KR101927059B1 - Cryogenic Deburring Method of Drill Hole Defect in CFRP - Google Patents

Abstract

Disclosed is a deburring method of forming an ice layer surrounding burrs in order to remove burrs generated while drilling a CFRP and cutting the burrs and the ice layer surrounding the burrs. Generally, it is difficult to remove burrs formed on the CFRP since the burrs are formed by protruding carbon fiber. The deburring method according to the present invention is effective since the ice layer supports the carbon fiber. Because the ice layer is used to support a defect part of the burrs, an increase of cutting load is very slight, there is no influence on tools and a workpiece, and the deburring method according to the present invention is an eco-friendly process capable of performing deburring very effectively. The deburring method according to an embodiment of the present invention can effectively remove the hole defect portion generated in an expensive CFRP, thereby preventing the workpiece from being wasted by drilling defects in order to enhance productivity.

Description

Technical Field [0001] The present invention relates to a method for deburring a CFRP hole using a cryogenic coolant,

The present invention relates to a method for removing pitting defects in CFRP using a cryogenic refrigerant. More particularly, the present invention relates to a process for forming an ice layer surrounding a defective portion by using a cryogenic coolant, and removing an ice layer and a defective portion together.

The contents described in this section merely provide background information on the present embodiment and do not constitute the prior art.

CFRP (Carbon Fiber Reinforced Plastic) composites show excellent mechanical properties and provide light weight compared to conventional metal materials, and are being applied to aviation, automobile and sports leisure. CFRPs, especially used as structural members, contain a large number of holes for connection to other components.

On the other hand, CFRP is characterized by the fact that carbon fiber, which is a highly rigid material, is included and the tool wear rate is fast. As the demand for CFRP increases, the cutting quality of CFRP is improved by special tools considering the cutting characteristics and diamond coating (CVD coating) tools for increasing the tool life. However, the abrupt initial wear of the tool insert is due to the tool life cycle It is possible to cause a hole machining defect even in the inside of the hole.

For aircraft CFRP parts, the percentage of deburring costs accounted for 30% of the total part manufacturing cost, and hole processing defects accounted for the largest portion of total manufacturing cost. For example, Boeing 747 has 300 million holes for connecting various components. For the Boeing 787, which has a full carbon composite material, the number of CFRP-related holes is 100,000. The price of CFRP is high, but not only the machining time required for hole machining but also the tool cost is considerable.

The types of defects that occur in hole machining of CFRPs include uncut fiber, edge wear, fraying, chipping, uncut resin, spalling and delamination. . The high stiffness of the carbon fibers, the rapidly changing properties of the resin at high temperatures, the low thermal conductivity of CFRP, and the localized defects inherent in the CFRP molding process are sources of these defects.

On the other hand, cutting processing using cryogenic coolant such as liquefied nitrogen is a field in which research and development are being carried out in earnest in the 21st century. Mainly, in machining hard material, the main part is to cool cutting area with cryogenic coolant to increase tool life and improve surface quality. In the processing of polymer materials, there is also a method (US 8,820,199) for improving the surface quality and for preventing the generation of burrs of polymer materials which are weak to heat, as a method of processing using cryogenic coolant.

United States Patent No. 8,820,199 (Registered on Mar. 02, 2014)

The present invention proposes a technique of forming an ice layer surrounding burrs to remove burrs generated in CFRP hole drilling, and then performing deburring by cutting burrs and ice layers together. Generally, burrs generated by CFRP are carbon fiber protruding forms, and deburring is difficult in such a composite in which carbon fiber and resin are combined. The present invention aims to provide an effective CFRP deburring process by ensuring the machining site rigidity required for cutting by wrapping such burrs, which are generally difficult to remove, in an ice layer.

According to an aspect of the present invention, there is provided a CFRP deburring method for removing burrs generated in a CFRP hole, comprising the steps of: injecting a liquid filler material into at least a part of a burr A charging step for attaching; A cooling step of cooling the filling material to solid state; A deburring step of removing a burr with a solid fill material by a cutting tool; And a separation step in which the solid filler material is liquefied or vaporized and removed from CFRP.

