Analysis of the requirements of cutting tools for modern CNC machine tools

With the increasing popularity of high-efficiency equipment such as CNC machine tools and machining centers, cutting operations have entered a high speed in the advanced development of aerospace, automotive, high-speed trains, wind power, electronics, energy, molds and other equipment manufacturing industries. The new era of high-speed machining, marked by high efficiency and environmental protection, is the stage of modern cutting technology.

High-speed cutting, dry cutting and hard cutting are important development trends of current cutting technology, and their important position and role are increasingly prominent. The application of these advanced cutting technologies not only doubles the processing efficiency, but also promotes the process of product development and process innovation. For example, the cavity of precision mold hard materials, with high speed, small feed and deep processing of snacks, can achieve high surface quality, but also eliminate grinding, EDM and hand polishing or reduce the time of the corresponding process, thus Shorten the production process and increase productivity.

In the past, some companies used to make complex molds, which basically took 3 to 4 months to deliver, but now they can be completed in half a month after high-speed machining. According to the survey, 60% of the machining capacity of general tooling can be achieved by high-speed machining.

High-speed machining requires not only high tool reliability, good cutting performance, stable chip breaking and chipping, but also high precision, and quick change or automatic replacement. Therefore, higher requirements are placed on tool materials, tool structures, and tooling.

Requirements for tool materials
The most outstanding requirement for high-speed machining tools is that they must have high hardness and high temperature hardness, and have sufficient fracture toughness. For this purpose, tool materials such as fine-grained carbide, coated cemented carbide, ceramic, polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) must be used – each with its own characteristics, adapted to the workpiece material and The cutting speed range is also different. For example, high-speed processing of non-ferrous metal parts such as aluminum, magnesium, and copper, mainly using PCD and CVD diamond film coated tools. High-speed machining of castings, hardened steel (50~67HRC) and chilled cast iron mainly use ceramic tools and PCBN tools.

Shanghai Volkswagen Automotive Co., Ltd. uses the cubic boron nitride CBN300 blade face milling cutter produced by Seco Tool (Shanghai) Co., Ltd. to high-speed milling the engine block plane (casting) on ​​the flexible production line with a cutting speed of up to 1600m/min and a feed rate of 5000mm/ Min. The cutting speed of aluminum alloy processed with PCD tool is generally 3000-4000m/min, and the highest is 7500m/min. The cutting speed of hardened steel and chilled cast iron with ceramic and PCBN tools has reached 200m/min.

1. Cemented carbide has entered the stage of fine-grained ultrafine grain
Coated cemented carbide tools (such as TiN, TiC, TiCN, TiAlN, etc.) have a wide range of materials for processing workpieces, but the oxidation temperature is generally not high, so it is usually only suitable to process steel in the cutting speed range of 400-500 m/min. Pieces. Ceramic and PCBN tools are available for Inconel 718 high temperature nickel based alloys. According to reports, Canadian scholars use SiC whisker toughen ceramic milling Inconel718 alloy, the recommended cutting conditions are: cutting speed of 700m / min, eating depth of 1-2mm, feed per tooth is 0.1-0.18mm / z.

At present, cemented carbide has entered the development stage of fine grain (1-0.5μm) and ultrafine grain (<0.5μm). In the past, fine grains were mostly used for K-type (WC+Co) cemented carbide. In recent years, P-type (WC+TiC+Co) and M-type (WC+TiC+TaC or NbC+Co) cemented carbides also progress toward grain refinement.

In the past, in order to improve the toughness of cemented carbides, the content of cobalt (Co) was usually increased, and the resulting reduction in hardness can now be compensated by refining the grains and increasing the flexural strength of the cemented carbide to 4.3 GPa. , has reached and exceeded the bending strength of ordinary high-speed steel (HSS), changed the popular belief that P-type cemented carbide is suitable for cutting steel, and K-type cemented carbide is only suitable for processing non-ferrous metals such as cast iron and aluminum. .

WC-based ultra-fine grain K-type cemented carbide can be used to process various steel materials. Another advantage of fine-grained carbide is the sharp edge of the tool, which is especially suitable for high-speed cutting of sticky and tough workpiece materials. Take the AQUA twist drill developed by Fujitsu of Japan as an example. It is made of fine-grained hard alloy and coated with a heat-resistant and friction-resistant lubricating coating. When cutting high-speed wet-processed structural steel and alloy steel (SCM), it cuts. The speed is 200m/min, the feed rate is 1600mm/min, the machining efficiency is increased by 2.5 times, the tool life is increased by 2 times; in dry drilling, the cutting speed is 150m/min and the feed speed is 1200mm/min.

2. Coatings are upgraded to the new stage of developing thick film, composite and multi-component coatings
Nowadays, coatings have entered a new stage of developing thick film, composite and multi-component coatings, newly developed TiCN, TiAlN multi-thin ultra-thin, ultra-multi-layer coatings (some ultra-thin coatings can be up to 2000 layers, The thickness of each layer is about 1nm) and the combination of TiC, TiN, Al2O3 and other coatings, plus the new plastic deformation resistant matrix, in improving the toughness of the coating, the bonding strength between the coating and the substrate, and improving the wear resistance of the coating. Significant progress has been made to improve the performance of cemented carbide.

