How to improve the cutting efficiency of CNC machining center？

 As a representative of the advanced productivity of modern manufacturing, CNC machining plays an extremely important role in machinery, aerospace and mold industries.

Since the 1990s, European, American, and Japanese countries have competed to develop and apply a new generation of high-speed CNC machine tools, accelerating the pace of high-speed machine tool development. The spindle speed of the motor in the high-speed spindle unit is 15000～100000r/min, the rapid traverse speed of the acceleration/deceleration moving parts is 60～120m/min, the cutting feed speed is up to 60m/min, and the feed speed of the high-speed machining center is up to 80m. /min, the dry running speed can reach about 100m/min.
The HyperMach machine tool from CINCINNATI of the United States has a maximum feed rate of 60m/min and a fast speed of 100m/min. The spindle speed has reached 60000r/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, and the precision machining center has increased from 3~5μm to 1~1.5μm, and the ultra-precision machining accuracy has begun to enter the nanometer level ( 0.01μm). 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. THE DIFFERENCE IN THE CUTTING LEVEL OF MACHINING CENTERS AT HOME AND ABROAD

At present, the cutting speed of turning and milling in advanced countries has reached 5000-8000m/min or more; the spindle speed of the machine tool is more than 30,000r/min (some as high as 100,000r/min).
For example, when milling planes, the cutting speed in China is generally greater than 1000-2000m/min, which is only equivalent to 1/12~1/15 of foreign countries, that is, 12-15 hours of domestic work is equivalent to 1 hour of foreign work. 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 to discuss. An investigation on the cutting efficiency of domestic CNC machining centers found that there are many problems such as low tool accuracy, large blade runout, low processing finish, and unmatched process equipment.

At present, new cutting processes 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 rate is increased by 20 times, far beyond the traditional cutting “forbidden zone”, the cutting principle has undergone a fundamental change.
The result is: 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%, the cutting heat remaining on the workpiece is greatly reduced, and the cutting vibration almost disappears ；
The cutting process has taken an essential leap. According to the current situation of machine tools, to give full play to the high-speed machining capabilities of advanced tools, high-speed machining is required to increase the volume of material removed per unit time (material removal rate Q).

While choosing a reasonable cutting amount, try to choose a close-tooth cutter (the number of cutter teeth per inch of the cutter diameter ≥ 3), increase the feed per tooth, improve productivity and tool life. Relevant experimental research shows that when the linear speed is 165m/min and the feed per tooth is 0.04mm, the feed speed is 341m/min and the tool life is 30 years.
If the cutting speed is increased to 350m/min, the feed per tooth is 0.18mm, the feed speed will reach 2785m/min, which is 817% of the original machining efficiency, and the tool life is increased to 117 pieces.

2. CHOOSE A TOOL MATERIAL WITH GOOD PERFORMANCE

In the cutting process of CNC machine tools, the role of metal cutting tools is no less than that of the steam engine invented by Watt. The material used to make the tool must have high high temperature hardness and wear resistance, necessary bending strength, impact toughness and chemical inertness, good manufacturability (cutting, forging, heat treatment, etc.), and not easy to deform.
At present, the main tool materials with good performance at home and abroad are: cermet, cemented carbide coated tools, ceramic tools, polycrystalline diamond (PCD) and cubic boron nitride (CBN) tools, etc. They have their own characteristics, and the range of workpiece materials and cutting speeds to which they are adapted is different.
CBN is suitable for cutting high-hardness hardened steel and hard cast iron. For example, ceramic tools and CBN tools are mainly used when machining high-hard steel parts (50～67HRC) and chilled cast iron. Among them, ceramic tools can be used for workpieces with a hardness of 60～65HRC. , And workpieces above 65HRC are cut with CBN tools;
PCD is suitable for cutting non-ferrous metals, alloys, plastics and glass steel, etc. When processing aluminum alloy parts, PCD and diamond film coated tools are mainly used; carbon tool steel and alloy tool steel are now only used as files, dies and Tools such as taps; carbide-coated tools (such as coated TiN, TiC, TiCN, TiAIN, etc.) have high hardness and a wide range of workpieces suitable for processing, but their oxidation resistance is generally not high, so the cutting speed is increased It is also restricted. Generally, steel parts can be processed in the range of 400～500m/min. The high temperature hardness of Al2O3 coating is high. When processed at high speed, its wear resistance is better than TiC and TiN coatings. 

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

3. SPEED ​​UP THE DEVELOPMENT OF COATING TECHNOLOGY

Since its inception, tool coating technology has played a very important role in the improvement of tool performance and the advancement of processing technology. Coated tools have become a symbol of modern tools, accounting for more than 50% of the tools.
At the beginning of the 21st century, the proportion of coated tools will further increase, and it is expected to break through the CBN coating technology technically, so that the excellent performance of CBN can be applied to more tools and cutting processes (including sophisticated and complex tools and forming tools). This will comprehensively improve the cutting level of processing ferrous metals.
In addition, the development and application of nano-scale ultra-thin and multi-layer and new coating materials will accelerate, and coating will become the main way to improve tool performance.

