Investigation On Key Technology And Theory Of Air-cooled Enhanced Heat Transfer In Narrow-deep-groove Grinding

Narrow-deep-groove is a unique part structure,which has the structural characteristics that the ratio of groove depth to groove width is greater than 2 and the width of the groove is generally less than 4 mm.The super-hard abrasive grinding wheel creep feed grinding narrowdeep-groove process solves the problems of low machining accuracy,high machining cost,severe wheel wear and low yield of traditional machining methods.However,the workpiece obstructs the narrow-deep-groove grinding area,so it is difficult for the externally supplied grinding fluid to reach the grinding zone.It is easy to cause burns if the cooling is not in the grinding zone at the right time.The material removal mechanism of each grinding zone is different.The traditional grinding theory gets challenging to explain this problem,limiting the popularization and application of the creep feed grinding technology in narrow-deep-groove grinding.Therefore,this paper develops the technology and theory of material removal and enhanced cooling for narrow-deep-groove creep feed grinding,studies the wear characteristics of the grinding wheel during narrow-deep-groove grinding,designs an air-cooled grinding wheel,and analyzes the airflow field characteristics of the grinding wheel.The material removal mechanism of the narrow-deep-groove grinding zone was studied,the temperature model of the narrow-deep-groove grinding zone based on the air-cooled enhanced heat transfer was established,and the surface integrity of the narrow-deep-groove grinding was studied.The main research work of this paper is as follows:(1)A single-layer c BN grinding wheel for narrow-deep-groove grinding was prepared by electroplating,the morphology of the grinding wheel was analyzed,and the wear characteristics of the grinding wheel during narrow-deep-grooves grinding were studied.The percentage concentration of abrasive grains on the surface of the grinding wheel is reasonable,the lateral distribution of abrasive grains is uniform,and the longitudinal distribution difference of abrasive grains on the grinding wheel is small according to the study.The stable wear stage of the grinding wheel accounts for 84.6% of the effective life cycle.The abrasive grains in the transition edge area are severely fractured and worn due to the alternating load.At the inner edge of the abrasive grain belt,the abrasive grains fall off causing the grinding force of the surrounding abrasive grains to increase.The high concentration of abrasive particles falling off in this area is obtained.The wear forms of the top edge area and the middle of the side edge area of the grinding wheel are micro-cracks,attrition wear and abrasive particles fall off.The main wear forms of c BN abrasive particles are attrition wear and cleavage fracture.The abrasive particles with a large protrusion height and a large wear platform area have a larger grinding force,which induces easy cleavage cracks on the surface of the abrasive wear platform or on the side of the abrasive particles.Cracks expand in the grinding process to produce cleavage fractures.The grinding wheel bonding layer wear forms include bonding surface scratches,joint surface breakage,displacement of grit,and cracks of bonding layer.The retaining strength of abrasive particles during grinding is reduced due to grinding wheel coating deterioration.The transition layer created in the grinding wheel’s bonding layer lowers the bonding layer strength between the coating and the substrate,causing portion of the coating to peel off or deform.The grinding wheel’s local burn begins at the top edge,where the workpiece material is stuck to and blocked.(2)The difficulty of cooling in grinding narrow-deep-groove induces burn,and the reason for the burn is deeply analyzed.An air-cooled grinding wheel is designed.The air-cooled grinding wheel’s internal flow field and external airflow field characteristics tests were conducted.The smoke wire flow visualization test results show that the ambient air enters the inner flow channel from the air inlet of the grinding wheel and finally exits from the air outlet in the radial direction.The design concept of the air-cooled grinding wheel is reasonable.The axial airflow field of the air-cooled grinding wheel is symmetrically distributed along with the thickness of the grinding wheel.Compared with a normal grinding wheel,the airflow velocity at the air outlet of the air-cooled grinding wheel increases by 35.2%.The ambient air is efficiently transported into the airflow channel of the grinding wheel as the opening of the wind guide wheel.The airflow velocity and pressure at the air outlet of the air-cooled grinding wheel gradually increase,when the grinding wheel speed increase.(3)The material removal mechanism of grinding narrow-deep-grooves is studied,and the grinding force models of each grinding zone of narrow-deep-grooves based on the single abrasive grain grinding force are established.The change law of contact arc length and groove side contact area are studied.Based on grinding zone partitions,a total grinding force model for narrow-deep-grooves is established.On the narrow-deep-groove section,the cutting depth of the abrasive grains in the top edge grinding area is equal,and the cutting depth of the abrasive grains in the transition edge grinding area gradually decreases along the direction closing to the side edge area.The grains only slide on the side surface grinding area or cut the higher raised part of the groove marks.The difference in the cutting depth of the abrasive grains in different grinding areas is the root cause of the surface gradient transition topography.The material removal rate increases with the increase of feed rate and narrow-deep-groove depth.The material removal rate is more sensitive to the change of the workpiece feed rate,which is the main factor affecting the material removal rate.The numerical calculation and the experimental results of grinding force show that the grinding force model can accurately predict the changing trend of the grinding force during creep feed grinding of narrow-deep-grooves,and the error between the calculated value and the experimental value of the grinding force is less than 10 %.(4)Different grinding areas of narrow-deep-grooves produce different intensities of grinding heat flux density.The top edge grinding area has the most significant heat flux density.The side edge grinding area has the most low heat flux density.Based on the experimental results of the grinding wheel’s airflow velocity,the airflow velocity of the air outlet of the aircooled grinding wheel located in the grinding area is studied.The air-cooled enhanced convection heat transfer model in the narrow-deep-groove grinding area is established.The theoretical formula of the grinding heat partition ratio introduced into the workpiece under the air-cooled condition is deduced,and the grinding temperature field models of different grinding areas of the narrow-deep-groove are constructed.By comparing the finite element simulation results with the grinding test results,it is found that the calculated value of the maximum temperature in the narrow-deep-groove grinding zone is in good agreement with the experimental value.Under the coupling effect of multiple heat sources,the grinding temperature of the side edge area of the narrow-deep-groove is the highest,the grinding temperature of the transition edge grinding area is second,and the grinding temperature of the top edge grinding area is lowest.(5)The impact of narrow-deep-groove grinding process parameters on surface integrity is investigated.Narrow-deep-groove grinding surfaces have a gradient transition surface morphology.The groove marks on the grinding surface progressively transition from deep and sparse to shallow and dense from the groove bottom surface to the transition fillet surface and then to the groove side surface.The surface roughness value is also decreased.A significant plastic deformation layer forms on the bottom surface of the narrow-deep-groove,and the crystalline grains are elongated in the grinding direction.On the transition fillet surface,the depth of the plastic deformation layer gradually decreases,and the crystalline grains on the side surface of narrow-deep-groove are only minimally distorted.The narrow-deep-groove’s work hardening layer is primarily spread on the groove bottom surface and the transition fillet surface.The microhardness of the the side surface of narrow-deep-groove is approximately equivalent to the initial state hardness of the workpiece material.The abrasive grains generate residual compressive stress on the grinding surface of the narrow-deep-groove,and the residual compressive stress on the bottom surface of the groove is higher than that on the side of the groove.