机械工程专业英语文本施平03

第 3 次课的教学整体安排
2.教学内容与讨论、思考题、作业部分可合二为一。

3 Machine Tool Frames
机床床身
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.
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). The frame material 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). 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.
Pros and Cons 优缺点
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.
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 wall thickness and putting the metal where it's needed is fairly easy.
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. 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.
Welded steel. Machine builders fabricate steel frames from welded steel sections when casting is impractical. Because steel has a higher modulus(弹性模量), it is usually ribbed(加筋) to provide stiffness. The number of welds is a design tradeoff: with welding, it’s easy to make large sections and add features even 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 increase damping by circulating coolant through the welded structure or adding lead or sand to frame cavities.
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 to provide strength in specific axes(某个方向上).
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 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 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 fasteners and rails if inserts are used. However, it isn't as strong as metals and diffuses heat less efficiently.
Designers must consider the different expansion coefficients(膨胀系数) 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.
Foundations基础
Foundations ensure the machine's stiffness, shock absorption and isolation are secondary considerations. If the machine isn't stiff enough, the foundation may have to supply the stiffness necessary. In selecting a foundation, designers must consider the machine's weight, the forces it generates, accuracy requirements, and the loads being transmitted to the ground by adjacent machines. Soil condition(土壤条件) can be a problem because long-term changes(长期的变化) can influence machine stability.
If a small machine tool is rigid enough, it doesn't 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(弹性安装钢板) in the machine base(底座) helps isolate it from other forces in the plant or shop. An independent mounting slab cast into(浇注在) the plant floor supports heavier machines. The most complex arrangement(结构) is a mounting slab isolated from the rest of the shop with passive or active shock-isolation elements.
Frame Design
The major considerations in frame design are loads, damping,
apertures(开孔), heat distortion, and noise.
Loads.Understanding the static and dynamic loads a machine generates is essential. The basic load is static: the mass(质量) 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(自励负荷) from load and vibration interaction.
Damping. Though frame material and design should handle damping, dampers are sometimes built into(安装在) frame sections to handle specific(具体的) 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 conditions may do more harm than good under dynamic conditions.
Apertures.Each frame face should be solid, but the machine needs openings(孔) for assembly and maintenance. The designer balances aperture number and size against stiffness and strength requirements.
Thermo considerations. Heat 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(介质), 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 heating should be minimized and kept constant.
Noise. Reduction of noise for health and safety reasons is a fairly recent concern. Air turbulence from moving parts and fans can be a particular(突出的、棘手的) problem. Enclosures prevent sound transfer though the machine,
and sound damping materials(吸音材料) help reduce objectionable(有害的) sound.。