Development of hybrid CNC machine tools (Figure)
1 Introduction Next page
Compared with the series mechanism, the parallel mechanism has the advantages of large bearing capacity (higher specific stiffness), fast response speed and high motion accuracy. Therefore, it is concerned by academics and industry, and attaches great importance to its theoretical research and application development research. Simple parallel mechanisms have long been used, such as planar two- or three-bar parallel mechanisms in truss members. As a mechanical equipment product, the parallel mechanism is first applied on the robot.
For the machine tool, the specific stiffness, response speed and motion accuracy are the important technical indicators that it pursues. Moreover, because the machine tool is the working machine, the requirements for stiffness and motion accuracy are much higher than those of the robot. Therefore, the advantages of the parallel mechanism have great appeal to the machine tool industry. The research and development of a parallel machine tool (also known as virtual axis machine tool) has become a hot topic. People place great expectations on parallel machine tools, predicting that parallel machine tools will become the 21st century CNC machine tools.
Although the parallel mechanism has the above-mentioned attractive advantages, it also has the disadvantage of having a small range of motion (work space), especially the limited range of return motion. From the general category of mechanical equipment products, the machine tool and the robot belong to the same family with coordinate motion (the difference is that the machine tool is mostly rectangular coordinate system type, and the robot is mostly joint coordinate system type). Therefore, a parallel machine can also be called a cutting robot. However, the machine tool is different from the general robot (such as handling robot, painting robot, welding robot, etc.), which not only has high precision and rigidity requirements, but also has high requirements for working space (linear motion space and rotary working space) and interference constraints. For example, when cutting a workpiece by relative motion between the tool and the workpiece, in order to avoid interference between the tool and the workpiece (especially complex internal surface and space hole machining), the constraint on the machine tool movement is required to be high. The processing of spatial surfaces or the processing of spatial holes requires a large range of rotary motion. For example, the pentahedron processing requires that the rotary motion of the table reaches 360°. The vertical and horizontal machine tool requires the spindle head to swing by 90°; some space curved or hole machining requires the spindle or workpiece to swing to 180°. The working space of the full parallel machine with all the parallels of each axis is limited. Therefore, the numerical control numerical ratio (or the price ratio of the working space) of the working space is low. We think that from the comprehensive numerical effects of numerical control axis ratio, function price ratio, performance price ratio, etc., the full parallel machine tool is not necessarily the best, and the hybrid CNC machine tool used in parallel and series is more practical. Based on our research and development experience, we will talk about some of the views on the development of hybrid CNC machine tools. 
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