Coated tools have become the hallmark of modern cutting tools, with a 60% use ratio in the tool. The products of coated carbide tools are now trending towards branding, diversification and generalization. For example, Schnell's ultra-long-life LL-coated end mill with nanotechnology has a tool life of 2-3 times longer when it is used to machine hardened steel with a hardness of more than 70 HRC.

Sweden's Sandvik's three new coated inserts (GC4225, GC4240, GC1030) have a wide range of versatility, GC4225 (Breakthrough No. 1) as an upgrade to the GC4025 (P25) grade, when it is used to process automotive crankshaft steel forgings, Tool life under the same cutting conditions can process 41 parts per cutting edge, while GC4025 can process 14 parts per cutting edge.

Seco Jabro's new solid carbide general-purpose milling cutter Solid2 series tools not only use new materials, but also introduce new coatings. The applicable processing temperature is increased from 800 degrees Celsius to 1100 degrees Celsius. Significantly improved processing efficiency and tool life. At the same time, the Solid2 series adopts the edge passivation treatment and the radial full-circumferential back-scraping technology, which makes the combination of the coating and the material of the tool more perfect, and the number of re-grinding of the tool is also greatly improved.

The H7 blade is a TiAlN coating from Kennametal, USA, designed for high speed milling of alloy steel, high alloy steel and stainless steel. The hole processing tool coating from the German company Guhring under the trade name "Fire" is a versatile composite coating - a combination of TiN, TiCN and TiAlN coatings. The advantages of the material are suitable for both dry and hard cutting as well as for normal cutting.

It is particularly worth emphasizing that the technology of coating diamond on the surface of cemented carbide developed in recent years has made the improvement of cutting efficiency not only in the field of ferrous metals but also in the field of non-ferrous metals. It can be seen that cemented carbide will continue to be the main matrix material for the production of high-speed machining tools.

At present, the United States, Sweden and Japan have successively introduced diamond-coated taps, drill bits, end mills and indexable inserts with chipbreakers (such as Sandvik's CD1810 and Kennametal's KCD25 grade). High-speed precision machining of non-ferrous and non-metallic materials. Another CBN coating suitable for processing steel materials has also been developed and is moving into the industrial trial phase.

Requirements for tool geometry and chipbreaker
Geometric parameter
The main causes of failure of the tool during high-speed and dry cutting are crater wear and thermal wear at the tip. This is caused by the high temperature at the interface between the tool and the chip and the contact area between the tool and the workpiece. Therefore, the high speed machining is slightly larger than the tool rake angle in the normal cutting process to lower the temperature of the cutting zone and to make a negative chamfer on the cutting edge.

In order to prevent thermal wear at the tip, the rounded or chamfered tip should be used at the joint of the primary and secondary cutting edges to increase the corner angle and increase the length of the cutting edge and the volume of the material in the vicinity of the cutting edge. Increase tool rigidity and reduce the probability of cutting edge breakage.

A new ME-13 carbide insert for dry cutting, introduced by Carboloy, USA, with a large rake angle (up to 34°), a stiffened edge and a ribbed rake face, which significantly reduces chipping The area of ​​contact with the rake face of the blade causes heat to be carried away by the chips. The blade is said to operate at a temperature 400°C lower than conventional blades, significantly reducing cutting forces and doubling tool life. The company uses a high-angle coated hard alloy crown end mill for high-speed milling of hardness of up to 55HRC die steel, cutting speed of 120m / min, feed rate of 7.6m / min, axial depth of 0.51mm, radial eat With a depth of 0.25mm and dry cutting, the tool life is up to 1.5h.

The spiral cutting edge milling insert with positive rake angle has been developed abroad to make the cutter have more reasonable geometric parameters. The blade has almost constant front angle along the cutting edge, and the back rake angle or side rake angle can be negatively corrected or From small to large, the cutting is lighter and smoother, and the cutting performance of indexable face milling cutters, end mills and slot milling cutters is improved to a new level, tool life can be increased by 50%-250%, and cutting efficiency is increased by 30%. -40%.

A company in the United States uses this new type of insert to dry milling the periphery of 17-4PH stainless steel. The cutting amount is: milling speed 304m/min, feed rate 1270mm/min, feed per tooth is 0.14mm/z, 20S removes the remaining 36cm3.

2. Chip breaker type
In order to stabilize chip breaking and chipping, a suitable chipbreaker type must be made on the blade. At present, the design and manufacturing technology of the three-dimensional curved chipbreaker on the indexable insert has been relatively mature. For the different workpiece materials and different cutting amounts, the corresponding common chipbreaker series has been developed.

For example, the Swedish, Sandvik company introduced the groove series of R, M and F (the roughing, semi-finishing and finishing of steel adopt the groove type of PR, PM and PF, and the groove type of MR, MM and MF when cutting stainless steel, The slotted versions of KR, KM and KF for cut castings and non-ferrous metals, as well as the slot design typical of Israel's Iscar with the "Overlord Knife" are unique.

The chipbreakers on these inserts have a wide range of chip breaking and good adaptability. They all have a spatial cutting edge and a curved rake face. The pre-angle on the cutting edge can be adjusted to zero or negative, and the working rake angle is a suitable positive value, so the cutting force is small, the cutting edge strength is high, and the high-speed resistance is high. The strong wear capacity indicates the direction of the development of the high-speed machining tool blade structure.