precision machining

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precision machining

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How much do you know about the finishing process?

A mold is composed of many parts. The quality of the parts directly affects the quality of the mold, and the final quality of the parts is guaranteed by the finishing. Therefore, it is important to control the finishing. In most domestic mold manufacturin

The processing of mold parts can be roughly divided into three categories according to the appearance and shape of the parts: plates, special-shaped parts and shafts. The common process is roughly: rough machining-heat treatment (quenching, quenching and tempering)- Fine grinding-electrical machining-fitter (surface treatment)-assembly processing.

1.Heat treatment of parts

In the heat treatment process of the parts, while obtaining the required hardness of the parts, it is also necessary to control the internal stress to ensure the dimensional stability of the parts during processing. Different materials have different treatment methods. With the development of the mold industry in recent years, the types of materials used have increased. In addition to Cr12, 40Cr, Cr12MoV, and cemented carbide, for some convex and concave molds with high working strength and severe stress, new materials powder alloy steel can be used. , Such as V10, ASP23, etc., this kind of material has high thermal stability and good organization state.

For parts made of Cr12MoV, quenching is performed after rough machining. After quenching, the workpiece has a large residual stress, which is likely to cause cracks 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℃, then cooled to 200-220℃, air-cooled, and then quickly returned to the furnace for tempering at 220℃. This method is called a primary hardening process, which can obtain higher strength and wear resistance. The mold, which is the main failure mode, works better. In production, some workpieces with many corners and complex shapes are encountered, and tempering is not enough to eliminate the quenching stress. Stress relief annealing or multiple aging treatments are required before finishing to fully release the stress.

For V10, APS23 and other powder alloy steel parts, because they can withstand high temperature tempering, the secondary hardening process can be used during quenching, quenching at 1050-1080℃, and then tempering at 490-520℃ for many times. With high impact toughness and stability, it is very suitable for molds with chipping as the main failure mode. The cost of powder alloy steel is relatively high, but its performance is good, and it is forming a trend of widespread use.

2.Grinding of parts

There are three main types of machine tools used in grinding: surface grinders, internal and external cylindrical grinders, and tool abrasives. During finishing grinding, grinding deformation and grinding cracks must be strictly controlled. Even very small cracks will be exposed in the subsequent processing and use. Therefore, the feed of the fine grinding should be small, not large, the coolant should be sufficient, and the parts with the size tolerance within 0.01mm should be ground as much as possible at a constant temperature. From the calculation, it can be seen that for a 300mm long steel, when the temperature difference is 3℃, the material will have a change of about 10.8μm, 10.8=1.2×3×3 (1.2μm/℃ per 100mm deformation), and each finishing process needs to fully consider this The influence of one factor.

It is very important to choose an appropriate grinding wheel during fine grinding. For the high vanadium and high molybdenum condition of the mold steel, it is more suitable to use GD single crystal corundum grinding wheel. When processing hard alloys and materials with high quenching hardness, organic bonding is preferred. The diamond grinding wheel and the organic binder grinding wheel have good self-grinding properties, and the roughness of the ground workpiece can reach Ra=0.2μm. In recent years, with the application of new materials, the CBN grinding wheel, that is, cubic boron nitride grinding wheel, has shown Very good processing effect, finishing on CNC form grinding, coordinate grinder, CNC internal and external cylindrical grinder, the effect is better than other types of grinding wheels. During the grinding process, attention should be paid to dressing the grinding wheel in time to keep the grinding wheel sharp. When the grinding wheel is passivated, it will rub and squeeze on the surface of the workpiece, causing burns on the surface of the workpiece and reducing its strength.

Most of the plate parts are processed by surface grinders. In the process, a long and thin sheet part is often encountered, which is difficult to process. Because during processing, under the action of magnetic force, the workpiece deforms and sticks to the surface of the worktable. When the workpiece is removed, the workpiece will return to deformation. The thickness measurement is consistent, but the parallelism cannot meet the requirements. Solution Magnetic isolation grinding method can be used. Contour blocks are used to pad under the workpiece during grinding. Blocks on all sides are used to resist the death. During processing, small feeds and multiple smooth knives can be used. After one side is processed, the contour blocks can be directly absorbed and processed. , This can improve the grinding effect and meet the requirements of parallelism.

