Research on the Influencing Factors of " Scale Veins " of Single Point Incremental Forming Workpieces Surface

To furthermore optimize the machining parameters and improve the surface quality of the workpieces manufactured by single point incremental forming method, the formation mechanism of the sacle veins on the metal incremental froming workpieces was studied through experiment method. The influence principle of the spindle speed, the feed speed and the material of tip of tools on the length of scale veins was obtained through analyzing the experimental results and building the mathematical model among the length of scale veins, feed speed and spindle speed through measuring the roughness of surfaces and observing the appearance of the forming workpieces. The experimental results showed that, the spindle speed, the feed speed and the material of tool tips have a significant effect on the scale veins formation on the surface of forming workpieces. Therefore, an appropriate group of spindle speed and feed speed can reduce the effect of scale veins on the roughness of single point incremental forming workpieces and furthermore improve the surface quality of forming workpieces.


INTRODUCTION
In the 1980s, Matsuhara Shio, Japanese scholar, proposed an incremental forming technology-a sheet metal mouldless flexible manufacturing technology.In this technology, depending on the hierarchical manufacturing concept, the three-dimensional entity is cut into slices hierarchically along the contour line to form a series of two-dimensional layers; each layer is formed in accordance with a certain track and is processed to form the final shape [1][2][3][4].This technology is independent of special moulds, and is high in flexibility and repeatability, so that the development period of metal sheet metal parts is shortened, the huge mould manufacturing costs are saved, and the application prospect is extensive [5][6][7][8].
Currently, the study on the surface quality of formed workpieces is the important direction of the study of the incremental forming technology.Wu Yan et al. from Nanjing Aeronautics and Astronautics University analyzed the change rule of parts in different forming angles via test comparison [9].Jeswiet et al. from Queen's University in Canada established quantitative relation between part shapes and increment step length [10].Durante et al. from University of Naples Federico in Italy derived relation between peak valley height and the radius and increment step length of forming tool [11].Song Xiucheng et al. from Shanghai Jiaotong University drew the mechanism of action and influence degree of each factor of influence of the surface quality of incremental forming parts [12].The study *Address correspondence to this author at the School of Mechanical and Precision Instrument Engineering, Xi'an University of Technology, 710048, Xi'an, China; E-mail: shipengtao@sust.edu.cn of the above persons is mainly focused on the influence of interlayer residual wave height on surface quality during the processing of incremental forming, but they were less focused on the study on the surface quality in each layer.
In this article, the forming mechanism of scale veins in each layer is analyzed via the experiment and the influence rule of various process parameters on scale veins is studied to provide theoretical basis for reducing the influence of scale veins and further improving the surface quality of formed workpieces.

EXPERIMENT PRINCIPLE
Under the further observation of the surfaces of sheet metal single-point incremental forming workpieces, we can find that the surfaces of formed workpieces have two veins affecting their surface quality.One is the transverse vein formed by residual wave between every two layers, and the other is scaly vein in each layer (hereinafter referred to as scale figure), as shown in the Fig. (1).The main influence factors of the first vein include interlayer spacing, the radius of tool head, forming angle, etc., and the influence of the first vein on the surface quality can be reduced by adjusting these parameters.When the influence of the first vein on the surface quality is reduced to a certain degree, scale veins in the layers are the major reason affecting the surface quality.In this article, the feed speed, rotating speed of spindle, spindle material, etc. are changed in the experiment process to obtain the workpiece surfaces with different surface quality.Surface roughness Rz value is measured by using the surface roughometer.At the same time, surface morphology is observed by using the stereoscopic microscope, and the formation mechanism of scale veins and the influence rule of scale veins on surface quality are explored via the analysis on numerical values and surface morphology.

Experiment Equipment and Material
In this experiment, the CY-VMC850 machining center is used as the main equipment of incremental forming.The clamp is fixed on the machining center by utilizing the flat tongs, as shown in the Fig. (2).The head of forming tool is the cylinder with diameter ofφ10, the end is in the hemispherical shape, and surface is polished.In order to reduce the abrasion of the tool head and prolong the service life of the tool head, the tool head is lubricated by using Kunlun Tianrun KR8 lubricating oil.2A112 plate of 150 mm×150 mm×1 mm is used as the experiment material.After machining, surface roughness Rz value is measured by using the Time TR200 surface roughometer, surface morphology of sample parts is observed by using the PXS5-T stereoscopic microscope, and the cycle length of veins is measured.

Parameter Setting
In this experiment, the inverted square bench is used as the experimental model, and is machined along the path of contour line, as shown in the Fig. (3).