Further, the filling material is water.

The cooling step also includes a cryogenic exposure step of exposing the fill material to cryogenic refrigerant.

Further, the cryogenic refrigerant is characterized by being liquefied nitrogen.

Further, the filling step is characterized in that the filling material is attached to the side surface including the edge of the hole where the burr and burr are formed.

The solid fill material is also attached to both the CFRP body and the burr, wherein the bur is attached from the CFRP body such that it is supported by the solid fill material.

In addition, the charging step includes a jig case formed so that at least a part of the bur is submerged in the liquid filling material.

Further, the filling step comprises a soft porous material formed to adhere a liquid filler material to at least a portion of the burr, wherein the porous material having the liquid filler material adheres to the side surface including the edge of the hole where the burr is formed .

Further, the deburring step is characterized in that the ultrasonic deburring is performed together.

The present invention provides a very high deburring performance by forming an ice layer surrounding burrs to remove burrs caused by CFRP hole drilling, followed by deburring by cutting burrs and ice layers together. In addition, as a member for supporting burrs, it is an eco-friendly process that is naturally removed after processing using an ice layer, and has no effect on workpieces. The deburring method using the cryogenic coolant according to the present invention can be effectively applied to the edge trimming of the CFRP.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a photograph showing a kind of hole machining defects generally generated in hole machining of a CFRP composite material.
Fig. 2 is a conceptual diagram for explaining a mechanism of occurrence of a typical hole defect in a CFRP composite material.
3 is a conceptual diagram illustrating a concept of a deburring process using an ice layer according to an embodiment of the present invention.
4 shows four examples of a deburring process using a cryogenic coolant.
5 is a photograph showing that the cryogenic coolant is sprayed to the CFRP wetted with water in the deburring process according to an embodiment of the present invention to cool the burr and the burr surrounded by the ice layer is deburred by the ultrasonic milling machine.
6 is a table showing the tools and cutting conditions used in the cutting test.
Fig. 7 is a photograph explaining quantitatively calculating the burr removal rate from the area occupied by the burr in the image of the hole.
FIG. 8 is a comparison result of a preliminary comparison experiment for selecting a test condition by a CFRP and a tool by comparing a deburring effect according to an amplitude of an ultrasonic wave with respect to a deburring instrument.
9 shows the result of performing ultrasonic deburring in a state where CFRP is directly cooled with a cryogenic coolant.
Fig. 10 shows the result of performing general deburring without ultrasonic excitation in the state where the processing holes are filled with water and CFRP and water are cooled with cryogenic refrigerant.
Fig. 11 shows the result of performing ultrasonic deburring in a state where the processing hole is filled with water and CFRP and water are cooled with a cryogenic refrigerant.
Fig. 12 shows the result of performing ultrasonic deburring in a state where the corner side surface including the machining hole and the machined defect portion is filled with water to a predetermined thickness and the CFRP and the water are cooled with the cryogenic coolant.
FIG. 13 is a photograph of a CFRP using a cryogenic coolant according to an embodiment of the present invention, taken and observed before and after a deburring process, using an electron microscope.

Hereinafter, some embodiments of the present invention will be described in detail with reference to exemplary drawings. It should be noted that, in adding reference numerals to the constituent elements of the drawings, the same constituent elements are denoted by the same reference symbols as possible even if they are shown in different drawings. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

In describing the components of the present invention, terms such as first, second, A, B, (a), and (b) may be used. These terms are intended to distinguish the constituent elements from other constituent elements, and the terms do not limit the nature, order or order of the constituent elements. Throughout the specification, when an element is referred to as being "comprising" or "comprising", it means that it can include other elements as well, without excluding other elements unless specifically stated otherwise . In addition, '... Quot ;, " module ", and " module " refer to a unit that processes at least one function or operation, and may be implemented by hardware or software or a combination of hardware and software.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a photograph showing a kind of hole machining defects generally generated in hole machining of a CFRP composite material.

