Cutting forces system for deep holes boring.

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

1. INTRODUCTION

The criteria used to establish the cutting capacity of the cutting tools are the followings: wear resistance or tool life, cutting efforts or specific cutting energy, roughness of the processed surface, dimensional accuracy and shape errors of the surface, dynamic stability during cutting process, reliability etc (Balan, 2006).

Research made in the cutting laboratories revealed that for specific cutting operation, cutting tool, quality of the manufactured workpiece, material etc., an appropriate system of criteria to evaluate the cutting tool performances can be used in order to define and compare several tools of the same type (Croitoru et al., 2007).

For the design of the technological system elements (machine tool-cutting tool-workpiece-device) the main criterion of the cutting capacity becomes the cutting efforts or specific cutting energy. Consequently, the cutting forces system for deep hole boring was considered, as shown in Fig. 1.

In case of deep holes borer having fragmented edges not along its diameter, considered as the general case, the cutting forces system was represented in the constructive reference system Oxyz of the cutting tool.

Correspondingly, the equilibrium equations will be:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

where:

--[[mu].sub.v] and [[mu].sub.s] are tangential and axial friction coefficients;

--[[kappa].sub.1] and [[kappa].sub.2] are positioning angles of the edges, complementary to the angles [[epsilon].sub.1] and [[epsilon].sub.2];

--[[beta].sub.1] and [[beta].sub.2] are positioning angles of the guides;

--[y.sub.1] and [y.sub.2] are distances to the Oz axis of cutting forces [F.sub.y];

--i = 4, 6, ... are the even and j = 3, 5, ... are odd edges;

--[phi] is the angle between the edge directions;

--r = d / 2 is the radius of the borer.

This equations system must be considered together with the relationships between some terms of the equations system:

--friction forces and normal reactions:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

--cutting forces in Oxy plane:

[F.sub.y1] = [F.sub.x1] tan [[epsilon].sub.1]; [F.sub.y2] = [F.sub.x2] tan [[epsilon].sub.2]. (3)

[FIGURE 1 OMITTED]

This cutting efforts system could be considered as general case for the static loads of the deep holes cutting borer:

--if in the equilibrium equations should make [phi] = 0, would be found the equilibrium equations of the borer having fragmented edges along its diameter;

--if in the equilibrium equations should make ([phi] = 0 and the cutting forces corresponding to the odd j and even i edges are considered equal to zero the obtained equilibrium system belongs to the "gun drill" (Wei Zhang, 2004).

After simplifications, we obtain the normal reactions:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

This equilibrium equation system is used to determine total cutting forces [R.sub.x], [R.sub.y], [R.sub.z] and total cutting torque [M.sub.T].

The same equation system can be used to determine the normal reactions on the borer guides [N.sub.1] and [N.sub.2] when total cutting forces [R.sub.x], [R.sub.y], [R.sub.z] and total cutting torque [M.sub.T] are known.

In the experimental research total cutting forces [R.sub.x], [R.sub.y], [R.sub.z] and total cutting torque [M.sub.T] are measured and the normal reactions on the borer's guides [N.sub.1] and [N.sub.2] are calculated.

This calculation is used to establish the borer guidance conditions (Mironeasa, 1997).

2. RESEARCH ON CUTTING EFFORTS

Cutting efforts included: cutting forces, cutting torque and consumed power during the cutting process (Constantinescu, 1993). Cutting forces and cutting torque were measured on a stand mounted on a lathe.

The measuring system consists of the following elements: borer, Wheatstone bridge made of resistive stress gauges glued on the borer fastening part, digital amplifier and computer with data acquisition system. Measurements were done for deep holes borers CARMESIN-BOTEK type, having diameters [d.sub.A] = 46 mm and [d.sub.B] = 32 mm, and workpiece material OLC 45.

