The influence of grinding on die life should be paid more attention
Die manufacture is a continuation of die design and a process of verifying the correctness of the design.Advanced, efficient and high precision machine tools and automatic production techniques are used in modern mold production.Grinding will take 25% to 45% of the total tooling time.China's mold industry has made great progress today, but there is still a large gap with foreign advanced level.The quality of the finished product is closely related to the precision of the mold making, especially to the precision and surface roughness of the mold cavity surface.
In actual production, factors affecting mold failure include:
Main mold structure;
Blank mold material;
The manufacturing process of cold and hot machining (forging, heat treatment, cutting, grinding, electrical machining, etc.);
Working conditions of the mold.In order to improve the die life, it is necessary to make a careful analysis of the causes of the die damage and various influencing factors, and develop the methods and measures to overcome them.
At present, there are two kinds of mould-making process routes in the world: one is to improve the precision and quality of machining and electrical machining, so as to minimize the workload of manual finishing, such as the development of high-precision machine tools and high-speed forming milling machines and their processing techniques, laying a foundation for the development of this process route.Second, the emphasis on finishing and grinding process, its processing time, and mechanical processing, edm processing time almost equal.A mould is composed of many parts, the quality of the part directly affects the quality of the mould, and the final quality of the part is guaranteed by finishing.In most domestic mold manufacturing enterprises, the finishing stage of the method is generally grinding, electrical processing and locksmith treatment.
Grinding effect on die life has not aroused people's attention, due to improper grinding process, cause the workpiece surface burning, grinding cracks, grinding trace and grinding stress, the follow-up process and die during the service of mechanical fatigue, cold and heat source of fatigue crack initiation, the serious influence the service life of mould.
1. Grinding of molds
The essence of the grinding process is the process in which the workpiece is ground onto the metal surface and deforms under the action of extrusion and friction at the instant of countless abrasive particles.The whole process of grinding is characterized by power and heat.
In the grinding process, the processed surface generates thermal expansion under the action of cutting heat. At this time, the base metal temperature is lower, so the surface generates thermal pressure stress.When the grinding is finished, the workpiece surface temperature decreases, because the surface has generated thermal plastic deformation and is restricted by the matrix, resulting in residual tensile stress on the surface and residual compressive stress on the inner layer.(TOOLOX tool steel has the characteristics of high strength, low internal stress and stable shape and size, and is easy to be ground.)
(2) grinding, grinding wheel and workpiece surface contact, grinding wheel is generated when cutting the plastic deformation and severe friction between wheel and workpiece, thus formed between grinding wheel and workpiece size is equal, in the opposite direction of grinding force, at the same time as surface material internal metal plastic deformation when the workpiece material and produce relative displacement between molecules, forming of internal friction and heat, external friction between wheel and workpiece and generate heat, this kind of grinding heat in the grinding zone can produce local instantaneous high temperature 1000 ℃, and the heat transfer of grinding wheel is not easy, so 80% of heat into artifacts and chip,And metal under solid state with the temperature change by a lattice into another lattice, the microstructure changes, when grinding of hardened steel, cooling, surface layer to produce secondary hardening, part of the retained austenite into martensite, and the martensite volume is larger, specific volume increases, the surface compressive stress, if the grinding is not good, or without coolant, surface tempering, martensitic transformation, surface tensile stress (such as gamma Fe into alpha Fe 1% volume swells,), these stress (residual stress can reach 500 ~ 1000 MPa), if exceed the material yield limit,In addition, if the mold is not tempered immediately after heat treatment and quenching, the quenching temperature is too high, and there is network carbonization. If martensitic or residual austenite is too much after tempering, it will produce phase transition and stress will cause cracks on the surface of the workpiece.Grinding crack is a kind of very fine surface crack. There are three types of grinding crack: parallel line, network crack, and octagon crack.The occurrence direction of the crack is related to the mold shape, and the development direction of the crack is related to the grinding direction of the grinding wheel on the surface of the workpiece, and its depth is within 0.03 mm.