Forming Angle
As the incremental forming process of the sheet metal is similar to forming by spinning, we can establish the shearing deformation mechanism in the incremental forming process by taking example of the deformation mechanism of forming by spinning [13][14][15][16].The shearing deformation mechanism considers that metal basically makes shear flow along the axial direction of workpieces in the incremental forming process [17,18], and after forming, the change in the thickness of sheet metal conforms to the cosine rule, as shown in the Fig. (4).
The formula ( 1) is substituted to obtain From the experiment, in order to avoid the cracks of 2Al12 sheet metal of 1 mm, the maximum thinning rate of the sheet metal shall not exceed 69.9% [11], and it is substituted into the formula (3) to obtain the forming angleα=72.5o , thus, the forming angle α shall be less than 72.5 o , and in this experiment, α=45 o to avoid cracks and even breakage.

Increment Step Length
In this article, the influence of scale veins in each layer on surface quality is studied, thus, a certain increment step length shall be selected to reduce the influence of interlayer residual wave height on surface quality, so that scale veins are the main factor affecting the surface quality.The relation [19,20] between interlayer residual wave height and increment step length is shown in the Fig. (5).4) and ( 5) to obtain As increment step length is 0.5 mm, h s =12.5μm, in the actual machining process, due to the influence of the vibration, friction, shearing, etc. of tool head, surface roughness Rz is 3.632μm [21], which is close to the surface roughness Rz of sheet metal without machining, and at the moment, scale veins are the main factor affecting surface quality.

Setting of Other Parameters
In order to study the scale veins better, we hope to obtain the distinct appearance of the scale veins.Therefore, in this experiment, the rotating speed of spindle is 50 to 1000 r/min, and the feed speed is 100 to 1000 mm/min.The scale veins are distinct to facilitate the study.

Experiment Scheme Design and Parameter Setting
In this experiment, the tool head is made of hard alloy material, the feed speed of spindle is 300 mm/min, and the machining path is in the counterclockwise direction.The spindle rotates at the speed of 50 to 950 r/min in the clockwise direction, and is tested at intervals of 100 r/min.After the experiment is finished, 6 different positions are selected randomly on each machined surface.Rz is measured by using the surface roughometer along the direction which is parallel to the contour line, its arithmetic mean value is valued, and at the same time, the surface morphology of sample parts is observed by using the stereoscopic microscope.

Experimental Result
The measuring results of surface roughness Rz at different rotating speeds are shown in the Table 1.Data in the Table 1 is subjected to Kruskal-Wallis inspection of multiple independent samples; significance is p=0.002＜ 0.05, indicating that the influence of the rotating speed of spindle on roughness Rz is significant within 95% confidence interval.And the higher the rotating speed of spindle is, the smaller the surface roughness Rz is .
The surface of the sample is observed by using the stereoscopic microscope, as shown in the Fig. (6).The scale veins on the surface are changed periodically, when the rotating speed is low, the cycle length of surface veins is large, and the wavy veins appear on the surface, as shown in the Fig. (6a); and when the rotating speed is high, the cycle length is reduced, the scaly veins appear on the surface, as shown in the Fig. (6b-d).The wavy veins and the scaly veins are essentially the same veins, the forming principle is the same, and they are in different shapes due to different cycle lengths.

Experiment Scheme Design and Parameter Setting
In this experiment, the tool head is made of hard alloy material, the feed speed of spindle is 300 r/min, and the machining path is in the counterclockwise direction, as shown in the Fig. (4).The spindle rotates in the clockwise direction, the feed speed is 100 to 1000 mm/min, and is tested at intervals of 100 r/min.After the experiment is finished, 6 different positions are selected randomly on each machined surface, Rz is measured by using the surface roughometer along the direction which is parallel to the contour line, its arithmetic mean value is valued, and at the same time, the surface morphology of sample parts is observed by using the stereoscopic microscope.

Experimental Results
The measuring results of surface roughness Rz at different feed speeds are shown in the Table 2.
Data is subjected to Kruskal-Wallis inspection of multiple independent samples; significance is p=0.008＜ 0.05, indicating that the influence of the feed speed on roughness Rz is significant within 95% confidence interval, and the feed speed is higher, the surface roughness Rz is larger, and the scale veins are more obvious.The feed speed is lower, the surface roughness Rz is smaller, and the scale veins are more unapparent.
The surface of the sample is observed by using the microscope, as shown in the Fig. (7).Under the overall consideration of Rz in the Table 2, when the feed speed is higher, the cycle length of surface veins is larger, and the surface roughness Rz is larger, and when the feed speed is lower, the cycle length of surface veins is smaller, and the surface roughness Rz is smaller.

