Linux教程(第2版)[孟庆昌等编著][电子教案]第5章

第一篇：Linux教程(第2版)[孟庆昌等编著][电子教案]第5章

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第5章 Linux内核简介 Linux内核简介

主要内容

Linux核心的一般结构 Linux核心的一般结构 ? 进程的概念、进程的调度和进程通信 ? 文件系统的构成和管理 ? 内存管理 ? 设备驱动及中断处理 5.1 概述

Linux系统大致可分为三层： Linux系统大致可分为三层：

靠近硬件的底层是内核，即Linux操作系统常驻内存部分。靠近硬件的底层是内核，即Linux操作系统常驻内存部分。? 中间层是内核之外的shell层，即操作系统的系统程序部分。中间层是内核之外的shell层，即操作系统的系统程序部分。? 最高层是应用层，即用户程序部分

从结构上看，Linux操作系统是采用单块结构的操作系统。从结构上看，Linux操作系统是采用单块结构的操作系统。? 一般说来，可以将操作系统划分为内核和系统程序两部分。

●进程控制系统用于进程管理、进程同步、进程通信、进程调度和内存

管理等。●内存管理控制内存分配与回收。●文件系统管理文件、分配文件空间、管理空闲空间、控制对文件的访问并为用户检索数据。●Linux系统支持三种类型的硬件设备：字符设备、块设备和网络设备。Linux系统支持三种类型的硬件设备：字符设备、块设备和网络设备。●核心底层的硬件控制负责处理中断以及与机器通信。5.2 进程管理

5.2.1 进程和线程的概念

1．进程及其状态 ? 简单说来，进程就是程序的一次执行过程。简单说来，进程就是程序的一次执行过程进程就是程序的一次执行过程。? 进程至少要有三种基本状态。这三种基本状态是：运行态、就绪态和

封锁态（或等待态）。

进程的状态可依据一定的条件和原因而变化 2．Linux进程状态 Linux进程状态 3．进程的模式和类型 ? 在Linux系统中，进程的执行模式划分为用户模式和内核 Linux系统中，进程的执行模式划分为用户模式用户模式和模式 ? 按照进程的功能和运行的程序来分，进程划分为两大类：按照进程的功能和运行的程序来分，进程划分为两大类：一类是系统进程，一类是系统进程，另一类是用户进程 4．Linux线程 Linux线程 Linux把线程定义为进程的“执行上下文” Linux把线程定义为进程的“执行上下文” ? 具有一段可执行的程序、专用的系统堆栈空间、私有的“线程控制块”（即thread_struct数据结构）线程控制块”（即thread_struct数据结构）

缺少自己的存储空间 5.2.2 进程的结构

1．task_struct结构 task_struct结构 ? task_struct结构包含下列几方面的信息： task_struct结构包含下列几方面的信息： ? ·进程状态 ? ·调度信息 ? ·标志符 ? ·内部进程通讯 ? ·链接信息 ? ·时间和计时器 ? ·文件系统 ? ·虚拟内存 ? ·处理器信息 2．进程系统堆栈 ? 每个进程都有一个系统堆栈，用来保存中断现场信息和进程进入内核

模式后执行子程序（函数）嵌套调用的返回现场信息。? 每个进程的系统堆栈和task_struct数据结构之间存在紧密联系，因而每个进程的系统堆栈和task_struct数据结构之间存在紧密联系，因而二者物理存储空间也连在一起

系统堆栈的大小静态确定，用户堆栈可在运行时动态扩展 5.2.3 对进程的操作 1．进程的创建

各个进程构成了树形的进程族系 ? 内核在引导并完成了基本的初始化以后，就有了系统的第

一个进程（即初始化进程，实际上是内核线程）。除此之外，所有其他的进程和内核线程都由这个原始进程或其子孙进程所创建。除初始化进程外，其他进程都是用系统调用fork()和除初始化进程外，其他进程都是用系统调用fork()和 clone()创建的。)创建的。

fork()是全部复制，而clone()有选择地复制)是全部复制

2．进程的等待 ? 父进程可用系统调用wait3()等待它的任一个子进程终止，也父进程可用系统调用wait3()等待它的任一个子进程终止，也可以用系统调用wait4()等待某个特定的子进程终止。可以用系统调用wait4()等待某个特定的子进程终止。wait3()算法如下：)算法如下：

（1）如果父进程没有子进程，则出错返回。（2）如果发现有一个终止的子进程，则取出子进程的进程号，把子进程的CPU使用时间等加到父进程上，释放子进程占用的把子进程的CPU使用时间等加到父进程上，释放子进程占用的 task_struct和系统空间堆栈，以供新进程使用。task_struct和系统空间堆栈，以供新进程使用。（3）如果发现有子进程，但都不处于终止态，则父进程睡眠，等待由相应的信号唤醒。

3．进程的终止 ? 进程可使用系统调用exit()终止自己进程可使用系统调用exit()终止自己 ? 其实现算法如下：（1）撤消所有的信号量。（2）释放其所有的资源，包括存储空间、已打开的文件、工作目录、信号处理表等。（3）置进程状态为“终止态”（TASK_ZOMBIE）。）置进程状态为“终止态” TASK_ZOMBIE）。（4）向它的父进程发送子进程终止的信号。（5）执行进程调度。

4．进程映像的更换 ? 改换进程映像的工作很复杂，是由系统调用execve()实现的，它用一个改换进程映像的工作很复杂，是由系统调用execve()实现的，它用一个可执行文件的副本来覆盖该进程的内存空间。

ELF可执行文件格式示意图 ELF可执行文件格式示意图

execve()系统调用的基本算法如下：)系统调用的基本算法如下：

（1）验证文件的可执行性，即用户有权执行它。（2）读文件头，检查它是一个可装入模块。（3）释放原有的内存空间。（4）按照可执行文件的要求分配新的内存空间，并装入内存。

5.2.4 进程调度

进程调度机制主要涉及到调度方式、调度时机和调度策略 1．调度方式

基本上采用“抢占式优先级”方式基本上采用“抢占式优先级”

2．调度策略——三种不同的调度策略调度策略——三种不同的调度策略 —— SCHED_FIFO——短实时进程，对时间性要求比较强 SCHED_FIFO——短实时进程，对时间性要求比较强 ? SCHED_RR——较长时间的实时进程，对应“时间片轮转法” SCHED_RR——较长时间的实时进程，对应“时间片轮转法” ? SCHED_OTHER——交互式的分时进程，这类进程的优先权取 SCHED_OTHER——交互式的分时进程，这类进程的优先权取

决于两个因素：一个因素是进程剩余时间配额；另一个是进程的优先数nice ——优先数越小，其优先级越高程的优先数nice ——优先数越小，其优先级越高

后台进程的优先级低于任何交互（前台）进程的优先级 3．调度时机（1）当前进程调用系统调用nanosleep()或者pause()，使）当前进程调用系统调用nanosleep()或者pause()，使自己进入睡眠状态，主动让出一段时间的CPU使用权。自己进入睡眠状态，主动让出一段时间的CPU使用权。（2）进程终止，永久地放弃对CPU的使用。）进程终止，永久地放弃对CPU的使用。（3）在时钟中断处理程序执行过程中，发现当前进程连续运行的时间过长。（4）当唤醒一个睡眠进程时，发现被唤醒的进程比当前进程更有资格运行。（5）一个进程通过执行系统调用来改变调度策略或者降低自身的优先权（如nice命令），从而引起立即调度。自身的优先权（如nice命令），从而引起立即调度。4．调度算法

