Incremental sheet forming tool, incremental sheet forming device, and incremental sheet forming method

Abstract

To provide consecutive change to pressing force while continuously bringing a tip part into contact with a sheet material to suppress flaw to the sheet material, extend a life of a tool itself, and suppress cracks of the sheet material.SOLUTION: An incremental sheet molding tool 3 is provided with a base shaft part 31 having a circular cross section and a tip shaft part 32 having an elliptical shape, where they are provided concentrically in the axial direction thereof. A peripheral surface of a tip part of the tip shaft part 32 is pressed to a sheet material 4 and has a shape having a positive curvature factor in the whole circumference about the axis thereof, where the positive curvature factor is not constant in the circumferential direction and has directionality.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to techniques for incremental sheet forming. [Background technology]

[0002] In recent years, a die-less incremental sheet forming (ISF) method has become known in which a rod-shaped forming tool is rotated and pressed against the sheet material to be plastically processed, and then moved relative to the material, for example, along contour lines, to form the material into a target shape (see, for example, Patent Document 1).

[0003] Patent Document 1 describes that when an incremental sheet forming tool with a hemispherical tip is used, depending on the angle of the inclined surface connected to the edge of the formed part, the amount of tension on the sheet material may increase, potentially causing cracks, so the cross-sectional shape of the tip is designed to be, for example, petal-shaped, semicircular, triangular or other polygonal, star-shaped, etc., so that the so-called peaks around the axis are intermittently pressed against the sheet material as the tool rotates. By pressing intermittently in this way, the amount of forming is divided, reducing the amount of forming per cycle, thereby suppressing cracks. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-153313 Summary of the Invention [Problem to be solved by the invention]

[0005] However, when the petal-shaped tip cross section of an incremental sheet forming tool, as shown in FIG. 1 of Patent Document 1, changes in the petal shape and the contact state with the sheet material as the tool rotates can cause the following problems. First, while the petal portion around the axis, i.e., the peak portion, faces the sheet material, the sheet material is pressed against the opposing peak portion. Next, as the tool rotates and faces the space between the peaks, i.e., the groove portion, the sheet material almost moves away from the groove portion and temporarily ceases to be pressed against it. Furthermore, as the tool continues to rotate, the sheet material impacts the next peak portion. This pressing action is repeated intermittently and with successive impacts during tool rotation, which can cause intermittent scratches on the formed sheet material and reduce the quality of the formed part. Furthermore, the repeated impact loads applied reflexively to the tool can cause deformation or damage to the tool, potentially reducing its lifespan. Furthermore, Figures 12, 13 and 15 of Patent Document 1 show a shape in which flat grooves are formed between the ridges of the tool, but the flat surface of this groove comes into contact with the sheet material at once, and immediately after that the next ridge comes into contact with the sheet material with an impact, and this action is repeated, which poses the same problem.

[0006] The present invention has been made in consideration of the above, and provides an incremental sheet forming tool, an incremental sheet forming device, and an incremental sheet forming method that continuously contact the sheet material while continuously changing the pressing force, thereby maintaining the quality of the formed part and extending the tool's lifespan. [Means for solving the problem]

[0007] The incremental sheet forming tool according to the present invention is a rod-shaped body, the peripheral surface of the tip portion of which has a shape with a positive curvature all around the axis, and the curvature has directionality in the circumferential direction.

[0008] The incremental sheet forming device according to the present invention also includes a mechanism for supporting the incremental sheet forming tool and pressing the tip end portion of the rod-shaped body against the sheet material to be formed.

[0009] In addition, the incremental sheet forming method according to the present invention is such that the incremental sheet forming tool is supported by a mechanism section, and the tip portion of the rod-shaped body is pressed against the sheet material to be formed.

[0010] According to these aspects of the present invention, the peripheral surface of the tip portion is continuously in contact with the sheet material and has a directional property around the entire circumference, i.e., it has a non-constant positive curvature in the circumferential direction, and the pressing force of the tip portion is continuously varied in strength. As a result, there is no impactful contact between the tip portion and the sheet material, and damage to the sheet material, the life of the tool itself is extended, and cracking of the sheet material is suppressed.

[0011] The rod-shaped body includes a base shaft portion and a tip shaft portion that are parallel to each other and connected in the axial direction, and the peripheral surface of the tip shaft portion has a shape with a positive curvature all around the axis of the base shaft portion, and the curvature has a directionality in the circumferential direction. With this configuration, since the rod-shaped body is composed of a base shaft portion and a tip shaft portion, different types of tools can be assembled by configuring the tip shaft portion to be detachable from the base shaft portion.

