机械制图制图的基本知识和技能（1）

　　今天我们学习《机械制图》——制图的基本知识和技能第一部分的内容。Today we are studying the first part of Mechanical Drawing - Basic Knowledge and Skills of Drafting.
　　No. 1 比例
　　比例是指图中图形与其实物相应要素的线性尺寸之比。Proportion is the ratio of the linear dimensions of the figure in a drawing to the corresponding elements of its physical object.
　　No. 2 字体
　　在图样上，用文字和数字来说明机件的大小，技术要求和其他内容。
　　在图样中书写的字体必须做到：字体工整，笔画清楚，间隔均匀，排列整齐
　　汉字应写长仿宋体，并采用国家正式公布推行的简化字。汉字的高度不应小于3.5mm，其宽度一般为h/√2。数字和字母可写成斜体或直体，常用斜体。
　　On the drawing, use words and figures to illustrate the size, technical requirements and other contents of the machine.
　　The typeface written in the drawing must be: neat, clear, evenly spaced and neatly arranged
　　Chinese characters should be written in long imitation Song font and use simplified characters officially announced by the state. The height of the characters should not be less than 3.5mm and their width is generally h/√2. Numbers and letters can be written in italics or straight, commonly used in italics.
　　No. 3 图线
　　在机械图样中采用粗，细两种线宽，它们之间的比例为2：1。
　　机械图样中常用的有以下几种：1、实线，分粗实线，可用作可见轮廓线。细实线，用作尺寸线，尺寸界线，剖面线、引出线等。
　　2、虚线，分细虚线，用作不可见轮廓和棱边线等。粗虚线，用作表面处理的表示线。
　　3、点画线，分细点画线，用作轴线，对称中心等。粗点画线，限定范围表示线。
　　4、双点画线，极限位置轮廓 线，假想投影轮廓 线，中断线等。
　　5、双折线，用作断裂处的边界线。
　　6、波浪线，用作断裂处的边界线，视图与局部剖视的分界线。等等。
　　In the mechanical drawing using thick, thin two line widths, the ratio between them is 2:1.
　　The following are commonly used in mechanical drawings: 1. Solid lines, pided into coarse solid lines, can be used as visible contour lines. Fine solid line, used as a size line, size boundary line, section line, lead line, etc.
　　2、Dashed lines, pided into thin dashed lines, used as invisible contours and prismatic lines, etc. Coarse dashed lines, used as surface treatment lines.
　　3、Dotted lines, pided into fine dotted lines, used as axes, centres of symmetry, etc. The coarse dotted line is used as a limit line.
　　4、Double dotted lines, limit position contour lines, hypothetical projection contour lines, mid-break lines, etc.
　　5、Double dash line, used as a boundary line at the fracture.
　　6. Wavy lines, used as boundary lines for fractures, piding lines between views and partial sections. etc.
　　No. 4 尺寸标法
　　基本规则：
　　机件的真实大小应以图样上所注的尺寸数值为依据，与图线的大小及绘图的准确度无关。
　　图样中的尺寸，以mm为单位时，不需标注单位符号或名称，如采用其他单位，则应注明相应的单位符号。
　　图样中所标注的尺寸，为该图样所示机件的最后完工尺寸。
　　机件的每一尺寸，一般只标注一次。
　　尺寸组成：
　　尺寸是由：尺寸界线、尺寸起止符号、尺寸线及尺寸数字组成的。
　　尺寸界线：尺寸界线用细实线绘制，并应由图形的轮廓线、轴线或对称中心线处引出。也可利用轮廓线、轴线或对称中心线作尺寸界线。尺寸界线一般应与尺寸线垂直，并超出尺寸线的终端2mm左右。
　　尺寸线：尺寸线用细实线绘制，不能用其它图线代替，一般也不得与其它图线重合或画在其延长线上。在圆或圆弧上标注直径或半径尺寸时，尺寸线一般应通过圆心或延长线通过圆心。
　　尺寸数字：线性尺寸的数字一般应注写在尺寸线的上方，也允许注写在尺寸线的中断处。
　　Basic rule.
　　The true size of the machine should be based on the dimensional values noted on the drawing, independent of the size of the drawing line and the accuracy of the drawing.
　　The dimensions in the drawing, in mm, do not need to be marked with unit symbols or names; if other units are used, the corresponding unit symbols should be indicated.
　　The dimensions marked in the drawing are the final finished dimensions of the part shown in the drawing.
　　Each dimension of a machine part is normally marked only once.
　　Dimensional composition.
　　Dimensions are made up of: dimensional boundaries, dimensional start/stop symbols, dimensional lines and dimensional figures.
　　Dimensional boundaries: The dimensional boundaries are drawn with fine solid lines and should be drawn from the contour lines, axes or symmetrical centre lines of the drawing. It is also possible to use the outline, the axis or the symmetrical centre line for the dimensional boundaries. The dimensional boundary line should generally be perpendicular to the dimensional line and extend about 2mm beyond the end of the dimensional line.
　　Dimensional lines: dimensional lines are drawn with thin solid lines and cannot be replaced by other lines, nor can they generally overlap with other lines or be drawn on their extensions. When dimensioning diameters or radii on circles or arcs, the dimension line should generally pass through the centre of the circle or be extended through the centre of the circle.
　　Dimensional figures: the figures of linear dimensions should generally be written on the top of the dimensional line, but are also allowed to be written at the interruption of the dimensional line.
　　参考资料：文字：百度；图片：微博；翻译：百度翻译
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