Cutting of CFRP differs from cutting of metal in many respects. Most of the ductile metal is cut by shear, but the composite material such as CFRP is cut by the following mechanism. That is, buckling of the fiber by compression, fracture of the fiber due to warping, shear-yield-crack of the composite, and peeling between the interfacial layers are combined to perform cutting. Particularly, in the case of CFRP in which a carbon fiber is bonded by a resin, it is preferable to use a cutting tool having a very low coefficient of friction. Since the resin used in the composite material sharply deteriorates even at a very low cutting temperature, the CFRP is coated with a chemical vapor deposition (CVD) method that can withstand abrasion due to friction with the carbon fiber, Diamond tools are used, and it is important to select cutting conditions that can maintain a low cutting temperature.

Carbon fibers have very high tensile strength, but they are easily broken without plastic deformation even in the case of small forces acting in the transverse direction. The resin bonding the carbon fibers has a much larger thermal expansion coefficient than the carbon fibers. The carbon fiber and the resin have different rigidity, so that the carbon fiber can be peeled off due to the dynamic load applied at the cutting edge during cutting. Peeling can also be caused by different thermal expansion characteristics due to local thermal stress.

In CFRP cutting, tool wear is very high due to carbon fiber. When referring to the machinability example of CFRP hole machining, CVD diamond coated tools show a tool life of 40 m, while diamond-like carbon (DLC) coated tools have a tool life of 5 m have. Hole machining defects in CFRPs tend to increase in burrs within the tool life cycle if the cutting edge wear increases rapidly above a certain level at the beginning of tool life.

As the machining time increases, the abrasion of the tool cutting edge increases due to the hard carbon fiber. As a result, the degree of cutting by the shearing, which is the basic mechanism in the cutting process, is reduced. Especially, in CFRP machining, the efficiency of machining into a fracture of fiber by compression or bending sharply decreases. In addition, the cutting load increases due to abrasion and the heat generated at the cutting portion increases, so that the quality of the resin bonding the carbon fibers increases and the cutting quality deteriorates rapidly due to the cutting force in a state where the adhesive force is decreased.

In particular, since both sides of the CFRP plate have no support on the outside, the carbon fibers are more easily spalled. This results in a peel-in burr at the entrance edge where the drill is inserted in the drilling, and a push-out burr occurs at the drill exit. In the case of the fill-inverts, the shape of the tool cutter can be prevented by an improved tool in the form of compressing the CFRP. However, the push-out burr is subjected to the force by the most abrasive tip wire in the drill tool, and the side layer is liable to be peeled off, and as a result, the uncut carbon fiber tends to be generated. Rapid tool edge wear at the beginning of cutting increases the axial cutting force in drilling, which makes this push-out burder easier. At this time, the protruded uncut carbon fibers are peeled off from the resin supporting the carbon fibers, so that the cutting force acting on the cutting edge at the inner circumferential surface of the hole is not transferred to the carbon fibers, so that even the deburring becomes difficult.

In general metal cutting, the shear strength of the material is lowered due to the increase in temperature at the local site where the shear occurs during cutting. However, the carbon fiber is decomposed at 3,600 ° C and there is no significant change in the mechanical properties at high temperatures. Rather, it is necessary to maintain a low temperature for cutting of CFRP. When the temperature of the resin in the composite increases, the rigidity and adhesion force for supporting the carbon fiber decreases, and the cutting force at the cutting edge of the tool is not transmitted to the carbon fiber properly while the carbon fiber is not supported by the resin. This tendency occurs most frequently at the drill exit side edge. FIG. 1 shows various types of hole machining defects caused by these effects collectively.

Fig. 2 is a conceptual diagram for explaining a mechanism of occurrence of a typical hole defect in a CFRP composite material.

