Effect of Conductive Heat Cutting on the Quality of Machining Surface

I. Introduction

　　 Conductive heating cutting is the use of tools and workpieces in the cutting process to form a circuit and pass a low voltage and high current, so that the material in the cutting zone will be softened due to the Joule heat generated by the resistance, thereby improving the cutting performance of the processing method. Conductive heating cutting can reduce cutting force, reduce tool wear, improve tool life, and significantly improve the machinability of difficult-to-machine materials [1]. In the study of conductive heating cutting, it was found that conductive heating cutting can also significantly improve the quality of the machined surface. However, in-depth systematic research is needed on the relationship between the size of the heating current and the quality of the machined surface under different cutting conditions, and how to adjust the heating current to maintain the best surface quality when the cutting amount changes.
When cutting plastic metal, the main obstacle to obtain a good surface quality is scaly thorns and built-up edge. According to the metal cutting theory [2], the main measures to inhibit scaly and built-up edge are high-speed cutting. Second, when the increase of cutting speed is limited, artificial heating measures can be applied to the cutting area. Conductive heating cutting is the best way to locally heat the cutting area. Due to the small heating area and high efficiency, it has good effects and outstanding advantages for suppressing the built-up edge and spurs caused by low cutting speed.

Second, the impact of conductive heating cutting on the surface quality

Single factor test was used to study the effect of conductive heating cutting on the quality of the machined surface. The main conditions of the test are as follows: Workpiece material: 45 steel (conditioner); Tool: YT15 does not regrind can be indexed cemented carbide insert; Tool geometry: γ0 = 25°, α0 = 5°, Kr = 45°, λs = 0; heating power supply: IGBT inverter power supply; processing machine: CM6140; surface quality measuring instrument: YCL-1 contact stylus profiler.

Surface quality of the heating current in relation <br> holding 1. Different cutting depth of cut (ap = 0.2mm) and the feed (f = 0.10mm / r) constant, corresponding to different cutting speed VC, using A set of different heating current values ​​are processed to obtain the relationship between the surface roughness value Ra and the heating current value I as shown in FIG. 1 . As can be seen from FIG. 1 , since the selected cutting speed is low, when the current is not passed, the surface roughness value is large, and the processing surface has scale marks; as the heating current gradually increases, the scale marks gradually disappear and the surface quality of the machining is obtained. improve. Under the cutting amount used in the experiment, the chip morphology began to show intermittent cuttings, and there were generally scaly thorns on the surface. As the heating current increased, the chips gradually became banded chips and the spurs disappeared. When the knife-chip contact area begins to turn red, the quality of the machined surface is best; continuing to increase the heating current, the knife-to-chip contact area becomes red and the quality of the machined surface drops again. If the heating current corresponding to the best processed surface quality is called the optimal heating current, the relationship between the optimal heating current Iopt and the cutting speed υc is shown in FIG. 2 .

Fig. 1 Relationship between surface roughness and heating current at different cutting speeds

Figure 2 Relationship between optimal heating current and cutting speed

2. The relationship between the machining surface quality and the heating current at different cutting depths <br> The cutting speed (vc=1.7m/s) and the feed rate (f=0.1mm/r) are constant, corresponding to different cutting depths ap. A set of different heating current values ​​are processed. The relationship between the surface roughness value Ra and the heating current value I is shown in FIG. 3 . Similar to the cutting speed test, at different depths of cut, the quality of the machined surface improves with the increase of the heating current, but when the current increases to a certain degree, the quality of the machined surface decreases. The relationship between the optimal heating current Iopt and the cutting depth ap is shown in FIG. 4 . As can be seen from Fig. 4, the optimal heating current is basically linear with the depth of cut. Using the experimental data for linear regression, the following empirical formula can be obtained:

Iopt=55.13+351.2ap (1)

The unit of Iopt is A, and the unit of ap is mm.

Fig. 3 Relationship between surface roughness and heating current at different depths of cut

Figure 4 The relationship between the optimal heating current and the depth of cut

Using formula (1) can estimate the appropriate heating current value under different cutting depths in actual processing. However, this formula is only applicable to the conductive heating cutting of 45 quenched and tempered steel and YT15 tool material. In practical applications, it must also take into account the effects of various factors such as feed rate and cutting speed.

3. The amount of feed at different surface quality of the heating current relationship <br> feed amount f great influence on surface roughness is calculated from the equation for surface roughness Ra = f / 4 (ctgKr + ctgKr '), Ry It is known that [2, 3] and [2, 3] indicate that the surface roughness is directly related to the feed amount, and the larger the feed amount f is, the larger the surface roughness theoretical value is. Although the surface roughness value formed by actual processing is also affected by factors such as built-up edge, scale, vibration, and tool wear, it is often much larger than the theoretical value of surface roughness, but it can also be seen that the amount of feed The importance of surface roughness effects.
Keep cutting speed (vc=0.66m/s) and depth of cut (ap=0.2mm) unchanged. For different feed amounts f, use a set of different heating current values ​​to process, obtain the surface roughness value Ra and The relationship between the heating current value I is shown in FIG. As can be seen from Fig. 5, after the current is processed by the electric current, the surface roughness value is much lower than when the electric current is not processed. This is mainly due to the reduction of the scale marks formed during the processing. When the cutting speed is low, as the feed amount increases, the cutting thickness ac (ac=fsinKr) also increases. After a certain degree of formation, nodular chips are formed. Although there is no built-up edge, it is easy. Generate burrs, and scaly height increases with the cutting thickness ac increases [2]. It can be seen that a large part of the surface roughness caused by a large feed rate is caused by burrs. Although conductive heating cutting itself does not reduce the theoretical value of surface roughness that is closely related to the feed rate, it can be The quality of the processed surface is improved by suppressing the generation of burrs. The relationship between the optimal heating current Iopt and the feed rate f is shown in FIG. 6 . From Figure 6, it can be seen that the optimal heating current and feed rate are also basically proportional to each other.

