Study the strength and stiffness of a boring and milling machine bed.

Dinu, Gabriela ; Baciu, Florin ; Vlasceanu, Daniel 等

1. INTRODUCTION

The machine tools are designated to generate the parts surfaces, using the technological process of cutting, following some conditions for dimensional accuracy, for surface quality and for productivity. The technological accuracy of a machine tool is a sum of multiple precision types: geometric, static, cinematic and dynamic accuracy (Gheorghiu et al., 2006).

The requirements imposed to a machine-tool are the processing accuracy, the operating safety and the operating and maintenance manageability (Budynas, 1999). The processing accuracy is defined by the stiffness of different elements of the structure and by the strains and displacements that occur during the operating period.

2. STUDY ON THE STIFFNESS COMPUTATION

This paper's goal is to study the strength and stiffness of a boring and milling machine bed. The sliding guide of the bed is represented in Fig.1, effecting some approximations as regards to the real sections of the bed and using simple sections and parameters on the entire length.

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The bed is put to flexion and torsion being loaded up according to the Fig. 2. The uniformly distributed load reacts on the bearing faces of the transit path but only on the length of the pedestal base slide. The operating version of the pedestal base slide laid on the studied bed is the socket head position which represents the most unfavorable situation.

The bed was studied by the finite elements method, using the performance program named "COSMOS/M" (Jiga et al., 2007). Fig. 3 presents equivalent displacements values and the displacements values on the uniformly distributed load direction are presented in Fig. 4.

The displacements into the vertical plane on the bed longitudinal direction are analyzed first of all. The machine tool clamping system is more important than those into the cross plane of the bed, because the former ones have a bigger influence for the machine tool working accuracy (Iliescu et al., 2005).

The range of values derived for the bed displacements is between 0.02-0.12 mm.

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Fig. 5 and Fig. 6 present the contact stress along the contact surfaces of the assembly slide-bed, especially for left and right guide-way(Atanasiu et al., 2007). As shown in the figures, the values of the contact stress on the bed guide-ways are in acceptable limits, but for a machine with a longer stroke on Y axis (that means a heaviest column), we suggested to use a bigger contact surface of the studied objects.

Using the same program, it is possible to observe in Fig.7, the equivalent Von Mises strains, which have acceptable values (Buzdugan et al., 1980).

On a plain model of the bed an experimental study has been performed, by tensiometric measurements, using an inductive transducer (Gheorghiu et al., 2007). The model was realized at a scale of 1:5 for the dimensions within the section and at a scale of 1:300 for the length. An L shaped plate was applied on the model bed by welding and sticking, exactly on the higher side, according to the Fig. 8.

For an equivalent force of F=200 kN a strain value of 0.069 mm in the measure has been obtained. A specialized company performed a technical research of the bed stiffness using high precision levels and a laser equipment and obtained real displacements during the operation between 0.01-0.15 mm.

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3. CONCLUSION

* The displacements values obtained by the operating version presented range within the normal limits for a correct operating and are comparable to one another;

* The difference between the displacements values obtained by experiments and those obtained using the numerical method is determined by the amendment of the study. Namely, the bed modeling is plain because it was difficult to make a space model;

* Based on the results of the study, the designer and the producer of the horizontal boring machine decided to find some technical solutions to decrease the influence of the bed deformations up to the top end of the tool: a strong foundation, supplementary point to sustain the slide, special technology for the column guide-ways grinding etc;

* For CNC machine tools, the value of contact deformations can be adjusted with some electronic corrections;

* For high precision machine tools (grinding machines, gear cutting machines, jig boring machines, machining centers), the deformations and contact displacements, like those between the bed and the clamping keys, can have a major influence for the machine tool accuracy;

* The values of deformations obtained with these two methods are comparable;

* The values of contact stress along the contact surfaces of the assembly slide-bed, especially for left and right surfaces range between the following limits: 2-104-3-105 Pa. Even such relatively small values can affect the precision of machine-tool;

* The research (with numerical methods and an experimental model) will be continued for the assemblies slide-column-headstock and bed-table, up to find the influence of the structural parts deformations at the tool cutting edge;

* The results are useful for the machine-tool's user, because the user can insert corrections so that the machine's accuracy will not be affected.

4. REFERENCES

Atanasiu, C., Hadar, A., Popescu, D. & Parpala, R. (2007). Computational Analysis of Closed-Cells Cellular Structures, The 18th International DAAM Symposium "Intelligent Manufacturing & Automation: Focus on Creativity, Responsability and Ethics of Engineers", October, Zadar, Croatia, p. 313-314, ISSN 1726-9679

Buzdugan, Gh. (1980). Strength of Materials, ed. XI-a, Ed. Tehnica, Bucuresti

Budynas, R.C. (1999). Advanced strength and applied stress analysis, 2nd ed., McGraw-Hill International Editions, New York

Gheorghiu, H. & Hadar, A., (2007). Determining Deformations and Existing Tensions in Structures Made from Bedded Composite Materials with Fibers, Proceedings of the 9th International Conference on Management of Innovative Technologies, 9-10 October, Fiesa, Slovenia, p. 191-195 and on C.D

Gheorghiu, H; Constantinescu, I.N.; Hadar, A. & Petre, C., (2006). Methodes numeriques pour le calcul des structures de resistance, Ed. BREN, Bucuresti

Iliescu, N.; Atanasiu, C. & Nastasescu, V. (2005). Combined Researches on Mechanical Behavior of Guide Block, 22nd DANUBIA-ADRIA Symposium on Experimental Methods in Solid Mechanics, Extended Abstracts, September 28-October 1, Monticelli Terme, Parma, Italia

Jiga, G., Vlasceanu, D.& Baciu, F. (2007). Analysis of a roll over protective structure using the FEM, Annals of DAAAM for 2007 & Proceedings of the 18th International DAAAM Symposium, October, Zadar Croatia, p.359-360