Optimization of external grooving operations with the defect elimination approach in the selection of machining strategy and parameters
Amir Alinaghizadeh , Armin Tabaghi , Mohammad Nemati
1- Faculty member of mechanical engineering department, Energy and Sustainable Development Research Center, Semnan Branch, Islamic Azad University, Semnan, Iran
1,2,3- Research and Development office, Technical and Engineering Department, Tabesh Abzar Pars Company, Tehran, Iran.
The machining process was affected by different factors. The proper adjusting of these parameters plays an important role especially in terms of workpiece quality. The grooving process is one of the most challenging operations in turning. This paper covers experimental tests in order to improve tool life in the grooving process. The optimization process was applied through adjusting cutting parameters - Cutting speed, Depth of cut and Feed rate per revolution) which were selected according to workpiece material and machining specification.
Besides that, machining strategy, regards to tool path, improved by using multiple-grooving method for roughing and a four-step technique for finishing process. Also, some hints were used to make machining conditions much better like pre-machining of oxidization layer (which was a result of workpiece material forging process) and changes in the cooling system. Test results have shown that the chosen strategy for grooving and other preparation can prevent insert breakage and led to improved tool life.
One of the most challenging machining operations on a lathe is grooving. The grooving term is referred to make a groove (usually narrow) with a certified depth . Grooving has different applications such as Machining O-ring seating, V-belt pulley, make a slot for lubrication, and machining the end part of the threading portion. Nowadays, indexable grooving tools are widely used.
Different types of inserts are used to make a groove related to machining parameters (cutting velocity, depth of cut, feed rate). Generally, grooving inserts are not suitable to perform other turning processes like external turning, facing, turning…, but some of them have different applications. They are also called “MDT” which is an abbreviation for Multi-Directional Turning that is applied for a variety of turning applications that usually have one or two cutting edges.
The important difference between external turning and external grooving is the cutting direction. External grooving is usually applied in the radial direction while in external turning operations, the cutting direction is parallel to the workpiece axis. According to that, usage of grooving inserts be limited and lower feed rate and depth of cut should be considered.
Problem Explanation, Innovations & Aims of Paper
Machining parameters that are not selected correctly, like cutting speed, depth of cut, feed rate, chip breaker, and tool material grade lead to poor tool life and insert breakage[1, 3]. Different type of chip breakers geometries is used related to machining condition (roughing or finishing). The most contributing factor for proper chip control in external grooving is the feed rate. Besides that, different shape of chip breakers is developed for a wide variety of materials. For example, a chip breaker with beams is suitable for gummy materials. (Fig. 1)
Fig. 1: Schematic view of chip breaker with beams
Different strategies are applied through external grooving. However, most of them result in tool damage because of avoiding machining rules. To perform external grooving by roughing method, three common strategies were suggested . First of all, in case the depth to width ratio is up to 1.5,
Fig. 2: Grooving strategy for roughing process regard to depth
to width ratio a) up to 1.5 b) more than 1.5
a grooving strategy is applied (Fig. 2a). Groove produce by radial movement of the tool, then return and feed in an axial direction and similar actions were performed again up to machining the desired width. In case a greater depth of cut was needed, this strategy applied in consequence passes. (Fig. 2b)
Secondly, in cases the depth to width ratio is more than 1.5, a multiple grooving strategy is applied. The desired width was produced by equal interval grooving and after that, the remaining flanges were machined . (Fig. 3)
Fig. 3:Multiple grooving strategy for roughing process 
The third strategy (Plunging strategy) is similar to external turning. Groove is produced by tool penetration in the workpiece and consequently axial movement. This process continues until reaching the desired depth of machining. The vital thing that should be considered is the depth of the cut. As grooving inserts are not applicable for external turning, the lower depth of the cut should be applied. Also, it is recommended to stop the axial movement before reaching the groove shoulders. (Fig. 4)
Fig. 4: Plunging strategy for roughing process[6, 8]
To achieve finishing dimensions, a 4-step strategy was applied. At first, the desired depth was machined. secondly, the grooving insert moves along the shoulder and goes toward another shoulder. then tool retracts back and moves along another shoulder (Fig. 5)
Fig. 5: Finishing process a) first step b) second step c) third step d) fourth step
Generally, insert breakage lead to tool damage[9, 10]. For improving machining results, the tool path was corrected firstly, then machining parameters (ap, Vc, f) were optimized and the shape of the chips was studied regard to feed rate range. These solutions lead to improved tool life, better machining quality, and preventing cutting tool holders from damage through insert breakage.
Research Method and Assumptions
In this study, the SCM420H was used as a workpiece material with 180-210 BHN hardness. which was forged and annealed. There was an oxidation layer on a workpiece surface due to the forging and annealing process which was performed on workpiece material that leads to surface hardening. Fig. 6 shows the workpiece drawing before and after machining process.
