CNC lathes in China entered the market in the early 1970s. Up to now, through the unremitting efforts of major machine tool manufacturers, cooperation with famous foreign machine tool manufacturers, joint venture, technology introduction, digestion and absorption of prototypes and other measures, China's machine tool manufacturing level has been greatly improved, and its output accounts for a large proportion in metal cutting machines. At present, the varieties and specifications of domestic CNC lathes are relatively complete, and the quality is basically stable and reliable, which has entered the stage of practical and all-round development. According to the different market positioning and demand, CNC lathes can be divided into economical CNC lathes and full-function CNC lathes.
(A) the status quo of CNC lathes
1. Bed and guide rail
(1) bed
Machine tool bed is the basic support of the whole machine tool and the main body of the machine tool. It is generally used to place important parts such as guide rails and headstock. The structure of the lathe bed has a great influence on the layout of the machine tool. According to the relative position between the guide surface of the lathe bed and the horizontal plane, there are five layout forms of the lathe bed: backward inclined lathe bed-inclined skateboard; Vertical bed-vertical skateboard; Flat plate-smooth flat plate; Front inclined bed-smooth plate; Flat inclined skateboard.
Generally speaking, inclined lathe bed and flat lathe bed inclined slide plate are mostly used in small and medium-sized CNC lathes, only large CNC lathes or small precision CNC lathes use flat lathe beds, and vertical lathe beds are less used. The flat bed has good manufacturability and is easy to process and manufacture. Because the tool rest is placed horizontally, it is beneficial to improve the motion accuracy of the tool rest, but the space under the lathe bed is small and it is difficult to remove chips. The cross slide of the tool rest is longer, which increases the width and size of the machine tool and affects the appearance. The flat bed has an inclined sliding plate structure and an inclined guide rail protective cover, which not only maintains the advantages of good manufacturability of the flat bed, but also does not make the width of the bed too large. Inclined bed and flat bed inclined slide plate structure are widely used in modern CNC lathes, because this layout has the following characteristics:
A. easy to realize electromechanical integration;
B. The machine tool is neat and beautiful in appearance and occupies a small area;
C. it is easy to set a closed protective device;
D chip removal is easy, and an automatic chip removal device is installed;
E. The hot chips cut from the workpiece will not accumulate on the guide rail, which will affect the accuracy of the guide rail;
F. Good comfort and easy operation;
G. it is convenient to install the manipulator and realize single machine automation.
(2) Guide rail
The guide rail of lathe can be divided into sliding guide rail and rolling guide rail.
The sliding guide rail has the advantages of simple structure, convenient manufacture and high contact rigidity. However, the traditional sliding guide rail has large friction resistance, fast wear, large difference between dynamic and static friction coefficients and easy crawling at low speed. Nowadays, CNC lathes do not use traditional slide rails, but use slide rails with wear-resistant tapes and new plastic slide rails. Good friction performance and long service life.
The rigidity of the guide rail, whether it is simple to manufacture, whether it can be adjusted, whether the friction loss is minimum, and whether it can maintain the initial accuracy of the guide rail depend largely on the cross-sectional shape of the guide rail. The cross-section shape of lathe sliding guide rail often adopts mountain-shaped cross-section and rectangular cross-section. The guide rail with this cross section has high guiding precision, and the worn guide rail will be automatically compensated by its own weight. The convex shape of the lower guide rail is beneficial to sewage discharge, but it is not easy to store oil. Rectangular section guide rail is easy to manufacture and maintain, and has large bearing capacity. The new guide rail has high guiding accuracy, but it cannot be automatically compensated after wear, and needs to be adjusted with inserts, which affects the guiding accuracy.
The advantages of rolling guide rail are small friction coefficient, close dynamic and static friction coefficient, no crawling phenomenon and grease lubrication. The guide rail travel of CNC lathe is generally long, so the rolling elements must circulate. According to the different rolling bodies, rolling guides can be divided into ball linear guides and roller linear guides, as shown in Figure 3. The latter has higher bearing capacity and stiffness than the former, but the friction coefficient is slightly larger. CK75C series full-function CNC lathes produced by Baoji Machine Tool Factory adopt ball linear guide, and CK535D full-function CNC inverted lathe adopts roller linear guide in X direction.
2. Spindle speed change system
Most economical CNC lathes can't change speed automatically. When it is necessary to change the speed, you can only stop the machine and then change the speed manually. The main drive system of full-function CNC lathe mostly adopts stepless speed change. At present, there are two kinds of continuously variable transmission systems: variable frequency spindle system and servo spindle system. Generally, DC or AC spindle motor is used to drive the spindle to rotate through belt drive, or through belt drive and reduction gear in the spindle box (to obtain greater torque). Because the spindle motor has a wide speed range and stepless speed regulation, the structure of the spindle box is greatly simplified. The spindle motor can output full power and maximum torque at rated speed.