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Cause analysis and solution of CNC tool chip breaking

    Time:2017-04-06    Click:3312
    Core prompt:1. Classification of chip shapeAccording to the specific conditions of the workpiece material, tool geometric parameters and cutting consumption, the chip shapes generally include:
    1. Classification of chip shape
    According to the specific conditions of the workpiece material, tool geometric parameters and cutting consumption, the chip shapes generally include: band-shaped chips, C-shaped chips, chipping chips, pagoda-shaped chips, clockwork chips, long screw chips, screw chips. Roll crumbs etc.
    (1) Band-shaped chips: When cutting plastic metal materials at high speed, if no chip breaking measures are taken, band-shaped chips are very easy to form. The chips are continuous and often entangled on the workpiece or the tool, which is easy to scratch or damage the surface of the workpiece. The cutting edge of the tool can even hurt people, so the formation of banded chips should be avoided as much as possible.
    But sometimes it is also desirable to obtain band-shaped chips so that the chips can be discharged smoothly. For example, when boring blind holes on a vertical boring machine.
    (2) C-shaped chips: When turning general carbon steel and alloy steel materials, it is easy to form C-shaped chips if turning tools with chip breakers are used. C-shaped chips do not have the disadvantages of band-shaped chips. However, most of the C-shaped chips are broken by hitting the flank face of the turning tool or the surface of the workpiece. The high frequency of chip breaking and breaking will affect the smoothness of the cutting process, thereby affecting the roughness of the machined surface. Therefore, it is generally not desirable to obtain C-shaped chips during finishing. However, I hope to get long spiral chips to make the cutting process more stable.
    (3) Clockwork-like chips: Turning steel parts with large cutting depth and large feed rate on a heavy-duty lathe. The chips are wide and thick. If C-shaped chips are formed, the cutting edge will be easily damaged, and the base will crash and hurt people. . Therefore, the radius of the bottom arc of the chip breaker is usually enlarged, so that the chips will collide and break on the machined surface in a clockwork shape, and fall by their own weight.
    (4) Long-tight crumbs: The formation process of long-tight crumbs is relatively stable and easy to clean. It is a better crumb shape on ordinary lathes.
    (5) Pagoda-shaped chip: When CNC machining, machine tool or automatic line processing, it is hoped to get this chip, because such chip will not be entangled on the tool and workpiece. And it's easy to clean up.
    (6) Chipping debris: When turning brittle materials such as cast iron, brittle brass, cast bronze, etc., it is easy to form needle-shaped or fragmented chippings, which are easy to splash and hurt people and damage the machine tool. If the chip rolling measure is adopted, the chips can be connected into short rolls.
    In short, the specific conditions of the cutting process are different, and the shapes of the desired chips are also different, but no matter what the shape of the chip, it must be reliable.
    Several commonly used chip breaking methods
    (1) Use chip breaker:
    As mentioned earlier, the chip breaker not only acts as an additional deformation on the chips. Moreover, the curling and breaking of chips can be controlled. As long as the shape and size of the chip breaker and the inclination angle between the chip breaker and the main cutting edge are appropriate, the chip breaking is reliable. Regardless of whether it is a welded tool or a machine-clamped tool, it can be a reground or non-reground tool.
    In order to apply different cutting parameters. The hard alloy indexable inserts are pressed with chipbreakers of various shapes and different sizes, which is convenient for selection, which is economical and simple. This method is the preferred method in cutting and is also the most widely used method.
    The disadvantage is that the determination of the reasonable geometric parameters of the tool is restricted by the requirements of chip breaking
    (2) Using chip breaker
     A chip guard 1 is installed on the front face of the turning tool. When the chips flow out along the front of the tool, they are blocked by the chip guard 1 and bend and break. The parameters Ln and α of the chip breaker can be designed and adjusted as required to ensure stable and reliable chip breaking under the given cutting conditions. Loosen the screw 3, under the action of the spring 4, the chip guard 1 and the pressure plate 2 can be lifted together, which is convenient for the adjustment of the chip guard and the quick indexing and replacement of the blade. This type of chip breaker is often used on the tools of large and medium-sized machine tools.
    (3) Using the method of pre-grooving on the surface of the workpiece:
    According to the diameter of the workpiece, one or several grooves are made in advance on the surface to be processed along the axial direction of the workpiece, the depth of which is slightly smaller than the cutting depth, so that the cut chips form a weak section and break. In this way, reliable chip breaking is ensured, and the roughness of the processed surface of the workpiece is not affected. Even when processing tougher materials, the chip breaking effect is very good. For example, when fine boring workpiece materials with greater toughness (such as 40Cr, etc.), when it is difficult to break chips by other methods, longitudinal grooves can be drawn on the processed surface and then boring. Using this method can show its unique advantages.
     Second, the principle of chip breaking
    In the process of metal cutting, whether the chips are easy to break is directly related to the deformation of the chips, so the study of the principle of chip breaking must start with the law of chip deformation.
    Due to the relatively large plastic deformation of the chips formed in the cutting process, its hardness will be increased, while the plasticity and toughness will be significantly reduced. This phenomenon is called cold work hardening. After hardening by cold work, the chip becomes hard and brittle, and it is easy to break when subjected to alternating bending or impact loads. The greater the plastic deformation of the chip, the more obvious the hard and brittle phenomenon, and the easier it is to break. When cutting high-strength, high-plasticity, and high-toughness materials that are difficult to break, try to increase the deformation of the chip to reduce its plasticity and toughness, so as to facilitate the purpose of chip breaking.
    The deformation of chips can be composed of two parts:
    The first part is formed during the cutting process, which we call basic deformation. The chip deformation measured during free cutting with a flat rake face turning tool is relatively close to the basic deformation value. The main factors affecting the basic deformation are the tool rake angle, negative chamfering, and cutting speed. The smaller the rake angle, the wider the negative chamfer and the lower the cutting speed, the greater the deformation of the chip, which is more conducive to chip breaking. Therefore, reducing the rake angle, widening the negative chamfer, and reducing the cutting speed can be used as measures to promote chip breaking.
    The second part is the deformation of the chips during the flow and curling process, which we call additional deformation. Because in most cases, only the basic deformation during the cutting process cannot break the chips, an additional deformation must be added to achieve the purpose of hardening and breaking. The easiest way to force the chips to undergo additional deformation is to grind (or press) a chip breaker of a certain shape on the rake face, and force the chips to curl and deform when they flow into the chip breaker. After the chip undergoes additional re-curling deformation, it is further hardened and embrittled. When it hits the workpiece or the flank surface, it is easily broken.
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