4. CHOOSE HIGH-PRECISION BLADES

The blade precision is low, the runout is too large, the surface finish of the face milling cutter will be reduced, and even grooves will appear. The runout of the blade on the high-precision CNC machine tool should be controlled within 2～5μm. With the development of CNC machine tools, the surface modification coating treatment of the blade correspondingly appeared (the substrate is high-speed steel, WCo-based cemented carbide, Ti-based cermet), which greatly improves the accuracy of the blade. At the same time, a variety of new indexable insert structures have appeared, such as high-efficiency scraping inserts for turning, complex-shaped front-angle milling cutter inserts, ball-end end milling cutter inserts, and anti-swing high-speed milling cutter inserts. Wait. Indexable inserts have entered a new stage of comprehensive development of materials, coatings and geometries. According to the processing materials and processing procedures, the functions of materials, coatings and geometries can be reasonably combined to develop inserts with the best processing results to meet Different requirements for high-speed, long-life cutting production technology.

5. IMPROVE THE QUALITY OF PROCESSED SURFACE

While maintaining the same cutting efficiency (ie the same Q value), increasing the cutting speed can improve the chip formation process and increase cutting damping, suppress chattering, and correspondingly reduce the feed amount of each tooth can reduce the residue formed by the cutting surface trajectory Height, improve the surface roughness, which is conducive to the processing of precision parts and molds.

6. ESTABLISH A REASONABLE TOOL RESERVE

The tools here refer to high cutting efficiency tools, and the prices of these tools are relatively high. For milling cutters with the same diameter, the price of good tools may be several times or even ten times that of ordinary tools.
If a company stores a large number of good knives for a long time, and these knives may not be used for a long time, it will cause a backlog of funds. However, if a tool is usually not stocked, or if the stock is too small, it will be used up soon, and new tools cannot be bought for a while, which will inevitably affect the efficiency of CNC machining.
The tool magazines of machining centers of most companies can hold more than 40 tools, and there are tool magazines with different numbers of tools such as 60, 90, 120, etc. to choose from. The tool exchange time is getting shorter and shorter. The tool change time of the BZ-26 of the German STEINEL company, the MCC86 of the Japanese MAKINO company, and the MAXIM500 type machining center of the American CINCINNATI company only needs 3 to 4 seconds.

7. DESIGN A SIMPLE TOOL SHARPENING FIXTURE

The machine-clamped milling cutter is highly efficient and easy to use. It is very popular among operators. However, the blade consumption is large and the use cost is high. In most cases, the damage of the blade is caused by the wear of the cutting edge. For factories, higher economic benefits can be obtained. Cemented carbide blades have high hardness and low grinding efficiency. The use of single-chip grinding will not achieve the purpose of saving. It is necessary to design an efficient and simple fixture to achieve multiple blades at once.

8. SELECTION OF PROCESSING METHODS

Machining methods can be divided into two types: down milling and up milling. The mechanical transmission system and structure of the machining center have high precision and rigidity, the friction coefficient of the relative moving surface is small, the gap of the transmission parts is small, the movement inertia is small, and there is an appropriate damping ratio, so down milling can be used. Way of processing to improve processing efficiency. In addition, according to machining experience, the tool life of down milling is more than doubled than that of up milling. Using asymmetrical end milling method, the tool life can be increased by 2 to 3 times.

9. CHOOSE A REASONABLE PROCESSING ROUTE

CNC machine tools, especially machining centers with 4 axes or more, generally perform one-time clamping and multi-directional machining, and all have tool magazines, which can automatically change tools and process shapes at one time. Therefore, determining the correct and simple processing route is the basis for ensuring processing quality and improving efficiency.
The main principles for determining the processing route during programming are:
It should be able to guarantee the requirements of the machining accuracy and surface roughness of the parts;
The processing route should be shortened as much as possible to reduce the idle movement time of the tool;
The numerical calculation should be simple and the number of blocks should be small to reduce the programming workload.
For example, for hole processing with high position accuracy and dimensional tolerance, the processing route for the hole diameter less than 18-20mm is: drilling center hole-drilling-reaming-reaming, and for processing the hole diameter greater than 18-20mm The process route is drilling-reaming-rough boring-fine boring.
In addition, through the comprehensive application of the processing technology, the number of installations of the workpiece can be reduced, and the handling and clamping time can be effectively shortened. For example, a five-sided, five-axis machining center and a vertical lathe are combined to form a universal machining center, which can complete most (or all) processing of parts at one time.

10. SELECTION OF WORKPIECE CLAMPING

Due to the concentration of processes during CNC machining, it is necessary to comprehensively consider the positioning of parts, the design of clamping, and the selection and design of fixtures. First of all, modular fixtures should be used as much as possible. Due to the poor flexibility of general fixtures and relatively low positioning accuracy, special fixtures can be designed when the product batch is relatively large and the processing accuracy is high. Secondly, when choosing tooling, it should facilitate tool exchange and online measurement to avoid collision and interference.

11. THE AUXILIARY EQUIPMENT OF THE PROCESSING CENTER SHOULD BE MATCHED

In the machining center, measuring devices such as tool presetters, automatic measuring devices, and precision detectors are used. When the automatic measuring device is used, the operator does not need to ensure very accurate positioning of the parts, and does not need the operator to move and adjust the parts all the time to match certain fixed coordinate systems of the processing program, which can reduce the clamping time. With the help of measurement, a process that originally required 2.5 hours, including the clamping time, was reduced to 1.5 hours. In addition, the application of these measuring devices can also reduce processing errors.