Two-point Incremental Forming Research Articles

Prediction of Negative Bulge in Two Point Incremental Forming of an Asymmetric Shape Part

The bulging effect in incremental sheet forming (ISF) has been explained by the build-up of compressive stresses during forming resulting in the material at the center to bulge inward or outward. In this study, effect of step size on the formation of a bulge in an asymmetric shape in two point incremental forming (TPIF) is examined. Experimentally validated finite element models were developed to simulate the forming at both the macro scale, and using a finer mesh to generate the surface profile. The formation of the bulge is determined to be due to a combination of compressive stresses and an instability created by the material in the underformed area during the forming operation.

Optimization of two-point incremental forming process of AA1050 through response surface methodology

Two point incremental forming (TPIF) process is used in the industry to enhance the dimensional accuracy of formed parts. However, the process limits the formability of metal sheets due to tension between two points i.e. tool and mandrel. Hence, selection of optimum TPIF process to enhance formability is a key issue in manufacturing of complex parts. In the present work, an experimental study based on response surface methodology was made to analyze formability of aluminum 1050 in two-point incremental forming process. Here, Box-Bhenken experimental design was utilized taking into account mandrel angle, tool nose diameter, sheet initial thickness and step down as input parameters as well as thinning ratio and maximum resultant force as output responses. Analysis of variances was also performed to find contribution of factors on the responses. From the results, it was found that the wall angle is the most influential factor for thinning ratio; while, initial thickness followed by step down have great impact on forming force. By performing multi-objective optimization, it is found that for simultaneous minimization of thinning ratio and forming force, setting of 45° wall angle, 15 mm tool nose diameter, 1 mm initial thickness and 0.5 mm step down should be selected. The obtained optimal solution was further verified by confirmatory experiment and results showed accuracy and predictability of proposed approach.

Multi-objective optimisation of profile accuracy in two point incremental forming using Taguchi-based grey relation analysis

This study investigates the effect of four key process parameters in two point incremental forming (TPIF) on the profile accuracy of the manufactured parts. The experiment was applied to A1050-H14 commercial aluminium sheets under different values for tool diameter, feed rate, vertical step size, and sheet thickness. The experiments were designed using the Taguchi L8 orthogonal array. Profile accuracy was estimated by measuring four quality parameters, namely, circularity error, waviness error, depth error, and side wall angle error. Analysis of variance (ANOVA) was used for studying the significance of each process parameter. Grey relational analysis (GRA) was then used to perform a multi-objective optimisation of the four quality parameters simultaneously. The results showed that while each quality parameter may be optimised separately, it is considerably difficult to optimise all of them together. In practice, a compromise must be made based on the profile feature necessary in a given situation.

Development of elastomer-based incremental sheet forming method for curved products

A novel methodology based on the elastomer-based incremental sheet forming method was developed to employ oblique two-point incremental forming. In order to evaluate the effectiveness of the develped method, several experiments have been conducted for curved products. Our experiments with the proposed method reveal that the material displacements do not only occur along the straight projection line, but also arise along the curved projection path.

Customized cranial implant manufactured by incremental sheet forming using a biocompatible polymer

PurposeThe purpose of this paper is to demonstrate the feasibility of incremental sheet forming (ISF), using the most common variants, single-point incremental forming (SPIF) and two-point incremental forming (TPIF), to produce prototypes of customized cranial implants using a biocompatible polymer (ultrahigh molecular weight polyethylene, UHMWPE), ensuring an appropriate geometric accuracy and cost.Design/methodology/approachThe cranial implant is designed based on computerized tomographies (CT) of the patient, converting them into a 3D model using the software InVesalius. To generate the toolpath for the forming operation computer-aided manufacturing (CAM) software is used. Once the cranial implant is manufactured, a 3D scanning system is used to determine the geometric deviation between the real part and the initial design.FindingsThe results corroborate that it is possible to successfully manufacture a customized cranial implant using ISF, being able to improve the geometric accuracy using the TPIF variant with a negative die.Originality/valueThis paper is one of the first research works in which a customized cranial implant is successfully manufactured using a flexible technology, ISF and a biocompatible polymer. The use of polymeric implants in cranioplasty is advantageous because of their lightweight, low heat conductivity and mechanical properties similar to bone. Furthermore, the cost of the implant has been calculated considering not only the raw materials and manufacturing time but also the environmental impact, revealing that it is a cheap process with a low lead-time.

