Contributions for increasing the productivity of CNC machines.

Lepadatescu, Badea ; Buzatu, Constantin ; Duicu, Simona 等

1. INTRODUCTION

The computer numerical control lathe is used to perform several operations on different surfaces of the workpieces with complex shapes. Usually these workpieces require a high surface finish and dimensional accuracy. In the program for numerical control are set up for different surfaces different cutting tools with different corner radius according with the accuracy that is asked by the documentation (Avallone & Baumeister 2003).

But for different reasons it is happen the operator is forced to use in machining a part with a tool with other corner radius that was initial in the machine program. In the paper are shown what will be happen if it uses this cutting tool radius and what will be the influences on the part accuracy and on the concordance between the dimensions of the different zones of the part. Also, there are shown the modifications that the operator has to make direct on the panel of machine tool to obtain a successfully machining in these new conditions.

In these cases are two interventions that have to be done. One is directly on the machine tool by the machine operator without changing the machine program and other is in the machine program itself when a great accuracy of machining is required. In both cases are shown the corrections that have to do direct on the machine panel by the operator, the deviations that appear after these corrections in linear and circular interpolations.

In the calculations was taken into account for the tool path the theoretical point of the cutting tool tip. It was noted that if is working with other corner radius that was in the initial machine program is not obtained the shape identically with that was in the part drawing. If the part tolerances permit these dimensional modifications it is not necessary to modify the initial machine program (Boothroyd & Knight 1989).

After were calculated the equations for corrections and deviations is given an example for a particular type of the tool insert.

2. THE IMPORTANCE OF THE TOOL CORNER RADIUS FOR THE MACHINING ACCURACY

The values of tool corner radius are very important when machining are the work pieces with surfaces in steps without undercutting. The values of the fillet radius is the identically with the corner radius of the cutting tool. In Fig.1a is shown the value of the deviation [DELTA] which appears when is used other corner radius [r.sub.[epsilon]2] instead that of [r.sub.[epsilon]1] which is use in the initial machine program. It is noted that if two parts are to be assembly, there can not make it completely, because the point B can't reach point A due to the different corner radius of the cutting tool. In Fig.1b is the same situation but for the machining parts with radius at corners.

In Fig.2 is presented an example of a part that have to be machined on CNC lathe and this has linear interpolation on the path AB and GH, circular interpolation on the path EF, turning to obtain diameters on the path BC, DE, HI and flat surfaces on the path CD and FG. For each of these trajectories have to make modifications to maintain the accuracy that is demanded for the shape of part.

In Fig.3 are shown the influences of the corner radius cutting tool modifications on the machining accuracy of a part without linear and circular interpolations. If is working with the cutting tool with corners radius [r.sub.[epsilon]2] instead of [r.sub.[epsilon]1] that was in the initial machine program is necessary to do some corrections by the machine operator on the machine panel in order to obtain the part accuracy that is required. These corrections are [A.sub.x] and [A.sub.z] that are given by the following equation, (Dumitrascu et al., 2007):

[A.sub.x] = ([r.sub.[epsilon]2] - [r.sub.[epsilon]1])(ctg [[epsilon].sub.r]/2 cos [gamma] - sin [gamma] - 1) (1)

[A.sub.z] = ([r.sub.[epsilon]2] - [r.sub.[epsilon]1])(ctg [[epsilon].sub.r]/2 sin [gamma] + cos [gamma] - 1) (2)

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In equations (1) and (2), the y is the angle between the part flat surface and the tool insert edge and [[epsilon].sub.r] is the tool included angle.

With these corrections will be obtained the diameters and flat surfaces as the part drawing is required, but the radius between the flat surface and cylindrical surfaces is now equally with [r.sub.[epsilon]2] which is bigger that was in the initial program.

In Fig.4 is shown the calculation of the deviations [D.sub.x] and [D.sub.z] which appears at linear interpolation after were made the corrections by the operator direct on the machine tool panel if is used an insert with corner radius [r.sub.[epsilon]2] instead of [r.sub.[epsilon]1] which was in the initial machine tool program (Reshetov & Portman 1989):

[D.sub.x] = ([r.sub.[epsilon]2] - [r.sub.[epsilon]1])(1 - tg [90.sup.0] - [alpha]/2) (3)

[D.sub.z] = ([r.sub.[epsilon]2] - [r.sub.[epsilon]1])(1 - tg [alpha]/2) (4)

In Fig.5 is presented the calculations of the deviation [D.sub.r] in the case of circular interpolation that have to be done by the operator direct on the machine tool panel when in used an insert with a corner radius [r.sub.[epsilon]2] instead of [r.sub.[epsilon]1] that was in the initial machine program ([D.sub.r] = [r.sub.[epsilon]2] - [r.sub.[epsilon]1]).

If after the modifications on the machine tool panel the part dimensions are not like in documentation will be necessary to make modifications in the initial machine program.

3. APPLICATION FOR THE CASE OF TURNING WITH TRIANGULAR INSERTS

If is used at turning a cutting tool with a triangular insert with the next values: [[epsilon].sub.r] = 60[degrees]; [r.sub.[epsilon]1] = 0.8; [r.sub.[epsilon]1] = 1.2; [alpha] = 30[degrees]; y = 0[degrees] will appear the next situations:

1. For the part surfaces when is not used linear or circular interpolation (Fig.3) the operator will do directly on the machine tool panel the corrections (Reshetov & Portman 1989), [A.sub.x] = 0.293 mm and [A.sub.z] = 0. The part will have the dimensions exactly with the drawing requirements, only the fillet radius will be changed to the value of [r.sub.[epsilon]2] = 1.2 mm.

2. If is used the linear interpolation (Fig.4) after is done the same corrections will appear the next deviations: [D.sub.x] = 0.169 mm and [D.sub.z] = 0.293 mm.

3. At turning with circular interpolations (Fig.5), after were made the corrections [A.sub.x] and [A.sub.z] the value of radius R will have a deviation [D.sub.r] = 0.4 mm (Simon & Mares, 2005).

If the deviations are inacceptable then it will be necessary to change the initial machine program.

4. CONCLUSIONS

In practical applications for different reasons are used other corner radius than were established in the initial machine program. In these cases, it is useful to know what they are the corrections which it has to be done by the operator direct on the machine tool panel to obtain a workpiece according with the technical documentations. The paper presents what are these corrections and in this case the productivity of machining will increase because the machine tool will not be necessary to be stopped till will be supplied with the inserts according with the initial machine program.

5. REFERENCES

Avallone, E.A. & Baumeister, TH. (2003). Mark's Standard handbook for Mecahanical Engineers, Tenth Edition, McGraw-Hill, New York

Boothroyd, G. & Knight, W.A. (1989). Fundamentals of Machining and Machine Tools, Edition New York: Marcel Dekker

Simon, A.E. & Mares, Gh. (2005). Surface Technology, "Transilvania" University of, Brasov

Dumitrascu, A.E.; Buzatu, C.; Duicu, S. & Enescu, I. (2007). Theoretical and applicative contributions to flexibility of technology at turning on CNC machine tools. "Recent Advances in Systems, Communications & Computers" Conference, pp. 325-329, ISSN 1790-5117, Hangzhou, China, April 6-8. Available from: http://www.wseas.org

Reshetov, D.N. & Portman, V.T. (1989). Accuracy of Machine Tools, New York, American Society of Mechanical Engineers.