THE DEPENDENCE OF THE CHIPS CONTRACTION COEFFICIENT ON THE NATURE OF THE CUT MATERIAL

BY

*MARIUS MILEA, *ANA-MARIA MATEI

Abstract. The paper deals with the dependence of the chips contraction coefficient Cd

on the nature of the material being cut. The hardness and the flow limit of the cutting material depend on its chemical composition (%C).

Keywords: cutting process, chips contraction

1. Introduction

The latest researches show two categories of results regarding the dependence of the chips contraction coefficient on the nature of the material being cut. The theoretical equations to determine the coefficient do not include any elements about the nature of the material being cut, equation [1].

[1]

Experimental results prove that the values of the chips contraction coefficient depend on the nature of the material being cut (more ductile or less ductile-fragile). This dependence is shown in Fig. 1, where the variation diagrams of the chips contraction coefficient for a series of materials depend on the main cutting speed as follows: carbon steel (1), copper (2), hard aluminum (3), lead (4), concrete iron (5), brass (6), low-grade steel (7), high grade steel (8), tin and titanium alloys (9) and cadmium (10). [1.1]

Fig. 1.1- Variation diagrams Cd=f(v) for different materials

The present paper deals with a series of specific elements regarding the dependence of the chips contraction coefficient, taking as examples two steel types: carbon steel and high carbon steel.

2. The dependence of the chips contraction coefficient on the nature of the cut material

The physical and mechanical properties and the carbon percentage of carbon steels (STAS 500/1-89) and high carbon steels (STAS 880-80) are shown in Table 2.1 and Table 2.2.

Table 2.1Properties of construction steels STAS 500/1-89 (OL)

Steel Type

Carbon Percentage C(%)

Yield PointReH N/mm2(σ0.2)

Tensile strength Rm N/mm2(σr)

Elongation at Yield A5%

a≤16 16<a≤40 40<a≤100

OL 32 0.15 180 170 160 310…390 33OL 34 0.17 240 230 190 330…410 31OL 42 0.19 260 250 210 410…490 25OL 37 0.20 240 230 210 360…440 25OL 44 0.31 280 270 230 430…540 22OL52 0.22 350 340 250 510…630 21OL50 0.22 290 280 270 490…610 21OL60 0.30 330 320 310 590…710 16OL70 0.40 360 350 340 min..690 11

Table 2.2Properties of steels used in manufacturing STAS 880-80 (OLC)

Steel Type

Carbon Percentage C(%)

Yield PointReH N/mm2 (σ0)

Tensile strength Rm (σr)N/mm2

Elongation at Yield A5%

Hardness HB max.

OLC 10 0,07…0,13 210 min 340 31 -OLC 15 0,12…0,18 230 min 380 27 -OLC 20 0,17…0,24 250 min 410 25 -OLC 25 0,22…0,29 270 min 450 24 180OLC 35 0,32…0,39 310 min 530 21 200OLC 45 0,42…0,50 360 min 610 18 229OLC 50 0,47…0,55 370 min 640 16 240OLC 55 0,52…0,60 390 min 670 14 245OLC 60 0,57…0,65 400 min 700 14 255

To better explain this dependence three steels have been chosen from each type: OL 37, OL52, OL60 in Table 2.3 and OLC32, OLC45, OLC60 in Table 2.4, respectively.

Table 2.3Steel Type Carbon percentage

%CYield Point Hardness HB

OL 37 0.17 230 330…410OL 50 0.22 280 490…610OL 60 0.30 320 590…710

The dependence of the hardness HB on the carbon percentage (%) is shown in Fig. 2.1 and Fig 2.2 for each type of steel.

Fig. 2.1- Hardness HB=f(C(%)) for OL

Table 2.4Steel Type Carbon percentage

%CYield point Hardness HB

OLC 35 0.22 270 200OLC 45 0.42 360 229OLC 60 0.57 400 255

The influence of the carbon percentage on the hardness HB is greater in the case of the carbon steel (STAS 500) as shown in Fig. 2.1 and Fig. 2.2. In both situations, the increase of carbon percentage leads to an increase in hardness.

Fig. 2.2- Hardness HB=f(C(%)) for OLC

The values of the chips contraction coefficient obtained experimentally in the case of straight turning show that an increase in the carbon percentage, as well as in hardness HB, for three types of carbon steel determines a decrease in the values of the chips contraction coefficient (Fig. 2.3). Experimental results prove that high values of steel hardness come with low values of the coefficient Cd as shown in Fig. 2.3 for OLC 60. Similar charts are obtained for other types of steel.

Fig. 2.3- Cd=f(HB) for OLC 60

By increasing the steel hardness as a result of a high carbon percentage, the values of the chips contraction coefficient decrease. It is

experimentally shown that a reduction in the ductility of steels (increase in hardness) is accompanied by low values of the chips contraction coefficient. The coefficient Cd has the rough value 1 when processing hard steels and fragile materials (ferrites), respectively. Theoretically speaking, according to the dislocation theory, there is no ferrite deformation in cutting when processing fragile materials on a microscopic scale and, therefore, Cd=1. [1,2,6]

3. Conclusions

1. The values of the chips contraction coefficient decrease at the same time with the increase of carbon percentage and hardness HB, respectively, in carbon steel and high carbon steel cutting.2. Knowing how the hardness of the cut material influences the chips contraction coefficient is necessary for a greater insight into the plastic deformation of metal cutting. 3. In the future, the chips contraction coefficient Cd may become a characteristic for metal classification (steels, pig irons, non-ferrous alloys) from the point of view of productivity in cutting, together with the carbon percentage C(%), HB hardness and tensile strength τ0,2.

Received: *Gheorghe Asachi Technical University of Iassy,RomaniaDepartment of machinery and tools

Iassy, Romania, e-mail:milea_marius@yahoo.com

REFERENCES

1. Cozmîncă, M., Panait S, Constantinescu, C., Bazele aşchierii, Ed. Gh Asachi, Iaşi, 1995

2. Cozmîncă, Mircea, Croitoru, Irina, Model pentru evaluarea deformaţiilor plastice la aşchierea metalelor, Construcţia de Maşini, 1999(51), nr. 11

3.McDonald, W.J., Murphey, B.F., The Deformation Process in Metal-Cutting, Journal of Engineering for Industry, vol. 82, nr.3, 1960, pp.253-257

4. ***Oţeluri de uz general pentru construcţii, STAS 500/1-89, STAS 500/2-80, STAS 500/3-80

5. ***Oţeluri carbon de calitate pentru tratament termic, destinate construcţiei de maşini, STAS 880-80

6. Segal, Rica, Contribuţii la studiul interdependenţelor dintre influenţele parametrilor de lucru asupra deformaţiilor plastice la aşchierea oţelurilor, Ph. Thesis, Univ. Tehnică “Gh. Asachi”, Iaşi, 1999

DEPENDENŢA COEFICIENTULUI DE DEFORMARE PLASTICĂ A AŞCHIILOR ÎN FUNCŢIE DE NATURA MATERIALULUI AŞCHIAT

(Rezumat)

În lucrarea de faţă se prezintă o serie de elemente specifice privind dependenţa coeficientului de deformare plastică de natura materialului aşchiat luând ca exemplu oţelurile carbon şi oţelurile carbon de calitate. Rezultatele experimentale arată că la creşterea durităţii HB a oţelurilor are loc scăderea valorilor coeficientului Cd. Experimental, s-a demonstrat că prin diminuarea ductilităţii oţelurilor (creşterea durităţii), valorile coeficientului de deformare plastică a aşchiilor scade.