5.2.5 shell基本工作原理 shell基本工作原理 ? ? ? ? ? ? 它不属于内核部分，而是在核心之外，以用户态方式运行。其基本功能是解释并执行用户打入的各种命令，实现用户与 Linux核心的接口。Linux核心的接口。5.3 文件系统

支持多种不同的文件系统，如：ext, FAT, ext2, ext3, MINIX, MS DOS, 支持多种不同的文件系统，如：ext, SYSV等。目前，Linux主要使用的文件系统是ext3 SYSV等。目前，Linux主要使用的文件系统是ext3。

5.3.1 ext2文件系统 ext2文件系统 ext2文件系统支持标准UNIX文件类型：普通文件、目录文 ext2文件系统支持标准UNIX文件类型：普通文件、目录文

件、特别文件和符号链接。

ext2文件系统可以管理特别大的分区。ext2文件系统可以管理特别大的分区。? ext2文件系统支持长文件名，最大长度为255个字符。ext2文件系统支持长文件名，最大长度为255个字符。? ext2文件系统为超级用户保留了一些数据块，约为5%。ext2文件系统为超级用户保留了一些数据块，约为5%。1．ext2文件系统的物理结构 ext2 2．块组的构造 ? 每个块组中包含超级块、组描述结构、块位图、索引节点位图、索引节点表

和数据块。

（1）超级块（Superblock））超级块（Superblock）? 超级块中包含有文件系统本身的大小和形式的基本信息。? 每个块组都一个超级块。? 超级块中包含:幻数、修订级别、安装计数和最大安装数、块组号码、数超级块中包含: 据块大小、每组数据块的个数、空闲块、空闲索引节点、第一个索引节点

（2）块组描述结构（Block Group Descriptor））块组描述结构（Block Descriptor）? 每个数据块组都有一个描述它的数据结构，即块组描述结构。? 包含以下信息：数据块位示图、索引节点位示图、索引节点表、空闲块数、空闲索引节点数和已用目录数。

3．索引节点（Inode）索引节点（Inode）? 索引节点又被称为I节点，每个文件都有惟一一个索引节点。ext2文件系统索引节点又被称为I节点，每个文件都有惟一一个索引节点。ext2文件系统

的索引节点起着文件控制块的作用，利用这种数据结构可对文件进行控制和管理。? 索引节点有两种形式：盘索引节点(如ext2_inode)和内存索引节点(如索引节点有两种形式：盘索引节点(ext2_inode)和内存索引节点(inode)。inode)。? 盘索引节点包括以下一些主要内容：（1）文件模式，描述文件属性和类型。（2）文件属主信息，包括文件主标志号和同组用户标志号。（3）文件大小，即文件的字节大小。（4）时间戳，包括索引节点建立的时间、最近访问时间、最后修改时等。（5）文件链接计数。（6）数据块索引表。利用多重索引表的结构存放指向文件数据块的指针。? 内存索引节点除了具有盘索引节点的主要信息外，还增添了反映该文件动态状态的项目 4．多重索引结构 5．ext2中的目录项 ext2 ? 当创建一个文件时，就构成一个目录项，并添加到相应的目录文件中。一个目录文件可以包含很多目录项，每个目录项(ext2文件系统的ext2_dir_entry_2)包含的信息有：录项(如ext2文件系统的ext2_dir_entry_2)包含的信息有： ●索引节点号 ●目录项长度 ●名字长度 ●文件类型 ●文件名字 6．位示图

利用一串二进位的值来反映该块组中数据块的分配情况，也称作位向

量(Bit Vector)法。Vector)法。? 设下列数据块是空闲的： ? 2，3，4，5，8，9，10，11，12，13，17，18，25，26，27，„„ 10，11，12，13，17，18，25，26，27，? 则块位示图的表示为：***100111111000„„ 则块位示图的表示为：***100111111000„„ 5.3.2 虚拟文件系统

1．VFS系统结构 VFS系统结构

VFS是建立在具体文件系 VFS是建立在具体文件系统之上的，它为用户程序提供一个统一的、抽象的、虚拟的文件系统界面。这个抽象的界面主要由一组标准的、抽象的有关文件操作构成，以系统调用的形式提供给用户程序

2.VFS超级块 2.VFS超级块 ? 每个安装的文件系统都有一个VFS超级块，其中包含以下每个安装的文件系统都有一个VFS超级块，其中包含以下主要信息： ●设备标识符 ●索引节点指针 ●数据块大小 ●超级块操作集 ●文件系统类型 ●文件系统的特殊信息 3．VFS索引节点 VFS索引节点 ? VFS中每个文件和目录都有一个且只有一个VFS索引节点 VFS中每个文件和目录都有一个且只有一个VFS索引节点

4．Linux文件系统的逻辑结构 Linux文件系统的逻辑结构

5．文件系统的安装与拆卸 ? 根文件系统一旦安装上，则在整个系统运行过程中是不能卸载的 ? 其他的文件系统（例如，由软盘构成的文件系统）可以根据需要（如从硬盘向软盘复制文件），作为子系统动态地安装到主系统中

6．VFS索引节点缓存和目录缓存 VFS索引节点缓存和目录缓存 ? 其基本思想是，VFS索引节点在数据结构上被链入不同的散列队列，其基本思想是，VFS索引节点在数据结构上被链入不同的散列队列，具有相同散列值的VFS索引节点在同一队列中。设置一个散列表，其具有相同散列值的VFS索引节点在同一队列中。设置一个散列表，其中每一项包含一个指向VFS索引节点散列队列的头指针。散列值是根中每一项包含一个指向VFS索引节点散列队列的头指针。散列值是根据文件系统所在块设备的标志符和索引节点号码计算出来的目录缓存也采用散列表的方法

进行管理。表中每一项都是一个指针，指向有相同散列值的目录缓存队列。散列值是利用文件系统所在设备的号码和目录名来计算的

7．数据块缓冲区 ? 采用多重缓冲技术，来平滑和加快文件信息从内存到磁盘的传输 ? 一个缓冲区由两部分组成：存放数据的缓冲区和一个缓冲控制块 ? 缓冲区和缓冲控制块是一一对应的。系统通过缓冲控制块来实现对缓

冲区的管理

所有处于“空闲”状态的buffer_head 都链入自由链中，它只有一条。所有处于“空闲”状态的buffer_head 具有相同散列值（是由设备的标志符和数据块的块号生成的）的缓冲区组成一条散列队列，可以有多个散列队列。5.4 内存管理

