretically for initial section.

Curves included in Figure 2 show cross section ([mm.sup.2]) values as a function of cutting speed for each feed applied.

[FIGURE 2 OMITTED]

4. CONCLUSIONS

The alloy presents a suitable adjustment with standard ISO 3685. This suitability is translated in clear geometric tendencies. Thus the chip appears to be more fragmented when increasing the feed. The geometric variability is favoured with the diminution of the feed. However, it can not be formulated a machinability criteria based on these results because there is not a good correspondence with quality surface (Sebastian et al., 2002; Abouelatta & Madl, 2001).

On the other hand, the analysis of the chips cross section has reported a low cutting speed influence and a significative dependence on the feed rate, although it could not be related to the machining process but the initial section value.

5. REFERENCES

Abouelatta, O.B. & Madl, J. (2001). Surface roughness prediction based on cutting parameters and tool vibrations in turning operations. J. Mater. Proc. Tech., 118, 269-277, 0924-0136

Carrilero, M.S. & Marcos, M., On the Machinability of Aluminium and Aluminium Alloys, J. Mech. Behav. Materials, 7(1996) 179-193

ISO 3685:1993, Tool-life testing with single-point turning tools, 1993.

Rubio, E.; Camacho, A.M.; Sanchez-Sola, J. M. & Marcos, M. (2006). Chip arrangement in the dry cutting of aluminium alloys. J. Achiev. Mat. Manuf. Eng., 16, 1-2, 164-170

Sebastian, M.A.; Sanchez, J.M.; Carrilero, M.S.; Gonzalez, J.M. & Marcos, M. (2002). Parametric model for predicting surface finish of machined AA2024 alloy bars. Int. J. Manuf. Sci. & Prod., 4, 181-188, 0793-6648

Tab. 1. Cutting parameter values

s (m/min) 43 65 85 125 170
f (mm/rev) 0.05 0.10 0.20 0.30