Generally, machining defects that occur during hole machining of a CFRP composite using a drill tool occur at the entrance to the tool entrance, and more frequently at the edge of the exit hole through which the tool penetrates. In the case of the hole entrance, a considerable solution is to be made by using a special type of compression tool, while in the case of the hole outlet, a CFRP exclusive end mill tool or a tool smaller than the hole diameter is used to reduce the cutting load It is a hole machining defect frequently caused by tool wear, except for machining.

On the other hand, the deburring for eliminating such drilling defects may be made in the opposite direction in which the drill tool is inserted, or may be approached and deburred in the direction in which the original drill was inserted by using the back deburring tool. However, CFRP parts in the aviation and automobile fields are often large, so it is desirable that the tool should enter the original direction and be deburred in the same direction as the original corrugation direction while being fixed to the original jig for drilling.

3 is a conceptual diagram illustrating a concept of a deburring process using an ice layer according to an embodiment of the present invention.

Fig. 3 (a) is a view showing a state in which a burr protruding into an ice layer formed on the inner circumferential surface of the hole is surrounded, and Fig. 3 (b) shows a form in which an inner circumferential surface of the hole and an outer surface of the edge where the burr is formed are surrounded by an ice layer.

3 shows the concept of deburring using a cryogenic coolant and an ice layer according to an embodiment of the present invention. Fig. 5 (a) shows a case in which a non-cutting burr is formed by using liquid nitrogen and water in the form of an ice layer on the inner circumferential surface of the hole. 5 (b) shows a case in which an ice layer is formed to a predetermined thickness on the side surface of the CFRP including the edge where the burr is generated. Burrs protruding toward the inner circumferential surface of the hole are firmly fixed by being surrounded by the ice layer and the phenomenon that the carbon fiber is not cut at the time of deburring but is merely pushed in the machining direction of the tool is prevented.

A CFRP hole machining defect deburring method using a cryogenic coolant according to an embodiment of the present invention is characterized by using an ice layer to support a defective site bur in the processed hole.

As described above, the burr of CFRP is most likely caused by the fact that the carbon fiber is peeled off from the resin and cutting becomes difficult. In one embodiment, such a carbon fiber is surrounded and supported by an ice layer to provide a situation in which the cutting edge of the tool can exert sufficient cutting force on the carbon fiber.

4 shows four examples of a deburring process using a cryogenic coolant. 6 (a) shows a first deburring method, (b) shows a second deburring method, (c) shows a third deburring method and (d) shows a fourth deburring method.

The first deburring method is an example similar to a conventional cryogenic refrigerant applying method, and the second deburring method forms an ice layer but does not perform ultrasonic excitation. In the second deburring method, a simple deburring process is performed after forming an ice layer to surround the projected burrs on the inner circumferential surface of the hole by spraying cryogenic coolant while spraying water on the inner circumferential surface of the burr and hole.

The third deburring method is a case where an ultrasonic milling main shaft is used for deburring in the second deburring method. In the fourth deburring method, an ice layer surrounding the inner circumferential surface of the hole is formed in the third deburring method, and water is sprayed on the side including the edge where the burr is formed, or a porous material, such as a sponge or cotton, And the water is buried in the burr, and the ice layer supporting the hole is formed, followed by ultrasonic deburring.

The deburring method according to an embodiment of the present invention includes a third deburring method and a fourth deburring method, and preferably means a fourth deburring method.

The fourth deburring method will be described in more detail as follows. For the sake of convenience, it is described on the basis of a CFRP in the form of a flat plate, and a normal technician will be able to apply the deburring method according to the idea of the present invention to a curved surface or other types of CFRP workpieces by appropriate modification.

The jig supporting the CFRP is provided with a jig case or a jig case which is open only in the upper part so as to be filled with water. When a porous material is used, it is preferable to use a soft or weak material in which the increase in cutting load due to deburring is insignificant in the state where it is cooled together with water by cryogenic refrigerant, and the local cutting is easily performed. In addition, when the porous material is attached to the curved surface of the CFRP, a supporting portion connecting the CFRP and the porous material is formed at a plurality of positions, so that when the porous material is cooled with water and solidified, it is firmly fixed to the CFRP So that it is not separated even in the axial load due to deburring.