Experimental Analysis of Several Variables Influencing Formed Thickness in Two-Point Incremental Forming Process

In the current paper, an experimental analysis on Al-sheet (AA 1050) with thickness 0.9 mm to reveal the effect of relevant forming factors on the formed thickness in two-point incremental forming (TPIF) process has been conducted. The formed thickness of pyramid-like shapes was analyzed by studying seven variables: die geometry, tool diameter, tool path, stepover, tool shape, lubricant and slope angle. The proposed analysis utilizes Box-Behnken design of experiment (BBD), main effects plot (MEP) and analysis of variance (ANOVA) for sake of studying the influences of the seven forming factors on the resulted thickness. The results of these analyses have indicated that the most significant factor affecting the formed thickness is the die geometry followed by tool shape, lubricant and stepover respectively for both slope angles of the pyramid. In addition, it has been found that the other variables have also significant effects on the formed thickness at both slopes of the pyramids produced.

Optimization of multiple quality characteristics in two-point incremental forming of aluminum 1050 by grey relational analysis

Incremental forming process is used to reduce the cost of sheet metal forming process when the low volume batches, customized parts or prototypes have to be manufactured. During recent years, some modifications were associated with this process to enhance formability, dimensional accuracy and product quality. In the present study, an experimental attempt was made to analyze formability of aluminum 1050 in two-point incremental forming process. Here, the effect of incremental forming parameters such as tool nose diameter, wall angle, spindle speed and deep step was studied on minimum thickness, surface quality, and springback. The obtained results showed that desirable thickness and corresponding formability are enhanced when the mandrel with lowest angle, i.e. 63° is utilized and tool tip diameter is 15 mm. It is also found that the increase in spindle speed causes reduction of formability due to exerting high temperature in the sheet material that increases thinning, while the increase in tool rotation improves both the dimensional accuracy and surface quality. Also, it was found that the increase in deep step and wall angle has a negative effect on quality characteristics. By performing multi-objective optimization through grey relational analysis, it is found that 15 mm tool nose diameter, 63° wall angle, 800 r/min spindle speed and 0.2 mm deep step are optimum parameter settings that cause 63% improvement in overall quality characteristic from initial parameter setting.

Part accuracy improvement in two point incremental forming with a partial die using a model predictive control algorithm

As a flexible forming technology, Incremental Sheet Forming (ISF) is a promising alternative to traditional sheet forming processes in small-batch or customised production but suffers from low part accuracy in terms of its application in the industry. The ISF toolpath has direct influences on the geometric accuracy of the formed part since the part is formed by a simple tool following the toolpath. Based on the basic structure of a simple Model Predictive Control (MPC) algorithm designed for Single Point Incremental Forming (SPIF) in our previous work Lu et al. (2015) [1] that only dealt with the toolpath correction in the vertical direction, an enhanced MPC algorithm has been developed specially for Two Point Incremental Forming (TPIF) with a partial die in this work. The enhanced control algorithm is able to correct the toolpath in both the vertical and horizontal directions. In the newly-added horizontal control module, intensive profile points in the evenly distributed radial directions of the horizontal section were used to estimate the horizontal error distribution along the horizontal sectional profile during the forming process. The toolpath correction was performed through properly adjusting the toolpath in two directions based on the optimised toolpath parameters at each step. A case study for forming a non-axisymmetric shape was conducted to experimentally validate the developed toolpath correction strategy. Experiment results indicate that the two-directional toolpath correction approach contributes to part accuracy improvement in TPIF compared with the typical TPIF process that is without toolpath correction.