Linux系统采用了虚拟内存管理机制，就是交换和请求分 Linux系统采用了虚拟内存管理机制，就是交换和请求分

页存储管理技术 5.4.1 请求分页机制

1．分页概念 ? 逻辑空间分页

内存空间分页

页面和内存块的大小是由硬件确定的 ? 逻辑地址表示 ? 内存分配原则 ? 页表 2．请求分页的基本思想 ? 请求分页提供虚拟存储器 ? 在每一个页表项中增加一个状态位表示一个页面是否已装入内存块 ? 如果地址转换机构遇到一个具有N状态的页表项时，便产生一个缺页如果地址转换机构遇到一个具有N 中断

3．Linux的多级页表 Linux的多级页表

Linux进程的虚存空间 Linux进程的虚存空间 ? Linux系统采用三级页表的方式 Linux系统采用三级页表的方式

4．内存页的分配与释放 ? Linux系统采用两种方法来管理内存页：位图和链表 Linux系统采用两种方法来管理内存页：位图和链表 ? 页组中内存页的数量依次按2的倍数递增页组中内存页的数量依次按2 5.4.2 内存交换

内核的交换守护进程kswapd 内核的交换守护进程kswapd ：有自己的进程控制块task_struct结构，有自己的进程控制块task_struct结构，它与其他进程一样受内核的调度。但是，它没有自己独立的地址空间，只使用系统空间，所以也把它称为线程。它的任务就是保证系统中有足够的空闲内存页。? 当系统启动时，交换守护进程由内核的init（初始化）进程启动。被当系统启动时，交换守护进程由内核的init（初始化）进程启动。被定时唤醒。

所做的工作主要分为两部分：将若干不常用的活跃内存页面变为不活

跃状态；清理不活跃的“ 跃状态；清理不活跃的“脏”页面，或者回收一些内存页，使之成为空闲的内存页。? 作为交换空间的交换文件实际就是普通文件，但它们所占的磁盘空间必须是连续的 5.5 进程通信 5.5.1 信号机制

1．信号概念 ? 信号（signal，称为软中断）机制是在软件层次上对中断机制的一种信号（signal，称为软中断）机制是在软件层次上对中断机制的一种

模拟

该机构通常包括三部分：

（1）信号的分类、产生和传送。（2）对各种信号预先规定的处理方式。（3）信号的检测和处理。2．信号分类

? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 信号号码 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 17 19 30 符号表示 SIGHUP SIGINT SIGQUIT SIGILL SIGTRAP SIGIOT SIGBUS SIGFPE SIGKILL SIGUSR1 SIGSEGV SIGUSR2 SIGPIPE SIGALRM SIGTERM SIGCHLD SIGSTOP SIGPWR 含义远程用户挂断输入中断信号(Ctrl+C)输入中断信号(Ctrl+C)输入退出信号(Ctrl+ 输入退出信号(Ctrl+)非法指令遇到调试断点 IOT指令 IOT指令总线超时浮点异常要求终止进程(不可屏蔽)要求终止进程(不可屏蔽)用户自定义越界访问内存用户自定义管道文件只有写进程，没有读进程定时报警信号软件终止信号子进程终止进程暂停运行电源故障 3．进程对信号可采取的处理方式 ? 进程彼此间也可用系统提供的系统调用（如kill()）发送信号进程彼此间也可用系统提供的系统调用（如kill()）发送信号 ? 普通进程只能向具有相同uid和gid的进程发送信号或向相同进程组中普通进程只能向具有相同uid和gid的进程发送信号或向相同进程组中的其他进程发送信号

进程接到信号后，在一定时机（如中断处理末尾）做相应

处理，可采取以下处理方式：

（1）忽略信号。进程可忽略收到的信号，但不能忽略SIGKILL和）忽略信号。进程可忽略收到的信号，但不能忽略SIGKILL和 SIGSTOP信号。SIGSTOP信号。（2）阻塞信号。进程可以选择对某些信号予以阻塞。（3）由进程处理该信号。用户在trap命令中可以指定处理信号的程序，）由进程处理该信号。用户在trap命令中可以指定处理信号的程序，从而进程本身可在系统中标明处理信号的处理程序的地址。当发出该信号时，就由标明的处理程序进行处理。（4）由系统进行默认处理。如上所述，系统内核对各种信号（除用户自定义之外）都规定了相应的处理程序。在默认情况下，信号就由内核处理，即执行内核预定的处理程序。

4．对信号的检测和处理流程 ? 信号的检测与处理的过程如图所示。图中的①～⑤标出处理流程的顺

序。从图中可以看出，信号的检测是在系统空间中进行，而对信号的处理却是在用户空间中执行。

5.5.2 管道文件

一个管道线就是连接两个进程的一个打开文件 ? 由系统自动处理二者间的同步、调度和缓冲。管道文件允许两个进程

按先入先出（FIFO）的方式传送数据，而它们可以彼此不知道对方的按先入先出（FIFO）的方式传送数据，而它们可以彼此不知道对方的存在每个管道只有

一个内存页面用做缓冲区，该页面是按环型缓冲区的方式来使用的。Linux系统也支持 Linux系统也支持命名管道

5.5.3 System V IPC机制 IPC机制

Linux系统也支持UNIX System V版本中的三种进程间通信 Linux系统也支持UNIX V版本中的三种进程间通信 ? ? 机制，它们是：消息通信共享内存

信号量

5.6 设备管理 5.6.1 设备管理概述

所有设备都作为特别文件，从而在管理上就具有下列共性：（1）每个设备都对应文件系统中的一个索引节点，都有一个文件名。（2）应用程序通常可以通过系统调用open()打开设备文件，建立起与目标设）应用程序通常可以通过系统调用open()打开设备文件，建立起与目标设备的连接。（3）对设备的使用类似于对文件的存取。（4）设备驱动程序是系统内核的一部分，它们必须为系统内核或者它们的子系统提供标准的接口。（5）设备驱动程序利用一些标准的内核服务，如内存分配等。另外，大多数 Linux设备驱动程序都可以在需要时装入内核，不需要时卸载下来。Linux设备驱动程序都可以在需要时装入内核，不需要时卸载下来。

设备驱动的分层结构

5.6.2 设备驱动程序和内核之间的接口

1．可安装模块 ? 可安装模块是可以在系统运行时动态地安装和拆卸的内核模块，即经

过编译但尚未连接的目标文件（后缀为.o）。过编译但尚未连接的目标文件（后缀为.o）。? 设备驱动程序或者与设备驱动紧密相关的部分（如文件系统）都是利用可安装模块实现的。

在通常情况下，用户利用系统提供的插入模块工具和移走模块工具来

装卸可安装模块。

2．字符设备 ? 用户对字符设备的使用就和存取普通文件一样。在应用程序中使用标

准的系统调用来打开、关闭、读写字符设备。

3．块设备 ? 对块设备的存取与对文件的存取方式一样，其实现机制也与字符设备

使用的机制相同。

5.7 中断、异常和系统调用 5.7.1 中断处理

1．中断响应 ? 一般说来，中断响应顺序执行下述三步动作：

(1)中止当前程序的执行；(1)中止当前程序的执行；(2)保存原程序的断点信息（主要是程序计数器PC和程序状态寄存器(2)保存原程序的断点信息（主要是程序计数器PC和程序状态寄存器 PS的内容）； PS的内容）；(3)从中断控制器取出中断向量，转到相应的处理程序。(3)从中断控制器取出中断向量，转到相应的处理程序。

