Ways to Improve the Cutting Efficiency of CNC Machining Centers

December 14, 2022

As a representative of the advanced productive forces of modern manufacturing, CNC machining plays an extremely important role in the industries of machinery, aerospace, and moulds. Since the 1990s, countries in Europe, the United States, and Japan have been competing to develop and apply a new generation of high-speed CNC machine tools, accelerating the pace of development of high-speed machine tools. High-speed spindle unit motor spindle speed 15000 ~ 100000r/min, high-speed and high acceleration / deceleration of the moving parts of the rapid transfer speed 60 ~ 120m/min, cutting feed speed up to 60m/min, high-speed machining center feed rate Up to 80m/min, air speed up to 100m/min. The HyperMach machine tool feed rate of CINCINNATI, USA is up to 60m/min, the speed is 100m/min, and the spindle speed has reached 60,000r/min. In terms of machining accuracy, in the past 10 years, the machining accuracy of ordinary CNC machine tools has increased from 10μm to 5μm, precision machining centers have increased from 3 to 5μm to 1 to 1.5μm, and ultra-precision machining precision has begun to enter the nanometer level ( 0.01 μm). The development and application of a new generation of high-speed CNC machine tools, especially high-speed machining centers, are closely related to ultra-high-speed cutting.

1. Differences in cutting level between machining centers at home and abroad

At present, the cutting speeds of cutting and milling in advanced countries have reached 5000-8000m/min or more; the number of spindle rotations of machine tools is more than 30,000r/min (some as high as 100,000r/min). For example, in the milling plane, the cutting speed in foreign countries is generally greater than 1000 to 2000 m/min, while the domestic equivalent is only 1/12 to 1/15 of the foreign country, that is, domestic dry 12 to 15 hours of activity is equivalent to 1 foreign dry. hour. According to the survey, the actual cutting time of many machining centers is less than 55% of the working time. Therefore, how to improve the processing efficiency and reduce the scrap rate has become a common issue for many companies. A survey of the cutting efficiency of CNC machining centers in China found that there are many problems such as low tool accuracy, large amount of blade run-out, low processing finish, and unmatched process equipment.

2. Ways to improve cutting efficiency

(1) Reasonable choice of cutting amount

New cutting technologies such as dry cutting and hard cutting represented by high-speed cutting have shown many advantages and strong vitality, and have become the main way for manufacturing technology to improve processing efficiency and quality and reduce costs.

Practice has proved that when the cutting speed is increased by 10 times and the feed speed is increased by 20 times, far beyond the traditional cutting “forbidden zone”, the cutting mechanism has undergone a fundamental change. As a result, the metal removal rate per unit power is increased by 30% to 40%, the cutting force is reduced by 30%, the cutting life of the tool is increased by 70%, and the cutting heat remaining on the workpiece is greatly reduced, and the cutting vibration is almost eliminated. The cutting process took an essential leap forward. According to the current situation of machine tools, in order to give full play to the high-speed machining capability of advanced tools, high-speed machining is required to increase the volume of material removed per unit time (material removal rate Q).

While selecting a reasonable amount of cutting, try to choose the dense cutter (the number of cutter teeth per inch diameter ≥ 3), increase the feed per tooth, improve productivity and tool life. Relevant experimental studies have shown that when the line speed is 165m/min and the feed per tooth is 0.04mm, the feed speed is 341m/min and the tool life is 30 pieces. If the cutting speed is increased to 350 m/min, the feed per tooth is 0.18 mm, and the feed rate is 2785 m/min, which is 817% of the original machining efficiency, and the tool life is increased to 117 pieces.

(2) Select a tool material with good performance

In the cutting process of CNC machine tools, the role of metal cutting tools is no less than steam invented by Watt. The materials used to make the tool must have high hardness and wear resistance at high temperatures, necessary bending strength, impact toughness and chemical inertness, good processability (cutting, forging, heat treatment, etc.), and are not easily deformed. At present, domestic and foreign tool materials with good performance include: cermets, hard alloy coated tools, ceramic tools, polycrystalline diamond (PCD) and cubic boron nitride (CBN) tools. They have their own characteristics, and they adapt to different workpiece materials and cutting speeds. CBN is suitable for cutting high hardness hardened steels and hard cast irons. For example, ceramic cutting tools and CBN cutting tools are used for machining high hardness steels (50 to 67HRC) and chilled cast iron. Among them, workpieces with a hardness of 60 to 65HRC or less can be used for ceramic cutting tools. , And 65HRC above the workpiece is used CBN cutting tool; PCD is suitable for cutting non-ferrous metals, and alloys, plastics and fiberglass, etc., when processing aluminum alloy parts, the main use of PCD and diamond film coating tools; carbon tools Steel and alloy tool steels are now used only for tools such as boring tools, dies, and taps; hard alloy coated tools (such as coated TiN, TiC, TiCN, TiAIN, etc.) have a high hardness and a wide range of workable workpieces. The anti-oxidation temperature is generally not high, so the improvement of cutting speed is also limited, generally in the range of 400 ~ 500m/min processing of steel parts, and Al2O3 coating high temperature hardness, processing in the high-speed range, its wear It is better than TiC and TiN coatings.

In addition, the geometric parameters of the cutting part of the cutting tool have a great influence on the cutting efficiency and the machining quality. In the high-speed cutting, the rake angle of the tool is generally 10° smaller than that of the ordinary cutting, and the back angle is 5°-8°. In order to prevent thermal wear at the tool tip, the tip of the main and auxiliary cutting edges should be used with a round tip or a chamfered tip to increase the local tip angle and increase the length of the cutting edge near the tip and the tool material volume. Improve tool rigidity and reduce tool breakage.

(3) Accelerate the development of coating technology

Since its inception, tool coating technology has played an important role in the improvement of tool performance and processing technology. Coated tools have become the symbol of modern tools, and the proportion of tools in the tool has exceeded 50%. At the beginning of the 21st century, the proportion of coated tools will further increase, and it is hoped that the CBN coating technology will be technologically advanced, so that the excellent performance of CBN will be applied in more cutting tools and cutting processes (including sophisticated and complex tools and forming tools). This will comprehensively increase the cutting level of processed ferrous metals. In addition, the development and application of nano-scale ultra-thin ultra-multilayer and new coating materials will accelerate, and coating will become the main way to improve tool performance.

(4) Select high precision blade

The low accuracy of the blade, the amount of run-out is too high, the surface finish of the face milling cutter will be reduced, and even a ditch will appear. The run-out of the blade on a high-precision CNC machine tool should be controlled at 2 to 5 μm. With the development of CNC machine tools, the surface modification coating treatment of the blade (the high-speed steel, WCo type hard alloy, Ti-based cermets) has appeared correspondingly, which has greatly improved the accuracy of the blade. At the same time, various new indexable insert structures have emerged, such as efficient squeegee blades for turning, complex-shaped reamer blades, ball end mill blades, and high-speed milling cutter blades that prevent flying Wait. Indexable inserts have entered the new stage of comprehensive development of materials, coatings, and grooves. According to the rational combination of materials, coatings, and groove types in the processing of materials and machining processes, blades with the best machining results can be developed to meet the requirements. Different requirements for high-speed, high-life cutting machining production technology.

(5) Improve the surface quality

While maintaining the same cutting efficiency (ie, the same Q value), increasing the cutting speed can improve the chip formation process and increase the cutting damping, suppress the flutter, and accordingly reduce the amount of feed per blade can reduce the formation of traces of the cutting surface track Height, improve the surface roughness, which is conducive to the processing of precision parts and molds.

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Author:

Ms. Mirada Kuo

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info@standdragon.com

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0964894641