ENHANCEMENT OF TOOL-LIFE OF HARD TURNING PROCESS VIA CRYOGENIC COOLANT AND MICROPATTERNED INSERT

Therefore, the proper development of textured surfaces on the cutting tool can improve the machining performance in the hard turning process. The cooling effects of the cryogenic refrigerant were included in the model by implementing an appropriate heat transfer coefficient.

INTRODUCTION

Motivation

So it is necessary to limit the temperature rise and reduce the friction between tool and workpiece during the hard turning process. The analytical solution for predictive modeling in hard turning will be needed in hard turning processes.

Research outline

Cutting forces, friction, chip morphology and wear of micropatterned insert tools are observed. Effects of feed rate and surface speed on cutting forces, friction, chip morphology and tool wear.

Research proposed plan

In this research, the numerical modeling of hard turning will be studied in effects of the patterns' shape, sizes and edge distances on cutting force, friction coefficients between tool-chip interfaces (Figure 4). Then the flank wear will be checked by tool wear experiments with variable cutting conditions.

Figure 3. Proposed research plan for predictive model of cryogenic cooling in hard turning process

LITERATURE REVIEW

Modeling of hard turning

However, the slip-line method is very complex and varies depending on the hodograph of the slip-line field. This model takes into account strain, strain rate, temperature of workpieces and is based on the slip-line field method.

Figure 6. (a) Proposed slip-line field for ploughing forces by Waldorf [35] and (b) Marchant’s forces diagram [39]

Modeling of cryogenic machining

This FE model predicted decreasing temperature, small grain size and increasing stress under cryogenic cooling (Figure 14 and Figure 15). In addition, the FE model (ANSYS, ABAQUS/Explicit, DEFORM 2D, AdvantEdge®) analyzes the grain size, residual stress, layers, cutting force and temperatures.

Figure 9. Problem domain for finite element simulation [61]

Experiment works in cryogenic machining

Hong, Shane et al [61] (Figure 19-(a)) and Bermingham [75] explained that cryogenic coolant delivered to the flank face can reduce the wear rate of the abrasive on the tool during the turning process of Ti-6Al-4V . Ghosh et al [79] reported a dramatic increase in tool life of alumina ceramics by cryogenic cooling (Figure 21).

Figure 19. Reduced temperature between tool-chip interface during machining of (a) Titanium alloy [61] (b) steel [77]

FEM based simulation/experiment work of patterned cutting tool

Yuan et al [96] designed surfaces with microgrooves which functioned to reduce friction due to the hydrodynamic effect, contact stress and additional lubrication (Figure 24 and Figure 25). Obikawa et al [98] investigated the effect of micro-patterning on the gouging surface of the tool in aluminum alloy machining (Figure 26).

Figure 24. Optical microscope image and 3D profile of the micro-grooves on the upper specimen [96]

Summary

PREDICTIVE MODEL OF CRYOGENIC COOLING IN HARD TURNING 33

Moving heat source model in shear and frictional zone

Heat source regions during the machining
Moving heat source model of primary zone
Moving heat source model of secondary zone

There are two heat source regions during machining: (1) Primary (cutting) zone; (2) Secondary (friction) zone for sharp-edged tool. The primary heat source is defined as a heat band present in the cutting plane (Figure 40). It is assumed that there is no thermal interaction between the primary heat source and the cutting tool.

The thermal energy of the primary heat source in the shear zone is represented in equation (11) and (12). Boundary condition for the thermal model of the primary heat source; the chip is assumed to be a continuous stream. The predictive power of the modified Oxley theory is converted into thermal energy in the primary heat source (equation (11).

In this model, it is assumed that the tool is not connected to the primary heat source. Where qshear and qfrictional are the heat intensity (energy) of the heat source in the shear and frictional regions.

Figure 41. Thermal model of the primary heat source in shear zone

Cutting force model under cryogenic cooling condition

A modified Oxley’s cutting theory model
Johnson-Cook plasticity model for workpiece
Effective ploughing force model considering tool edge geometry
Temperature model considering flank wear length
Cutting force model considering flank wear length

According to Waldorf plow cutting power (Pcut), plow thrust (Pthrust) is defined as Equation (48). Finally, calculate Equation to total cutting force (Fcs) and thrust force (Fct) taking into account edge geometry. The flowchart for prediction of cutting force and temperature in cryogenic coolant (This model based on the modified Oxley's cutting theory).