2．中断处理 ? 核心对中断处理的顺序主要由以下动作完成： ⑴ 保存正在运行进程的各寄存器的内容，把它们放入核心栈的新帧面中。⑵ 确定“中断源”或者核查中断发生，识别中断的类型确定“中断源”（如时钟中断或者是盘中断）和中断的设备号（如哪个磁盘引起的中断）。系统接到中断后，就从机器那里得到一个中断号，它是检索中断向量表的位移。中断向量因机器而异，但通常都包括相应中断处理程序入口地址和中断处理时处理机的状态字。⑶ 核心调用中断处理程序，对中断进行处理。⑷ 中断处理完成并返回。中断处理程序执行完以后，核心便执行与机器相关的特定指令序列，恢复中断时寄存器内容和执行核心栈退栈，进程回到用户态。如果设置了重调度标志，则在本进程返回到用户态时做进程调度。

5.7.2 系统调用

系统调用象普通C函数调用那样出现在C程序中。只发生在系统调用象普通C函数调用那样出现在C ? ? ? 用户空间。能把进程的状态从用户态变为核心态，Linux的系统调用能把进程的状态从用户态变为核心态，Linux的系统调用是通过中断指令“ 是通过中断指令“INT 0x80”实现的。Linux内核在系统调 0x80”实现的。Linux内核在系统调用时是通过寄存器而不是堆栈传递参数。所有的系统调用都集中在系统调用入口表中统一管理。系统调用入口表是一个函数指针数组（大小为256），以系统调用入口表是一个函数指针数组（大小为256），以系统调用号为下标在该数组中找到相应的函数指针，进而就能确定用户使用的是哪一个系统调用。当CPU执行到中断指令“INT 0x80”时，硬件就作出一系列 CPU执行到中断指令“ 0x80” 响应，其动作与上述的中断响应相同 5.8 网络系统

在Linux网络中，网络数据从用户进程传输到网络设备需要经历四个层次 5.8.1 socket 一个套接字就是与网络的一个连接 ? socket在逻辑上有三个特征（或要素）： socket在逻辑上有三个特征（或要素）：

（1）网域。它表明一个插口用于哪一种网络。（2）类型。它表明在网络中通信所遵循的模式。网络通信中有两种主要的模式，一种称为“有连接” 中有两种主要的模式，一种称为“有连接”模式，一种称为“无连接”模式。无连接”（3）协议。它表明具体的网络规程。

5.8.2 网络分层结构

第二篇：大学体验英语综合教程2 第三版 Unit 2 电子教案

Unit 2

Jobs and Careers Objectives:

★ first listen, and then learn to ask about job opportunities and go for job interviews

★ read about online job applications and dream jobs

★ write about how to find a potential job

★ practice the use of unreal conditions

★ write your own résumé

★ visit Culture Salon to learn to tell the difference between job and career

I Passage A

Your Dream Job: A Click Away 1.Summary

Since most students will choose to work after graduation, job-hunting is of great importance to them.They will write plenty of résumés about their education and send them to the companies they want to work for.It takes both time and energy to do so.Now, thanks to the development of the Internet, job-hunting has become much easier.Job-hunters can find a job just by clicking a mouse on the computer.Many of them make their dream come true through the Internet.This article describes the experiences of five people who used online sources to look for new jobs.Theresa Smith used the JOB-TRAK website to find an administrative assistant’s job at a university.Steven Tools used the CareerBuilder website to find a marketing management position.Madeline Gragg used Yahoo!to find a job teaching English in Japan.Nedzad Dozlic used a newspaper website to find a driver’s job with a car dealership.Wendy Mello used CareerBuilder to find a position in human resources for a media-information-services company.Mello also used another website to calculate the cost of living in her new location and to decide what salary to request.All these are typical examples of online job searches, since many different types of jobs can now be found this way.2.Language Points

1.career: a job or profession for which one is trained and which one intends to follow for the whole of one’s life

Examples:

There are many more careers open to women now than fifty years ago.Florence Nightingale made nursing her career.2.refer to: 1)send(someone or something)to(usually someone else)for decision or action Examples:

The Local Court has referred the whole case to the High Court.The dispute between the two countries was referred to the United Nations.2)mention;speak about Examples:

Don’t refer to the matter again.The speaker referred to his past experience.3.criteria:(pl.)an established rule, standard, or principle, on which a judgment is based Examples:

What criteria do you use when judging the quality of a student’s work?

There are several criteria of a good school.4.salary: fixed(usually monthly)pay for regular work Examples:

My father draws his salary at the end of every month.Don’t spend all of your salary.Try to put something away each month.5.resident: a person who lives(in a place)and is not just a visitor Examples:

The local residents were angry at the lack of parking spaces.The residents of the town are proud of its new library.6.come across: meet or discover, especially by chance Examples:

I have just come across a beautiful poem in this book.She came across some old letters in the course of her search.7.curious: eager to know or learn Examples:

The boy was curious about everything he saw.Miss Matfield threw a curious glance at her.8.fill out / in: put in(whatever is needed to complete something)Examples:

After Tom passed his driving test he filled out an application for his driver’s license.The policeman filled out a report of the accident.9.fill up: make or become completely full Examples:

The room soon was filled up with people.The rain has filled up the ditches again.10.available: able to be got, obtained, used, etc.Examples:

A limited number of seats are still available.There were no tickets available for Friday’s performance.11.elevate: make better, higher, or more educated Examples:

The clerk was elevated to a managerial position.The government is trying to elevate the living standards of the people.12.scan: look through quickly Examples:

He scanned the newspaper while having his breakfast.He scanned the articles that might give the information he needed.13.procedure: an action or set of actions necessary for doing something Examples:

Writing a check is quite a simple procedure.We have worked out a new set of procedures for using this machine.14.spot: pick out, recognize, see(one person or thing out of many)Examples:

He was the first to spot the danger.We spotted the winner of the beauty contest the moment she appeared.15.variety: number or group of different things Examples:

Everyone arrived late at the party for a variety of reasons.The college library has a wide variety of books.16.potential: that can or may come into existence or action Examples:

Although this area is very poor now, its potential wealth is great.We should always be on the lookout for potential dangers.17.learn of: become informed of Examples:

How did you learn of our product? Was it through our advertisement?

I learned of your new address from your parents.18.via: by means of;using Examples:

I’ve read this French play via an English translation.I sent a message to Mary via her sister.19.annual: of one year Examples:

The annual farm output was to be increased by 4 to 5 percent.Mr.White’s annual income is $36 000.20.financial: connected with money Examples:

In that case they would receive financial aid from the state.They are now confronted with a serious financial crisis.21.current: of present time Examples:

They suggested measures to overcome current difficulties.In some schools children study current affairs as a subject.22.flash: show for a moment Examples:

The news flashed on television.He flashed a $10 note at the man by the door.23.detailed: with a lot of facts given Examples:

He gave me a detailed account of his work.He kept a detailed diary of the meetings.3.Important sentences 1.The career placement center referred the liberal-arts major to JOB-TRAK, an Internet site listing 45 000 entry-level positions.The career placement center advises the student majoring in liberal arts to visit JOB-TRAK, a website containing 45 000 jobs for new workers.2.Smith is one American who clicked her way into a job.Smith is one American who found a job online.3.“The Internet is like hiring a personal assistant,” says Tools.“Effortlessly you can become aware of opportunities that may elevate your career.”