(3) grinding, grinding wheel is not sharp, large amount of feed, and poor cooling make workpiece surface temperature of 300 ℃, cause of burn surface phenomenon.
2. Measures to reduce defects in grinding
In addition, the grinding dosage is reasonably selected, and the fine grinding method with a small amount of radial feed and even fine grinding is adopted.If the radial feed amount and grinding wheel speed are appropriately reduced, and the axial feed amount is increased, the contact area between grinding wheel and workpiece is reduced, and the heat dissipation condition is improved, so as to effectively control the increase of surface temperature.
(2) reasonable selection and dressing grinding wheel, using white corundum grinding wheel is better, it is the performance of hard and brittle, and easy to produce the new cutting edge, so the cutting force is small, the grinding heat is small, use on the granularity of medium size, such as 46 ~ 60 mesh is better, on the grinding wheel hardness by using soft and soft (ZR1, ZR2 and R1, R2), namely, coarse granularity, low hardness of the grinding wheel, good self excitation can reduce heat cutting.Of selecting the appropriate fine grinding wheel is very important, in view of the condition of the die steel with high vanadium high molybdenum, choose GD is suitable for single crystal corundum grinding wheel, when processing hard alloy, high quenching hardness of materials, the priority of using organic binder diamond grinding wheel, the organic binder grinding wheel grinding, grinding the workpiece roughness of Ra0.2 mu m, in recent years, with the application of new materials, CBN (cubic boron nitride) grinding wheel shows very good processing effect, both inside and outside the CNC forming grinder, jig grinding machine, CNC grinding machine on the finish machining, grinding wheel is better than other species.In the grinding process, we should pay attention to dressing the grinding wheel in time and keep the grinding wheel sharp. When the grinding wheel is passivated, it will rub and squeeze on the surface of the workpiece, resulting in burn on the surface of the workpiece and reduction in strength.
The proper use of cooling lubricating fluid is made to perform the three functions of cooling, washing and lubrication and keep the cooling and lubrication clean, so as to control the heat of grinding within the allowable range and prevent the thermal deformation of workpiece.Improve the cooling conditions during grinding, such as using oil sand soaking wheel or internal cooling wheel.The cutting fluid is introduced into the center of the grinding wheel. The cutting fluid can enter directly into the grinding area to play an effective cooling role and prevent the surface of the workpiece from being burned.
The quenching stress after heat treatment is reduced to the minimum, because the quenching stress and reticulated carbonized tissue under the action of grinding force, the microstructure produces phase change and easily cracks the workpiece.In order to eliminate the residual stress of grinding, the high precision mould should be treated with low temperature aging after grinding to improve the toughness.
5. Eliminate grinding stress can also will die in 260 ~ 315 ℃ of salt bath at 1.5 min, and then in 30 ℃ oil cooling, so can drop 1 HRC hardness, residual stress reduced by 40% ~ 65%.
For precision grinding of precision molds with size tolerance less than 0.01 mm, moulds should pay attention to the influence of ambient temperature and require constant temperature grinding.By calculating, the 300 mm long steel, temperature 3 ℃, the change of the material there are about 10.8 microns, (10.8 = 1.2 * 3 * 3, 1.2 mu per 100 mm deformation m / ℃), various finishing processes are required to fully consider the influence of this factor.
Electrochemical grinding is used to improve the precision and surface quality of die manufacturing.During electrolytic grinding, the grinding wheel scrapes the oxide film: instead of grinding metal, so the grinding force is small and the grinding heat is small. There will be no burr, crack or burn.In addition, the abrasion of grinding wheel is small, for example, when grinding hard alloy, the abrasion of silicon carbide grinding wheel is about 400% ~ 600% of the weight of polished hard alloy. When using electrolytic grinding, the abrasion of grinding wheel is only 50% ~ 100% of the abrasion of hard alloy.