Fig. 6: Workpiece drawing a) before machining process b) after machining process
As shown in Fig. 7, a groove with 9mm width and 5mm depth was machined on the workpieces by performing two different steps, roughing and finishing. The roughing process was applied by a strategy which was shown in Fig. 8a. This includes 3 steps which were performed from right to left respectively. Each step has 3mm width and 5mm depth. For the finishing process, as shown in Fig. 8b, the grooving tool moves along the shoulder and wall of the groove and retract back respectively.
Fig. 7: Workpiece a) before grooving b) after grooving
Fig. 8: Workpiece machining strategy a) roughing b) finishing
Fig. 9 shows the clamping system of the workpiece which would benefit from three special jaws for proper clamping of the workpiece.
Fig. 9: Workpiece clamping system
In this study, the insert and tool holder were Mitsubishi products. The insert for grooving is GY2M0300F020N-MM MY5015 with 3mm width which was mounted on the Monoblock tool holder GYQL 2020K00-F06 as shown in Fig. 10.
Fig. 10: Grooving holder and insert
Machining setup was done and cutting parameters - ap=5, Vc=330, fn=0.05 – adjusted. During the test, 650 pieces of workpiece were machined by the first cutting edge. After 500 pieces of workpiece machining by the second cutting edge, the insert broke and the holder damaged as shown in Fig. 11.
Fig. 11: broken insert and holder
To solve this problem, the metallurgical and mechanical properties of the broken holder were checked to assured that there was no matter in the holder manufacturing process. Besides that, the machining strategy was improved by changing the tool path (multiple grooving for roughing) as shown in Fig. 12. Symmetrical machining steps led to equal contact of the grooving insert’s shoulders and reduction in the grater component (Fp) of machining forces (Fp as a radial component and Fc as a tangential component). This results in a lower amount of vibration and consequently improved tool life.
Fig. 12: Machining strategy for improving rough process 
Also, for the finishing process, a strategy like the one which was shown in Fig. 13 was used. This strategy makes it possible to machine the groove shoulder perfectly .
Fig. 13: Schematic view of suggestion strategy for finishing
In case the grooving tool moves in a radial direction, the radial component of machining force is dominant and others can be ignored. So, by increasing cutting speed, no change occurred in chip formation. But the possibility of tool wear will be increased. Therefore, cutting speed reduced from 330 m/min in both roughing and finishing process to 240 m/min in roughing and 250 m/min in the finishing process. Also, the feed rate was increased from 0.05 mm/rev to 0.16 mm/rev to better chip breaking conditions.
Results and Discussion
As explained above, the broken holders were first tested for hardness and metallurgical properties (spectrometry test). The mentioned experiments were performed by Razi Metallurgical Laboratory, the results of which are as follows.
As a result, both grooving holders have the same alloy elements. Consequently, the hardness test was applied and the same hardness value (45 Rc ) was obtained.
Table 1: Spectrometry test result for a) ordinary holder b) breakage holder
Table 2: Hardness test result for a) ordinary holder b) breakage holder
According to the hardness and spectrometry test, it is shown that both holders have the same metallurgical properties. So, the machining strategy should be optimized. Fig.14 shows the chip which was obtained by a previous machining strategy that is not a proper chip shape.
Fig. 14: Chip which was produced by the previous strategy
Cutting data due to tool breakage shown in Table 3 and related tool life shown in Table 4.
The optimization progress was performed by cutting data correction according to tool supplier suggestion which is shown in Table 5.
Figure 15a shows the chip deformation regard to a feed rate of 0.1 mm/rev. It is known that the chip formation improved after the changes in the machining parameters of the process. However, since the chips were long and did not break, the feed rate increased in four steps until it was finally crushed at 0.16 mm/rev., Figure 15b shows the final chip formation. In addition, in the tests using improper coolant flushing, the chips were not broken but were successfully evacuated from the groove. The insert wear was considered by flank wear which led to poor insert lifetime. When adding high-pressure coolant to the flank face, the chip broke successfully, and correspondingly the tool life increased meaningfully.
After the corrections, the process was performed under constant conditions and the test was continued with 4 inserts without damaging the holder. The related data are shown in Table 6.
According to the test result, it was suggested to change insert cutting edge per 600 workpiece machining to prevent excessive insert wear which may lead to tool breakage.
In this paper, grooving insert breakage and tool holder damaged were studied. According to experimental test,
It was shown that tool life enhanced through tool path strategy improvement and cutting data optimization. The former was performed by using multiple grooving strategy and the latter was done by adjusting the machining parameters (Vc, ap, fn). At last but not least, the chips were broken successfully by excessive feed rate and using proper coolant flood.
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