What is the difference between rapid prototyping and CNC ?

Rapid prototyping (SLA) is a discrete/stacking process. It proposes a new thinking mode dimension model from the molding principle, that is, the three-dimensional model of the parts made on the computer is meshed and stored, and layered to obtain the two-dimensional contour information of each layer section. The contour information automatically generates a processing path, and the forming head selectively cures or cuts a layer of the forming material under the control of the control system to form a profile of each cross-section, and sequentially stacks the three-dimensional blanks. The post processing of the blank is then performed to form the part.

The process of rapid prototyping is as follows:

l) Construction of a three-dimensional model of the product. Since the RP system is directly driven by the 3D CAD model, the 3D CAD model of the workpiece being machined is first constructed. The 3D CAD model can be directly constructed using computer-aided design software (such as Pro/E, I-DEAS, Solid Works, UG, etc.), or a 2D pattern of an existing product can be converted to form a 3D model, or a product entity. Laser scanning and CT tomography are performed to obtain point cloud data, and then the inverse engineering method is used to construct the three-dimensional model.

2) Approximate processing of the three-dimensional model. Since the product often has some irregular free-form surfaces, the model should be approximated before processing to facilitate subsequent data processing. Because the STL format file format is simple and practical, it has become a quasi-standard interface file in the field of rapid prototyping. It uses a series of small triangular planes to approximate the original model. Each small triangle is described by 3 vertex coordinates and a normal vector. The size of the triangle can be selected according to the accuracy requirements. The STL file has two output forms, binary code and ASCll code. The binary code output form occupies much less space than the ASCII code output file, but the ASCII output form can be read and checked. Typical CAD software comes with the ability to convert and output STL format files.

3) Slice processing of the 3D model. According to the characteristics of the processed model, the appropriate machining direction is selected, and the approximated model is cut in a series of spaced planes in the forming height direction to extract the contour information of the section. The interval is generally 0.05mm~0.5mm, and 0.1mm is commonly used. The smaller the interval, the higher the molding accuracy, but the longer the molding time, the lower the efficiency, and the lower the accuracy, but the higher the efficiency.

4) Forming processing. According to the cross-sectional profile of the slicing process, under the control of the computer, the corresponding forming head (laser head or nozzle) performs scanning motion according to the profile information of each section, and the materials are stacked layer by layer on the workbench, and then the layers are bonded. And finally get the prototype product.

5) Post-processing of molded parts. The molded part is taken out from the molding system, polished, polished, coated, or placed in a high-temperature furnace for post-sintering to further increase its strength.

Rapid prototyping has several important features:

l) can create arbitrarily complex three-dimensional geometric entities. Due to the principle of discrete / stacked forming. It simplifies a very complex 3D manufacturing process into a superposition of 2D processes, enabling the machining of arbitrarily complex shaped parts. The more complex the part, the better the RP technology is. In addition, the RP technology is especially suitable for parts that are difficult to manufacture or even impossible to manufacture by conventional methods such as complex cavities and complex profiles.

2) Rapidity. Design and processing information for a new part can be obtained by modifying or reorganizing a CAD model. Parts can be manufactured from hours to tens of hours, with outstanding features for rapid manufacturing.

3) Highly flexible. Complete complex manufacturing processes without the need for any special fixtures or tools to quickly manufacture tooling, prototypes or parts

4) Rapid prototyping technology has achieved two advanced goals pursued by the mechanical engineering discipline for many years. That is, the extraction of materials (gas, liquid solid phase) process and manufacturing process integration and design (CAD) and manufacturing (CAM) integration

5) Combine with Reverse Engineering, CAD technology, network technology, virtual reality, etc., and become a powerful tool for product development.
Therefore, rapid prototyping technology plays an increasingly important role in the manufacturing field and will have an important impact on the manufacturing industry.

CNC–machine machining center, also called CNC system, is the control part of the machining center machine tool. It generates CNC machining program according to the input and drawing part information, process and process parameters, and then passes the number of electric pulses. Then, the servo drive system drives the machine components to perform corresponding movements.
On the traditional CNC machine tool (NC), the machining information of the parts is stored on the CNC paper tape, and the information on the CNC paper tape is read by the photoelectric reader to realize the machining control of the machine tool. Later, it developed into computer numerical control (CNC), and the function was greatly improved. All the information of one processing can be read into the computer memory at one time, thus avoiding frequent startup of the reading machine. More advanced CNC machines can even remove optical readers, program directly on the computer, or directly receive information from CAPP for automatic programming. The latter CNC machine tool is the basic equipment of the computer integrated manufacturing system. Modern CNC systems often have the following features:

(1) Multi-axis linkage control;

(2) Tool position compensation;

(3) System fault diagnosis;

(4) Online programming;

(5) Machining and programming parallel operation;

(6) Machining simulation;

(7) Tool management and Monitoring;

(8) Online detection.

The use of CNC machine tools is more and more widely used in aerospace, automotive, food industry, tool and die, electrical appliances, furniture and other industries. Common CNC machine tools are CNC lathe machine, CNC milling machine, CNC drilling machine, CNC machining center, CNC EDM die-sink, CNC EDM wire-cut, CNC water-jetting machine, CNC laser-cutting machine, CNC punch, CNC Router, etc. .
The most commonly used CNC machining technology consists of CNC programming, setup and adjustment, and operation. The NC program can be generated by the G code and CAD/CAM software for CNC machine tools. Common CAD/CAM software includes MASTERCAM, PRO/E, UG/II, CATIA, IDEAS, CIMATRON, EDGECAM, etc. If you want to master the CNC machining technology, you must have knowledge and skills in machine tools, machining processes, machining tools, fixtures, measuring tools, G code, CAD/CAM software.