A designed workholding device will allow for the stationary positioning of a tool while the workpiece is moved to machine the desired convex shape, enabling better control over the machining process and resulting in better machining and shape accuracy.

The machining of convex surfaces is a common requirement in many industries, including aerospace, automotive, and manufacturing. However, the process can be challenging due to the need for precise workholding and tool positioning. Inaccurate work holding can result in poor surface finish and dimensional inaccuracies, leading to increased production costs and reduced product quality. This work aims to develop a workholding device for milling convex surfaces. The aim of this work is to improve the efficiency and accuracy of the machining process by providing a stable and adjustable workholding solution. This device allows the machining of a maximum radius of 180mm. The designed workholding device allows the stationary positioning of the tool while the workpiece is moved to machine the desired convex shape. This approach enables better control over the machining process and results in better machining and shape accuracy. The proposed device provides exceptional rigidity and ensures accurate milling of convex surfaces.

1 Introduction

Milling, a versatile and indispensable machining process in the realm of manufacturing, involves the meticulous removal of material from a workpiece through the controlled rotation of a cutting tool known as a milling cutter. Milling machines are available in various configurations such as vertical and horizontal mills, along with the highly automated CNC milling machines that can carry out intricate operations with exceptional accuracy. These machines are used across many industries, from aerospace and automotive to woodworking, enabling the production of complex shapes, intricate components, and precise parts. Overall, milling remains an essential and ever-evolving process, shaping the world by producing intricate, precise, and customized components that define modern manufacturing. A sensor-integrated milling vice using a MEMS accelerometer as a non-invasive monitoring solution for chatter detection with the use of advanced signal process algorithms for the demodulation of the vibration signal, along with the use of artificial intelligence, led to a high-performance system at a low cost [1]. Using ANSYS effectively, 3D-Finited Element Analysis was used by researchers in the past to evaluate the residual stress and yield strength for the weldability of SS304 and SS316L materials [2,3].

A design and manufacture of a distinct collet system for the specific workholding needs of forged components employed for the precision in manufacturing allow machines to operate with minimal errors and human intervention. Implementing these mechanisms is a simple and effective way to cut labor costs, decrease cycle times, and enhance both quantity and quality of production [4]. A study focusing on the topography of Cr12MoV die steel after CNC milling, particularly shaping convex surfaces, unveiled the predicted topography carries an error range of 10-16%. Subsequent ball milling accentuated tool wear and inaccuracies, especially as the milling tool moved away from the curve’s highest point [5].

The milling experiments on AISI D6 hardened steel focusing on tool path strategy and impact of tilt angle was carried out and tested with two approaches (upward and downward). Results showed a significant influence on surface roughness, with the downward strategy providing a smoother finish. This research emphasizes the importance of selecting the right tool path strategy and tilt angle for desired surface quality [6].

In another research work, a ball end mills’ cutting mechanism introduced for machining spherical surfaces using the contour path method, aiming to improve precision in mold-and-die production. By varying tool path radii and lead angles, the earlier researchers calculated cutting area of cross section, measured forces, and discussed results to identify optimal cutting conditions [7]. The smooth curved convex surface was performed by using various machining strategies such as raster, 3D-offset, radial, and spiral techniques, while considering cutting parameters such as feed rate, cutting speed, and step over. The earlier experiments were conducted on low curvature convex surfaces of stainless steel, employing tool paths including spiral, radial, 3D-offset, and raster methods for finish milling operations [8].

Tool wear and tool life examination during ball nose end milling of Inconel 718 under minimum quantity lubricant (MQL) conditions using physical vapor deposition coated carbide inserts for both up-milling and down-milling operations lead to reduced tool wear in down-milling [9]. The nondestructive testing methods can also be employed for evaluating materials such as SS304L using magnetic particle and ultrasonic testing of welded samples [10]. The unconventional machining can also be used for making some convex surfaces of material such as Ti-6Al-4V alloy. A study of material removal and tool wear rate was done for desirable surface finish [11].

ANSYS Fluent could effectively be used in the analyses of scramjet combustor flow dynamics. The investigation revealed that decreasing temperature and increasing pressure during injection leads to the transition mode [12]. A short review by earlier investigators has shown an enhancement of heat transfer through various fins using variable geometric and thermos physical parameters such as perforated and porous fins [13]. The optimized process parameters for surface roughness in milling En-31 steel material using Taguchi robust design methodology employed brass-coated carbide inserts with a 16mm diameter and found that the optimum parameters were a Spindle Speed of 1,094 RPM, a Feed of 100 m/min, a Depth of cut of 1 mm, and a maximum optimum coolant flow of 90 liters/min to achieve improved surface finish [14].

From the literature study, it was found that there is no workholding device that exists for milling convex surfaces. This research aims to design and analyze a workholding device/vice for convex milling in horizontal and universal milling machines. Also, this work ensures the proposed milling vice can effectively handle a range of materials from metals to plastics, as well as different machines, with minimal error.