Fig. 5 Relationship between surface roughness and heating current at different feed rates

Figure 6 Relationship between optimal heating current and feed rate

three. Analysis and discussion

1. Analysis of the best current <br> From the above test results, it can be seen that for a given tool material, workpiece material and a set of determined cutting amounts, the quality of the machined surface gradually improves as the heating current increases; When the heating current reaches a certain value, the best machining surface quality can be obtained; but when the heating current continues to increase, the quality of the machining surface will deteriorate. The reason for this phenomenon is mainly the effect of temperature changes in the cutting zone. The mechanism of conductive heating cutting is to influence the quality of the machined surface by increasing the cutting zone temperature. In low-speed cutting, because the cutting temperature is not high, if the heating current is small, the heat generation is small, and the compensation of the temperature in the cutting zone is insufficient to suppress the generation of the built-up edge and burr, thereby improving the quality of the processing surface. This condition is called “temperature under compensation”; if the heating current is continued to increase, the temperature in the cutting zone is further increased, the built-up edge and scale blisters are gradually suppressed, and the hardness ratio and strength ratio of the tool relative to the workpiece are further increased. Abrasive wear and adhesive wear on tools that dominate in low-speed cutting have also been reduced [1]. On the one hand, the tool life has been improved, and on the other hand, the tool's edge-retaining ability has been increased, so that smaller tools can be obtained. The surface roughness value; if the heating current is further increased afterwards, the temperature in the cutting zone will continue to increase, and the diffusion wear and oxidation wear of the tool will be gradually increased [1], and the tool's durability and edge shape retention ability will be rapidly reduced, resulting in The quality of the machined surface deteriorates again. This phenomenon is called "temperature over compensation." It is due to the existence of "temperature under compensation" and "temperature over compensation" phenomenon, so there is an optimal heating current value in the conductive heating cutting, under which the optimal surface quality can be obtained under the action of the heating current. The essence of the optimum heating current is the optimum cutting temperature.

2. The factors influencing cutting zone temperature during the discussion of best <br> EHM current relationship between the cutting amounts are: a heating resistor (1) the cutting zone; (2) requires heating of the chips by mass; (3) heat condition. All three factors are closely related to the amount of cutting. According to Joule's law, when the current is applied to the cutting zone, the total heat is

Where i(t) - the instantaneous current through the cutting deformation zone (A)
Rz - resistance in the cutting zone (Ω)
t - heating time (s)
The Joule heat causes the average temperature rise in the cutting zone to

ΔT=Q/cm (3)

Where c is the specific heat of the chips (J/kg°C)
m——The quality of chip (kg)
Taking into account the existence of other resistances in the conductive heating cutting circuit (such as the resistance of the cable, between the carbon brush and the lead-in copper ring, between the chuck and the workpiece, the contact resistance between the tool and the lead-in copper, internal resistance of the power supply, etc.), The variation ΔRz of the cutting zone resistance Rz caused by the change in the cutting amount is negligible, so it can be considered that the heating current in the loop is basically unchanged [4]. From equation (2), we can see that when the current is basically constant and the heating time is the same, the total heating value of the cutting zone depends on the resistance Rz of the cutting zone. The larger the Rz, the greater the heat generation.
Cutting speed is the main factor that affects the cutting temperature. With the increase of cutting speed, the cutting temperature will increase significantly [2]. At the same time, because the general metal material has a positive temperature coefficient of resistance, the resistance Rz of the cutting area will also increase. As a result, the total heat generated in the cutting area increases, and at this time, the optimum heating current needs to be reduced to maintain the machining effect (see Fig. 2). Otherwise, "temperature over-compensation" will occur. When the cutting speed increases to a certain degree, the heating current becomes zero, and this becomes the ordinary cutting mode.
When the power is not applied, the depth of cut has little effect on the cutting temperature. Even if the depth of cut is significantly increased, the increase in cutting temperature is not significant [2]. However, the cutting depth has a significant effect on the cutting temperature during power machining. The greater the depth of cut, the smaller the resistance Rz of the cutting area, which is an approximately inverse relationship [4], resulting in a reduction of the total current heat generation in the cutting area; As the mass of the chips needed to be heated increases, and the length of the cutting edge that participates in the cutting improves the heat dissipation conditions [2], it is necessary to increase the optimum heating current to ensure the heating effect (see Figure 4), otherwise there will be "temperature Under compensation."
As the feed rate increases, the resistance Rz in the cutting zone decreases [4], the total heat generation in the cutting zone also decreases, and at the same time, the mass of the chip that needs to be heated increases, so the temperature in the cutting zone decreases. Although the increase in feedrate will cause the cutting temperature to increase, its influence on the cutting temperature is far less significant than the change in the cutting speed. Therefore, the average temperature rise in the cutting zone is not obvious [2]. Therefore, the heating must be increased. The current can keep the temperature of the cutting area basically unchanged, thus ensuring the heating effect (see Figure 6).

IV. Conclusion

　　 Based on the results of the experimental study, the following conclusion can be drawn: In the conductive heating cutting, in order to obtain the best surface quality, there is an optimum heating current value, which should decrease with the increase of the cutting speed, with the cutting depth and feed The increase in the amount of increase, the essence of which is to maintain the optimal cutting temperature by the change of the heating current. Under normal circumstances, when the workpiece material and tool material are determined, to determine the optimal heating current value, a large number of cutting tests must be established to establish the corresponding relationship between different cutting amounts (vc, f, ap) and the optimal heating current, and then establish the conductive heating Cutting the optimal heating current database for actual production use.

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