Recent Approaches for the Manufacturing of Polymeric Cranial Prostheses by Incremental Sheet Forming

This paper presents recent research experiences developed with the aim of manufacturing cranial prostheses in polymeric sheet using Incremental Sheet Forming (ISF) technologies. With this purpose, different approaches have been carried out in Single-Point Incremental Forming (SPIF) and Two-Point Incremental Forming (TPIF) in order to produce customized cranial implants using different polymeric materials. In this context, this research work provides a methodology to design and manufacture polymer customized cranial prostheses using the ISF technologies starting from a patient's computerized tomography (CT). The results demonstrate the potential of manufacturing polymeric cranial prostheses by ISF in terms of the high formability achievable and show the appropriate geometrical accuracy at affordable manufacturing costs provided by these processes.

Analysis into Differences between the Buckling in Single-Point and Two-Point Incremental Sheet Forming of Components for Self-Supporting Sheet Metal Structures<sup></sup>

In the architecture and construction sector the trend for individualization is often expressed in complex-shaped freeform buildings. Due to missing universal and mature construction methods for freeform buildings, they are usually realized with customized solutions that often include massive, material-consuming substructures, while the visible skin has neither structural nor functional properties. In this context a new concept for self-supporting lightweight structures for the realization of free-form surfaces and the production of the corresponding components has recently been proposed. Taking into account the large part dimensions and the varying part geometries in this application a flexible production chain based on incremental sheet forming has been developed and optimized. It has been validated by producing six-sided large-scale pyramids in 140 similar variants which were assembled to a self-supporting free-form demonstrator. Two-point incremental sheet forming (TPIF) was used with a universal partial supporting tool with the goal to produce all variants without dedicated tooling. Although the majority of pyramids was produced successfully with the applied TPIF strategy, there was a small number of parts with a very asymmetric shape that showed severe buckling in the side walls. For a detailed analysis of this observation the asymmetry was quantified using a wall angle ratio. Subsequently, a single-point incremental sheet forming (SPIF) strategy was successfully applied as an approach to avoid buckling. The results confirm the assumption that the circumferential expansion in SPIF suppresses buckling due to tensile stresses in the side walls, whereas the circumferential compression in TPIF triggers buckling due to the compressive stresses in the side walls.

Numerical simulation and experimental investigation to improve the dimensional accuracy in electric hot incremental forming of Ti–6Al–4V titanium sheet

Electric hot incremental forming is feasible and easy to control to form hard-to-form sheet metals, but the limited accuracy is a major deficiency. In order to find methods to improve precision, single-point electric hot incremental of Ti–6Al–4V titanium sheet was numerically simulated using MSC.Marc, and experimental investigations were also carried out in this paper. Through numerical analysis, distributing laws of temperature, thermal strain, stress, and equivalent strain were revealed, and impacts of cold contract and thermal strain on forming were also revealed. Analysis showed that electric hot incremental forming is a complex pyroplastic deformation, and there is a large internal stress in single-point electric hot incremental forming. The incremental sheet forming region can be divided into three parts: bending deformation at the beginning, shear forming at middle, and reverse bending at last; it is important to enhance the accuracy of the bending part and the reverse bending part, and adequate support must be provided in the beginning to reduce the bending part. In order to form a workpiece with small angle, two-point incremental forming was adopted at first because the gravity of clamp can reduce the reverse bending, then single-point electric hot incremental forming was adopted to enhance the accuracy and reduce internal stress of workpiece.

Fabrication of a biopsy meso-forceps prototype with incremental sheet forming variants

This study aimed at developing an application and comparison of performance between the two main incremental sheet forming (ISF) variants, single-point incremental forming (SPIF) and two-point incremental forming (TPIF), on a scaled prototype of a biopsy micro forceps (BMF). Experimental tests were carried out initially with the SPIF set-up. Rupture of specimens and low accuracy of them due to bending of the material led to the more complicated TPIF variant. Prior to the physical support tool construction, FEM simulations allowed validating the precision gains. After several adjustments to the tool-path in the new configuration, the BMF was successfully manufactured by TPIF on AA1050-H24 and AISI 304 05 mm thick. Therefore, the presented results demonstrate the feasibility to use ISF for the rapid prototyping of sheet metal medical devices.

