[Abstract] The rolling guideways of a machining center serve as guides and load-bearing components. All moving parts of the machine tool move along the rolling guideways of the bed and column in a specified manner. The rolling guideways enable the moving parts to move along a linear trajectory along a certain spatial axis, ensuring the relative position and accuracy of each moving part, and bearing the weight of the moving parts and the cutting forces. The quality of the rolling guideways determines the rigidity, accuracy, and stability of the machining center .
1. Introduction
Rolling guides possess advantages such as high guiding accuracy, good wear resistance, high rigidity, and excellent friction characteristics, making them widely used in machining centers. This study of the development and characteristics of rolling guides provides a theoretical foundation for vocational school students' CNC machine tool course design experiments, enriching the course content and laying a solid foundation for the development of high-precision feed mechanisms. Through a case study of single-stage feed rolling guide life calculation, this paper analyzes various factors affecting guide life, providing reference data for future rolling guide research.
The development of rolling guides in machining centers can be broadly divided into four stages. The first stage primarily achieved the excellent characteristics of rolling guides and enabled smooth, unobstructed linear guiding motion. The second stage, due to new requirements in installation and use, led to research into rolling guide pairs with bearings and cages to prevent ball leakage. The third stage, with the increasing application of rolling guides in high-speed applications above 35 m/min, eliminated friction and collision between balls during reciprocating motion at high speeds, achieving low noise. The fourth stage, due to the rapid application of rolling guides in heavy-duty, high-precision, and high-speed machining centers, where the rigidity and load-bearing capacity of the circulating balls were insufficient, led to the development of heavy-duty roller-type guides. These inherited the advantages of the former and truly met the requirements of machining centers for guide pairs.
Machining centers place high demands on rolling guides. Firstly, they must possess excellent guiding accuracy. The geometric accuracy of the rolling guide directly affects the guiding accuracy of moving parts. Therefore, the inspection standards for rolling guides clearly specify the straightness of the guide and the parallelism of the contact surfaces between the two guides. The main factors affecting its accuracy are manufacturing precision, assembly quality, rigidity of the rolling guide and its supporting components, and the degree of thermal deformation.
Secondly, the rolling guide rail must have good wear resistance. Long-term movement of the moving contact surface along the fixed guide surface will cause uneven wear on the rolling guide rail, thereby compromising its motion guidance accuracy and ultimately affecting machining accuracy. The wear resistance of the rolling guide rail directly affects the stability of the machine tool's machining accuracy. The wear resistance of the rolling guide rail is a key aspect of design and manufacturing, and an important indicator of the quality of a machining center. Improving the wear resistance of the rolling guide rail in a machining center, reducing uneven wear, and enabling automatic compensation or adjustment after uneven wear are also crucial.
Furthermore, the rolling guides of a machining center must have high rigidity. When subjected to external forces, the rolling guides deform, affecting the relative positions of the moving parts and their guiding accuracy. Therefore, high rigidity is essential for the rolling guides in a machining center. The deformation of the rolling guides under stress depends primarily on their type, dimensions, connection method with the support components, and the stress conditions.
Finally, the rolling guide must have excellent friction characteristics. Creep is prone to occur when moving parts move at low speeds. Creep during feed motion increases the roughness of the machined surface, and creep during positioning motion reduces positioning accuracy. Therefore, the rolling guide of the machining center must be very smooth during low-speed movement. In addition, the coefficient of friction of the rolling guide should be low to reduce frictional resistance and thermal deformation of the rolling guide itself.
The rolling guide of a machining center is mainly composed of rolling guide blocks and linear rolling guide pairs. The rolling guide blocks of a machining center generally use circulating cylindrical rollers, which cooperate with the guide rails on the machine tool bed. Its advantage is that it is not limited by the stroke length and has relatively high rigidity.
The load life of the rolling guideway in a machining center is related to the lifespan of the machining center itself. Calculating its lifespan is an indispensable part of the design and manufacturing of machining centers, and it is also the basis for selecting rolling guideways. Furthermore, it is related to the reputation of product designers. Below, we will determine the lifespan of the rolling guideway based on examples.
Design and usage conditions: A laboratory workbench with horizontal guide rails, using two guide rails with a center distance of 180mm and a stroke length of 300mm. The workbench weighs 50kg and has a load of 2.5kN. The center of gravity is in the center of the workbench. The guide rails move at a constant speed, with n=3 round trips per minute. The expected service life is 10 years.
Pre-selected guide rail model: Taekyong guide rail (Korean model SBG15SL)
Selection and calculation of safety factor: Since the experimental workbench operates without impact or vibration, a safety factor of fi=1.5ft=1fc=1fa=1 can be selected from the mechanical handbook.
Assuming the experimental workbench operates on 270 working days per year, with 8 hours of use per day, its utilization rate is 90%. Therefore, its expected lifespan is 23490 ÷ 270 ÷ 8 ÷ 0.9 = 12.08 years. Thus, this rolling guide rail meets the design requirements of the experimental workbench.
The rigidity of the rolling guideway in a machining center is closely related to the machining and manufacturing precision of the machine tool bed guideway and rolling elements. If the machining and manufacturing quality is very high, the elastic displacement of the machine tool bed guideway and rolling elements will result in very good rigidity of the rolling guideway.
It has been proven that the greater the load on the rolling guide, the shorter the life of the rolling guide slider pair. The rated dynamic load of the rolling guide also determines the life of the guide slider pair. The greater the dynamic load it bears, the longer the life of the rolling guide slider pair. The more sliders there are, the longer the life of the rolling guide slider pair. The longer the stroke of the rolling guide, the shorter the life of the guide slider pair. Conversely, the shorter the stroke, the longer the life.
2. Conclusion
The research on the sliding block of the rolling guideway in machining center has profound significance for inspiring students' course design. It can not only help students understand the design of rolling guideways, but also play a huge guiding and promoting role in the future development of the sliding block of the rolling guideway in machining centers.
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