The thermal model calculates the temperature in the flank wear zone with combined cryogenic thermal convection coefficients. a) Initial tool cryogenic cooling length (b) Flank wear length and worn tool cryogenic cooling length. Calculations of flank wear effects are important for predicting the cutting temperature in cryogenic cooling. The increasing cutting force (Equation (56)) due to flank wear length can be calculated using the Waldorf wear model [119].

The 2D cutting force and thrust force can be transferred to 3D oblique cutting force by equivalent side cutting edge Cs*, equivalent slope angle i*(=ηc*) and equivalent cutting edge rake angle. The 2D cutting force and thrust force can be transferred to 3D oblique cutting force by equivalent side cutting edge Cs*, equivalent slope angle i*(=ηc*) and equivalent cutting edge rake angle.

Figure 44. The element of paralleled sided shear zone

Tool wear rate model under cryogenic cooling condition

Abrasive wear model
Adhesive wear model
Diffusive wear model
Composition of volume-loss wear rate modeling

During machining, they become trapped between the tool-workpiece interfaces, as in Figure 51. Three-body abrasive wear occurs, causing the softer surfaces to break and roll into the surfaces with the abrasive particles. The volume loss modeling of three-body abrasive wear mechanisms was developed by Rabinowicz et al. [120]. The abrasive armor of particles in CBN tool materials can be expressed in equation (65) [123].

The area of the interface of the side of the arm and the workpiece is made in the welding of the agile junction (Figure 52) due to high temperatures and stress during processing. P , asperity hardness depends on temperature, stress, strain rate, properties of soft surfaces and diffusive layers in the wear zone of the wing. However, CBN dust may be released because CBN tool fasteners are unstable in simultaneous machining.

The objective function is to minimize least square errors (Equation (79)) and find the optimized model with five wear coefficients by genetic algorithm. The cutting force and temperature of the flank wear zone continue the wear rate modeling with calibrated wear constants.

Figure 51. A schematic of abrasive wear mechanisms in machining

Summary

VALIDATION WORKS OF CRYOGENIC COOLING IN HARD TURNING 59

Constants of Johnson-cook plasticity model in workpieces

Model validation set-up for 2D orthogonal cutting
Comparison of cutting force in cryogenic cooling/dry condition
Comparison of temperatures in cryogenic cooling/dry condition
Conclusions
Model validation set-up for 3D oblique cutting
Influence of cryogenic cooling on cutting force in fresh tool
Influence of cryogenic cooling on temperatures in fresh tool
Predicted shear angle and strain in shear zone
Conclusions

Comparison of predicted and measured cutting forces (Fc) in cryogenic and dry condition during 2D orthogonal test. Comparison of predicted and measured thrust forces (Ft) in cryogenic and dry condition during 2D orthogonal test. Thermocouple located temperature in tool-chip interfaces under cryogenic cooling and dry condition during 2D orthogonal test.

Predicted temperature in tool-chip interfaces under cryogenic cooling and dry condition during 2D orthogonal test. Modified Oxley's cutting theory predicts the cutting force and temperature in thermocouple location and tool chip interfaces under cryogenic and dry condition. Increase in cutting speed slightly reduces the P1, P2 and P3 in cryogenic and dry condition (deviation is 10 N).

The cutting force model also included the cryogenic cooling condition to predict the cutting forces P1, P2 and P3. Cryogenic coolant was able to reduce the temperature between tool-chip interfaces by as much as 17% compared to dry conditions.

Table 7. Cutting conditions of cryogenic 2D orthogonal hard cutting test

Tool wear of cryogenic cooling condition in hard turning

Predicted cutting force for flank wear
Predicted tool wear lengths in cryogenic cooling condition
Conclusions

Comparison of predicted and measured thermocouple temperature in cryogenic cooling and dry condition with increasing processing time for experiment no. 5. Estimated tool flank wear lengths in cryogenic cooling and dry condition [experiment no. 1 (V100m/min, f 0.1 mm rev/. Estimated flank) length of tool wear in cryogenic cooling and dry condition [experiment no. 2 (V175m/min, f 0.1mm rev./.

Predicted wear lengths of the flank tool in cryogenic cooling and dry conditions [experiment no. 3 (V250m/min, f 0.1 mm rev/). Predicted wear lengths of the flank tool in cryogenic cooling and dry conditions [experiment no. 4 (V100m/min, f 0.2 mm rev/.Predicted wear lengths of the flank tool in cryogenic cooling and dry conditions [Experiment No. 5 (V175m/min, f 0.2 mm rev/.

Predicted flank tool wear lengths in cryogenic cooling and dry condition [experiment # 6 (V250m/min, f 0.2mm rev/. Tool wear rate modeling explains that lowered temperature at the tool-workpiece interface can reduce flank wear lengths under cryogenic cooling conditions.

