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      1. 1

        CNC grinding machine base and guide rail

        Time:2019-09-16 Visits:699
        Base and guide rail

        1 Y-axis base design

        1.1 Basic requirements for Y-axis base structure

        The base is the basic support component for the entire Y-axis, usually used to place important components such as guide rails. In order to meet the requirements of high speed, high productivity, high reliability, high precision, and high automation of CNC machine tools, and compared with ordinary machine tools, CNC machine tools need to have higher static and dynamic stiffness, as well as good vibration resistance. The base mainly puts forward requirements in five aspects.

        1. High precision and durability. There are many machining surfaces for mounting components and guide surfaces for moving components on the base, which require high precision and positional accuracy and can be maintained for a long time. In addition, during cutting, all dynamic and static loads are often transmitted to the base, making the force on the base very complex. Therefore, in order to ensure the mutual position or relative motion accuracy between components, in addition to meeting the geometric dimension position accuracy, it is also necessary to meet the technical requirements of static and dynamic stiffness, vibration resistance, thermal stability, and so on.

        2. It should have sufficient static and dynamic stiffness, including the structural stiffness, local stiffness, and contact stiffness of the base. Corresponding measures should be taken to achieve a high stiffness mass ratio.

        3. Dynamic stiffness directly reflects the motion characteristics of the base. In order to ensure that the base has high resistance to deformation and forced and self-excited vibrations under alternating loads, measures such as increasing damping and increasing natural frequency can be taken to reduce resonance and noise caused by thin arm vibration.

        4. For machine tools, the thermal stability of high-precision CNC machine tools has become a prominent issue. It is necessary to reduce the thermal deformation of the entire machine or minimize the impact of thermal deformation on machining accuracy in the design.

        5. High strength and wear resistance. Each base has sufficient strength to withstand external loads without exceeding the allowable deformation. However, when it comes to areas with high external loads and minimal deformation requirements, attention should still be paid to their strength. As for the parts where the base contacts the moving parts, namely the guide rail, excellent wear resistance is required.

        2 Guide rail design

        2.1 Requirements for guide rails

        The requirements for the guide rail include the following points:

        1. It should have a certain degree of guidance accuracy. 2. It should have good wear resistance

        3. Sufficient stiffness is required. 4. To reduce the impact of thermal deformation

        5. To make the movement light and stable, 6. There must be a certain degree of craftsmanship

        2.2 Sliding guide rail

        According to the friction situation of the contact surface, guide rails can be divided into sliding guide rails, rolling guide rails, hydrostatic guide rails, etc

        Three major types. The Y-axis of the CNC grinder designed this time adopts a sliding guide rail.

        The sliding guide rail has a series of advantages, such as simple and convenient manufacturing structure, large bearing area, good contact stiffness, and good vibration resistance. Sliding guide rails are usually used in open-loop systems with low positioning accuracy. As the servo system designed in this project is a closed-loop system, a feedback system is provided. The positioning accuracy of the guide rails is not affected due to error compensation, so sliding guide rails are used.

        2.3 Plastic sliding guide rail

        Sliding guides have the advantages of simple structure, convenient manufacturing, and high contact stiffness. However, traditional sliding guides have high friction resistance, fast wear, large differences in dynamic friction coefficients, and may experience crawling at low speeds. Therefore, a new type of plastic sliding guide rail is used.

        The plastic guide rail used in this design is a polytetrafluoroethylene guide rail soft belt.

        1. The characteristic of PTFE guide rail soft belt is good friction: the static and dynamic friction coefficients of cast iron quenched guide rail pairs differ greatly, almost twice the difference. However, the static and dynamic friction coefficients of metal PTFE guide rail soft belt generally remain unchanged. So this good friction performance can reduce low-speed crawling, resulting in smooth motion and high positioning accuracy.

        Good wear resistance: In addition to low friction coefficient, the PTFE guide rail soft belt material contains blue molybdenum disulfide and graphite, so it already has lubrication effect. The oil supply requirements for lubrication are low, so intermittent oil supply is sufficient. In addition, plastic has a relatively soft texture, even if embedded with metal debris, dust, etc., it will not damage the metal guide surface and the soft belt itself, and can also extend the service life of the guide pair.

        Good damping performance: Plastic has high damping performance, and its damping and noise reduction function can improve the relative performance of friction pairs

        Dynamic speed has a high significance.

        Good processability: It can reduce the hardness and surface quality requirements of the metal guide rail substrate for pasting plastic, and

        Due to the ease of processing plastic, the contact surface of the guide rail pair achieves excellent surface quality.

        At the same time, there are also advantages such as high chemical stability, convenient maintenance, and good economy.

        2. The initial process for using the guide rail soft tape is to process the surface of the guide rail to a roughness of Ra3.2-1.6. Sometimes, in order to achieve positioning, the guide rail adhesive surface needs to be processed into grooves of 0.5mm to 1mm deep. Clean the guide rail adhesive surface with gasoline and metal or acetone, then use adhesive to bond the guide rail soft tape. After initial curing under pressure for 1-2 hours, close it onto the paired fixed guide rail and apply a certain amount of pressure. Cure at room temperature for 24 hours, remove the remaining adhesive, Oil grooves can be opened and precision machining can be carried out, as this type of guide rail uses a pasting method and is generally referred to as a "plastic pasting guide rail".
        2.4 Guide rail structure
        The Y-direction uses a V-shaped guide rail, which has high guiding accuracy. After the guide rail is worn, it can automatically compensate by sinking under its own weight. At the same time, the lower guide rail adopts a concave shape, which can also facilitate oil storage, and the top angle is 90 degrees.
        2.5 Guide rail design

        The thrust exerted by the grinder on the moving parts is parallel to the axis of the moving parts. In order to prevent the moving parts from getting stuck and to ensure flexible movement, the V-shaped guide rail needs to be h/L<1 Where h is the distance from thrust F to the axis, and L is the width of the guide rail. In this design, the V-shaped guide rail, h=115mm, L=440mm, and h/L=0.26<1, meets the requirements.

        2.6 Materials of guide rails

        Plastic guide rails are used on the moving guide rails of the guide rail pair, and the matching guide rails are made of cast iron to form a cast iron plastic guide rail pair. Cast iron is made of wear-resistant cast iron with the grade HT3054, and the surface quenching hardness is HRC45-55. After grinding, the quenching layer depth should be kept at 1.0-1.5mm