The Internet is like a personal helper.It presents you with information about possible chances to improve your career.4.Most major newspapers and trade publications have online versions of their classified listings, enabling job-seekers to scan for work available across town, in another state, or around the world.Most major newspapers and specialist journals have their job advertisements online, to help jobseekers quickly spot job vacancies anywhere in the world.5.A refugee of the war in Bosnia, Dozlic had had a variety of jobs but was now ready for something new.Dozlic, who escaped from the war in Bosnia, had done several different jobs but was now ready to find a new one.6.To find out more about the company, she clicked on to Artitron’s home page and that of its parent company, Ceridian Corp., where she reviewed an annual report and the company’s financial performance.To learn more about the company she entered its website and the website of its headquarters, and studied the company’s yearly report and financial situations.7.By accessing an online real estate service, she saw color photos of rental properties...Through an online real estate site, she saw pictures of homes which a person can rent rather than buy...4.Practical Writing

Reference Key

There are several things you cannot ignore when tracking down information on job openings.First, you should consult your friends, relatives, or neighbors, who may offer information unavailable elsewhere.Second, you should read the classified section in the newspapers often.Next, remember to register with two or three recruitment agencies.Finally, don’t forget to make use of the Internet.In today’s society the Internet is a quick and cost-effective method for employers to announce their employment needs.Therefore, if you log onto the Internet, you just might find your dream job.II Passage B

Dream Jobs: College Students Make Their Picks 1.Summary For recent college graduates, finding a job is a lot like finding a match when dating.In its annual survey of U.S.college students, the research firm Universum USA asked its more than 60 000 respondents to identify the characteristics they associate with dream employers, and broke down the results into seven personality types: careerists, entrepreneurs, explorers, harmonizers, hunters, idealists and leaders.Google continued to rank as the big favorite, taking the No.1 spot in five of the personality types and taking a top three spot in all seven.Apple was in the top three with six of the personality types, and Disney was in the top three for five, including idealists, who ranked it No.1 ahead of Google.Banks, oil companies, and the Big Four accounting firms lost some of their luster among business students, but government and nonprofit employers had a good year among careerists.Disney and Apple, perceived as offering dynamic, challenging work settings, gained ground in this year’s survey with several personality types.Petter Nylander, chief executive of Universum, says it’s clear that young people want employers who share their values and are a good fit for their personality.“What you see is young employees selecting companies that confirm their views of themselves,” he says.Language Points

1.personality: characteristics and qualities of a person seen as a whole Examples:

Parents have great influences on the development of a child’s personality.I’m deeply impressed both by your personality and talent.2.genuine: authentic;sincere Examples:

I was shocked to know that such an expensive painting was not genuine.Having been working here for 20 years, I do have a genuine love for the campus.3.connection: relationship Examples:

My conclusion is that there is no connection between the two incidents.We can see a clear connection between one’s attitude and one’s performance.4.arm candy:(informal)a sexually attractive person who accompanies another at social events Examples:

Tony usually had two pretty girls accompany him to parties as arm candy.She’d already had mini-roles in eight movies when she turned up as George Sanders’ arm candy in the party scenes of this film.5.survey: investigation using a pool or questionnaire Examples:

A recent survey found that 36% of the women asked did not feel safe walking alone at night.They ran a survey of the most popular television programs.6.identify: recognize or distinguish(as being the specified person or thing)Examples:

This is a course that teaches us to identify plants and flowers.Can you identify your umbrella among this lot?

7.characteristic: distinguishing feature Examples:

A person of your characteristic is sure to be popular with young people like college students.Generosity is one of his nicest characteristics.8.associate: connect in thought, memory or imagination Examples:

Whisky is usually associated with Scotland.People will always associate the name of Steve Jobs with Apple.9.break(something)down(into): be divided or divide into parts Examples:

Expenditure on the project breaks down as follows: raw materials $1m, equipment $2m, wages $2m.Water is broken down into hydrogen and oxygen.10.prestigious: having or bringing prestige;having high status Examples:

Yale University is one of the world’s most prestigious universities.Only the most prestigious scholars deserve such a title.11.recruit: enroll(someone)as a member or worker in an organization;enlist Examples:

The coach recruited nine boys for the baseball team.Our university recruited more staff this year than last year.12.assignment: a duty or piece of work that is given to someone to do Examples:

Five staff members have agreed to take on the one-year assignment in Africa.This is a difficult assignment for him, but he has made up his mind to complete it.13.balance: a state in which opposite or competing forces are evenly matched or given equal importance Examples:

She tried to keep her balance but still fell off the bike.Maintaining a favorable balance of trade is a difficult task for the country.14.secure: feeling safe, stable, and free from fear or anxiety Examples:

Many young people today don’t feel secure about their future.His position in the company is quite secure.15.employment: a person’s trade or profession Examples:

According to the report, 85% of the college graduates have found employment.The newly-built factory can provide employment for many people.16.competitive: of or involving competition;able to do as well as or better than others Examples:

This supermarket offers more competitive prices, so its business is very good.The country boasts a car industry that is competitive with any in the world.17.prospect: picture in the mind or imagination, especially of a future event;the possibility or likelihood of some future event occurring

Examples:

He is so excited by the prospect of having his own house in such a big city.When he thinks of the prospect of a holiday abroad, he feels motivated to work hard.18.earnings: money which is earned by working Examples:

The husband has to give most of his earnings to his wife every month.After a month’s work my earnings are just 1800 Yuan.19.ethical: of morals or moral questions;morally correct Examples:

This is an ethical issue that we should never neglect.His behavior is not ethical.20.designate: mark or point out clearly;choose(someone / something)for a special purpose Examples:

The boundaries between the two countries are designated on the map.The eastern part of the city has been designated as a development zone.21.overall: total;general Examples:

My overall impression of him is that he is very talkative.Can you tell me the overall cost of the project?

