可焊接的结构钢【精品-】课件

1、Lesson 3 Machine Tool FramesThe frame is a machines fundamental element. Most frames are made from cast iron, welded steel, composite, or concrete. The following factors govern material choice.The material must resist deformation and fracture. Hardness must be balanced against elasticity. The frame

2、must withstand impact, yet yield under load without cracking or permanently deforming. frame 床身cast iron 铸铁welded steel 可焊接的结构钢composite 复合材料govern 决定hardness 硬度be balanced against 与平衡The frame material must eliminate or block vibration transmission to reduce oscillations that degrade accuracy and t

3、ool life. It must withstand the hostile shop-floor environment, including the newer coolants and lubricants. Material expansion must be understood (in order) to minimize forces needed to move slides. The material must not build up too much heat, must retain its shape for its lifetime, and must be de

4、nse enough to distribute forces throughout the machine. Pros and Cons Either castings or welded sections can be used in most applications. The decision on which is best depends on the costs in a given design situation.Cast iron. Almost all machine tool frames were traditionally made of cast iron bec

5、ause features difficult to obtain any other way can be cast in. Castings have a good stiffness-to-weight ratio and good damping qualities. Modifying wall thickness and putting the metal where its needed is fairly easy.Although cast iron is a fairly cheap material, each casting requires a pattern. La

6、rger sizes are a limiting factor because of pattern cost, problems with bolted joints, and the need to anneal castings, which is difficult and costly with larger sections. pros and cons 优缺点casting 铸件stiffness-to-weight ratio 刚度重量比damping quality 减震性能pattern 模型bolted joint 螺栓连接anneal 将退火 section 截面尺寸

7、Smaller, high-volume machines usually have cast iron frames because they more easily absorb pattern cost. Welded frames may be cheaper for lower volume machines.Welded steel. Machine builders fabricate steel frames from welded steel sections when casting is impractical. Because steel has a higher mo

8、dulus, it is usually ribbed to provide stiffness. The number of welds is a design tradeoff: with welding, its easy to make large sections and add featureshigh-volume 大批量absorb 分摊lower volume 小批量modulus 弹性模量rib 筋、加筋weld 焊缝tradeoff 衡量因素distortion 变形frame cavity 床身内空穴Composites. Advanced forms of these

9、 materials, including those with polymer, metal, and ceramic matrices, may change machine tool design dramatically. Both matrix and reinforcing material can be tailored to provide strength in specific axes.Reinforced concrete. Though conventional reinforced concrete in simple sections increases mass

10、 and reduces vibration, another form, actually a polymer matrix composite made of crushed concrete or granite bound in a polymer matrix, is more popular. The composite has better damping characteristics than cast iron, can be cast into almost any shape, needs no stress relieving, and can accommodate

11、 fasteners and rails if inserts are used. However, it isnt as strong as metals and diffuses heat less efficiently.Designers must consider the different expansion coefficients between the composite and the metal sections to which it is joined. The most common applications for this material are high-a

12、ccuracy machine tools and grinders. polymer 高分子的 ceramic 陶瓷的matrix 基体tailor 定制 surface roughness 表面粗糙度brittle 易碎的crush 粉碎granite 花岗岩stress relieving 去应力退火accommodate 容纳fastener 紧固件rail 导轨insert 嵌件expansion coefficient 膨胀系数grinder 磨床FoundationsFoundations ensure the machines stiffness, shock absorpti

13、on and isolation are secondary considerations. If the machine isnt stiff enough, the foundation may have to supply the stiffness necessary. In selecting a foundation, designers must consider the machinesIf a small machine tool is rigid enough, it doesnt need the extra stiffness supplied by a base. F

14、or light loads, machines need only a 6-8(152-203 mm) -thick concrete floor. Usually an elastic mount in the machine base helps isolate it from other forces in the plant or shop. An independentmounting slab cast into the plant floor supports heavier machines. The most complex arrangement is a mountin

15、g slab isolated from the rest of the shop with passive or active shock-isolation elements.Frame DesignThe major considerations in frame design are loads, damping, apertures, heat distortion, and noise.Loads Understanding the static and dynamic loads a machine generates is essential. The basic load i

16、s static: the mass of the machine and its work piece. The dynamic load adds all that happens once the machine is running. This includes the forces of acceleration, deceleration, tool action, irregular loads caused by an unbalanced condition, or self-exciting loads from load and vibration interaction

17、.aperture 开孔static 静态的dynamic 动态的unbalanced 不平衡的self-exciting load 自励负荷interaction 相互作用Damping Though frame material and design should handle damping, dampers are sometimes built into frame sections to handle specific problems. They work best in machine areas where little dynamic force is at work an

18、d are effective only when the designer has a good understanding of all the loads involved. For example, a damper that works well under static conditions may do more harm than good under dynamic conditions.Apertures Each frame face should be solid, but the machine needs openings for assembly and main

19、tenance. The designer balances aperture number and size against stiffness and strength requirements.Thermo considerationsHeat from external or internal sources can be a major cause of error if the frame distorts. External sources include ambient shop conditions, cooling and lubricating media, and th

20、e sun. The machine also has its own heat sources: motors, friction from machine motion, and the cutting action of the tool on the work piece. Ideally, frame heating should be minimized and kept constant.NoiseReduction of noise for health and safety reasons is a fairly recent concern. Air turbulence

21、from moving parts and fans can be a particular problem. Enclosures prevent sound transfer though the machine, and sound damping materials help reduce objectionable sound.damper 减震器maintenance 维修ambient 周围的media 介质 turbulence 涡流sound damping material 吸音材料objectionable 有害的e+VHte0WItf1WIug1XJvg2YKvh3ZK

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