Although the method of enclosing water in the CFRP is not disclosed in the detailed and various embodiments in describing the present invention, if it is a structure that can fill the water with a sufficient thickness to surround the bur, for example, several times the thickness of the burr, Or a porous material such as a porous material, for example, may be used. The shape and hole position of the CFRP may vary widely, and it would be difficult for a typical engineer to form the jig and jig case in a manner consistent with the technical idea of the present invention.

5 is a photograph showing that the cryogenic coolant is sprayed to the CFRP wetted with water in the deburring process according to an embodiment of the present invention to cool the burr and the burr surrounded by the ice layer is deburred by the ultrasonic milling machine.

6 is a table showing the tools and cutting conditions used in the cutting test.

In the comparative experiment for verifying the deburring process, which will be described later, the other conditions were set the same, and the conditions of the cryogenic coolant, the ice layer formation position, and the presence or absence of the ultrasonic wave were changed.

Fig. 7 is a photograph explaining quantitatively calculating the burr removal rate from the area occupied by the burr in the image of the hole.

Referring to Fig. 7, the ratio of all the burrs existing in the hole is defined as a ratio of the sum of the areas occupied by burrs divided by the area of the holes. The removal ratio of burrs is defined as the rate of change of the ratio of all burrs in the holes before and after deburring.

FIG. 8 is a comparison result of a preliminary comparison experiment for selecting a test condition by a CFRP and a tool by comparing a deburring effect according to an amplitude of an ultrasonic wave with respect to a deburring instrument.

Referring to FIG. 8, in the combination of UD (unidirectional) type CFRP and cutting tool used in the experiment, the erbium excitation amplitude was 4.318 ㎛, and the ultrasonic excitation amplitude was 4.318 ㎛ . It can be seen that the burr removal rate of the CFRP hole is not 20% even though the deburring process using the ultrasonic milling spindle is carried out.

9 shows the result of performing ultrasonic deburring in a state where CFRP is directly cooled with a cryogenic coolant.

Fig. 10 shows the result of performing general deburring without ultrasonic excitation in the state where the processing holes are filled with water and CFRP and water are cooled with cryogenic refrigerant.

Fig. 11 shows the result of performing ultrasonic deburring in a state where the processing hole is filled with water and CFRP and water are cooled with a cryogenic refrigerant.

Fig. 12 shows the result of performing ultrasonic deburring in a state where the corner side surface including the machining hole and the machined defect portion is filled with water to a predetermined thickness and the CFRP and the water are cooled with the cryogenic coolant.

In Fig. 12, the process of filling the side surface with the predetermined thickness of water may be performed when the above-described jig case is used, when the porous material with water is adhered thereto, and when water is filled in the processing hole and cooled with the cryogenic coolant, It may also be in a cooled form by spraying water. The method of forming the ice layer so as to surround the corner side surface including the machining hole and the machining defective portion may include various methods other than the disclosed method and any method may be used provided that the ice layer is formed in the form of surrounding the entire burr portion.

Referring to the deburring results of FIGS. 9 to 12, it can be seen that the burr removal rate is higher in the first to third deburring methods than in the fourth deburring method. That is, as in the fourth deburring method, the ice layer completely covers the side of the CFRP including the burr and burr-formed corners, and when the deburring is completed before the ice layer is formed by heat, You can remove most of the burrs. Referring back to FIG. 6, it can be seen that the drill used in the verification test is a non-coated general drill and can be deburried at a very high efficiency even though it is not an exclusive expensive tool that is CVD coated for CFRP processing.

10 to 12, it has been experimentally determined that the burr removal rate is relatively low when the feed is low under the cutting condition. This is because the cutting heat caused by the friction or the like becomes longer as the contact time between the tool and the workpiece becomes longer It can be judged that the ice layer serving as a back up plate is melted or peeled off from the carbon fiber to fail to provide sufficient support.