[0012] The peripheral surface of the tip shaft portion has an elliptical shape. According to this configuration, by making the cross section of the tip shaft portion elliptical, a peripheral surface of positive curvature having directionality around the circumference is formed even though the tip shaft portion is provided coaxially.

[0013] In addition, the axis of the tip shaft portion is eccentric with respect to the axis of the base shaft portion. With this configuration, the change in the positive curvature in the circumferential direction with respect to the base shaft portion changes depending on the degree of eccentricity, making it versatile. [Effects of the Invention]

[0014] According to the present invention, by continuously contacting the tip portion with the sheet material while continuously changing the pressing force, it is possible to prevent scratches on the sheet material, extend the life of the tool itself, and prevent cracks in the sheet material. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a schematic configuration diagram showing an embodiment of an incremental sheet forming device according to the present invention; [Figure 2] 1A and 1B are perspective and bottom views of a conventional general-purpose tool, and FIG. 1C and 1D are perspective and bottom views of one embodiment of a tool according to the present invention. [Figure 3] 1 is a table showing the results of Experiment 1 regarding forming limits. [Figure 4] 10 is a diagram showing the results of Experiment 2 regarding heat generation temperature. [Figure 5] 10 is a diagram showing the results of Experiment 3 regarding in-plane strain. [Figure 6] 10 is a diagram showing the results of Experiment 4 regarding dimple fracture surfaces. [Figure 7] 10A and 10B are views showing another embodiment of an incremental sheet forming tool, in which (A) is a perspective view and (B) is a bottom view. [Figure 8] FIG. 10 is a bottom view of another embodiment of an incremental sheet forming tool. [Figure 9] 10A and 10B are views showing another embodiment of an incremental sheet forming tool, in which (A) is a side view and (B) is a view taken along the line BB'. DETAILED DESCRIPTION OF THE INVENTION

[0016] Fig. 1 is a schematic diagram showing an embodiment of an incremental sheet forming device 1 according to the present invention. Fig. 2 shows an incremental sheet forming tool 3, in which (A) and (B) are a perspective view and a bottom view of a known general tool, and (C) and (D) are a perspective view and a bottom view of an embodiment of the tool according to the present invention.

[0017] As shown in FIG. 1, the incremental sheet forming apparatus 1 (hereinafter referred to as apparatus 1) includes a drive unit 10 and a support unit 20. The drive unit 10 includes a control unit 11, an XYZ movement mechanism 12 controlled by the control unit 11, and a rotation mechanism 13. A rod-shaped incremental sheet forming tool 3 (hereinafter referred to as tool 3) is attached vertically to the tip of the rotation mechanism 13. The support unit 20 is disposed below the XYZ movement mechanism 12 and includes support columns 21 and a presser member 22 that vertically clamp multiple locations on the edge of a sheet material 4 of a predetermined size to be plastically processed using unillustrated fasteners. The XYZ movement mechanism 12 includes a known slider that moves in the X direction on one horizontal parallel plane and in the Y direction on the other plane, and also includes a mechanism for raising and lowering the rotation mechanism 13 in the vertical Z direction. The XYZ movement mechanism 12 and the rotation mechanism 13 are driven by, for example, motors.

[0018] The control unit 11 includes a processor that controls the machining operations of the XYZ movement mechanism 12 and the rotation mechanism 13. The control unit 11 is previously provided with forming data for the formed part, converts the forming data into drive signals, and outputs the signals to the corresponding XYZ movement mechanism 12 and rotation mechanism 13 to move and rotate the tool 3. Various methods can be used to move the tool 3 during machining, and the tool 3 may be moved in a circular manner as indicated by the white arrows, for example, along contour data (tool paths) set at predetermined intervals included in the forming data. The control unit 11 also outputs a motor rotation signal to the rotation mechanism 13 to control the rotation of the tool 3 at a predetermined rotation speed.

[0019] Next, the tool 3 will be described with reference to FIG. 2. A conventional general-purpose tool To is a rod-shaped body having a predetermined diameter, as shown in (A) and (B), and has a hemispherical tip portion that is pressed against the sheet material 4. On the other hand, the tool 3 is equipped with a base shaft portion 31 and a tip shaft portion 32 that are coaxial and integral (or semi-integral) in the axial direction, as shown in (C) and (D). The base shaft portion 31 is a rod-shaped body having a circular cross section with a predetermined diameter. The tip shaft portion 32 is a rod-shaped body having an elliptical cross section perpendicular to the axial direction, and the peripheral surface of its tip portion is formed into a semi-elliptical sphere. The tool 3 is rotatably attached to the rotation mechanism 13 via the base shaft portion 31.

