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Milling technology for hardened steel molds

There are three main types of mold processing: Soft machining, hard machining and EDM. Depending on the structure and hardness of the mold, it is decided which method or combination of them.
When the mold is large and deep, rough machining and semi-finishing before softening are used. Hard machining for finishing after quenching; Small, shallow dies can be milled at once after quenching. If the mold wall is thin and the cavity is deep, electromachining is used.

I. Selection of tool
When machining hardened molds, it is important to choose the right milling cutter. There are three basic types of milling cutters: Ball end mills, round corner flat end mills and sharp corner flat end mills. Usually, the preferred tool is a ball end mill. The large arc shape of the ball head can disperse the cutting force and cutting heat acting on the blade during high-speed machining of hardened metal, and the processed surface is closer to the desired shape.

If the mold cavity has a large and flat bottom surface, after rough machining with a ball end mill, a rounded end mill should be used. This tool has a worse dispersion of force and heat than a ball end mill. The light-angle flat-blade milling cutter is used for processing the parts that need root clearance. The root-blade end mill or round-corner flat-end milling cutter has cut out as much margin as possible before cleaning the root; The light angle of this tool is easy to chip.

The rigidity of the tool is important: In order to increase the rigidity of small-diameter milling cutters, the diameter of the tool holder is made much larger than the diameter of the tool to improve the processing finish and extend the tool life; The overhang of the tool clamping should be as short as possible.

Secondly, the shape of the shank should be adapted to the structure of the mold. Normally, a half-degree gap between the milling cutter and the side of the workpiece should be maintained. For example, the side of the workpiece is a 3 ° bevel, the shape of the tool holder is made 5/2 ° to obtain maximum rigidity. If the side of the workpiece is a 90 ° straight face, the tool holder should have a thin neck structure.

II. Reduced cutting heat

Excessive cutting heat will deform the workpiece and reduce machining accuracy. One way to reduce heat is to control the spacing between each pass.
For roughing, the cutting distance S should be equal to 25% to 40% of the milling cutter diameter. For finishing, the cutting distance can be calculated based on the given tool mark height H.

The cutting distance determines the length of time that each cutting edge participates in cutting in each revolution. In other words, the time of not participating in cutting, that is, the cooling time, determines the amount of heat accumulated in the tool. The distance between the cutting tools is large, the cutting time of the blade is more in each revolution, the cooling time is less, the heat is continuously accumulated, and the tool temperature is increased; Conversely, a small cutting distance can limit the generation and accumulation of heat. Therefore, by adjusting the cutting distance, the heat and tool temperature can be controlled, and the cutting speed can be further increased so that the cutting temperature is still lower than the maximum temperature that the coating can withstand.

On the other hand, you can also choose new coatings, so that the tool can withstand higher cutting temperatures, even higher cutting speeds. For example, the maximum workpiece temperature for TICN coating is 400 ° C (720F), while TIAIN is 800 ° C (1470F). Because of its good heat resistance, TIAIN coating is more suitable for high-speed machining of hardened molds.

Cutting speed and feed are also key factors in controlling heat. Thick chips remove more heat, leaving less heat in the remaining processed parts. If the chips are too thin, the tool's pressing friction on the workpiece can cause the workpiece to heat up. In addition, large cutting thickness sections can extend tool life and increase productivity.

III. Tool durability and clamping
The blunt tool must be replaced in time, so how to judge the wear of the tool? It can usually be observed with the naked eye: When the tool is blunt, the tip will turn red when cutting, indicating that both force and temperature have been overloaded. At first, this glowing red color only appeared at the point of the blade or where the amount of material removal was large, sometimes it was absent. You can see continuous redness of the blade when it becomes blunt. To make it easier to see this red, the machine lighting can be turned off during observation.

Correct clamping of the milling cutter is very important for the processing of the hardened mold, which involves the tolerance of the shank, the cooperation of the shank and the holder, the radial jump after installation and other factors. Proper fitting tolerances of tool holders and holders can ensure clamping rigidity, accuracy and consistency. For this reason, the manufacturing tolerance of the tool holder should be -0.0025MM to -0.005MM, and the structure should be suitable for heat shrinkable clamping; The tolerance specified in the standard is as high as -0.0125MM, which will cause excessive radial runout. In addition, the roundness of the tool holder should be maintained at least ± 0.00625MM.

The radial runout after clamping results in uneven cutting load, some cutting edge loads are large, and other cutting edge loads are small. Vibration caused by runout can cause machine vibration and tool chipping, so the runaway of the tool must be strictly controlled. Care should be taken not to polish the holder, as polishing the holder will reduce the reliability of the clamping.

IV. High-performance machine tools
In the efficient processing of hardened molds, the requirements for machine tools should not be ignored. Although hardened workpieces can also be machined on a backward, low-speed machine, they are inefficient. If the spindle speed of the machine tool is low, its feed speed is correspondingly low. Therefore, good results can only be obtained by using a rigid machine with high accuracy.

The numerical control system of the mold processing machine needs to process a large amount of data. When considering buying a new machine, pay attention to the performance of the CNC. At high feed rates, the system should have high acceleration and deceleration compensation capabilities. The processing efficiency of a machine tool is also related to factors such as the speed of data processing, the speed of response of the servo system, the speed of interpolation operations, the resolution of the feedback system, and the quality of moving parts.

V. Programming points
When machining a mold, the way the tool cuts in, that is, the force on the tool depends on the CNC programming. Therefore, programming is one of the keys to efficient machining of hardened steel. The path of the tool cutting into the mold should use spiral interpolation, so the cutting process is relatively stable. Where incisions cannot be made from the side or spiral, oblique wave incisions should be used to avoid axial incisions. Programming also determines the size and depth of the radial pass.

In short, to achieve efficient machining of hardened steel, it is necessary to rationally apply machine tools, tools, tool holders and programming techniques. Taking all factors into consideration can achieve the desired effect.
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