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1、Lesson 3 Machine Tool FrameslThe frame is a machines fundamental element.Most frames are made from cast iron,welded steel,composite,or concrete.The following factors govern material choice.lThe 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.l frame 床身床身lcast iron 铸铁铸铁lwelded steel 可焊接的结构钢可焊接的结构钢lcomposite 复合材料复合材料lgovern 决定决定lhardness 硬度硬度lbe balanced against 与与平衡平衡lelasticity 弹性弹性limpact 冲击冲击lcracking 裂纹裂纹ldeforming 永久性变形永久性变形lThe frame material must elimi
3、nate or block vibration transmission to reduce oscillations that degrade accuracy and tool life.It must withstand the hostile shop-floor environment,including the newer coolants and lubricants.lMaterial expansion must be understood(in order)to minimize forces needed to move slides.The material must
4、not build up too much heat,must retain its shape for its lifetime,and must be dense enough to distribute forces throughout the machine.lvibration 震动震动loscillation 震荡震荡ldegrade 降低等级降低等级ltool life 刀具寿命刀具寿命lwithstand 耐受耐受lhostile 恶劣的恶劣的lcoolant 冷却液冷却液llubricant 润滑剂润滑剂l slide 滑动部件滑动部件(滑块滑块)lPros and Con
5、s lEither 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.lCast iron.lAlmost all machine tool frames were traditionally made of cast iron because features difficult to obtain any other way can be cast in.Cast
6、ings have a good stiffness-to-weight ratio and good damping qualities.Modifying wall thickness and putting the metal where its needed is fairly easy.lAlthough cast iron is a fairly cheap material,each casting requires a pattern.Larger sizes are a limiting factor because of pattern cost,problems with
7、 bolted joints,and the need to anneal castings,which is difficult and costly with larger sections.lpros and cons 优缺点优缺点lcasting 铸件铸件lstiffness-to-weight ratio 刚度重量比刚度重量比ldamping quality 减震性能减震性能lpattern 模型模型lbolted joint 螺栓连接螺栓连接lanneal 将将退火退火l section 截面尺寸截面尺寸lSmaller,high-volume machines usually h
8、ave cast iron frames because they more easily absorb pattern cost.Welded frames may be cheaper for lower volume machines.lWelded 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 sti
9、ffness.The number of welds is a design tradeoff:with welding,its easy to make large sections and add featuresleven after 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 inc
10、rease damping by circulating coolant through the welded structure or adding lead or sand to frame cavities lhigh-volume 大批量大批量labsorb 分摊分摊llower volume 小批量小批量lmodulus 弹性模量弹性模量lrib 筋、加筋筋、加筋lweld 焊缝焊缝ltradeoff 衡量因素衡量因素ldistortion 变形变形lframe cavity 床身内空穴床身内空穴lComposites.Advanced forms of these material
11、s,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.lCeramics.The Japanese introduced experimental machine tools with ceramic frames in the 1980s.Ceramics offer
12、 strength,stiffness,dimensional stability,corrosion resistance,and low surface roughness,but they are brittle and expensive.Their lack of conductivity can be an advantage or not.Both composite and ceramic use is limited.lReinforced concrete.Though conventional reinforced concrete in simple sections
13、increases mass 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 linto almost any shape,needs no stress relieving,and can acc
14、ommodate fasteners and rails if inserts are used.However,it isnt as strong as metals and diffuses heat less efficiently.lDesigners 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
15、high-accuracy machine tools and grinders.lpolymer 高分子的高分子的 lceramic 陶瓷的陶瓷的lmatrix 基体基体ltailor 定制定制 lsurface roughness 表面粗糙度表面粗糙度lbrittle 易碎的易碎的lcrush 粉碎粉碎lgranite 花岗岩花岗岩lstress relieving 去应力退火去应力退火laccommodate 容纳容纳lfastener 紧固件紧固件lrail 导轨导轨linsert 嵌件嵌件lexpansion coefficient 膨胀系数膨胀系数lgrinder 磨床磨床lFou
16、ndationslFoundations 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 selecting a foundation,designers must consider the machineslweight,the forces it generates,accu
17、racy 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.lIf a small machine tool is rigid enough,it doesnt need the extra stiffness supplied by a base.For light loads,machines need
18、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 independentlmounting slab cast into the plant floor supports heavier machines.The most complex arrangement is a mounting slab isolated from the rest of
19、the shop with passive or active shock-isolation elements.lFoundation 基础基础lelastic mount 弹性安装钢板弹性安装钢板lisolate 隔离隔离lmounting slab 安装钢板安装钢板lcast into 浇注在浇注在larrangement 结构结构lshock-isolation 隔震隔震lFrame DesignlThe major considerations in frame design are loads,damping,apertures,heat distortion,and noise.
20、lLoads lUnderstanding 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.lThis includes the forces of acceleration,deceleration,tool action,irregular load
21、s caused by an unbalanced condition,or self-exciting loads from load and vibration interaction.laperture 开孔开孔lstatic 静态的静态的ldynamic 动态的动态的lunbalanced 不平衡的不平衡的lself-exciting load 自励负荷自励负荷linteraction 相互作用相互作用lDamping lThough frame material and design should handle damping,dampers are sometimes built
22、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 ldesigner 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 u
23、nder dynamic conditions.lApertures lEach 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.lThermo considerationslHeat from external or internal sources can be a major caus
24、e of error if the frame distorts.External sources include ambient shop conditions,cooling and lubricating media,and the sun.lThe 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 an
25、d kept constant.lNoiselReduction 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.ldamper 减震器减震器lmai
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