Two-Point Incremental Forming with Partial Die: Theory and Experimentation

This paper proposes a new level of understanding of two-point incremental forming (TPIF) with partial die by means of a combined theoretical and experimental investigation. The theoretical developments include an innovative extension of the analytical model for rotational symmetric single point incremental forming (SPIF), originally developed by the authors, to address the influence of the major operating parameters of TPIF and to successfully explain the differences in formability between SPIF and TPIF. The experimental work comprised the mechanical characterization of the material and the determination of its formability limits at necking and fracture by means of circle grid analysis and benchmark incremental sheet forming tests. Results show the adequacy of the proposed analytical model to handle the deformation mechanics of SPIF and TPIF with partial die and demonstrate that neck formation is suppressed in TPIF, so that traditional forming limit curves are inapplicable to describe failure and must be replaced by fracture forming limits derived from ductile damage mechanics. The overall geometric accuracy of sheet metal parts produced by TPIF with partial die is found to be better than that of parts fabricated by SPIF due to smaller elastic recovery upon unloading.

Analysis of forces, accuracy and formability in positive die sheet incremental forming

Sheet Incremental Forming (IF) is a forming technology which consists of a sheet clamped along its edges by a suitable blank-holder while an hemispherical head punch is moved along a defined path and locally deforms the sheet. To improve the part geometrical accuracy, a die (which can be positive or negative) is placed behind the sheet with respect to the punch position. In this case, the process is called Two Point Incremental Forming (TPIF). In the present paper TPIF with positive die was studied through an experimental tests campaign using deep drawing steel sheets. The die geometry used in this research was chosen so to be representative of the process and it was tested using different tool paths. Forming forces were measured using self designed instrumented punch and table while the final workpiece geometry was detected using a coordinate measuring machine (CMM). The collected data allowed to study the effects of the different tool paths on the maximum forming forces, the geometry errors, the maximum reachable wall angle and the drawing depth during forming and after springback. The results were also compared with a previously performed experimental campaign where similar tests with negative die were conducted.

Comparison of some Final Part Geometrical Characteristics of Cylindrical Cups Manufactured by Deep-Drawing and Two-Point Incremental Sheet Forming

Asymmetric incremental sheet forming (AISF) is a new sheet metal forming process in which sheet metal parts are produced by CNC-controlled movements of a simple ball-headed forming tool. Despite its flexibility and successful application in many cases, AISF has not yet been established in an industrial context due to some still existing process limits such as severe thinning, which strongly depends on the inclination of the part surface, as well as a limited geometric accuracy due to springback. Furthermore, there is little knowledge available about the properties of parts produced by AISF, especially in comparison to deep-drawn parts. The aim of the present paper is to compare cylindrical cups manufactured by deep-drawing and AISF regarding the resulting strain and thickness distribution. For AISF, different forming strategies were applied. Comparisons of the wall thickness and surface strain distributions show similar results for the cup produced by deep-drawing and the best cup produced by AISF, but the surface strains and the sheet thinning in the parts formed by AISF were larger than in the deep-drawn part.

The mechanics of incremental sheet forming

The deformation mechanism of incremental sheet forming (ISF) is examined experimentally through forming specially prepared copper sheets. Strain distributions through the thickness of the sheets are measured for two configurations of ISF: two-point incremental forming (TPIF) and single-point incremental forming (SPIF), and a comparison is made to pressing. The measurements show that the deformation mechanisms of both SPIF and TPIF are stretching and shear in the plane perpendicular to the tool direction, with shear in the plane parallel to the tool direction. Strain components increase on successive laps, and the most significant component of strain is shear parallel to the tool direction. Increasing stretching and shear perpendicular to the tool direction account for differences between the sine law prediction and measured wall thickness for both SPIF and TPIF. The observed mechanisms of SPIF and TPIF differ from a mechanism of pure shear that has previously been assumed.