2 Methods and materials

2.1 Convex surface milling vice

In the intricate and ever-evolving world of machining and precision engineering, development of specialized tools and fixtures plays a pivotal role in meeting the demands of today’s complex workpiece geometries and intricate machining processes. One remarkable innovation that has emerged to address these challenges is the creation of a convex surface milling vice. This engineering marvel represents a paradigm shift in how we approach the milling of convex or curved workpieces, offering good unprecedented precision and versatility in the manufacturing landscape.

As industries continue to push the boundaries of what is possible, the demand for finely tuned components with complex, curved surfaces has become increasingly common. Whether it is in aerospace, automotive, medical device production, or countless other fields, the need to efficiently and accurately machine these complex workpieces has never been more crucial. The convex surface milling vice stands at the forefront of this machining revolution, a tool that embodies innovation, precision, and adaptability. In this comprehensive exploration, we embark on a journey to delve deep into the concept, design, applications, and the advantages that the convex surface milling vice brings to the table.

From its inception to its pivotal role in modern manufacturing processes, this remarkable tool will become an indispensable asset in the hands of engineers and machinists, enabling them to achieve new heights in precision and efficiency. The convex surface milling vice offers a remarkable capability to machine convex surfaces with ease, versatility, and precision. Its design and functionality enable machinists to tackle a wide range of convex surfaces, including those with different radii. Whether in producing intricate components with complex curves or crafting precision parts with varying convex profiles, this innovative tool provides a practical solution that simplifies the machining of diverse workpieces. Its adaptability to handle different radii and curved geometries makes it an invaluable asset in the manufacturing world, where meeting the demands of today’s intricate designs and exacting tolerances is a constant challenge.

2.2 Working

A trammel of Archimedes is a mechanism that generates the shape of an ellipse. It consists of two shuttles, confined (“trammeled”) to perpendicular channels or rails and a rod attached to the shuttles by pivots at fixed positions along the shaft. As the shuttles move back and forth, each along its channel, all points on the rod move in elliptical paths. The motion of the rod is termed elliptical motion. The semi-axes of the ellipses have lengths equal to the distances from the point on the rod to each of the two pivots. Similarly, the table moves on the two links between the central pivot and the circular slots.

The two slots act like the perpendicular channels, allowing the table to take an elliptical/circular path and move the work piece in a circular path. The workpiece can be machined by a stationary rotating tool that removes the material from the work piece. The table is moved by two links that travel forward when the threaded rod is rotating. It acts like a screw and moves the table the distance of the pitch for every rotation of the threaded rod. The threaded rod has a handle to allow it to rotate by hand.

2.3 Design considerations

The exploded view of the proposed workholding device for convex milling is shown in Figure 1. The design was done in SolidWorks using various features such as sketch, extrude, extrude cut, etc. Each individual part (See Figure 2a to 2j) was designed separately and then assembled in SolidWorks. Additional standard parts such as the threaded rod and bolts are imported from the SolidWorks Toolbox ISO library (See Table 1).

Figure 1: Exploded view of work holding device for convex milling.
Figure 2a: Side plate; Figure 2b: Base plate; Figure 2c: Rotating handle; Figure 2d: Table top; Figure 2e: Rotating handle holder; Figure 2f: Link.
Figure 2g: Stand rod; Figure 2h: Arm; Figure 2i: Translator; Figure 2j: Table leg.
Table 1: Bill of materials.

2.4 Design analysis

The analysis was done in SolidWorks 2023 using a simulation add in. Solidworks is chosen for the simulation because the design and assembly are done in SolidWorks itself. Simulating an assembly in any other software needs to define the mates and references again. In such cases, it loses the design parametric flexibility. A load of 1,000N was chosen and applied on the surface of the table. The bottom surface of the base was fixed, and all the other constraints remain unchanged.

Figure 3 shows the model after applying the boundary conditions i.e., a load of 1,000N on the table surface, a face fixed at the bottom of the base plate, and the motion constraints in place. Also, the meshing is done using SolidWorks built in Mesher. The dynamic mesh provided by the SolidWorks automatically assigns the fine mesh where the surfaces or features are in smaller and coarser mesh everywhere on the model (see Figure 4). This helps to reduce the computational load on the computing processor and achieves a higher quality analysis. Figures 5, 6, and 7 show the results.

Figures are listed left to right;, top, middle and bottom: Figure 3: Boundary conditions; Figure 4: Meshed assembly; Figure 5: Stress plot; Figure 6: Displacement plot; Figure 7: Strain plot; Figure 8: Meshed model with boundary conditions; Figure 9: Stress due to torque; Figure 10: Displacement due to torque; Figure 11: Strain due to torque.

