Overview in PDF format - Tallinna Tehnikaülikool

2.3. Prediction of machining accuracyDynamical phenomena of vibrations are caused by external factors on the strainedsystem of the lathe. In the turning operations, tool vibrations influence both productquality and productivity and may also have a negative influence on the workingenvironment. The relative dynamic motion between cutting tool and workpiece willaffect machining results, the surface finish in particular (Hakansson 1999). Duringthe machining of a material, all disturbances finally lead to relative displacementsof the cutter and the blank. It allows us to link the parameters of surface roughnessto the relative vibrodisplacements of the cutter and the blank (Sharma, 2004). In thecalculation of dynamics characteristics, the real elastic system of the lathe wasreplaced by a system with finite degrees of freedom. In the case of insufficientaccuracy of the underlying data, complicated calculation schemes can lead tosignificant errors in the calculation (Bayard, 2003). Therefore we used simplifiedschemes, composed on the basis of experimental investigations.The internal friction is due to imperfect elasticity of vibrating bodies – the mainfactor of damping in the lathes. Internal friction can be determined by non-elasticdamping factor of frequency of cyclical strains. In many cases structural dampingdepends on the type of machine tool construction. It was assumed, therefore, thatnon-elastic damping coefficient includes also the losses caused by structuraldamping. Experimental measurements were performed on the lathes of type 1K62at different cutting speeds, feeds and depths of cut. After every cutting, surfaceroughness was measured with a profilograph Surtronic 3+.In order to simplify the dynamic model, the factors of a minor effect on theresults were eliminated. In this study only dynamic models with one and twodegrees of freedom were investigated (Fig. 2.8).a)y=ybsinptyOlϕyAxb) y=y b sinptz=zbsinptOzylAxFigure 2.8 Dynamic models with one (a) and two (b) degrees of freedomIn machining of a detail on the lathe, the cutting force F (Fig. 2.9) is not constant.It is determined by several factors, as the change in the thickness of the cut-offchips, the change in the mechanical properties of the blank material and tool wear.The input of the lathe system is the cutting force F as the function of time and theoutput is the displacement of the cutter or the blank.23