Key Factors For Successful Milling Of Hardened Steel

In the past, when rough milling hardened steel, only very low cutting speed and feed rate, as well as large cutting depth and tool walking distance can be used. This processing method is slow and time-consuming, and it is possible to form deep stepped tool marks on the workpiece. Therefore, it is necessary to carry out multiple subsequent semi finish milling and finish milling. Another alternative is to perform rough milling on workpieces with low hardness, then perform heat treatment on them, and then re clamp the hardened workpieces on the milling machine to complete semi fine milling and fine milling through multiple clamping. Another method is electric discharge machining (EDM) for hardened steel, but this process is also very time-consuming and costly.

Now, the high-speed hard milling technology with small cutting depth and large feed is increasingly replacing these time-consuming and laborious traditional processes. The machining workshop can first drill holes and waterlines on the die blank, then carry out heat treatment, and then adopt the high-speed milling strategy to complete rough milling and fine milling through one-time clamping. Hard milling has a high metal removal rate. Because the near net shape workpiece can be obtained after rough milling, the workload of semi fine milling and fine milling can be greatly reduced. The machined surface finish of hard milling can reach 10-12rms. This process can significantly improve the production efficiency and reduce the cost of multiple clamping and repeated processing of workpieces.

However, in order to successfully apply high-speed hard milling technology, it is necessary to thoroughly understand and fully consider the key factors affecting the process.


Hardness And Machinability Of Workpiece Materials

The hardness range measured for a typical hardened steel is usually hrc48-65. However, when considering the machinability of actual machining, Rockwell hardness does not represent everything. For example, the hardness of D2 die steel is about hrc60-62, but its high chromium content (11%-13%) increases the toughness of the material, so its machinability is closer to the workpiece material with hardness of hrc62-65. For D2 die steels and similar multi-component alloy steels, the cutting parameters provided by the tool supplier and generally applicable to higher hardness materials must be used.

Maintain Constant Chip Load

In milling (especially in high-speed milling of hardened steel), the key to prolong tool life and improve part quality is to maintain the consistency of chip load borne by the cutting edge of the milling cutter. Chip load = feed rate ÷ spindle speed × Number of blades. If the chip load changes too much or inappropriately (too large or too small), the milling cutter will be worn, broken or damaged too quickly.

It is particularly difficult to maintain a constant chip load when milling the common three-dimensional profile in die production. Conventional programming methods usually adopt linear high cutting speed and large feed tool path, but when milling complex profiles, the load borne by the tool is constantly changing, and the machine tool may not be able to maintain the required chip load. For example, when the milling cutter reaches the 90 ° angle, its cutting angle will be doubled, and the cutting force will also increase. If the feed rate is not reduced, the milling cutter will be rapidly worn or damaged. In order to mill the changing die profile, the machining technician may manually reduce the feed rate through the feed overload controller, or the cam machining program and the machine tool control system may jointly reduce the feed rate to a reasonable level.

By loading the cam machining program and tool onto the machine tool and setting the z-direction height of the milling cutter at about 25.4mm above the workpiece, the machining technician can determine whether the specified feed rate can be reached. The actual feed rate can be known through one trial run. The basic principles of physics make it impossible to maintain the required feed rate and chip load at all times. A useful rule of thumb is that if the holding time of the programmed feed rate is less than 80% of the total processing time, the spindle speed must be reduced accordingly to ensure that the chip load is consistent.

Reduce Tool Runout

In milling, another important but often overlooked factor is tool runout. Generally speaking, if the runout is greater than 0.01mm (1/7 of the diameter of human hair), the tool life may be shortened by half. It is very important to reduce the tool runout as much as possible when using milling cutters with very small specifications. For some small-diameter milling cutters, a runout of 0.01mm will double the chip load acting on a single tooth, resulting in accelerated wear of the cutting edge of the milling cutter. Although some processing workshops use expensive machine tools and high-grade tools, they use low-cost tool chucks with low precision, which is an important reason for many processing problems. High precision tool chucks (including hot fitting chucks, hydraulic chucks, etc.) can basically eliminate the negative impact of tool runout.

Adopt Advanced Programming Software

The machining programming software is essential to maintain a constant chip load. Compared with the lower level programming system, the high-end cam system can use more data points to define the tool path. This kind of cam program can also control the cutting in and cutting out of the tool, so that the cutting force acting on the blade is kept at a reasonable level. Although high-end CAM software is usually more expensive, its benefits can generally exceed the higher initial purchase cost.

The function of machine tool control system also plays an important role in efficient milling. In order to effectively implement the high-speed milling strategy, the machine tool must have strong computing power to predict in advance and smoothly handle the rapid changes of machining parameters specified by the cam program. In high-speed milling, in order to track and execute complex machine motion commands, the control and servo system of the machine tool is required to process a large number of code blocks at high speed, but the old controller and server may not meet this requirement.

Manage Tool Life

For the tool life of high-speed milling, as long as the chip load, tool runout and other problems (such as machine tool rigidity) are carefully considered, unexpected results can be produced. When milling hardened steel, the correct application of milling cutter can prolong the tool life. Of course, the definition of tool life is also one of the factors to be considered. The customer’s requirements for the surface finish of the die may limit the service time of the milling cutter before replacing the tool.

High cutting temperature will have an adverse impact on the tool life. Therefore, in high-speed milling, using a small cutting depth can increase the time for the milling cutter to exit cutting, so that the cutting edge can be cooled, thus extending the tool life. When milling workpiece materials with hardness greater than HRC 48, in order to avoid thermal shock to the tool, air jet cooling or oil mist / air mist can usually be used to replace the coolant. Although in some cases, the coolant flow can wash away the chips and avoid secondary cutting, jet cooling is undoubtedly a better option because it eliminates the need for the tool to withstand rapid and severe temperature changes.

Select Appropriate Cutting Tools

Like the development trend of the whole industry, mold products have higher and higher requirements for dimensional accuracy, and these requirements are reflected in the cutting tools used to process molds and their parts. A few years ago, the typical radial dimension tolerance of a ball end milling cutter was 10 μ m. Now it is close to 5 μ M. It is difficult to achieve high fitting accuracy for parts machined by a ball end mill with low forming accuracy. In the mold manufacturing industry with strict accuracy requirements (for example, the mismatch error of liquid silicone extrusion mold is reduced to 2 μ m) It is very important to avoid errors caused by tools.

Because milling hardened materials will produce a lot of cutting heat, many carbide end mills used for hard milling use thermal barrier coating (such as AlTiN coating). In order to improve heat resistance and strength, these milling cutters usually adopt high hardness microcrystalline cemented carbide matrix (cobalt content 8%), and the cutting edge adopts negative rake angle to prevent edge collapse. In fine milling, cubic boron nitride (CBN) cutters can be used, and blade end mills are very suitable for rough milling.

Micro milling cutter can process micro features which can only be realized by EDM before. At present, milling cutters with diameters as small as 0.1mm are available, and even such a small cutter can be effectively applied to high-speed milling as long as a shorter groove length is adopted.

Comprehensively Balance Various Factors

In order to maximize the production efficiency and machining quality of milling hardened steel, we should comprehensively apply precision tools, advanced CAM software, high-performance machine tools, high-precision tool collets and take other measures (such as replacing coolant). Tool, machine tool and workpiece material suppliers are usually willing to provide their own expertise and skills to help the processing workshop achieve a real process balance and achieve its productivity goals

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