Chip forming and forms in milling zinc alloys.

Minciu, Constantin ; Croitoru, Sorin Mihai ; Constantin, George 等

1. INTRODUCTION

The fact that chip formation and forms are important in manufacturing ability evaluation is proved by the attention of many researchers (Popescu, 1970; Popescu, 1974; Oprea, 2007) especially when it influences the quality of the cutting process.

Chip evacuation, together with cooling and lubricating the cutting tool, are done by the coolant liquid brought in the active cutting zone. In order to evacuate the chips from this zone they must have dimensions corresponding to the evacuation channels.

Fragmentation and crushing the chips are imposed by the necessity to continuously evacuate them with the coolant liquid, in order to avoid their blockage in the evacuation channels of the cutting tool. In case of fragile materials, as cast iron is, this problem is self solved, because these materials give breaking chips--small chips, not linked--which can be easily evacuated without blockages.

In case of tenacious materials, like steel, there are slip chips, linked, in some cases continuous (flow chips) which, in order to be evacuated and for protection reasons as well, must be fragmented and crushed. In these cases the cutting tool edge must have chip breakers, edged being fragmented along the chip width. Slip chips, depending on the plastic deformation capacity and the ratio between normal and tangential stresses are divided into fragmented slip chips (having relatively small length), jointed slip chips (bonded fragmented slip chips) and flow chips (continuous, having big lengths).

An analysis of chip forming and a classification of chip forms considering the mechanical properties of the workpiece material and the influences of the cutting regime parameters are presented in technical literature influenced by the former USSR specialists--Davidenko-Friedman diagram--(Duca, 1969; Bobrov, 1975; Oprean et al., 1981; Minciu & Predincea, 1992).

American technical literature (Perry & Lissner, 1972; Camman, 1986), without specifying the theoretical support, presents a classification of chip forms only for aluminium alloys cutting, using five groups:

A--very short, crushed chips, excellent finish;

B--curved, helical, short chips, very good finish;

C--continuous, not helical, long chips, good finish;

D--continuous, helical, long chips, satisfactory finish;

E--not uniforme, long, jagged chips, unsatisfactory cutting.

This classification does not consider the properties of the workpiece material, but indicates the quality of cutting operation. It must be underlined the quality diminishes when the length of the chip increases.

In technical literature there are other ways to classify the chip forms, with little applicability, for example the international standard ISO 3685.

Quantitative description (measurement) of chip forming and forms can be done by determination of chip crush coefficients, which give the measure of the plastic deformation energy needed to transform the cutting layer into chip(s).

Cutting theory uses three plastic deformation coefficients to characterize plastic deformations of the real chip, corresponding to the nominal dimensions of the chip:

* shortening coefficient, [k.sub.1] = l/[l.sub.1] = (1.5 ... 4.5);

* widening coefficient, [k.sub.b] = [b.sub.1] / b = (1 ... 1.5);

* thickening coefficient, [k.sub.a] = [a.sub.1] / a = (1.5 ... 4.5).

where l, b, a represent the length, wideness and thickness of the theoretical chip (cutting layer) and [l.sub.1], [b.sub.1], [a.sub.1] represent the length, wideness and thickness of the real chip (detached chip). Just for information, the usual values of the three coefficients are given in the brackets. It must be underlined that along length and thickness of the chip deformations are more important than along the wideness of the chip.

2. EXPERIMENTAL RESEARCH

The experiments to evaluate chip forming and forms at milling zinc alloys were performed together with experiments to determine cutting efforts and surface roughness. During each experiment samples of resulted chips were preserved.

The experimental stand is presented in Fig. 1.

The experiments were performed in the following conditions:

* workpiece material was plates of zinc alloy ZnAl4Cu1T;

* four end milling cutters were chosen (symbolized A, B, C and E), having the diameters of 16 mm, 20 mm, 22 mm and 25 mm; the milling cutters are made of HSS Rp3 and Rp4, having two inclined teeth;

* the used machine tool was a milling machine TOS Type FN32 Cehoslovakia;

[FIGURE 1 OMITTED]

* no cooling during end milling cutting process;

* cutting regimes were chosen considering the possibilities of the machine tool. For each milling cutter diameter were chosen three values for the cutting speed, three values for the feed and two values for the axial cutting depth, as following:

* [d.sub.A] = 16 mm: [v.sub.c] = 40.2/50.3/62.8 m/min;

[f.sub.z] = 0.04/0.063/0.1 mm/tooth, [a.sub.p] = 2/4 mm;

* [d.sub.B] = 20 mm: [v.sub.c] = 50.3/63/78.6 m/min;

[f.sub.z] = 0.04/0.063/0.1 mm/tooth, [a.sub.p] = 2/4 mm;

* [d.sub.C] = 22 mm: [v.sub.c] = 55.3/69.2/86.4 m/min;

[f.sub.z] = 0.04/0.063/0.1 mm/tooth, [a.sub.p] = 2/4 mm;

* [d.sub.E] = 25 mm: [v.sub.c] = 49.5/62.8/78.5 m/min;

[f.sub.z] = 0.04/0.063/0.1 mm/tooth, [a.sub.p] = 2/4 mm.

A smaller cutting depth (ap = 1 mm) does not influence significantly the chip form. This was proved experimentally. As well, in such case it would be difficult to evaluate the chips type (slip chips or breaking chips).