22.favorite: a person or thing that is loved above all others Examples:

Hollywood blockbusters are my favorites.He is a favorite with his grandfather.23.rank: give(someone or something)a rank or place within a grading system Examples:

Almost all the schools rank the students according to their grades.I rank Google higher than Apple.24.diversity: variety Examples:

There was a diversity of opinions on whether a nuclear plant should be built near the city.The tolerance of diversity is the key to the survival of the world in the 21st century.25.critical: of or at a crisis;decisive;crucial Examples:

The unemployment problem in that country is very critical at present.At the critical moment he jumped into the river and saved the boy.26.reflect: embody or represent(something)in a faithful or appropriate way Examples:

Her remarks at today’s meeting reflected her personality.The article reflected the author’s opinion on the matter.27.belief: something one accepts as true or real;a firmly held opinion or conviction Examples:

Nothing can shake my belief in his integrity and honesty.He has a strong belief in that country’s higher education.28.ethnicity: state of belonging to a social group that has a common national or cultural tradition Examples:

We welcome all talent, regardless of their background, gender and ethnicity.In this university you must learn to get along with people of different ethnicities.29.attractive: having the power to attract Examples:

The mountain is very attractive at this time of year.He has got three very attractive job offers.30.for starters: first of all, to start with Examples:

For starters, we don’t have enough time.He is not suitable for the task.For starters, he hasn’t got any experience in dealing with emergency.31.luster: soft brightness of a smooth or shining surface;glory, distinction Examples:

She couldn’t eat, and her hair lost its luster.Brave deeds add luster to one’s name

32.consequence: a result or effect of an action or condition;importance Examples:

Drunk driving can have very bad consequences, so make sure that you never violate the law.This is an issue of tremendous consequence for the country.33.in addition: as an extra person, thing, or circumstance Examples:

In addition, there is a severe drought in many provinces in the country.The international language school teaches English and mathematics in addition.34.account for: make up(a specified amount or proportion)Examples:

The local people just account for 1/3 of the workers in the factory.Food and clothing account for a large portion of the family’s monthly expenditure.35.category: a class or division of people or things regarded as having particular shared characteristics Examples:

The books fall into two categories: those of social sciences and those of natural sciences.We can divide the products of the company into three categories.36.appealing: attractive, charming, interesting Examples:

The film is not appealing to me.The actor’s voice makes him very appealing to his fans.37.achievement: something successfully finished or gained, especially through skill or hard work Examples:

The scientist was rewarded by the government for his great achievement in rice hybrids research.The successful launch and return of the manned spacecraft was a great achievement of our country’s manned space program.38.further: help the development of(something);promote Examples:

This is a good opportunity for you to further your career.The aim of this activity is to further our cause of helping the disabled.39.significant: of noticeable importance Examples:

The annual report of the company is significant for the public.Your success in this position will be significant for your future.40.network: interact with other people to exchange information and develop contacts, especially to further one’s career Examples:

He shows great skill in networking with people of different personalities.For your self-development, you should learn how to network with your boss.41.come in: finish in the stated place in a race or competition Examples:

George came in third in the 100-meter race.Where did our team come in — second or third?

42.likely: probable;that is expected Examples:

I think it likely that we will win the match.The likely outcome of the negotiation is a compromise made on both sides.43.as well as: in addition to Examples:

There are young trees as well as flowers in his garden.He is hard-working as well as intelligent.44.reputation: the beliefs or opinions that are generally held about someone or something Examples:

The doctor has a reputation for being very patient.The expert has lost his good reputation because he often speaks for the privileged.45.executive: a person with senior managerial responsibility in a business organization Examples:

He is one of the company’s senior executives.As the chief executive he spends much time on decision making.46.confirm: reinforce someone in(an opinion, belief, or feeling);make definite Examples:

The information provided by Mr.White confirmed my judgment of the company’s financial status.The couple’s joint statement in today’s newspaper confirmed the rumor that they had divorced each other.47.identity: the fact of being who or what a person or thing is Examples:

Show me your identity card, please.The reporter concealed his identity so that he could make the investigation smoothly.48.build(something)on(something): base(something)on(something);use something as a foundation for making further progress Examples:

Your accusation is not built on evidence.The company drew the conclusion by building on its own survey.2.Sentence Explanation 1.Everybody wants a genuine connection, and a little arm candy doesn’t hurt.Everybody wants a true and sincere relationship, a good match to their personality in locating a job, but it is not bad to have something to help make you look more appealing when “dating”.2.The employees must refl ect our users.Our employees must be people with a diversity of beliefs, background and ethnicity to refl ect our users, who are people of diverse cultural backgrounds.3.For starters, banks, oil companies, and the Big Four accounting fi rms lost some of their luster among business students.First of all, banks, oil companies, and the Big Four accounting firms are not as excellent and appealing as before in the eyes of business students.4.“What you see is young employees selecting companies that confirm their views of themselves,” he says.“What you see is young employees selecting companies that recognize and reinforce their views,” he says.5.“Your identity today is built on where you work.It sends a strong message about who you are.”

“Your identity today is recognized according to where you work.Where you work tells people a lot about your social position, your economic status, etc.”

III General Writing: Unreal Conditions To make predictions, you may find unreal conditional clauses useful.Unreal conditions are either impossible or unlikely to be realized.In unreal conditional sentences, a contrary-to-fact condition exists.Examples: Present Time:

If this was allowed to happen, the dam would collapse.If the dam collapsed, many acres of good farmland would disappear.Past Time:

If this had not been allowed to happen, the dam would not have collapsed.Past Time with Present or Future Result:

If they had passed the law, the economy would be in better shape now.

第三篇：机电专业英语第2版电子教案Unit 2

Unit 2 Machine Elements I.Lead in Information related to the text Do you know what is referred to as a machine element? Which elements are associated in pairs? Can you give an example? What is the most common machine element? How can the supporting structure be assembled? Why does the individual reliability of machine elements become the basis for estimating the overall life expectancy of a machine? Maybe you have seen and you are much interested in some machine elements, of course also interested in these questions.In this unit, you’ll learn some machine elements and know how they are widely used in industry.Now, let’s study new words and phrases together.II.Word Study 1.individual [,indi'vidjuəl] adj.个别的n.个人,个体 1）These styles can be adapted to suit individual tastes.这些式样均可改动以适应个人不同的爱好.2）Who was the individual champion? 谁是个人项目的冠军？

3）Am I a Business or Individual account? 我采用企业帐户还是个人帐户？ 2.dismantle [dis'mæntl] v.拆除．．．的设备, 分解

1）To dismantle or raze;tear down.拆除，毁坏：拆除，摧毁；拆毁 2）Before mounting, You can not dismantle the plug of port in case the dirty entering.装机前不得将各油口堵塞拆掉，以防脏物进入。

3）The pressure test is over.Now decrease the pressure , dismantle the pipe and check the inside of the pipe.压力试验通过了，把压力放掉，拆开管子，检查管子内部。3.perform [pə'fɔ:m] v.执行,表演,做

1）Lasers can be used to perform operations nowadays.现在激光可以用来做手术。

2）To perform operations on data in a process.对一个过程﹝进程﹞中的数据进行操作。

3）Able to assemble and disassemble molds and perform mold repair work as instructed.能够组装和拆卸注塑模具，并按规定进行模具维修工作。

4.associate [ə'səuʃieit] n.同伴, 伙伴

v.联合, 联想

adj.副的 1）I don't want to associate myself with them any more.我不愿再和他们交往了。

2）She associated happiness with having money.她把幸福和有钱联想到一起。

3）We should associate ourselves with the large firm.我们应该和这家大公司联合。

4）Medias associate tightly with the society, closes to personal life, people can touch the pulse of the community.传播媒介于社会联系密切，与人民生活息息相关，人们可感觉到社会的脉搏。

5.evolve [i'vɔlv] v.进展，进化，展开

1）The American constitution was planned;the British constitution evolved.美国宪法是精心制定的, 英国宪法是约定俗成的.2）No one can process and evolve for another.没有人能为别人推移和进化。