[0020] An example of the shape of the tip shank 32 is shown in Figure 2(D). The radius of the base shank 31 of the tool 3 is 10 mm (denoted as D10). The major axis dimension of the tip shank 32 is D10, the minor axis dimension is D8, and the major axis dimension of the tip portion is D5, and the minor axis dimension is D4. Thus, at least the tip portion of the tip shank 32 is formed with a positive curvature (i.e., a convex shape) over the entire circumferential surface. Furthermore, the circumferential curvature continuously and repeatedly changes between maximum and minimum every quarter of a revolution around the axis O, and has a directional characteristic rather than being constant. Therefore, during machining, the pressing force of the tool 3 against the sheet material 4 changes continuously (gently) between strong and weak every quarter of a revolution. In this way, the tool 3 is constantly in contact with the sheet material 4 over the entire circumference, maintaining a state of pressing against the sheet material 4, and the pressing force changes continuously between strong and weak.

[0021] FIG. 3 is a diagram showing the results of Experiment 1 regarding the forming limit.

[0022] Figure 3 specifically shows the difference in the forming limit of sheet material due to differences in tools, and fracture depth and thinning rate were used as parameters to indicate the difference. The forming depth refers to the depth just before cracks or other damage occur on the surface of the sheet material 4 when the surface of the sheet material 4 is pressed in the depth direction. The thinning rate refers to the ratio of the initial thickness to the thickness just before cracks or other damage occur on the surface of the sheet material 4 when the sheet material 4 is pressed in the surface direction. The thinning rate was measured using a digital caliper (manufactured by Monotaro Corporation).

[0023] In Experiment 1, Comparative Examples 1 and 2 and Examples 1 and 2 were used. Comparative Examples 1 and 2 used tool To shown in Figures 2(A) and (B), with Comparative Example 1 being a case where processing was performed without rotation and Comparative Example 2 being a case where processing was performed with rotation at 3,000 rpm. Examples 1 and 2 used tool 3 shown in Figures 2(C) and (D), with Example 1 being a case where processing was performed with rotation at 1,500 rpm and Example 2 being a case where processing was performed with rotation at 3,000 rpm. The sheet material used in Experiment 1 was made of aluminum and had a square shape with a thickness of 0.5 mm and a side length of 240 mm. The shape formed on the sheet material was a concave truncated cone with an inclination angle of 0° to 90°.

[0024] 3 showing the results of Experiment 1, first, focusing on the forming depth, it was 29.0 mm in Comparative Example 1 and 35.0 mm in Comparative Example 2, but 37.5 mm in Example 1 and 42.5 mm in Example 2, and it was found that pressing was possible to a relatively deep position compared to Comparative Examples 1 and 2. Note that the photograph of the formed part shown at the top of Figure 3 is shown upside down for ease of explanation.

[0025] Furthermore, focusing on the thickness reduction rate, it was 54.0% in Comparative Example 1 and 66.0% in Comparative Example 2, but 76.0% in Example 1 and 84.0% in Example 2, demonstrating that it was possible to process to relatively thinner dimensions compared to Comparative Examples 1 and 2. Furthermore, comparing Example 1 and Example 2, it was observed that the values ​​of the forming depth and the thickness reduction rate increased as the rotation speed of the tool 3 increased. This is thought to be because, for example, in FSW (Friction Stir Welding), the metal material on the surface of the sheet material 4 is scraped out by the shape and rotational motion of the tool 3 and mixed with the neighboring material, thereby suppressing the formation of cracks. Various factors that explain the results of Experiment 1 will be discussed below in Experiments 2 to 4.

[0026] FIG. 4 is a chart showing the results of Experiment 2 regarding exothermic temperature.

[0027] Figure 4 specifically shows the difference in heat generation in sheet material due to differences in tools, with the horizontal axis representing the indentation depth (Z-displacement (mm), corresponding to the tool pass (see Figure 1)) and the vertical axis representing temperature (°C). Generally, when a metal exceeds its yield point, the strain in response to stress undergoes plastic deformation. This yield point is known to be temperature-dependent, decreasing the higher the temperature. When the temperature of the sheet material rises due to frictional heat generated by the tool movement, the yield point decreases, increasing the likelihood that the pressing force from the tool will cause cracks or deformation in the sheet material. Therefore, it is desirable to minimize the temperature rise in the sheet material during processing and maintain a high yield point.

