INCREASING TOOL LIFE DURING TURNING WITH A VARIABLE DEPTH OF CUT POVEČANJE ZDRŽLJIVOSTI ORODJA PRI STRUŽENJU Z VARIABILNO GLOBINO REZA Marek Sadilek, Robert Čep, Zuzana Sadilkova, Jan Valicek, Lenka Petrkovska VŠB-Technical University of Ostrava, Faculty of Mechanical Engineering, 17. listopadu 15/2172, 708 33 Ostrava, Czech Republic marek.sadilek@vsb.cz Prejem rokopisa - received: 2012-09-05; sprejem za objavo - accepted for publication: 2012-09-27 The article deals with the improvement of cutting-tool durability by using CAD/CAM systems. It proposes new roughing turning cycles where a variable depth of cut is applied. The experimental part verifies theoretical prerequisites when a flange is being machined with a sintered-carbide cutting tool. It compares the turning where the standard roughing cycle is used and the turning where the proposed roughing cycle with a variable depth of cut is applied. Keywords: variable depth of cut, durability, CAD-CAM systems, turning Članek obravnava izboljšanje zdržljivosti orodij za odrezavanje z uporabo sistema CAD/CAM. Predlagani so novi cikli grobega struženja, kjer se uporablja spremenljivo globino rezanja. V eksperimentalnem delu so potrjeni teoretični prvi pogoji pri struženju prirobnice z rezilnim orodjem iz sintranih karbidov. Primerjano je struženje, kjer je uporabljen standarden cikel odrezavanja, s tistim struženjem, kjer je uporabljen predlagan cikel odrezavanja s spremenljivo globino rezanja. Ključne besede: spremenljiva globina rezanja, zdržljivost, sistem CAD-CAM, struženje 1 SUPPOSED PROCESS OF TOOL WEAR Tool wear depends on numerous factors, for example: workpiece material, tool material and geometry, cutting parameters (cutting speed, feed, cutting depth), used process liquid, cutting machine and many others1. It is proved that cutting the depth does not have the greatest influence. Since it has its share in the intensity of tool wear (a growing cutting depth increases the tool wear as well) it makes sense to deal with cutting depth and we can consider it as a significant factor of tool wear. pronounced notch over the maximum possible length of the tool's cutting edge. Figure 2 below describes the used marking of the tool wear according to ISO 3685.6 In applying a variable depth of cut (in the sintered carbide) the durability-improvement effect is expected to occur in the cutting edge provided the wear shaped as a notch on the back is distributed over the longer part of the cutting edge. These prerequisites were verified with the experiments during the practical machining of the concerned part - the flange. 1.1 Tool wear of a selected type of cutting tools The most used cutting materials in the area of CNC machining are sintered carbide and cutting ceramics2,3. As seen in Figure 1, the tool-wear method depends on the applied tool materials. The tool wear of the cutting ceramics is rather linear without any marked stepped increases. The tool wear will therefore grow with an increasing depth of cut. It is, thus, not advisable to apply a variable depth of cut to the ceramics with such behavior. The types of cutting ceramics, such as the nitride ceramics, in which a more pronounced notch on the back is formed during the machining may be used exceptionally4,5. Sintered carbide tends to form a pronounced notch on the face and main back. This notch could be very advantageously used in the roughing cycles with a variable depth of cut. Efforts will be made to distribute this 1 - surface of wear on the back Aa - main back 2 - crater wear Aa - minor back 3 - notch wear on the main back Ay - face 4 - notch wear on the minor back 5 - notch on the face Figure 1: Cutting-ceramics wear process and the tool-wear process of sintered carbides4,5 Slika 1: Proces obrabe rezilne keramike in obrabe orodja iz sintranih karbidov4,5 Figure 2: Tool wear according to ISO 3685:1993 Slika 2: Obraba orodja skladno z ISO 3685:1993 Both types of the cutting material were tested in practice by experimental machining of the selected turning parts. The theoretical assumption that the cutting ceramic does not tend to form a notch on the back was proven and it is essential for this tool-life improvement method. That is why only the experimental works where sintered carbide is used are stated below. • hole machining, • parting off (cutting off), • rest cycle (residual turning), • hand setting of tool motion. The conventional roughing cycles in turning, where a cutting tool performs a constant depth of cut can be adapted and extended with the cycles when the tool cuts with a variable depth of cut. The proposed roughing cycles are as follows: • rough cycle "decreasing of engagement" (Figure 3), • roughing by creating a conical surface, • roughing with the use of nonlinear methods, etc. Figure 4 depicts a commonly used roughing cycle. A constant depth of cut is used in this roughing cycle. The machining process results in the wear