3）Your raising of consciousness through love and responsibility nourishes us, replenishes us, and expands our consciousness so that we may evolve further.通过爱和责任滋养我们、补充我们，我们意识提升，并且扩展我们的意识以便我们能进一步进化。

6.expectancy [ik'spektənsi] n.期待(公算)1）A quiver of expectancy ran through the audience.全场引颈以待, 群情鼎沸.2）Women have a higher life expectancy than men.女人比男人的预期寿命长.3）Greatly increased life expectancy and control board system reliability.大大提高控制线路板寿命和系统可靠性。

III．Language points

1.However simple, any machine is a combination of individual components generally referred to as machine elements or parts.译文：无论怎样简单的机床,都是由通称为机械零件或部件的单一元件组成的.说明：“be referred to as” means “be known as” or “be regarded as”(称为、叫作)，e.g.1)PMO can also be referred to as a “program management office, ” “project office, ” or "program office.项目管理办公室也可以叫作“计划管理办公室”或“计划办公室”。

2)Including credit cards, debit cards, online payment gateway and all other non-cash, to pay the bills, should be referred to as electronic payments.包括信用卡、借记卡、线上网关支付等一切非现金、票据支付的方式，都应该称为电子支付。

2.If a machine is completely dismantled, a collection of simple parts remains such as nuts, bolts, springs, gears, cams and shafts—the building block of all machinery.译文：如果把机床完全拆开, 就可以得到像螺母,螺栓,弹簧,齿轮,凸轮及轴等简单零件—所有机器的组合元件.说明：“dismantle” means “take sth.to pieces”(拆开, 拆除)

“a collection of ” means “a group of ” e.g.1)Installation & Dismantle：展台搭建和撤展，常简称为“I &D”。2)The new government dismantled their predecessors' legislation.新政府废除了前任政府的立法。

3)Trouble is, after the inspection, people take down the fences or dismantle the alarms.问题是，在通过检查后，人们会拆掉护网或警报器。

3.A machine element is, therefore, a single unit designed to perform a specific function and capable of combining with other elements.译文：因此，机械零件就是用来设计可以执行某一特定功能，且能与其它零件配合的单一元件。

分析：“designed to perform a specific function and capable of combining with other elements” is a past participle phrases used as attribute to modify “a single unit”, it has the same function as an attributive clause.说明：“perform” means “carry out”（执行，完成）e.g.1)When to perform update, progress input is control by process, not software.何时执行更新，进度输入是过程控制，不是软件控制。2)You can also perform a test.你也可以进行一个测试 3)Perform other tasks as required.完成要求的其他任务； “be capable of” means “be able to do”

A teacher must be capable of great patience.This is largely a matter of self-training.一名教师必须具有极大的耐心，在很大程度上这是个个人修养问题。4.The most common example of a machine element is a gear, which, fundamentally, is a combination of the wheel and lever to form a toothed wheel.译文：机器零件中最常见的是齿轮,它实际上是由轮子和杆组成的带

有齿的轮子。

分析：which introduces a nonfinite attributive clause to modify gear.The infinitive phrase “to form a toothed wheel” is used as adverbial to express result.5.Other fundamental machine elements have evolved from wheel and lever.译文：其他基本机械零件是由轮子和杆逐渐发展而来的.说明： “evolve from” means “develop from”

evolve [i'vɔlv] vt.发展，进化；进化；使逐步形成；推断出 vi.发展，进展；进化；逐步形成 e.g.1)Strategic goals evolve from the mission of the organization.策略的目标从组织的任务进展。

2)Great historical achievements often evolve from simple concepts.历史上的壮举，往往是从朴素的概念演变过来的。

3)No one can process and evolve for another.没有人能为别人推移和进化。

6.standardize ['stændədaiz] v.标准化，使合于标准

1)Should perfect administration further to hearing system, make it standardized and legal.应当进一步完善行政听证制度，使其规范化、法定化。

2)This paper aims at building different types of software fault models to manage and standardize these faults.这篇文章旨在建立不同类型的故障模型，以便于管理和标准化这些故障。

7.establish [is'tæbliʃ] v.建立，确立，创办

1)Our company was established in 1994.我们的公司成立于1994年。2)We wish to establish business relations with you.我们希望能和你们建立业务关系。

3)Prices branch, only prices branch has the right to establish the price.物价部门，只有物价部门有权利制定价格。

IV.Translating Skills

词义的确定

英汉两种语言都有一词多类、一词多义的现象。一词多类是指一个词往往属于多个词类，具有几种不同的意义。如display一词，既可做名词“显示（器）”，又可以作形容词“展览的、陈列用的”，还可做动词“显示、表现”等意思。一词多义是说同一个词类中，往往有几个不同的词义。如power 一词，做名词时，其汉语意思为“电力、功率、次方”等。在英汉翻译过程

中，我们在弄清句子结构后就要善于选择和确定句子中关键词的意义。选择和确定词义通常从以下几个方面入手：

一、根据词类确定词义

选择某个词的词义时，首先要确定这个词在句中应属于哪一种词类，然后再进一步确定其词义。下面以display为例：

Here, you have the option of defining your own display variants.这里，你有权定义你自己的显示变式。（display为名词）Often, it is best to display materials on an information table.通常，最好是把资料放在提供各类资讯的桌子上展示。（display为动词）

The reverse side of a control panel, display panel, or the like: the side with the interconnecting wiring.（display为形容词）

控制面板、显示面板或类似的面板的反面，即带有互连接线的那一面。

二、同一词类表达不同词义

英语中同一个词，同一类词，在不同的场合，往往具有不同的含义。此时，必须根据上下文的联系及整个句子的意思加以判断和翻译。例如as 这一连词：

Wear resistance improves as cutting tool hardness increase.当切削刀具的硬度越高时，其耐磨性就越好。（as引导时间状语从句）

As heat makes things move, it is a form of energy.因为热能使物体运动，所以热是能的一种形式。（as引导原因状语从句）

三、根据单词搭配情况确定词义

英译汉时，不仅必须根据上下文的联系理解词义，还需要根据词的搭配情况来理解词义。尤其在科技文献中，由于学科及专业不同，同一个词在不同的专业中具有不同的意义。比如：

The fifth power of two is thirty-two.二的五次方是三十二。（数学）With the development of electrical engineering, power can be transmitted over long distances.随着电气工程的发展, 电力能输送得非常远。（电学）Friction can cause a loss of power in every machine.摩擦能引起每一台机器功率的损耗。（物理学）

同一个词与不同的词搭配，可以产生不同的词义。如： make contact 闭合电路

make metals熔炼金属 make good

修理补偿

make a circuit接通电路

make up

补偿，配制

make over转让，移交

四、根据名词的单复数选择词义

英语中有些名词的单数或复数表达的词义完全不同。例如：名词

单数词义

复数词义

facility

简易，灵巧

设施，工具 charge

负荷，电荷

费用 spirit

精神

酒精 Although they lost, the team played with tremendous spirit.那队输是输了，但却表现了极其顽强的精神。Whisky, brandy, gin and rum are all spirits.威士忌、白兰地、杜松子酒和朗姆酒都是烈酒。

