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Abstract : By deeply analyzing the mechanism of grinding burn during slow grinding, this paper puts forward a new idea of ​​high-pressure jet impact strengthening of heat transfer in grinding arc region, and shows the heat transfer effect through grinding experiment of intermittent slow grinding jet lateral impact arc region.
Keywords: intermittent grinding enhanced heat transfer jet impact
Abstract: This paper provides a new method of enhancing heat transfer of grinding contact zone through jet impinging on the basis of the analysis on the mechanism of burn during the creep feed grinding, and shows the effect of heat transfer through grinding experiment of jet flank impinging on Grinding contact zone during intermittent creep feed grinding.
Keywords: intermittent creep feed grinding enhancing heat transfer jet impinging
In recent years, intermittent grinding has attracted people's attention due to its unique cooling effect and obvious advantages of comprehensive process. Due to the intermittent cooling effect, the accumulation of grinding heat is significantly reduced, so that the thermal damage of the surface layer of the workpiece is effectively avoided, and the grinding of difficult-to-machine materials is more advantageous. Combining the advantages of intermittent grinding and cooling, the wear characteristics of CBN grinding wheels and the low-temperature characteristics of slow grinding, the intermittent grinding process of grinding wheels shows great potential in solving burn-in burns.
The study on the mechanism of slow-burning burns has revealed that when the grinding heat flux exceeds the critical value, film boiling occurs in the arc region, and the grinding fluid will no longer be able to contact the surface of the workpiece due to the film barrier, so it can be vaporized by the grinding liquid. The large amount of grinding heat taken away was forced to divert into the workpiece, causing the workpiece surface to rapidly increase in temperature and burn quickly. The problem of workpiece burn caused by the high heat flux density in the face of high heat flux density has been solved effectively in the thermal engineering field through the use of enhanced heat transfer technology. Therefore, according to the idea of ​​enhanced heat exchange, combined with intermittent grinding process of grooved grinding wheel, a grinding fluid filling method is proposed that uses a high-pressure jet to directly impact the surface of the workpiece in the arc region, thereby breaking the formed gas film and making the arc. The zone can still exert the advantage of nuclear boiling vaporization heat transfer under high heat flux density, and the heat transfer efficiency is raised to a new level. The experimental study on the grinding performance by using the lateral jet impinging on the surface of the arc zone during the intermittent slow grinding shows that the jet impact enhanced arc zone heat transfer technology has broad application prospects.
1 Experimental conditions and test system
The grooved grinding wheel is easier to introduce the grinding fluid into the arc zone than the whole grinding wheel, but it is difficult to achieve the direct impact of the jetting part on the arc zone work. Therefore, this paper uses the lateral jet direct impact method shown in Fig. 1. Due to the use of a high-pressure plunger pump and a special experimental rectangular nozzle, the jet exit velocity can reach 80m/s, while the width of the grinding specimen is less than the width of the grinding wheel. The specific experimental conditions are shown in the table below. The trials used the titanium alloy TC4 with low thermal conductivity and easily burned aerospace materials to perform grinding performance experiments. The effects of jet impact on heat transfer in arc region during intermittent grinding were studied.
Fig. 1 Jet Lateral Impact
Experimental condition table
Grinding Machine MMD7125 Precision Plane Feeder Grinding Machine Grinding Wheel Plating Grooving CBN Grinding Wheel, Particle Size 80, Concentration 200, Number of Slots 177 Grinding Fluid 5% Emulsion, Coolant Flow 90L/min, Maximum Supply Pressure 7MPa Trimming Mode Dual Electrodes Electrolytic trimming In order to test the temperature change in the arc zone in real time, the semi-artificial thermocouple was used to measure the temperature in the experiment. The signal was directly recorded by the 3562 signal analyzer, and the spindle power of the grinder was recorded by an XY recorder.2 Experimental results and analysis
2.1 Comparison of Ordinary Tangential and Jet Lateral Jet Cooling Effects in Arc Zones
Fig. 2 shows the effect of tangential feed and lateral jet impact on the grinding temperature at the same wheel speed Vs, table feed speed VW, and different depths ap. Fig. 3 is the curve of grinding temperature change captured by the signal analyzer under the same grinding conditions, under the conditions of ordinary liquid supply method and side jet impact heat exchange. Cooling features.
Fig. 2 Comparison of grinding temperature of two cooling modes
Figure 3 Measured grinding temperature wave
Figure 4 shows the influence of tangential feed and lateral jet impact on grinding power at the same grinding wheel speed, table feed rate, and different cutting depths. It can be seen that the lateral injection is lower than the corresponding tangential liquid supply under the condition that the grinding power is not much different. Since the grinding heat flux q is proportional to the grinding power N
In the formula, J-heat work equivalent;
R- grinding heat into the workpiece ratio;
L-contact arc length;
B-workpiece width.
Fig. 4 Comparison of grinding power in two cooling modes
In other words, under the condition that the grinding heat flux density is similar, the corresponding low temperature of the lateral jet impact and the heat exchange efficiency are improved.
2.2 Experimental comparison of heat transfer between lateral high-speed jets and low-speed jet arcs
Fig. 5 shows the effect of the high-speed lateral jet and low-velocity jet impact on the grinding temperature under the same grinding wheel speed, table feed speed, and different cutting depths. From the diagram, high-speed jet impact has a significant cooling advantage, and no burns occur even under large depths of cut.
Fig. 5 Comparison of high and low speed jet grinding temperatures
3 Conclusion
Experiments show that jet impact enhanced heat transfer technology is an effective method to improve heat transfer efficiency in arc region. The higher the jet velocity, the better the heat transfer effect. It can break through the obstacles of film-forming boiling. Even at high heat flux, the surface temperature of the workpiece can maintain the critical temperature for film formation and boiling of the grinding fluid to be below 120-130°C (water-based grinding fluid), indicating deep cutting. The material removal rate without burns at the time of gentle grinding increases synchronously. However, due to the limitation of experimental conditions, the lateral jet impinging on the workpiece surface in the arc region is far less efficient than the jet impinging on the surface of the workpiece along the radial direction of the grinding wheel. Therefore, the research and development can be applied in production. The new grinding liquid supply device for the radial high-pressure jet impact heat transfer in the arc region is of great significance to effectively avoid workpiece burn and improve the grinding efficiency.
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