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 finishing, so it is important to control the finishing. In most of the domestic die manufacturing enterprises, the method used in the finishing stage is generally grinding, electrical processing and pliers processing. At this stage, it is necessary to control the deformation, internal stress, shape tolerance and dimensional accuracy of parts and many other technical parameters. In the specific production practice, the operation is more difficult, but there are still many effective experience methods worth learning.

The manufacturing of precision molds, finishing things...

Die parts processing, according to the appearance and shape of parts, parts can be roughly divided into three categories: plate, special-shaped parts and shaft, the common process is roughly: rough processing - heat treatment (quenching, tempering) - fine grinding - electrical processing - pliers (surface treatment) - assembly processing.

1. Heat treatment of parts

The heat treatment process of the parts, while making the parts obtain the required hardness, also need to control the internal stress, to ensure the stability of the size of the parts processing, different materials have different treatment methods. With the development of die industry in recent years, the types of materials used have increased. In addition to Cr12, 40Cr, Cr12MoV, hard alloy, for some convex and concave dies with high work intensity and harsh force, new materials can be selected such as powder alloy steel, such as V10, ASP23, etc., this kind of material has high thermal stability and good organization state.

The manufacturing of precision molds, finishing things...

For the parts made of Cr12MoV, quenching treatment is carried out after rough machining. There is a large residual stress in the workpiece after quenching, which is easy to lead to cracking during finishing or working. After quenching, the parts should be tempered while hot to eliminate the quenching stress. The quenching temperature is controlled at 900-1020℃, and then cooled to 200-220℃ for air cooling, and then quickly returned to the furnace for tempering at 220℃. This method is called single hardening process, which can obtain higher strength and wear resistance, and has a better effect for the mold whose main failure form is wear. For some workpiece with many corners and complex shapes encountered in production, tempering is not enough to eliminate the quenching stress, and stress relief annealing or multiple aging treatments should be carried out 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 high temperature tempering at 490-520℃ for many times, can obtain higher impact toughness and stability, it is very suitable for the die whose main failure form is breakage blade. Powder alloy steel has a high cost, but its performance is good, and it is forming a wide application trend.

2. Grinding of parts

There are three main types of machine tools for grinding: surface grinding machine, internal and external grinding machine and tool grinding tools. When finishing grinding, the generation of grinding deformation and grinding crack should be strictly controlled. Even if the crack is very small, it will be revealed in the subsequent processing. Therefore, the feed of fine grinding should be small, not large, the coolant should be sufficient, and the parts with size tolerance within 0.01mm should be grinding at constant temperature as far as possible. According to the calculation, when the temperature difference of 300mm steel is 3℃, the material has a change of about 10.8μm, 10.8=1.2×3×3(deformation per 100mm 1.2μm/℃), each finishing process should fully consider the influence of this factor.

The manufacturing of precision molds, finishing things...

It is very important to select appropriate grinding wheel during fine grinding. GD single crystal corundum grinding wheel is more suitable for the condition of high vanadium and high molybdenum of die steel. When processing hard alloy and materials with high quenching hardness, diamond grinding wheel with organic binder is preferred. With the application of new materials, CBN grinding wheel, also known as cubic borax nitride wheel shows very good processing effect, finishing on CNC forming grinding, coordinate grinding machine, CNC internal and external grinding machine, the effect is better than other kinds of grinding wheel. During grinding, pay attention to dressing the grinding wheel in time to keep the sharpness of the grinding wheel. When the grinding wheel is passivated, it will slip and squeeze on the workpiece surface, resulting in burns on the workpiece surface and reduced strength.

Most of the processing of plate parts are processed by surface grinding machine. In the process of processing, we often encounter a long and thin sheet parts, which is difficult to process. Because of the processing, under the adsorption of magnetic force, the workpiece deformation, close to the surface of the table, when the workpiece, the workpiece will produce recovery deformation, thickness measurement is consistent, but the parallelism can not meet the requirements, the solution can be used in magnetic isolation grinding method, grinding to the height of the block pad under the workpiece, four blocks dead, processing small feed, multi-light knife, after processing a good side, The grinding effect can be improved and the parallelism can be achieved by direct adsorption processing without pad of equal height.

Shaft parts with rotary surface, its processing is widely used internal and external cylindrical grinder and tool grinder. In the process of processing, the head frame and the top is equivalent to the bus, if there is a pulsation problem, the workpiece processed will also produce this problem, affecting the quality of the parts, so before processing to do a good job of the head frame and the top detection work. When grinding the inner hole, the coolant should be poured fully to the grinding contact position to facilitate the smooth discharge of grinding. Processing thin-wall shaft parts, it is best to use the clamping process table, clamping force can not be too large, otherwise it is easy to produce "inner triangle" deformation on the circumference of the workpiece.

3. Electric machining control

Modern mold factory, can not lack of electrical processing, electrical processing can be processed for all kinds of special-shaped, high hardness parts, it is divided into two kinds of wire cutting and electric discharge.

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

The manufacturing of precision molds, finishing things...

When processing punch, the choice of cutting position and path should be carefully considered. The best effect can be achieved by drilling and threading. High precision wire cutting machining, usually cut four times, can ensure the quality of parts. When processing the concave die with taper, in line with the fast and efficient position, the first rough machining straight edge, the second taper processing, and then finish the straight edge, so there is no need for X vertical finishing, only finish the straight edge of the cutting edge section, which saves time and cost.

Electric discharge machining first to make electrodes, electrodes have coarse, fine points. Finishing electrode requirement shape conformity is good, it is best to use CNC machine tool processing. Electrode material selection, copper electrode is mainly used for general steel processing. Cu-W alloy electrode, good overall performance, especially in the process of processing consumption is significantly smaller than copper, with sufficient amount of scouring liquid, very suitable for difficult material processing and the section shape of complex parts finishing. When making electrodes, it is necessary to calculate the amount of electrode gap and the number of electrodes. When large area or heavy electrode processing is carried out, the workpiece and electrode clamping should be firm to ensure sufficient strength and prevent loose processing. During deep step machining, attention should be paid to electrode loss and arc discharge caused by poor discharge of liquid.

4. Surface treatment and assembly

The surface of the parts left in the processing of the knife mark, wear mark is the place where the stress is concentrated, is the source of crack growth, so at the end of the processing, the surface of the parts need to strengthen, through the pliers grinding, processing hidden dangers. Some of the workpiece edge, acute Angle, hole blunt, R. Generally, the electromachining surface will produce about 6-10μm metamorphic hardening layer, the color is grayish white, hardening layer brittle and with residual stress, before use to fully eliminate the hardening layer, the method is surface polishing, grinding to remove the hardening layer.

The manufacturing of precision molds, finishing things...

In the process of grinding and electric processing, the workpiece will have a certain magnetization and weak magnetic force, which is very easy to absorb some small things. Therefore, before assembly, the workpiece should be demagnetized and the surface should be cleaned with ethyl acetate. In the assembly process, first look at the assembly drawing, find all the parts, and then list the order of the equipment between the parts, list the matters needing attention, and then proceed to assemble the mold, assembly is generally installed first guide column guide sleeve, and then install the mold frame and the mold, and then the gap, especially the gap between the mold for adjustment, after the completion of the assembly to implement the mold detection, Write the overall situation report. For the problems found, we can use reverse thinking method, that is, from the post-process to the forward process, from finishing to rough processing, check one by one, until we find out the crux and solve the problem.

Practice has proved that good finishing process control can effectively reduce parts out-of-tolerance, scrap, and effectively improve the success rate and service life of the die.