[0028] In Experiment 2, Comparative Example 1 and Example 1 were used. In Comparative Example 1, the tool To shown in Figures 2(A) and (B) was used, rotating at 3,000 rpm to perform processing. In Example 1, the tool 3 shown in Figures 2(C) and (D) was used, rotating at 3,000 rpm to perform processing. The sheet material used in Experiment 2 was the same as in Experiment 1, and the target shape for forming was also the same. In Comparative Example 1, the indentation depth was 30 mm, and in Example 1, the indentation depth was 40 mm.

[0029] Referring to Figure 4 showing the results of Experiment 2, in Comparative Example 1, when the indentation depth exceeded 10 mm, the temperature rose to approximately 150°C, and then maintained at 170-180°C until the indentation depth reached 30 mm. On the other hand, in Example 1, although a slightly high temperature was observed at the beginning of indentation, it was then observed to maintain a stable temperature of around 100°C until the indentation depth reached 40 mm. Thus, in Comparative Example 1, where the heat generation temperature rose to 170-180°C, the yield point decreased, making the sheet material more susceptible to cracking during processing. On the other hand, in Example 1, the heat generation temperature was maintained relatively low, on the low side, so the decrease in yield point was not a problem and cracking was suppressed.

[0030] FIG. 5 is a diagram showing the results of Experiment 3 regarding in-plane strain.

[0031] Specifically, Fig. 5 shows the difference in in-plane strain due to differences in tools, with the vertical axis representing the vertical displacement ratio (D M / D0) on the horizontal axis, and the horizontal displacement ratio (D C More specifically, Figure 5 shows the susceptibility to cracking due to torsional deformation caused by the difference in forming dimensions between the surface (inner) against which the tool is pressed and the back surface (outer).

[0032] Experiment 3 employed Comparative Examples 1 and 2 and Example 1. Comparative Example 1 was performed using tool To shown in Figures 2(A) and (B) at a rotational speed of 0 rpm, i.e., no rotation, while Comparative Example 2 was performed using the same tool To at a rotational speed of 3,000 rpm. Example 1 was performed using tool 3 shown in Figures 2(C) and (D) at a rotational speed of 3,000 rpm. The sheet material used in Experiment 3 was the same as in Experiment 1, and the target shape of the forming was also the same.

[0033] In Experiment 3, first, identical circles (initial holes) of a predetermined diameter were marked on the front and back surfaces of the sheet material at appropriate positions before processing, and then the tool was operated to perform forming. After the forming operation was completed, the diameter of the previously marked circle was measured, and the ratio to the diameter of the initially drawn circle was defined as the vertical displacement ratio (D M / D0), lateral displacement ratio (D C / D0).

[0034] In Figure 5, which shows the results of Experiment 3, the three oval diagrams in the lower half of the figure show, from left to right, the plotted inner side (solid line) and outer side (dashed line) of Comparative Examples 1 and 2 and Example 1 after molding. As shown in Figure 5, in Comparative Example 1, as indicated by the black and dashed circles, both ratios significantly exceed 1.0, making it prone to cracking. Comparative Example 2, while improved over Comparative Example 1, also exhibits both ratios exceeding 1.0. In contrast, in Example 1, as indicated by the black and dashed triangles, both ratios are close to 1.0, and the black triangle (inner side) in particular is smaller than 1.0, suggesting that cracking is suppressed.

[0035] FIG. 6 is a photograph showing the results of Experiment 4 regarding the dimple fracture surface.

[0036] Specifically, Figure 6 shows the difference in dimples (depressions) on the machined surface due to differences in tools. Experiment 4 employed Comparative Example 1 and Example 1. Comparative Example 1 was formed using tool To shown in Figures 2(A) and (B) at a rotation speed of 3,000 rpm. Example 1 was formed using tool 3 shown in Figures 2(C) and (D) at a rotation speed of 3,000 rpm. Figure 6(A) shows the dimpled fracture surface of Comparative Example 1, where many dimples are formed. Figure 6(B) shows the dimpled fracture surface of Example 1, where it was observed that the size of the dimples was relatively smaller than that of Figure 6(A).