Shaft parts have revolving surfaces, and internal and external cylindrical grinders and tool grinders are widely used for processing. During the processing, the headstock and the center are equivalent to the bus bar. If there is a runout problem, the processed workpiece will also have this problem, which will affect the quality of the parts. Therefore, the headstock and the center must be inspected before processing. When grinding the inner hole, the coolant should be poured to the grinding contact position to facilitate the smooth discharge of the grinding. For processing thin-walled shaft parts, it is best to use a clamping technology table, and the clamping force should not be too large, otherwise it is easy to produce "inner triangle" deformation on the circumference of the workpiece.

3.Electrical machining control

Modern mold factories cannot lack electric machining. Electric machining can process all kinds of special-shaped and high-hardness parts. It is divided into two types: wire cutting and electric spark.

The machining accuracy of slow-moving wire cutting can reach ±0.003mm, and the roughness is Ra0.2μm. When the processing starts, check the condition of the machine tool, check the deionization degree of the water, the water temperature, the perpendicularity of the silk, the tension and other factors to ensure a good processing state. Wire-cutting processing is to remove processing on a whole piece of material, it destroys the original stress balance of the workpiece, it is easy to cause stress concentration, especially at the corners, so when R<0.2 (especially sharp corners), you should The design department makes suggestions for improvement. The method to deal with stress concentration in processing can use the principle of vector translation. Before finishing, leave a margin of about 1mm, pre-process the rough shape, and then perform heat treatment to release the processing stress before finishing to ensure thermal stability.

When processing the punch, the cutting position of the wire and the selection of the path should be carefully considered. The best effect is achieved by perforating and threading. High-precision wire-cutting processing, usually the number of cutting passes is four, which can guarantee the quality of the parts. When processing taper dies, based on a fast and efficient standpoint, the first round of rough machining of straight edges, the second pass of taper machining, and then the finishing of straight edges, so that X-segment vertical finishing is not required, only Finishing the straight edge of the cutting edge section saves time and cost.

EDM first needs to make electrodes, which can be coarse and fine. The precision machining electrode requires good shape conformity, and it is best to be processed by CNC machine tools. In terms of electrode material selection, copper electrodes are mainly used for general steel processing. The Cu-W alloy electrode has good overall performance, especially the consumption during processing is obviously smaller than that of copper. With a sufficient amount of scouring fluid, it is very suitable for the processing of difficult-to-process materials and the finishing of parts with complex cross-sectional shapes. When making electrodes, it is necessary to calculate the amount of electrode gap and the number of electrodes. When processing large areas or heavy electrodes, the workpiece and electrode should be clamped firmly to ensure sufficient strength to prevent processing looseness. When performing deep step processing, pay attention to the wear of the electrode and the arc discharge caused by poor drainage.

4.Surface treatment and assembly

The tool marks and wear marks left on the surface of the part are the place where the stress is concentrated and the source of the crack propagation. Therefore, after the processing is completed, the surface of the part needs to be strengthened and polished by a fitter to deal with hidden processing hazards. Some edges, acute angles, and orifices of the workpiece are dulled and rounded. Generally, the electro-processed surface will produce a metamorphic hardened layer of about 6-10μm. The color is off-white, the hardened layer is brittle and contains residual stress. The hardened layer must be fully eliminated before use. The method is surface polishing and grinding to remove the hardened layer.

In the process of grinding and electrical processing, the workpiece will be magnetized to a certain extent, with weak magnetic force, and it is easy to attract some small things. Therefore, before assembly, the workpiece should be demagnetized and the surface should be cleaned with ethyl acetate. During the assembly process, first refer to the assembly drawing, find the parts, and then list the equipment sequence between the parts, list the items that should be noted, and then proceed to assemble the mold. The assembly generally first installs the guide pin and guide sleeve, and then installs the mold Frames and convex and concave molds, and then adjust the various gaps, especially the gap between the convex and concave molds. After the assembly is completed, the mold inspection should be carried out and the overall situation report should be written. For the problems found, the reverse thinking method can be used, that is, from the back process to the previous process, from finishing to roughing, checking one by one until the crux is found and the problem is solved.

Practice has proved that good precision machining process control can effectively reduce the over-tolerance and scrapping of parts, and effectively improve the first-time success rate and service life of the mold.

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