that prevails in one point of the cutting edge only. During the roughing strategy - the cut decrement -each chip removal is performed with a different depth of cut so a different cutting part of the tool is under stress during each cutting operation. This method of machining can be time-consuming due to several passes. This is compensated for by an increased tool life, a lower loading of the machine spindle and a reduced machine noise. The depth of cut is reduced when the final diameter is being approached. The maximum wear point is therefore moved outwards from the cut, prolonging the cuttingtool durability. This type of roughing cycle is already contained in the advanced CAM systems (EdgeCAM, 2 POSSIBILITIES OF A ROUGHING TOOL PATH IN CAM SYSTEMS The basic types of the used machining cycles and operating stages for the 2-axis turning are as follows: • straight (rectangle) turning, • face turning, • rough turning, • finish turning, • profile turning, • groove turning, • pocket turning, • thread turning, Figure 4: Roughing cycle with a constant depth of cut Slika 4: Cikel odvzemanja s konstantno globino reza 6 6 3 EXPERIMENTAL WORK tool paths rapid paths Figure 6: Roughing-cycle simulation when a conical surface is made during an internal turning Slika 6: Simulacija cikla odvzemanja, pri katerem nastaja konična površina pri notranjem struženju turning to profile). An application of this feature, when longer parts are turned, is advantageous for the elimination of the ever-decreasing workpiece stiffness. The cutting, with which a conical surface is formed, starts with the deepest depth of cut which decreases in the feeding direction, as shown in Figure 5. The second cut is programmed to be parallel with the workpiece axis. This provides for an efficient removal of the conical surface formed in the previous cut. Thanks to this strategy, the tool wear moves along the cutting edge from the maximum to the minimum depth of cut (apmax to apmin). Figure 6 shows a simulation of the machining (a programmed tool path) when turning the flanges, for which the experimental part was done. The non-linear roughing-cycle method also ensures a variable depth of cut. For example, a tool path's wavy profile (Figure 7) will achieve the same effect as the previous methods. During both the first and the second cut, the machined material is on a gradual increase and decrease and a variable depth of cut is thereby achieved. It is also possible to shift the machined surface and, in doing so, change the depth of cut. However, this requires an advanced CAD/CAM system joined with the CNC cutting machines. Two strategies were used during turning: the conventional method with a constant depth of cut and the cone-forming machining. 3.1 Experimental characterization The verification of theoretical presumptions was carried out in practice. For this purpose we used the following cutting machine: Mori Seiki SL - 65 B with the drive system Fanuc and the spindle power of P = 71 kW. The worpiece material was the austenitic stainless steel 1.4401 that corresponds to DIN X5CrNiMo17-2-2 with a hardness of 180 HB. This material is mainly used in chemical industry, apparatus engineering, pulp industry and food industry. The dimensions of the worpiece (the flange) were as follows: its external diameter D = 350 mm, internal diameter d1 = 56 mm, length L = 73 mm and the internal diameter of the semi-product before finishing d2 = 157 mm. The cutting tool was an internal radius turning tool (the Sandvik company) with the cutting inserts: CNMG 12 04 12 -MR 2025 with the CVD coating. This tool is suitable for longitudinal medium roughing and roughing. During the cutting the cutting fluid was used. The cutting conditions differed only in the cut size and depth shape, as shown in Tablel. The different depths of cut (stock removals) in the internal roughing cycle are shown in Figure 8. Here a gradual removal of the material from the first tool path to the last one (nth) is shown. The total number of tool paths (n) depends on the size of the workpiece and the technological possibilities of the insert as well. A representative sample of the tool wear has been selected from all of the performed experiments focusing Figure 7: Roughing cycle - the nonlinear method Slika 7: Cikel odvzemanja - nelinearna metoda Figure 8: Tool paths: a) a constant depth of cut, b) a variable depth of cut (i <1,n> = number of tool paths; n = total number of tool paths; L, D, d1 = dimensions of the workpiece; L, D, d2 = dimensions of the semi-product before finishing) Slika 8: Poti orodja: a) konstantna globina reza, b) spremenljiva globina reza (i <1,n> = število poti orodja, n = celotno število poti orodja, L, D, d1 = dimenzije obdelovanca, L, D, d2 = dimenzije nedokončanega polproizvoda) Figure 9: Comparison of the tool-face and back wear: a) a constant depth of cut, b) a variable depth of cut Slika 9: Primerjava obrabe