INVESTIGATION OF SHAPE CORRECTNESS OF DIAMOND BURNISHED LOW ALLOYED ALUMINIUM COMPONENTS

. Conventional machining methods such as turning or milling can cause surface irregularities, defects such as tool traces and scratches, resulting in energy dissipation (friction) and surface damage (wear). In contrast, the environmentally friendly chipless burnishing process clearly improves the integrity of the machined surface and largely considered in industrial cases in order to restructure surface characteristics. In this paper influence of different burnishing parameters, such as burnishing speed (v), feed rate (f) and burnishing force (F) are examined. Based on theoretical considerations, we use full factorial experimental design method to determine the optimal combination level of the different parameters in the given interval. The measurement of the shape correctness was executed with Taylor Hobson Talyrond 365 measuring equipment at the Institute of Manufacturing Science.


INTRODUCTION
More and more intensive and/or varied [1-2] technologies are emerging in the machining of components. At the same time, the productivity of the manufactured parts can only be increased if the accuracy (shape, size, position) and surface roughness of the parts can be ensured even under the conditions of the applied processes [2-3]. So finishing processes have always been important in manufacturing of all kinds of parts and in engineering it is obligate to improve the surface quality of different machine parts to ensure their durability and reliability. As a post finishing operation, the aim of applying burnishing can be increasing surface smoothness and dimensional accuracy of the elements [4][5][6] such resistance against fatigue strain [7][8]. This is particularly important when machining aluminum alloys because it allows the production of lightweight and high-strength components which are made mass production in the automotive (and aerospace) industry [3]. As a result, research is under way in both machining and cold plastic forming to achieve better results. In this work a comparing analysis of cylindricity deviations of low alloyed aluminium components is presented focusing on the determination of chosen burnishing parameters such as burnishing speed (v), feed rate (f) and burnishing force (F) using the full factorial experimental design method [9][10], which is valid between the applied maximum and the minimum of the above mentioned parameters.

EXECUTING BURNISHING PROCEDURE ON EXTERNAL CYLINDRICAL SURFACES
Burnishing process is one of the micro plastic manufacturing methods, when a special tool compresses the surface and causes plastic deformation in the subsurface layer, while the pressure must exceed the yield point of the material of the workpiece and flattens asperities from previous machining processes [6,11]. Experimental work was carried out to determine the effect of the burnishing process parameters on the shape correctness of the external cylindrical surfaces. Before burnishing the components were fine turned on a universal lathe with f1 = 0.2 mm/rev, than f2 = 0.15 mm/rev. The schematic illustration of the component with its dimensions can be seen on Fig. 1. Burnishing of outer cylindrical surfaces can be executed on conventional universal lath or up-to-date CNC lathe. In our experiments the latter one was used as can be seen on Fig. 2, when the burnishing tool is pressed against premachined surfaces to plastically deform peaks into valleys. The deformation is different depending on a lot of things, e.g. among them the magnitude of the force pressing against [12].

IMPLEMENTATION OF THE EXPERIMENT 3.1. The applied experimental design method
In order to achieve the optimum set of burnishing parameters for a given specific workpiece requires a large number of experiments. For reducing the total number of the experiments the use of factorial experimental design method is advantageous, which experimental design method is effective and active. The aim of this method is to determine the function relationship between the dependent variable (shape correctness) and the independent variables (burnishing parameters). Each independent variable, called factor can take more values, which are called levels.

The applied burnishing parameters
During planning of technological process of manufacturing machine components, the method of burnishing, the machining conditions, etc. should be selected [5]. So, in accordance with the full factorial experimental design method the values of the selected factors were set to two (minimum and maximum) levels, which are summarized in Table 1. In determination of the numerical values, we have taken into consideration the results of previous theoretical and practical research works.

Measuring of cylindricity deviations
Measuring process of the shape correctness was done with a circular and position error measuring equipment type Talyrond 365. Basic functions of this device include a profile scan with a measuring element fixed in the desired vertical position on the rotating workpiece. This function is suitable for detecting and evaluating circular errors and cylindricity deviations.
In this investigation inductive sensor made from artificial ruby was applied for measuring before and after burnishing in 2 mm distances on 18 mm length of the external cylindrical surface of the component. In all 16 cylindricity indices was analysed and 3 of them was chosen and examined which mostly determine operating properties. These are so called CYLp and CYLv that write down maximal difference from cylindricity as peaks and valleys, CYLt shows the total distance between peaks and valleys.
For the complete characterization of the cylindricity deviation, a 3D representation of the measured values of the specimen is also required. An example can be seen in Fig. 3 which shows the changing of shape correctness of the component marked 5.

EVALUATION OF RESULTS
In our investigation we have created dimensionless ratios to make the changing of shape correctness more obvious, which are shown in formula (1) and (2): (1) where: ρCYL Improvement ratios of cylindricity parameters (ρCYLp, ρCYLv, ρCYLt). These are dimensionless ratios, that texture the changes occurring because of burnishing, CYLburnished Cylindricity remain after burnishing, CYLturned Cylindricity remain after turning, ρ% The percentage value of the improvement ratios.
The lowest the values of ρ%, the greater the improvements due to burnishing. Measured calculated data are summarized in Table 2.  (Fig. 4-6) were prepared by using "MathCAD 15.0" software.

SUMMARY AND DISCUSSIONS
The paper deals with the experimental analysis of sliding burnishing when the material of the workpiece was low-alloyed aluminium. Experimental parameters were the burnishing speed, feed rate and burnishing force. The aim of the experiments was to determine how these parameters have effect to characterising values of shape correctness.
On the base of the present research work it can be stated: -Contrary to theoretical research and expectations, the diamond burnishing process has not brought such an improvement as it should have. This phenomenon may be due to improper pairing of material quality and burnishing parameters, of course, a more detailed examination of this belongs to our future research plans. -Examining the evaluated results it can be stated that applying the smaller burnishing parameters is more positive in these ranges and the most appropriate improvement ratio was resulted when the burnishing parameters were as follows: v = 15 m/min f = 0.001 mm/rev F = 20 N

ACKNOWLEDGEMENTS
"The described study was carried out as part of the EFOP-3.6.1-16-00011 "Younger and Renewing University -Innovative Knowledge Cityinstitutional development of the University of Miskolc aiming at intelligent specialisation" project implemented in the framework of the Szechenyi 2020 program.