机身材料具有消除或阻止振动（oscillation）传递的 性能。以减轻降低精度和刀具寿命的振荡。它必须面 对不利的车间环境，包括冷却液和润滑剂。
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Material expansion must be understood (in order) to minimize forces needed to move slides. The material must not build up(聚集) too much heat, must retain its shape for its lifetime, and must be dense enough to distribute forces throughout the machine.
材料必须能够抵抗变形和断裂。硬度必须与其弹性相 平衡。机身必须能承受冲击且在载荷作用下不能裂al must eliminate or block vibration transmission to reduce oscillations(震 荡) that degrade accuracy and tool life(降低精 度和刀具寿命). It must withstand the hostile(恶 劣的) shop-floor environment, including the newer coolants(冷却液) and lubricants(润滑剂).
上一课回顾
单词复习： Lubrication wear weld viscosity Friction contaminant corrosion Dampen sealing asperity 边界润滑 绝缘体 黄油
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Lesson 18 Machine Tool Frames 机床床身
材料的膨胀不能妨碍滑动部件的运动。在正常工作期 间材料不能产生太多的热量，应能保护其形状，且具 有足够大的密度以支承机床的其它部件。
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Pros and Cons 优缺点 Paragraph 3:
Either castings or welded sections(焊接结构) can be used in most applications. The decision on which is best depends on the costs in a given(一定的) design situation.
铸件和焊接件应用都很广泛，哪种形式最好由给定条 件下的成本来决定。
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Paragraph 4:
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-toweight ratio and good damping qualities. Modifying wall thickness and putting the metal where it's needed is fairly easy.
重点词汇： frame/bed impact/yield/hostile/dense volume fabricate impractical/practical dramatically mass/granite/diffuse
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Paragraph 1
The frame is a machine's fundamental element. Most frames are made from cast iron, welded steel(可焊接的结构钢), composite, or concrete. The following factors govern(决定) material choice.
机身是机器的基础部件。大多数机身是铸铁（cast iron）、焊接结构钢、复合材料、混凝土 （concrete）材料制作而成。材料选择由下列因素 来决定。
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Paragraph 2:
The material must resist deformation and fracture. Hardness must be balanced against elasticity(必须解决硬度合弹性之间的矛盾). The frame must withstand impact, yet yield(产生) under load without cracking(裂纹) or permanently deforming(永久性变形).
尽管铸铁是一种便宜的材料，但是每个铸件都需要一 个铸型。铸型要用螺栓紧固联接，且铸件需要退火处 理使得大尺寸铸件受到限制，这些条件也使得大尺寸 铸件制作困难，花费大。
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Smaller, high-volume(大批量的) machines usually have cast iron frames because they more easily absorb pattern cost. Welded frames may be cheaper for lower volume machines.
铸铁：几乎所有传统机床的床身都是由铸铁制成，因 为其他方式难以获得的形状都能铸造而成。铸件有好 的刚度与重量比和好的减震(damp)性能。各种壁厚 都容易浇注而成。
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Paragraph 5
Although cast iron is a fairly cheap material, each casting requires a pattern(模型). Larger sizes are a limiting factor because of pattern cost, problems with bolted joints, and the need to anneal(将…退火) castings, which is difficult and costly with larger sections.