cela in začelja orodja: a) konstantna globina reza, b) spremenljiva globina reza Table 1: Cutting conditions: the constant-depth-of-cut strategy and the conical-surface strategy Tabela 1: Pogoji rezanja: strategija s konstantno globino rezanja in strategija s konično površino rough turning Cutting conditions constant variable depth of depth of cut cut cutting speed Vc m min-1 180 180 feed f mm r-1 0.3 0.3 depth of cut ap mm 4 apmin = 3 apmin = 5 on the wear in the form of a notch on the face and on the back. With regard to the measurement during the production, the tool wear was measured with a microscope with an installed digital camera. To read the wear values the Micrometrics SE Premium software, version 7.2, was used. The upper part of Figure 9 shows the tool wear on the face. The lower part of the same figure shows the tool wear on the back. The strategy with a constant depth of cut reports, for the same period of time (machining i a I I OOllif e crite rion V B„ = 0 .25 mi tn / / t • constant depth of cut ■ variable depth of cut / 10 12 14 16 18 20 Machining timet fminj Figure 10: Dependence of the VBn tool wear on time under the roughing strategy - a constant depth of cut and a variable depth of cut Slika 10: Odvisnost obrabe VBn orodja od časa odrezavanja - konstantna globina reza in spremenljiva globina reza time t = 18.6 min), a greater tool wear than the strategy with a variable depth of cut. Figure 9 reveals a visible notch on the face (with the length Xcons of 1.03 mm) and on the back formed while using the constant depth of cut. The notch wear on the face is greater than in the case of the strategy with a variable ap. There is also a visible notch on the back, VBn = 0.25 mm. The strategy using a variable depth of cut causes no notch on the back, as shown in Figure 9, the bottom b part). With the variable depth of cut, the notches are distributed over the tool's longer cutting edge corresponding to the variable depth of cut. The notches shift in relation to the changing depth of cut (from 3 mm to 5 mm depth of cut), ^variable = 1.49 mm. The dependence of the tool wear on time was recorded and entered into the "Dependence of VBn tool wear on time" chart, as shown in Figure 10. The cutting process was interrupted in order to measure the tool wear after the tool path (between the cuts) so as not to disturb the cutting process by starting a cut and getting out of a cut and the measuring interval was kept to last for about 2 min. This chart clearly implies that the roughing method with a variable depth of cut causes a lower wear VBn in the same time and under the same cutting conditions. The wear has a slighter inclination in the second area of the chart (in this area the tool wear increases uniformly). The tool wear criterion was set to be VBn = 0.25 mm. The durability of the cutting edge in the roughing cycle with a constant depth of cut was, on average, 18 min. In the newly designed cycle with a variable depth of cut, the durability of the cutting edge was 26 min, i.e., the durability increased by 44 %. 4 DISCUSSION AND CONCLUSIONS The proposed manufacturing technology of the flange and shaft components ensures a reduced tool wear, i.e., an increased turning-tool durability and life. There is a more favorable distribution of wear on the replaceable tool insert when employing the proposed turning technology. An application of the new roughing cycle resulted in a decrease in the spindle load by 10 %. This reduction was monitored directly on the cutting machine - it was displayed on the indicator of the spindle load. This change caused a reduction in the energy demand of the machine tool. The roughing cycle with a variable depth of cut was applied in the company of JohnCrane a.s. It gave excellent results in the form of an increased durability of the cutting edge by 44 % in the maintained machining time. The increased durability of the tool significantly reduces the total costs for the cutting tools. These costs are also reduced by less frequent downtimes when a worn tool is replaced. However, the disadvantage of this technology is a more complicated tool-path programming for the roughing cycle with a variable depth of cut. The suggested roughing cycles may not have a positive influence on the machining process. It is necessary to carefully consider all the aspects associated with the proposed cycles. These can include a reduced rigidity of the machine tool caused by both the tool's simultaneous movement in the directions of the two axes and the requirements of the software (CAM system) and the CNC programmer. Acknowledgement We would like to thank the company of JohnCrane a.s. for allowing us to perform our experiments. This paper was supported by the Czech Science Foundation, Student Grant Competition (SP2011/23), VSB-Technical University of Ostrava. 5 REFERENCES 1 A. Antic, B. P. 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