An economical precision rapid prototype manufacturing technology

1 Introduction The most mature rapid prototyping technology (RP) is currently researched: Stereolithography (SLA), 3D-Printing, Laminated Manufacturing (LOM), Selective Laser Sintering (SLS), Melt Stacking Law (FDM). As far as the existing RP technologies are concerned, there are some problems that need to be researched and improved in engineering practice: Accuracy and resolution are limited to 0.1mm or more. Edge thermal damage and burrs caused by heat fusion during laser cutting result in poor contour accuracy and surface roughness of the formed parts. In order to improve this situation, it is necessary to increase the milling process. For example, in the "3D-Plotting" technology of Sanders Corporation, after the formation of each layer, the upper surface of the layer is milled to control the Z-direction dimension. The laser rapid prototyping technology at the Fraunhofer Institute of Technology in Germany, after forming a single layer of parts, uses a high-speed milling head to light up the part's contours. In this way, the entire forming system is made more complicated and its price is high. With the micro-hole injection and extrusion molding technology, the micro-holes are easy to be blocked, and the working reliability needs to be improved. Some processes using vertical cutting have to minimize the thickness of each layer in order to reduce the step effect and improve the accuracy, which limits the manufacturing speed. Most processes are limited to one or a few materials, for example: SLS limited to paraffin, plastics, metals, ceramics and other powdered thermoplastic materials: SLA limited to thermosetting photosensitive resin: LOM is limited to paper, metal foil, plastic film and other thin materials: FDM Limited to waxes, thermoplastic resins, low melting point metals, etc. 2 Thick-Layer Manufacturing Technology Principles and Features of Cut-Cutting-Cutting Contours These shortcomings hinder the commercial application of RP. Therefore, in the next few years, RP technology with high accuracy, low cost, and wide range of applicable materials will be the mainstream of research and development in this field. To this end, we started the study of thick-layer stack manufacturing technology based on the tangential cutting contour of the tool. The principle is to layer the three-dimensional CAD model of the part, then use the CNC milling machine to process the layers, and then stack, bond, and assemble the layers, after appropriate post-processing (such as hot pressing, roasting, etc.), and finally The formed parts not only have a precise geometry, but also a good functional structure (Figure 1). Compared with the general lamination manufacturing with laser cutting profiles and other rapid prototyping technologies, the thick layer manufacturing technology based on the tangential cutting contour of the tool we are studying has high accuracy, high production efficiency, low cost, and a wide range of materials. Etc. details as follows:

Figure 1 Working process of a thick layer stack manufacturing system

Figure 2 Comparison of vertical and tangential cuts When the CAD model of a part is layered, unequal thickness stratification is performed according to the surface complexity of the part. Each single layer is processed using tangential cutting to eliminate the step effect (Figure 2). Because of the use of tangential cutting, the thickness of each single layer can be larger (2-5mm), and the accuracy is higher than that of normal vertical cutting. Due to the use of a tool (ordinary small-diameter end mill) to cut the contour, dimensional accuracy and surface roughness can achieve the effect of general CNC machining. Various engineering materials, including plastic plates, epoxy plates, ceramic plates, metal plates, etc., can be used as raw materials. Compared to laser-based RP systems, the entire mechanical device is relatively small, light, cheap, and easy to maintain. System Composition Scheme In order to improve the accuracy of CNC machine tools, we use thicker layers and tangential cutting tools. Therefore, the CNC milling machine used requires five-axis linkage. The five coordinates are: X, Y, Z, A (rotation around the X axis), B (rotation around the Y axis). In order to reduce the cost, we have reconstructed an old three-axis Drilling and Milling Machine: Add two turntables (Figure 3) on the X and Y tables to form A and B coordinates. Install the cutting table on the small turntable. Stepping motors are installed on the five axes X, Y, Z, A, and B. The movement is controlled by a computer numerical control system.

1. Y stepping motor 2.X stepping motor 3. Support plate 4. Z stepping motor 5. Small turntable 6. Large turntable 7. U type connection plate 8. Large turntable support plate 9. Base plate 10. Cutting table Figure 3 Axis milling machine

1. Cutting table 2. Backing plate 3. Machined workpiece single layer 4. Bolt Figure 4 Clamping scheme

Clamping scheme In order to facilitate clamping, a number of process holes are machined on the cutting table (Figure 4). During processing, the individual single-layer clamps of the parts can use these process holes: or, according to the specific parts, a special fixture is designed. . The stacking and assembling scheme processes the assembly process holes while cutting each single layer profile. When stacking, use bolts or positioning pins to assemble and use adhesives for bonding between layers. This part is temporarily hand-operated. When the entire processing process is verified and feasible and matured, automatic assembly is performed. We use Pro-Engineering for 3D CAD modeling software. The software uses a unified database management technology and parametric feature description methods, with a strong three-dimensional solid modeling capabilities, you can easily construct a variety of complex models. Slicing software The current rapid prototyping system slicing software first converts 3D CAD files into STL file format (first developed by 3D Systems, USA, it is relatively simple, and has nothing to do with the CAD system), and then sliced ​​into a rapid prototyping system. The required data. Since we use tangential cutting, in addition to obtaining the geometric data of each single-layer contour and obtaining the tangent information of each single layer around the slices of the 3D 9CAD model, we use the method of directly slicing the 3D CAD model. When the 3D CAD model of a part is layered, unequal thickness stratification is performed according to the surface complexity of the part, and thickening stratification organically combines efficiency and precision. We used polyvinyl chloride plates and epoxy plates for preliminary forming experiments. The model has a surface roughness of Ra 3.2 μm and a dimensional accuracy of 0.5 mm. Through the transformation of the three-axis drilling and milling machine for the five-axis CNC milling machine, the use of existing three-dimensional CAD software, configure the slicing software, to achieve the purpose of reducing costs and improving accuracy and efficiency.

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