FIG. 13 is a photograph of a CFRP using a cryogenic coolant according to an embodiment of the present invention, taken and observed before and after a deburring process, using an electron microscope.

Fig. 13 is a comparative photograph confirming that water used as a filler material for wrapping and supporting the bur part does not adversely affect the carbon fiber, the resin, and the bonding portion between the CFRP composite and the CFRP composite. The CFRP test specimens used in the tests were laminated in the UD format. No areas where carbon fibers or resin were abnormally peeled off after deburring were not found. In general, an epoxy using an amine-based curing agent used in CFRP is hydrophilic before curing and is completely dried, thereby lowering physical properties due to moisture penetration, but there is no problem caused by water contact after curing. Also, in the CFRP deburring process using the cryogenic refrigerant according to an embodiment of the present invention, since the time in which the water in contact with the CFRP is in a liquid state is short and the process proceeds in a state of being frozen with the cryogenic refrigerant, It can be judged.

The present invention grasps the occurrence of burrs as an image in the jig-coupled state for hole machining, forms an ice layer by cryogenic coolant, enables the work to proceed, and enables deburring without changing the setting state of the workpiece, thereby consuming unnecessary setting time none. In addition, since the same drill tool can be used, it shows a high burr removal rate, and there is no need to exchange the tool, so that the cost reduction due to the shortening of the machining time can be expected.

The foregoing description is merely illustrative of the technical idea of the present embodiment, and various modifications and changes may be made to those skilled in the art without departing from the essential characteristics of the embodiments. Therefore, the present embodiments are to be construed as illustrative rather than restrictive, and the scope of the technical idea of the present embodiment is not limited by these embodiments. The scope of protection of the present embodiment should be construed according to the following claims, and all technical ideas within the scope of equivalents thereof should be construed as being included in the scope of the present invention.
[Sasa]
This study was conducted as part of the core technology development project of the Machinery Industry of the Ministry of Industry, Trade and Industry. [10053248, Project: Carbon Fiber Composite (CFRP) Processing System Development]
[Acknowledgment]
This work was supported by the Technology Innovation Program (10053248, Development of Manufacturing System for Carbon Fiber Reinforced Plastics Machining (CFRP) funded by the Ministry of Trade, Industry & Enegry (MOTIE, Korea).

Claims (9)

A CFRP deburring method for removing a burr generated in a CFRP (Carbon Fiber Reinforced Plastic) hole,
A filling step of attaching a liquid filling material to at least a part of the burr;
A cooling step of cooling the filler material and solidifying the filler material;
A deburring step of removing the bur buried with the solid fill material with a cutting tool; And
A separation step in which the solid filler material is liquefied or vaporized and removed from the CFRP;
Wherein the CFRP deburring method comprises: The method according to claim 1,
Wherein the filling material is water. The method according to claim 1,
Wherein said cooling step comprises a cryogenic exposure step of exposing said fill material to cryogenic coolant. The method of claim 3,
Wherein the cryogenic refrigerant is liquefied nitrogen. The method according to claim 1,
In the charging step,
Wherein the filler material is attached to a side surface including the edge of the burr and the burr hole. 6. The method of claim 5,
Wherein the solid fill material is attached to both the CFRP body and the burr, wherein the bur is adhered from the CFRP body to be supported by the solid fill material. 6. The method of claim 5,
In the charging step,
And a jig case formed so that at least a part of the bur is submerged in the liquid filler material. 6. The method of claim 5,
In the charging step,
And a soft porous material formed to adhere the liquid filler material to at least a portion of the burr, wherein the porous material having the liquid filler material adheres to a side surface including an edge of the hole in which the burr is formed Lt; RTI ID = 0.0 > CFRP < / RTI > The method according to claim 1,
The deburring step may include:
Wherein ultrasonic deburring is performed together.

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