V.Key to Exercises of the text.I 1.A machine element is a single unit designed to perform a specific function and capable of combining with other elements.The most common machine element is a gear.2.Other fundamental machine elements have evolved from wheel and lever.3.In the automotive industry only fasteners, bearings, bushings, chains, and belts are standardized.II.1.F

2.T

3.T

4.T

5.T

6.F III.1.最常见的机械零件是齿轮，实际上是由轮子和杆组成的跌有齿的轮子。

2.齿轮的坚硬性决定了它的耐磨能力。

3.制造业的工程师们集中精力，研制标准化的零件。

4.这些零件是以大批量、高规格和降低成本的条件下生产的。Key to Exercises of the reading 1.The most important properties of cutting tools are hardness at high temperature, wear resistance, and impact strength.2.The hardness of the tool and the degree to which it remains its hardness at high temperature are important in the selection of a cutting-tool material.Because all metal cutting tools begin to lose hardness when heated to sufficiently high temperatures.3.The various materials from which most metal cutting tools are made can be classified under the following principal headings: Carbon tool steel，High-speed steel, Cast alloys, Cemented carbides, Ceramics, Diamonds.4.In order to detect the time when a cutting tool should be changed.VI.阅读材料参考译文

切削刀具

金属切削刀具必须具有许多不同的性能，才能在各种恶劣条件下切削不同的金属。为满足这些要求，制作刀具需要用许多种材料。

切削刀具的性能

切削刀具最重要的性能是在高温下能保持的硬度以及耐磨损耐冲击和韧性好。

当刀具进行切削时，刀具刃口处的压力和摩擦产生了高温。达到一定高的温度时，所有的金属切削刀具便开始软化。在刀口因高温而软化的同时，刀具刃口便开始磨耗并损坏。不同的切削刀具材料在不同的温度点开始软化。因此，刀具的硬度和它在高温下所能保持的硬度是选择切削刀具材料时的重要因素。

如果切削刀具的刃口和刀面耐磨，刀具就不易磨损。切削刀具的硬度越高，耐磨性就越好。

切削刀具还必须具有高强度才能抗振动和抗冲击。切削刀具材料的强度并不都是与硬度成正比的。有些硬的刀具材料由于太脆而缺乏强度。

切削刀具材料

大多数金屑切削刀具的制作材料可分为以下几大类：

碳素刀具钢；高速钢；铸合金；硬质合金；陶瓷；金刚石。

刀具寿命，即切削刀刃在需要重新打磨之前能加工的零件数，是生产某个零件或产品的重要成本因素。当切削刀具开始变钝时必须重新打磨。如果在这时还要继续用下去，刀具会很快地磨损。

要知道什么时候该换刀具，大部分现代机床都安装了指示器来显示切削加工所耗的功率。当刀具用钝时，加工需要消耗更多的功率，并由指示器显示出来。一旦发生这种情况就应当立即打磨刀具。

第四篇：大学体验英语综合教程2 第三版 Unit 4 电子教案

Unit 4

Calamities and Rescues Objectives:

★ first listen, and then talk about traffic accidents ★ read about calamities and rescues

★ write to describe how an airplane crash takes place ★ practice the use of subject clauses ★ write to apply for holiday insurance ★ visit Culture Salon for an introduction to the Red Cross

I Passage A

Death of a Dream

1.Summary

In 1961 the 18 members of the US figure skating team boarded a plane to travel to Belgium on their way to the world championships in Czechoslovakia.As the plane approached Brussels, the weather was good, but something was wrong with the plane.Twice it descended as if to land but pulled up and ascended again.The second time it exploded and crashed to the ground.All 72 people on the plane were killed and there were ten families that had lost at least two dear members.The crash site was a scene of total destruction.Later three pairs of melted skates were found dangling from one of the wings.The competition in Prague was canceled to honor the dead.Never before had such a terrible tragedy occurred in the sport of skating.2.Language Points 1.championship: a competition held to determine the champion;position of a champion Examples:

An American team won the pairs championships.They won the men’s and women’s singles championships respectively.2.beam: smile brightly and happily Examples:

He is beaming with delight.He beamed inside.3.dazzling: showing skill, qualities or beauty Examples:

She gave him a dazzling smile.She has a dazzling diamond.4.senior: older in years;higher in rank, authority, etc.Examples:

Mr.Gray is a senior officer in this bank.He is too senior to try for a young man’s job.5.bound: ready to start, having started(for)Examples:

They were on the New York express, bound for Maine.That ship is bound for South America.6.distress: a state of danger or great difficulty Examples:

If the storm continues on the mountain, the climber will be in distress by morning.The lifeboat went out to rescue a ship in distress.7.signal:

(n.)something intended to warn, command, or give a message Examples:

A red light is often used as a danger signal.American Indians used to occasionally send smoke signals.(v.)send a signal or signals to Examples:

The general signaled to his officers for the attack to begin.She was signaling wildly, waving her arms.8.contact: get in touch with somebody Examples:

I shall contact you by telephone.I must contact my lawyer before I make my fi nal decisions.9.lower: move or let down in height Examples:

Lowering the window shade will keep out the sun.He sat quite still, with his gaze lowered to the carpet.10.approach:(n.)movement towards or near to something Examples:

Our approach drove away the wild animals.With the approach of the Spring Festival the weather turned cold.(v.)come near or nearer Examples:

Walk softly as you approach the bed.I saw a figure approaching towards me.11.collision: an accident in which two or more people or vehicles hit each other while moving in different directions Examples:

The liner is reported to have had a collision with an oil tanker.The two cars were broken into pieces in the collision.12.in any case: whatever happens Examples:

In any case, I shall return in a day or two.In any case, I would insist upon your being paid.13.crash:

(n.)a violent vehicle accident Examples:

There have been a lot of crashes lately.All the passengers were killed in the plane crash.(v.)fall or strike suddenly, violently and noisily Examples:

I heard the dinner tray crash to the floor.Standing on the beach, I could hear the waves crashing against the rocks.14.rear: raise;lift up Examples:

A lion suddenly reared its head from among the tall grass.The skyscraper rears above the neighboring buildings.15.explode: burst or cause to burst violently and noisily Examples:

The boiler exploded and many people were injured by the hot steam.He pumped the ball up too much and it exploded.16.scatter: separate or cause to separate widely Examples:

A flock of birds scattered when the shot was fired.The government scattered the factories instead of concentrating them in a single area.17.stun: shock into helplessness Examples:

He was stunned by the unfairness of their judgment.She was stunned by the news of her father’s death 18.tragic: very sad;unfortunate Examples:

The tragic accident took eight lives.The driver of the car made a tragic mistake.19.comb through: search something thoroughly Examples:

The students spent many hours in the library, combing through old books looking for facts they wanted.He combed through the files searching for evidence of fraud.20.wreckage: the broken parts of a destroyed thing Examples:

After the accident, the wreckage of the cars was removed from the highway.The shore was covered with the wreckage of the destroyed ship

1.championship: a competition held to determine the champion;position of a champion Examples: