A research on a new structure of forming tool in Single Point Incremental Forming (SPIF)

Use your smartphone to scan this QR code and download this article ABSTRACT Single Point Incremental Forming (SPIF) is really a new technology of forming metal sheet nowadays and in recent decades. Although it was invented in, 1967 by Lezak, an American inventor, but the applications of the innovative technology were broad from 1990 because of the advance of controlling technology. This technology is especially adapted to small batch, unique or single productions. There are many forming parameters that influence to the formability of the metal sheet workpiece such as diameter of tool, the revolution per minute of the tool tip, the vertical feed rate after each orbit, the velocity of tool in horizontal plane.... Among of them, in our own experiences, we recognize that in almost all cases, the revolution per minute of the forming tool when forming ferricmaterial sheet such asmild steel, stainless steel, hard steels... should be as small as possible to get the biggest ability of deformation of theworkpiece sheet to get rid of failure on the lateral edge of the sheet. The tangential velocity of forming point on the spherical tool tip should be selected to attain the situation of rolling but no sliding of the surface of the spherical tool on the one of the sheet material. The paper recommends a new version of a forming tool in which the tip of the tool is a very hard ball (such as the quenched ball in a ball bearing) that is freely rotate by the friction to modify the contact point on the spherical surface of the tool to avoid the abrading and keep the spherical shape and the situation of rotating but no sliding on the surface of the workpiece sheet as mentioned above. Themanufacture of the innovative forming tool is performed and then empirical processes verified it. Themodels formed by the typical tool are better in comparisonwith the ones of normal forming tool.


INTRODUCTION
According to almost all researches, 1-3 the forming mechanics in Incremental Sheet Forming (ISF) differs from conventional forming process as the material is mainly deformed by local deformation at contact area between tool and workpiece sheet. Due to the forming mechanism of the Forming Limit Curve (FLC) Figure 4 in ISF is higher than the one in conventional processes, and typically defined by a negative slope straight line. During the forming process the sheet suffers a combination of shear, stretching and bending that lead to thinning, approximated by the sin law: t f = t 0 × sin(90 − ϕ ). Figures 3 and 4 The failure conditions in SPIF mostly occur with uniform thinning until fracture. Failure results on cracking that typically fallows a zigzag propagation. Aside from fracture, failure may include skinning 4 , essentially when dealing with larger forming forces 1 . The purpose of this paper is a recommendation of way of enhancing the formability of the Single Point Incremental Forming (SPIF) by a new structure of tool and spindle to eliminate or to decrease the friction between tool and workpiece sheet.

METHOD: SCHEME OF RECOMMENDATION A NEW STRUCTURE OF SPIF TOOL AND RESULT
Normally a traditional ISF tool is hold via a suitable collet in the spindle of CNC milling or specialize ISF machine as in Figure 1. It rotates with the RPM of the spindle that is selected with a suitable value according to the sheet material, coarse or finishing session. Because tool have no cutting edge so we do not known the effect of its rotation on the ability of forming metal sheet. We carried out an experiment on 3 typical materials sheets: • Aluminum A 1059 H14 stands for nonferrous metal, • SS330 steel stands for popular mild steel, • SUS304 stainless steel stands for popular metal used in industry and daily life.
In order to evaluate the effect of revolution of the tool when forming sheet by SPIF technology, a set of 4 in-   In order to measure the angle a that makes of the tangent line with the sheet profile and the horizontal line at the first tore point on the sheet, a model profile is selected in Figure 2. Herein: R radius of model profile, z depth at random point on profile. The angle α that stands for the formability 2,4 is calculated by α = arc cos . All the experiments were achieved in Specialized SPIF machine ( Figure 3) in CAD-CAM workshop of DCSELAB.

Empirical process and result
In the Design of Experiment (DOE), 2 limited values are used in partial DOF of 4 factors, so the number of experiments for each typical metal is 24-1=8. Calculated number of iteration is 3. Therefore, the number of models for each typical material is 8×3=24. The initial limited values of each parameter are selected according to the ability of machines and the stiffness of tool and metal sheet: The average results of measuring the angle of deformation of 3 typical materials: From the results of angle a that stand for the formability of 3 materials under the influence of 4 parameters in Table 2, with the help of Minitab software, we attempt to form 3 recursion equations of the angle of formability:

Science & Technology Development Journal -Engineering and Technology, 2(SI1):SI157-SI163
The partial differential of n is derived from the above equation to define the influence of revolution n to α ∂ α ∂ n = 0.00023 > 0 Hence, in case of aluminum or other nonferrous metal, the formability is covariant with the revolution of the tool. We can say that the higher the revolution of the tool, the bigger the formability of the product. Recursion equation of mild steel SS330: Recursion equation of stainless steel SUS304: From (1) and (2) we recognize that the formability of mild steel and stainless steel or more generality other ferrous metal is contra-variant with the revolution of the tool n. That means that when forming the popular ferrous metal technician should select the value of revolution of spindle or tool as small as possible.
Recommendation a new structure of SPIF tool for ferrous metal sheet Figure 4 illustrates the structure of SPIF tool that is designed for ferrous metal sheet only under our own experience in forming sheet by SPIF technology with following specifications: • The thermal treated ball tip with the diameter of ∅5 get the hardness of 60-62 HRC that is assembled on the end of the taper HSS body by cooper braze weld. • HSS cylinder tool body with diameter of ∅12 is hold with a couple of angular ball bearing Koyo 6000 via diameter of ∅10m 7 • The angular ball bearing are fixed inside a bushing which has outside diameter of ∅36 • The bushing is hold by DIN 6388 collet OZ-32 5 inside the arbor of the spindle that illustrated in Figure 5 The structure of designed SPIF tool is represented in Figure 4. This innovative SPIF tool consists of a couple of thrust angular bearing and a shoulder of ∅44 that could support the huge axial force that the workpiece sheet applies on the tool. With the selected bore of ∅32, the designed tool could be perfectly held by standard OR-32 collet 5 .

Result of the new structure of SPIF tool for ferrous metal sheet
The application of the new structure of the tool for popular mild steel is useful to increase the formability of these ferous sheet materials as in Figure 6.

DICUSSION
The innovative structure of SPIF tool could be directly assembled to the arbor of the 3D CNC milling machine or the current specialist 3D ISF machine in DC-SELAN. In the future, when the effective performance of the new designed tool will be proved, it could be assembled to a future structure head tool that will reach the convertible modification to 5D CNC ISF machine.

CONCLUSION
This design tool is applied on y for ferrous sheet. In operation, the spindle of CNC milling machine or specialized SPIF machine have to be set fixed and stable. Because of the friction between the spherical tool tip and the sheet material, the ball rotates no sliding on the surface of the formed sheet to get the smallest revolution to satisfy the condition to enhance the formability of almost all ferrous material sheets.