3. RESULTS AND CONCLUSIONS

The samples of resulted chips were collected to appreciate the chip forms, especially qualitatively and less quantitatively, in the known conditions. For this purpose there were organized some exhibition boards with the samples of chips preserved from each experiment.

From the very beginning, at an overall appreciation it must be underlined the material ZnAl4Cu1T is tenacious, with a great plastic deformation capacity, because in all cases the chips were of slip chip type, of all three divisions (fragmented, jointed, continuous). On all chips can be seen the flow lines, which do not occur in case of breaking chips. This conclusion leads to the fact that zinc alloys have a good or very good cutting ability.

Analyzing the forms and dimensions of the chips shows the chip forms change from fragmented chips to jointed and even flow chips with the increase of the cutting regime parameters. Breaking chips did not occur at all.

In all cases, on the detached chip occurred visible slip lines, which means along these directions tangential stresses had values above the flowing stress. Cutting speed increase leads to more visible slip lines. As well cutting depth, feed and milling cutter diameter increase lead to more visible slip lines.

At the increase of the milling cutter diameter some aspects occur:

--the length of the jointed chips increase, the chips type change from fragmented chips to jointed, even flow chips; the phenomenon is more visible with the increase of the cutting depth;

--with the increase of the cutting regime parameters, like feed and cutting speed, crowded chips occur. Because of the increase of cutting speed the cutting temperature increase, and if the chips volume is enough the chips are crowded, even bonded. In this respect, it would be useful to use cutters with bigger evacuation channels between the teeth of the cutting tool instead the standardized cutting tools;

--with the increase of the diameter from 16 mm to 25 mm it was observed the detached chip segment has a trend to become from curved to straight.

As in other tenacious materials cutting cases, it was observed the chip surface in contact with the rake face of the cutting tool is shiny, having a small roughness. This means there is great friction between the chip and rake face of the cutting tool, with great mechanical power consumed.

It was also observed the length of the jointed or flow chips increase with the increase of both feed and cutting depth. For a specific value of feed and cutting depth the length of the jointed chips increase with the increase of the cutting speed. This means the increase of cutting speed leads to the increase of the plastic deformation capacity of the zinc alloy, which is explained by the increase of the cutting temperature.

A quantitative approach was used upon the aspects related to chip forming and forms, after the qualitative approach presented before. This quantitative approach means the determination of the chip crush coefficients, which depend on the mechanical power used for plastic deformation along chip's dimensions.

In this research the widening and thickening coefficients were determined. For this purpose, the real wideness and thickness of the real chip were measured, and compared to the nominal cutting depth and feed per tooth. The crush coefficients ranges are presented in Table 1.

Correct measurement of the real chip thickness was altered by the existence of the slip lines on the chip, even if their height is relatively small.

It was determined that along the wideness of the chip the widening coefficient was relatively small, showing small deformations, but along the thickness of the chip the thickening coefficient is relatively big.

Generally, the shortening coefficient is equal to the thickening coefficient.

The methodology to determine the chip forms and forming presented in this paper is used to evaluate the cutting ability of any workpiece material, not only zinc alloys, and also to evaluate cutting capacity of any cutting tool.

Considering all the observations and conclusions presented in this research, the influence of the cutting regime parameters upon the chip forming and forms and the classification of the chips versus the mechanical properties of the workpiece material it can be stated the zinc alloy ZnAl4Cu1T has a very good cutting ability.

4. REFERENCES

Bobrov, B. F. (1975). Osnovy teorii rezania metallov (New concepts on metal processing theory), Masinostroienie

Camman, J., (1986). Untersuchungen zur Verschleissminderung an Scherschneidwerzeugen der Blechbearbeitung durch Einsatz geeigneter Werkstoffe und Beschichtungen (Studies on wear reduction in shear sheet metal cutting tools using appropriate materials and coatings), Dissertation, T.H. Darmstadt

Duca, Z., (1969). Bazele teoretice ale prelucrarii pe masiniunelte (Theoretical bases of manufacturing on machine tools), Edit. Tehnica, Bucharest

Minciu, C. & Predincea, N., (1992). Bazele aschierii si generarii suprafe{elor (Fundamentals of Surface Cutting and Generation), Polytechnical Institute of Bucharest

Oprea, D. (2007). Contribufii privind studiul prelucrabilitatii prin aschiere la gaurirea aliajelor de aluminiu (Contributions to Cutting Ability Study in Case of Aluminium Alloys Drilling), PhD Thesis, University "Politehnica" of Bucharest

Oprean, A.; Sandu, I.Gh.; Minciu, C., Deac, L., Oancea, N. & Giurgiuman, H. (1981). Bazele aschierii si generarii suprafetelor (Fundamentals of Surface Cutting and Generation), Edit. Didactica si Pedagogica, Bucharest

Perry, C.C. & Lissner, H.P., (1972). The Strain Gauge Primer, McGraw-Hill Book Co., N.Y

Popescu, I. (1970). Contributii la stabilirea tehnologiei optime de prelucrare a aliajelor de aluminiu pe strunguri automate (Contributions to Setting the Optimum Cutting Technology of Aluminium Alloys on Automatic Lathes), PhD Thesis, Polytechnical Institute of Bucharest

Popescu, I. (1974). Aschierea aliajelor de aluminiu (Cutting of Aluminium Alloys), Edit. Tehnica, Bucharest

Tab. 1. Crush coefficients of the measured chips

Widening coefficient, [k.sub.b] 1.1 ... 1.75
Thickening coefficient, [k.sub.a] 1.4 ... 4.5