Experiment Scheme Design and Parameter Setting
In this experiment, the rotating speed of spindle is 300 r/min, and the machining path is in the counterclockwise direction, as shown in the Fig. (4).The spindle rotates in the clockwise direction, the feed speed is 300 mm/min, and Q235, 40Cr, 06Cr19Ni10 and 45 are used as the materials of the tool head respectively to be tested.After the experiment is finished, 6 different positions are selected randomly on each workpiece.Rz is measured by using the surface roughometer along the direction which is parallel to the contour line, and its arithmetic mean value is valued, and at the same time, the surface morphology of sample parts is observed by using the stereoscopic microscope.

Experimental Results
Sample parts are machined by using the tool head made of different materials, and the results of the measured surface roughness Rz are shown in the Table 3. Data is subjected to Kruskal-Wallis inspection of multiple independent samples; significance is p=0.005＜0.05,indicating that the influence of the hardness of the tool head on roughness Rz is significant within 95% confidence interval, and when the hardness is larger, the surface roughness Rz is smaller; and when the hardness is smaller, the surface roughness Rz is larger.The photographs of the scale veins under the stereoscopic microscope are shown in the Fig. (8), when the tool heads are made of different materials, the cycle lengths of the machined surface veins are basically the same, the material hardness is larger, surface veins are more unapparent, the surface roughness Rz is smaller; and when the material hardness is smaller, surface veins are more obvious, and the surface roughness Rz is larger.

Analysis of Formation Reason of Scale Veins
As shown in the Fig. (9), during incremental forming, tool tips may be bent and deformed under the resistance in the reverse direction in the machining process.Simultaneously, the spindle rotates, so that the size and direction of the force of tool tips are changed continuously.The size and direction of the extrusion force of the formed surface of sheet metal are also changed periodically with the rotation of tool tips under the retroaction of force, so that the surfaces of the formed workpieces are extruded to form periodic veins, namely scale veins.

Cycle Length of Scale Veins
From the above analysis, the formed surface of sheet metal is subjected to the extrusion force which is changed periodically in size and direction.The periodic change of the extrusion force occurs as the tool tips rotate by a circle to form the scale veins on the formed surface of sheet metal.Therefore, cycle length is L=vt (7) where, v̶Movement speed of spindle (mm/s) t̶Time required by every cycle (s) v=F/60 (8) where, F̶Feed speed (mm/min) t=1×60/S (9) where, S̶Rotating speed of spindle (r/min) Combine the formulas ( 7), ( 8) and ( 9), L=F/S (10)   From the formula (10), in the case of regular rotating speed S, the feed speed F is higher, and the cycle length of the scale veins is larger.In the case of regular feed speed F, the rotating speed S of spindle is larger, and the cycle length of the scale veins is smaller.The cycle lengths of the above experiments are measured respectively, as shown in the Fig. (10), and values are consistent with the results derived from the above formulas.

Influence of Rotating Speed of Spindle and Feed Speed on Surface Roughness
In the case of the same feed speed, rotating speed is higher, the feed distance of the tool tip within one cycle is smaller, and deformation is smaller.At the same time, the change in the extrusion force of the tool tip on the formed surface of the sheet metal is smaller, and surface roughness is smaller.On the contrary, surface roughness is larger.
When the rotating speed of spindle is regular, the feed speed is lower, the feed distance of the tool tip within one cycle is smaller, the deformation of the tool tip is smaller, change in the pressure on the formed surface is smaller, and surface roughness is smaller.On the contrary, surface roughness is larger.

Influence of Tool Head Material on Surface Roughness
From experimental data, surface roughness Rz is different with difference in the tool head materials in the case that the feed speed F is equal to the rotating speed S of spindle.The reason is that when the feed speed F is equal to the rotating speed S of spindle, force of the tool head is the same; and when the hardness of the tool head materials is smaller, deformation is larger, change in pressure on the formed surface is larger, the scale veins are more obvious, and surface roughness Rz is larger.When the hardness of the tool head materials is larger, deformation is smaller, change in pressure on the formed surface is smaller, the scale veins are more unapparent, and surface roughness Rz is smaller, as shown in the Fig. (11).

CONCLUSION (1)
During the incremental forming of sheet metal, the formed surface may be subject to the periodically-
Fig. (7).The surface morphology of the Sample under different feed speed.
Fig. (8).The surface morphology of the Sample under different material of tool tips.