Key component technology for ultra-precision machining machines (3)

4 Feed and micro feed system

The most commonly used in the feed system are various feed screws. In ultra-Precision Machining machines, the ball screw has been widely used due to its small backlash and high transmission efficiency. More precise hydrostatic screws and friction drives are also being used in ultra-precision machine tools.

The ball screw of an ultra-precision machine tool has a general accuracy class of C0. Thanks to the closed-loop control, the highest level of positioning accuracy of 0.01 μm is obtained with the best grade of ball screw. The ball screw does not require the attachment necessary for the static screw, and is a screw that is extremely convenient to use. However, as a feed screw of a submicron ultra-precision machine tool, it is necessary to consider problems such as slight vibration due to the rotation of the balls and the contact sliding between the balls, and poor vibration damping characteristics compared with the sliding screw. The ball screw used in HCM-I ultra-precision machining machine is to ensure the air-floating area of ​​the slide and improve its air float on the basis of strictly ensuring the connection between the servo motor and the screw, the screw and the nut and the base and the slide. Stiffness, thereby reducing the influence of the error of the lead screw on the movement accuracy of the slide. Moreover, the screw nut and the slide plate adopt a floating connection structure, thereby reducing the instantaneous or permanent deformation of the lead screw caused by the fluctuation of the ball screw caused by the fluctuation of the ball screw. At the same time, the movement error of the slide plate caused by the rotation of the screw rod caused by the bending of the ball screw itself is avoided, so that the minimum displacement resolution of the movement is ≤0.01 μm.

Figure 2 Friction drive schematic

Since the screw and nut of the static screw pair are not in direct contact, but have a layer of high-pressure liquid film separated, there is no creep and back gap caused by friction, so the accuracy can be maintained for a long time, and the feed resolution is higher; Moreover, since the oil film has a homogenization effect, the feed precision can be improved, and the nano-scale positioning resolution can be achieved over a long stroke. However, the static pressure screw device is large, and there are many auxiliary devices such as an oil pump, an accumulator, a liquid circulation device, a cooling device, and a filtering device, and there are also environmental pollution problems.

The friction drive converts the rotary motion of the servo motor into a linear motion of the driven rod by friction, and realizes the gapless transmission. The working principle is shown in FIG. 2 . Microscopically, the oil film between the pinch wheel and the follower rod is in a liquid lubrication state, and the shearing characteristics of the lubricating oil determine the traction system. Therefore, it is necessary to select a lubricating oil with a higher coefficient. Feed is achieved when the pinch roller rolls, and the feed resolution depends on the number of steps the servo motor makes one revolution. An important problem with friction drive feed is preloading. If the preload is too small, the contact surface may slip; if the preload is too large, it is difficult to achieve proper drive due to elastic deformation. In addition, due to the presence of pre-stress, it is also prone to wear problems. New research has shown that the Å (A) level positioning can be achieved by a twisted roller friction drive.

The various feed screws and friction drive characteristics are shown in Table 2.

Micro-feed mechanisms are also widely used in ultra-precision machine tools to meet the requirements for higher positioning accuracy and feed resolution. Commonly used methods include a method of using a rolling screw feed and an elastic feed, and a method of combining a coarse adjustment and a fine adjustment of a piezoelectric element. The HCM-I ultra-precision machining machine uses a piezoelectric micro-feeding tool holder.

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