Considering the requirements in the technical literature and of the manufacturers, the following cutting regimes were used:

a) for the borer having [d.sub.A] = 46 mm:

--spindle speed [n.sub.c] = (230; 305; 380) rpm, which corresponds to the following linear speeds [v.sub.c] = (33.23; 44; 55) m/min;

--feed s = (0.06; 0.1; 0.16; 0.2) mm/rev;

b) for the borer having [d.sub.B] = 32 mm:

--spindle speed [n.sub.c] = (305; 380; 460) rpm, which corresponds to the linear speeds [v.sub.c] = (30.67; 38.20; 46.25) m/min;

--feed s = (0.06; 0.1; 0.16; 0.2) mm/rev.

For each experiment the diagrams showing the variation of the total feed force and total cutting torque were obtained. There are 24 diagrams for axial force and 24 diagrams for the cutting torque (12 + 12 diagrams for the borer with [d.sub.A] = 46 mm and 12 + 12 diagrams for the borer with [d.sub.B] = 32 mm).

For example, in Fig. 2 are presented 2 diagrams showing the cutting torque and axial force for the borer with [d.sub.A] = 46 mm (experiment A3). After processing the diagrams as shown in Fig. 2, the axial force and total cutting torque were determined. The results were organized in tables. Using these experimental results the consumed power during the cutting process and the main cutting force were calculated.

Using all these results the variation diagrams of the measured or calculated cutting efforts versus the cutting speed and feed were drown. Two of such diagrams are presented in Fig. 3. The measured and calculated data and their variation diagrams underline the influence of the cutting speed and feed upon the

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

In case s = constant,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)

Consumed power during the cutting process was measured on deep holes boring machine MGA 250 x 1000 using a wattmeter. The measured data were used to calculate the main cutting force and the total cutting torque.

Variation diagrams of the three cutting efforts were drown and Taylor functions depending on the cutting speed and feed were determined in case of workpiece material OLC 45 steel and a borer having the diameter d = 46 mm, as following:

[F.sub.z] = 1.145.6 [s.sup.1.013] daN; [M.sub.T] = 26.368 [s.sup.1.013] daNmm,

[P.sub.c] = 18.1 [s.sup.1.013] kW. (7)

3. CONCLUSIONS

Compared to other cutting operations (turning, milling, grinding) in case of boring deep holes the total cutting force includes components that are not produced by the cutting process itself. These components are contact forces between the drill and the workpiece and their values are comparable to the effective cutting forces. As a consequence, the cutting forces system in case of boring deep holes is very complex.

Measurements of the cutting efforts are useful for both designer and manufacturer of the machine tool. The cutting efforts decrease with the increase of the cutting speed. However, this conclusion is valid only in case of cutting without cutting fluids. In real cases, the coolant is a necessity.

The experimental stand is an original one, using a lathe instead of a deep hole boring machine. The experimental results confirmed the theoretical research and were confirmed by other experiments using a real deep holes boring machine.

4. REFERENCES

Balan, M. (2006). Contributii privind cresterea capacitatii de aschiere a burghielor pentru alezaje lungi (Contributions regarding the increase of cutting capacity of the long drills), PhD Thesis, University "Politehnica" of Bucharest, Romania.

Constantinescu, C. (1993). Cercetari privind optimizarea constructiva si geometrica a burghielor pentru prelucrarea alezajelor lungi (Researches regarding the geometric and constructive of the drills for long holes procesing), PhD Thesis, Technical University of Iasi, Romania.

Croitoru, S.M.; Minciu C. & Balan, M. (2007). Experimental Research on Measuring Forces and Moments at Deep Holes Machining, The 5-th International Conference on Advanced Manufacturing Technologies ICAMaT 2007, pp. 367-370, ISSN 1843-3162, AGIR Publishing House, Sibiu, Romania.

Mironeasa C., (1997). Cercetari privind optimizarea autoghidarii burghielor utilizate la aschierea alezajelor lungi (Researches regarding the optimization of self-guiding of the drills used in long holes cutting), PhD Thesis, Technical University of Iasi, Romania.

Wei Zhang, Fengbao He, Dilin Xiong (2004). Gundrill life improvement fordeep-hole drilling on manganesse steel, International Journal of Machine Tools and Manufacturing, No. 44.