The induced stress transferred to the bolts, and they are the parts that have to withstand stress to hold the device together. The yield strength of the device is 3.5×108N/m2 or 350 MPa. The stress is more on the bolts that support the table and the links. Another simulation was also done by applying a Torque of 1,000Nm. The torque acts on the surface of the table leads to the bottom of the fixed base plate. The torque is applied counter clockwise, shown in Figure 8. Figures 9, 10, and 11 show the stress, displacement, and strain due to torque. Both stress plots shown in Figures 5 and 9 reveal there are no effects on the workholding device, even as the force applied is 1,000N. Similarly, the strain plots shown in Figures 7 and 11 clearly indicate there is no elongation of material. But the displacement plots shown in Figures 6 and 10 disclose there are possible movements on the table top, which may affect the precision convex milling. This can be overcome by properly tightening the arms, side plates, and base plate.

2.5 Materials

The material used was mild steel sheets of 2mm. Mild steel is a type of carbon steel with a low amount of carbon – it is also known as “low carbon steel.” Although ranges vary depending on the source, the amount of carbon typically found in mild steel is 0.05% to 0.25% by weight. The higher carbon steels typically contain carbon content from 0.30% to 2.0%. If there is a further addition of carbon content, the steel becomes cast iron. Mild steel is not a steel alloy and does not contain other elements other than iron; you will not find vast amounts of chromium, molybdenum, or other alloying elements in mild steel.

Since its carbon and alloying element content are relatively low, there are several properties that differentiate it from higher carbon and alloy steels. Less carbon means mild steel is typically more ductile, machinable, and weldable than high carbon and other steels; however, it also means it is nearly impossible to harden and strengthen through heating and quenching. The low carbon content also means it has very little carbon and other alloying elements to block dislocations in its crystal structure, generally resulting in less tensile strength than high carbon and alloy steels.

Mild steel also has a high amount of iron and ferrite, making it magnetic. The lack of alloying elements such as those found in stainless steels means the iron in mild steel is subject to oxidation (rust) if not properly coated. But the negligible amount of alloying elements also helps mild steel to be relatively affordable when compared with other steels. It is the affordability, weldability, and machinability that make it such a popular choice of steel for consumers. Additionally, the bolts used for joining the parts together are M5 standard bolts of different lengths. They include M5 bolts of lengths 100mm and 15mm. A M12 threaded bolt was used to move the table and is welded to a handle to facilitate the manual rotation of the bolt.

3 Results and discussion

The convex surface milling vice shown in Figure 12 has great potential in machining convex surfaces. To test its accuracy, a wooden workpiece had been machined on the vice, and the results are compared with the expected values.

Figure 12: Working model of the vice.

The path of the radius of curvature was traced in AutoCAD using the drawings of the device with a pointed workpiece placed on the table. The drawings of different configurations and positions were taken and combined to a single drawing. Later, a circle was fitted to the workpiece points to create a circle of the path that the workpiece will be following (see Figures 13 and 14).

Figure 13: Radius of curvature of actual surface.
Figure 14: Radius of curvature of measured surface.

The radius of curvature of the workpiece was measured in a different but similar way. Three points on the surface were measured, and a local coordinate system was established. The points are P1 (0, 2.5), P2 (40, 7), and P3 (81.5, 0). These points were plotted in AutoCAD, and a circle was fitted between these points, and the radius of curvature of that circle was known and compared with the expected radius. The machined surface has 17.97% error, i.e. it is 82.21% accurate. The error can be further minimized by machining the parts with higher accuracy and making sure the tolerances are as close as possible.

4 Conclusions

The current workholding device is capable of machining maximum radius of 180mm. The maximum radius achieved can be changed by increasing the arc lengths of the circular slots during the fabrication of the device. The table of the device can accommodate the workpiece length of 150mm x 50mm in length and breadth. While the present workholding device has an error of 17.79%, it can be minimized by precision machining the parts with closer tolerances. This workholding can be more efficient in the future by automating the movement of the table through motors and gears for precise control over the machining speed and feed. Furthermore, the adjustment of the links in the circular slots can be automated using internal gear mechanisms for a precise transition between the radius. The convex surface milling device, designed to streamline the machining of convex surfaces, has shown promising results, although it is yet to achieve commercialization. Our device is entirely novel and innovative, bringing a fresh and simple solution to the table. This vice offers an elegant and practical solution, particularly for small-scale industries that still rely on conventional milling machines. It effectively produces convex shapes with minimal errors and a good surface finish, positioning it as a potentially invaluable asset for a range of small-scale industries. Further, the precise convex milling on the workpiece can be ensured by incorporating angular scale on the side plates and by the proper tightening of arms of the proposed workholding device.

Acknowledgements

The authors are grateful to the management of the MLR Institute of Technology Hyderabad for the permission extended to bring out this investigation in the machine tools and manufacturing process lab .

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Published under licence by IOP Publishing Ltd. This article (https://iopscience.iop.org/article/10.1088/1742-6596/2837/1/012061) is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). It has been edited to conform to the style of Gear Solutions magazine.