Cubic boron nitride (CBN) is synthesized by hexagonal boron nitride and catalyst under high temperature and high pressure. After artificial diamond, GE Corporation first announced another new superhard material synthesized by high temperature and ultra high pressure equipment in 1957. . It has high hardness, thermal stability and chemical inertness. Its hardness is second only to diamond, but its thermal stability is much higher than that of diamond, which has greater chemical stability to iron-based metal elements. The following small series will take you to understand the causes and solutions of chipping, peeling and breaking of cubic boron nitride (CBN) tools.
When the cubic boron nitride tool is continuously cut, the cause of the damage is mainly wear, so reasonable wear and durability should be determined according to the processing method and processing requirements. Since the ceramic tool has good high temperature performance and is most suitable for high speed cutting, the superiority of the ceramic tool wear durability should be fully considered. At the same time, the ceramic tool is suitable for the machine clamp indexable insert, and the tool change is convenient. For machining that requires strict accuracy and roughness, and for forced tool change, wear durability should be determined according to specific requirements. When the cubic boron nitride tool is cut intermittently, the main form of tool damage is damage. There are two types of tool damage in the early and late stages. Early breakage is damage that occurs immediately after cutting or after a short cut. At this time, the front and back flank surfaces have not produced significant wear. This type of breakage occurs most often when cutting high-hard materials or interrupted cutting. Late damage is damage caused by fatigue of the tool material after a certain period of processing.
Causes of brittle fracture of cubic boron nitride cutter
In actual production, the workpiece is far from regular and uniform, regardless of its geometry or the physical and mechanical properties of the material. For example, irregular blank geometry, uneven machining allowance, uneven surface hardness, and grooves, grooves, holes, etc. on the surface of the workpiece, all of which make the cutting process more or less always with intermittent cutting properties. Some other processing methods, such as end milling, are interrupted cutting. Under intermittent cutting conditions, it must be accompanied by strong mechanical shock and thermal shock. In addition, the cubic boron nitride cutter has high hardness and brittleness, and belongs to the powder sintered material. The structure is uneven, and there are many defects and Void. Therefore, it is easy to cause damage to the cutter, especially early damage is more common. The main causes of damage are impact, mechanical fatigue and thermal fatigue. Early damage is the damage caused when the flank face has not produced significant wear. At this time, the cutting edge is subjected to a small number of cutting cycles. Mechanical fatigue and thermal fatigue are not the main contradictions. The stress caused by mechanical impact exceeds the permission of the tool material. The strength causes a brittle fracture to occur.
The shape of brittle fracture of cubic boron nitride cutters is as follows:
Crushing
This is the most common type of early damage in the case of cubic boron nitride tools for continuous or intermittent cutting of steel and cast iron. It is a small gap created in the cutting edge, with a few small gaps in the blade or a small piece that has collapsed. Generally, the gap size is equal to or slightly larger than the feed rate. Micro-cracked tools continue to cut within the allowable wear limits.
2. Peeling off
When end-milling steel and cast iron, shell-like spalling occurs most often on the rake face. When the cubic boron nitride tool is end milled hardened steel (HRC58-65), a large area of ​​shell-like peeling occurs on the rake face, such wear occurs, which can improve the cutting speed, reduce the feed rate, and use negative Chamfered, blade with a passivated edge.
Broken
Small pieces or large pieces of fracture occur on the cutting edge, and normal cutting cannot be continued. Cubic boron nitride tools often break through high-hard materials or interrupted cutting. When turning hard steel, the cutting speed is too high and small cracks occur at the tip of the tool. Usually, it can be reground and repaired. . When the cubic boron nitride tool is intermittently turned (40MnNi3CrMo), the low alloy steel has a large fracture at the tip of the tool. In this case, the tool cannot be reground. This is due to the excessive impact load during intermittent cutting and the early damage that occurs after a short time of cutting. Although the cutting conditions are appropriate, after cutting for a long time, there is no timely tool change, which occurs due to the fatigue of the tool material. The break.
Crack
After a long time of intermittent cutting, the cubic boron nitride tool has mechanical fatigue cracks parallel to the cutting edge caused by mechanical impact, and cracks perpendicular to or inclined to the cutting edge due to thermal shock. When these cracks are continuously expanded and merged, the blade is broken or broken.
When cubic boron nitride tools are cut intermittently, the relationship between tool damage durability and cutting conditions should be established based on the damage law. Thereby determining the appropriate cutting amount of the tool and the tool geometry parameters.
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