[0037] When the tool is rotated to continuously apply a uniform molding stress to the surface of the sheet material, as in Comparative Example 1, the material on the surface of the sheet material is peeled off, forming dimples, which are likely to become crack initiating points. On the other hand, when the pressing force is continuously (slowly) changed while maintaining contact with the sheet material, as in the tool of Example 1, the surface metal material nearby the formed dimples moves locally, i.e., the metal materials are mixed together, eliminating the dimples and suppressing the occurrence of cracks.

[0038] FIG. 7 shows another embodiment of an incremental sheet forming tool, with (A) being a perspective view and (B) being a bottom view. The tool 3A includes a cylindrical base shaft 31A having an axis O and a cylindrical tip shaft 32A having an axis Q at its tip, which is eccentrically mounted. As shown in FIG. 7(B), the axis O is positioned within the cross section of the tip shaft 32A, so that the tip shaft 32A is in continuous (constant) contact with the sheet material 4. The diameter of the cylindrical tip shaft 32A may be smaller or larger than the diameter of the base shaft 31A. This configuration allows the pressing force to be continuously changed while the tip shaft 32A is in continuous contact with the sheet material 4, thereby preventing scratches on the sheet material 4, extending the life of the tool 3A, and suppressing cracks in the sheet material 4.

[0039] FIG. 8 is a bottom view showing another embodiment of an incremental sheet forming tool. The tool 3B includes a cylindrical base shaft 31B having an axis O and an elliptical cylindrical tip shaft 32B having an axis Q at its tip, which is eccentrically disposed relative to the base shaft 31B. The axis O is positioned within the cross section of the tip shaft 32B, and the tip shaft 32B is configured to be in continuous (constant) contact with the sheet material 4. The diameter of the cylindrical tip shaft 32B may be smaller or larger than the diameter of the base shaft 31B. This configuration allows the pressing force to be continuously changed while the tip shaft 32B is in continuous contact with the sheet material 4, thereby preventing scratches on the sheet material 4, deformation of the tool 3B, and cracking of the sheet material 4.

[0040] FIG. 9 shows another embodiment of an incremental sheet forming tool, with (A) being a side view and (B) being a view taken along the line B-B'. Tool 3C is configured with hemispherical portions 311C and 312C, each having a different positive curvature at half the circumference of the tip of rod-shaped body 31C. Hemispherical portion 312C is formed with a larger curvature than hemispherical portion 311C. Note that hemispherical portion 312C may have a shape that covers half the circumference, or may be more or less than half the circumference of hemispherical portion 311C. Even with this configuration, by continuously (always) keeping rod-shaped body 31C in contact with sheet material 4 and continuously changing the pressing force between hemispherical portions 311C and 312C, it is possible to prevent scratches on the sheet material 4, deformation of tool 3B, and cracking of sheet material 4.

[0041] Although the present embodiment has been described as a configuration in which the tool 3 rotates (spins on its axis), the present invention can also be applied to a configuration in which rotation does not occur. Even in a configuration in which rotation does not occur, by rotating the tool 3 along the shape of the molded part, for example, along a contour line (tool pass), the phase of the tool 3 around the axis that abuts against the sheet material 4 sequentially moves and completes a full rotation. Therefore, at least the same effects as those of the above embodiment can be achieved. [Explanation of symbols]

[0042] 1. Incremental sheet forming device 10 Drive unit 20 Support part 3,3A,3B,3C Tools 31, 31A, 31B Base shaft (rod-shaped body) 31C Rod-shaped body 32,32A,32B Tip shaft part (rod-shaped body)

Claims

1. An incremental sheet forming tool, wherein the rod-shaped body has a base shaft portion and a tip shaft portion, the tip portion of the tip shaft portion has a peripheral surface having a shape with a positive curvature around the entire axis, the curvature having directionality in the circumferential direction, the tip shaft portion rotates coaxially with the base shaft portion, and the tip portion of the tip shaft portion has multiple positive curvatures when viewed from the side.

2. 2. The incremental sheet forming tool according to claim 1, wherein the tip end portion of the tip shaft portion has a tapered shape toward the tip end.

3. An incremental sheet forming device that supports the incremental sheet forming tool described in claim 1 or 2 and has a mechanism that presses the tip portion of the rod-shaped body against the sheet material to be formed.

4. An incremental sheet forming method in which a mechanism supports the incremental sheet forming tool described in claim 1 or 2 and presses the tip portion of the rod-shaped body against the sheet material to be formed.

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