Talking about the Analysis of Precision Machining Control of Mould Parts
A mold consists of many mold parts. The quality of non-standard mold parts directly affects the quality of molds, and the final quality of non-standard mold parts is guaranteed by finishing. Therefore, the control of finishing is very important. Practice has proved that good finishing process control can effectively reduce the out-of-tolerance and scrap of parts, and effectively improve the one-time success rate and service life of the mold.
In most mold manufacturers in my country, the methods used in the finishing stage are generally grinding, electrical machining and fitter machining. At this stage, we should control many technical parameters such as part deformation, internal stress, shape tolerance and dimensional accuracy. In specific production practice, the operation is difficult, but there are still many effective experiences and methods worth learning from.
1. Mold finishing process control
For the processing of mold non-standard parts, the general guiding ideology is to adapt to different materials, shapes and technical requirements. It has certain plasticity and can achieve good processing effect by controlling processing.
According to the different appearance and shape of the parts, the parts can be roughly divided into three categories: shafts, plates and special-shaped parts. The common process flow is roughly: roughing – heat treatment (quenching, tempering) – fine grinding – electrical machining – fitter (surface treatment) – assembly processing.
2. Heat treatment of parts
The heat treatment process of the parts should not only obtain the required hardness of the parts, but also control the internal stress to ensure the dimensional stability of the parts during processing. Different materials have different processing methods. With the development of the mold industry in recent years, in addition to the development of the mold industry, there are more and more types of materials. Cr12, 40Cr, Cr12MoV, in addition to cemented carbide, for some convex and concave dies with high working strength and harsh force, new material powder alloy steel can be selected, such as V10, ASP23, which have high thermal stability and high thermal stability. Well organized.
For parts made of Cr12MoV, quenching is performed after rough machining. After quenching, the workpiece has a large residual stress, which is easy to cause cracking during finishing or work. After quenching, the parts should be tempered while hot to eliminate quenching stress. The quenching temperature is controlled at 900-1020°C, then cooled to 200-220°C, air-cooled, and then quickly returned to the furnace for tempering at 220°C. This method is called hardening process, which can obtain higher strength and wear resistance. The die in the form of failure has a better effect. Tempering is not enough to eliminate quenching stress, and workpieces with complex shapes are not enough to eliminate quenching stress. Before finishing, stress annealing or multiple timely treatments should be performed to fully relieve stress.
For powder alloy steel parts such as V10 and APS23, since they can withstand high temperature tempering, a secondary hardening process can be used during quenching, quenching at 1050-1080 °C, and then tempering at 490-520 °C for several times to obtain higher impact toughness and stability, it is very suitable for molds with chipping as the main failure mode. Powder alloy steel has high cost, but good performance, and is forming a wide range of application trends.
3. Grinding of parts
There are three main types of grinding: surface grinders, internal and external cylindrical grinders and tool grinders. In the process of finishing grinding, the generation of grinding deformation and grinding cracks should be strictly controlled, and even very small cracks will be revealed in subsequent processing and use. Therefore, the fine grinding knife should be small, not large, the coolant should be sufficient, and the dimensional tolerance should be within 0.01mm to grind the internal parts at a constant temperature. For a 300mm long steel piece, when the temperature difference is 3°C, the material changes by about 10.8μm, 10.8=1.2×3×3 (deformation per 100mm is 1.2μm/°C). The influence of this factor should be fully considered for each finishing process.
It is very important to choose the right grinding wheel when fine grinding. According to the high vanadium and molybdenum conditions of die steel, it is more suitable to choose GD single crystal corundum grinding wheel. When processing hard alloy and high quenching hardness materials, organic binder diamond grinding wheel is preferred. The organic binder grinding wheel has good self-grinding ability. The roughness of the grinding workpiece is Ra=0.2μm. In recent years, with the application of new materials, the CBN grinding wheel, that is, the cubic boron nitride grinding wheel, has a better finishing effect than other types of grinding wheels in CNC forming grinding, coordinate grinding machines, and CNC internal and external cylindrical grinding machines. During the grinding process, attention should be paid to repair the grinding wheel in time to keep the grinding wheel sharp. When the grinding wheel is passivated, it will slide and squeeze on the surface of the workpiece, causing burns on the surface of the workpiece and reducing the strength.
The processing of plate parts is mostly processed by a surface grinder. In the process of processing, long and thin plate parts are often encountered, which are difficult to process. Because in the process of processing, under the action of magnetic adsorption, the workpiece is deformed and close to the surface of the worktable. When the workpiece is removed, the workpiece will be deformed back, and the thickness measurement is consistent, but the parallelism cannot meet the requirements. The solution can be magnetic grinding. Method, the grinding block is placed under the workpiece, and the four-sided block is dead. When processing small knives and multi-blade knives, the processing does not require the block and other blocks. Direct adsorption processing can improve the grinding effect and meet the parallel requirements.
The shaft parts have rotary surfaces, which are widely used in internal and external cylindrical grinders and tool grinders. In the process of processing, the headstock and the top are equivalent to the busbar. If there is a runout problem, the machining workpiece will also have this problem, affecting the quality of the part. Therefore, the inspection of the headstock and the top should be done before processing. When grinding the inner hole, the coolant should be fully poured to the grinding contact position so that the grinding can be discharged smoothly. It is best to use a clamping process table for the processing of thin-walled shaft parts, and the clamping force should not be too large, otherwise the inner triangle deformation will easily occur around the workpiece.