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1、Lesson 3 Machine Tool Frames,The 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 must
2、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,elasticity 弹性impact 冲击cracking 裂纹deforming 永久性变形,The frame material must eliminate or block vibration transmission to red
3、uce oscillations that degrade accuracy and tool 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
4、shape for its lifetime,and must be dense enough to distribute forces throughout the machine.,vibration 震动oscillation 震荡degrade 降低等级tool life 刀具寿命withstand 耐受hostile 恶劣的coolant 冷却液lubricant 润滑剂 slide 滑动部件(滑块),Pros and Cons Either castings or welded sections can be used in most applications.The decisi
5、on 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 because features difficult to obtain any other way can be cast in.Castings have a good stiffness-to-weight ratio and good damping qualities.Modifying wa
6、ll thickness and putting the metal where its needed is fairly easy.,Although cast iron is a fairly cheap material,each casting requires a pattern.Larger 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
7、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,Smaller,high-volume machines usually have cast iron frames because they more easily absorb pattern cost.Welded frames may be cheaper for lower volume mach
8、ines.,Welded steel.Machine builders fabricate steel frames from welded steel sections when casting is impractical.Because steel has a higher modulus,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 features,even a
9、fter the initial design is complete,but the heat can introduce distortion and also adds cost.Welds also help block vibration transmission through the steel frame.Builders sometimes increase damping by circulating coolant through the welded structure or adding lead or sand to frame cavities,high-volu
10、me 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 materials,including those with polymer,metal,and ceramic matrices,may change machine tool design dramatically.Both matrix and reinforcing material can be tailored
11、 to provide strength in specific axes.,Ceramics.The Japanese introduced experimental machine tools with ceramic frames in the 1980s.Ceramics offer strength,stiffness,dimensional stability,corrosion resistance,and low surface roughness,but they are brittle and expensive.Their lack of conductivity can
12、 be an advantage or not.Both composite and ceramic use is limited.,Reinforced concrete.Though conventional reinforced concrete in simple sections increases mass and reduces vibration,another form,actually a polymer matrix composite made of crushed concrete or granite bound in a polymer matrix,is mor
13、e popular.The composite has better damping characteristics than cast iron,can be cast,into almost any shape,needs no stress relieving,and can accommodate fasteners and rails if inserts are used.However,it isnt as strong as metals and diffuses heat less efficiently.,Designers must consider the differ
14、ent expansion coefficients between the composite and the metal sections to which it is joined.The most common applications for this material are high-accuracy 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
15、 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 absorption and isolation are secondary considerations.If the machine isnt stiff enough,the foundation may have to supply the stiffness necessary.In selectin
16、g a foundation,designers must consider the machines,weight,the forces it generates,accuracy requirements,and the loads being transmitted to the ground by adjacent machines.Soil condition can be a problem because long-term changes can influence machine stability.,If a small machine tool is rigid enou
17、gh,it doesnt need the extra stiffness supplied by a base.For 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 independent,mounting slab cast into the plant floor supports heav
18、ier machines.The most complex arrangement is a mounting slab isolated from the rest of the shop with passive or active shock-isolation elements.,Foundation 基础elastic mount 弹性安装钢板isolate 隔离mounting slab 安装钢板cast into 浇注在arrangement 结构shock-isolation 隔震,Frame DesignThe major considerations in frame de
19、sign are loads,damping,apertures,heat distortion,and noise.,Loads Understanding the static and dynamic loads a machine generates is essential.The basic load is 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 forc
20、es of acceleration,deceleration,tool action,irregular loads caused by an unbalanced condition,or self-exciting loads from load and vibration interaction.,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 damp
21、ing,dampers are sometimes built into frame sections to handle specific problems.They work best in machine areas where little dynamic force is at work and are effective only when the,designer has a good understanding of all the loads involved.For example,a damper that works well under static conditio
22、ns may do more harm than good under dynamic conditions.,Apertures Each frame face should be solid,but the machine needs openings for assembly and maintenance.The designer balances aperture number and size against stiffness and strength requirements.,Thermo considerationsHeat from external or interna
23、l sources can be a major cause of error if the frame distorts.External sources include ambient shop conditions,cooling and lubricating media,and the 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 h
24、eating should be minimized and kept constant.,NoiseReduction of noise for health and safety reasons is a fairly recent concern.Air turbulence 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,