Figure 71 presents comparison of predictive and measured thermocouple temperature following machining time

FEM BASED SIMULATION OF MICROPATTERNED INSERT

Boundary condition in FEM based simulation
Properties and geometries for workpiece and tool
Process parameters in FEM simulation
Results and Discussion

Effects of texture shape
Effects of edge distances
Effects of pitch size
Effects of texture heights
Effects of friction factor

Conclusions

Six percent reduction in the cutting force was predicted in the perpendicular texture than the flat tool. The perpendicular texture therefore had the effect of reducing the cutting force by curling the chip away from the rake face. 2 in Table 14 was considered for the study of the edge distance on the cutting forces, effective friction coefficient and the chip flow angle.

Cutting force data indicate that there are no changes in cutting force when the edge distance changes to 300 µm. The cutting force is predicted to decrease as the pitch size increases from 50 to 100. The minimum value of the cutting force is about 130 N at the highest friction coefficient of 0.6 in both pattern types.

The friction force applied to the rake face has a component in the direction opposite to the cutting force. In addition, the friction factor at the tool-chip interface was varied to demonstrate the effect on cutting force and chip flow angle.

Figure 82. (a) Model assembly and boundary conditions and (b) Mesh configuration

EXPERIMENT WORKS OF MICROPATTERNED INSERT

Fabrication of the micropatterned insert
The configuration of hard turning experiments
Friction coefficient model of chip-morphology
Criterion and measurement of tool wear in micropatterned tool
Results and Discussion

Effects of feed rates in micropatterned insert
Effects of surface velocity in micropatterned tool
Evaluation of tool life in hard turning process

Mechanism of reduced friction in micropatterns on CBN tool rake surfaces
Conclusions

The micro-patterned insert produced a larger shear angle and smaller coefficient of friction than the non-patterned tool. Increasing the feed rate reduces deviations of force, the coefficient of friction between micropattern and non-pattern insert. Calculated cutting ratio, shear angle, friction angle and friction coefficient with increases in feed rates between non-patterned and micropatterned tools [138].

The chip cutting angle using the patterned tool is greater than that of the non-patterned tool, as shown in Table 19. While the friction coefficients increased slightly using the non-patterned tool, those of the micropatterned insert increased dramatically , from a surface velocity of m/min. It also increased the difference in coefficient of friction between unpatterned and micropatterned inserts.

Calculated cutting ratio, cutting angle, friction angle and friction coefficient with an increase in surface velocity between unpatterned and micropatterned tools (fixed undeformed chip thickness 152µm) [138]. Tool wear of the micropattern insert arm is reduced compared to the non-pattern insert.

Figure 100. Movement of the tool electrode and workpiece: (a) conventional EDM milling and (b) layer-by-layer EDM machining

SUMMARY AND RECOMMENDATIONS

SUMMARY

Furthermore, friction coefficients can be calculated from the observations of the chip thickness obtained from the modelling. In addition, the results of the wear test suggested that the micropattern tool can reduce the flank wear lengths as much as 9~11%. Overall, as shown in Figure 116, this thesis contributes to the field of machining science by showing the various ways to improve tool life and machinability during the hard turning process.

Furthermore, FEM and experimental work showed that micropatterned cutting tool can reduce the generation of tool wear in hard turning process.

RECOMMENDATIONS

Suri, "Cooling Techniques for Improved Productivity in Turning," International Journal of Machine Tools and Manufacture, vol. Zheng, “An examination of the fundamental mechanics of shear force coefficients,” International Journal of Machine Tools and Manufacture, vol. Newman, "State-of-the-art cryogenic processing and machining," International Journal of Computer Integrated Manufacturing, vol.

Ding, “Cooling approaches and cutting temperatures in cryogenic machining of Ti-6Al-4V,” International Journal of Machine Tools and Manufacture, vol. June, “Performance of self-lubricating textured tools in dry cutting of Ti-6Al-4V,” The International Journal of Advanced Manufacturing Technology, vol. Lei, “FEM evaluation of microhole textured cutting tool performance in dry machining of Ti-6Al-4V,” The International Journal of Advanced Manufacturing Technology , vol.

Lei, “3D numerical investigation of microgroove textured cutting tool performance in dry machining of Ti-6Al-4V,” International Journal of Advanced Manufacturing Technology, vol. Liang, “CBN tool wear in hard turning: a research progress study,” The International Journal of Advanced Manufacturing Technology, vol.