今天我们学习《机械制图》——点，直线，平面的投影第二部分的内容。今天的主要内容是直线的投影。Today we are studying Mechanical Drawing - Projection of Points, Lines and Planes Part 2. Today's main topic is the projection of a Straight line.
No.1 直线及直线上的点的投影特性
一.直线及直线上的点的投影特性
1.不垂直于投影面的直线的投影，仍为直线；垂直于投影面的直线的投影，积聚成一点。
2.直线上点的投影，必在直线的同面投影上；不垂直于投影面的直线段上的点，分割直线段之比，在投影图上仍保持不变。
I. Projection properties of straight lines and points on straight lines
1. The projection of a straight line not perpendicular to the projection plane remains a straight line; the projection of a straight line perpendicular to the projection plane accumulates into a point.
2. The projection of a point on a straight line must be on the same projection of the line; the ratio of the points on a straight line segment not perpendicular to the projection plane to the segment of the line remains the same on the projection diagram.
No.2直线对投影面的各种相对位置
二.直线对投影面的各种相对位置
1.直线按对投影面的相对位置可分为三类，分别是一般位置直线，投影面平行线和投影面垂直线。后两类又可再分为三类，分别是正平线，水平线，测平线；正垂线，铅垂线，侧垂线。
2.直线与它的水平投影，正面投影，侧面投影的夹角，分别称为该直线对投影面H，V，W的倾角α ，β，γ。当直线平行于投影面时，倾角为0°；垂直于投影面时，倾角为90°；倾斜于投影面时，则倾角大于0°，小于90°。
II. The various relative positions of straight lines to the projection plane
1. Straight lines can be divided into three categories according to their relative position on the projection plane: general position lines, parallel lines on the projection plane and perpendicular lines on the projection plane. The latter two categories can be subdivided into three categories, namely, orthogonal, horizontal and level lines; and orthographic, plumbline and lateral plumb lines.
The angle between a line and its horizontal, frontal and lateral projections is called the inclination of the line to the projection plane H, V and W. When the line is parallel to the projection plane, the inclination is 0°; when it is perpendicular to the projection plane, the inclination is 90°; when it is inclined to the projection plane, the inclination is greater than 0° and less than 90°.
No.3两直线的相对位置
三.两直线的相对位置
1.两直线的相对位置有三种：平行，相交，交叉（既不平行，又不相交，即不在同一平面上，称这两直线不共面，亦称异面）
2.平行：三对同面投影都分别互相平行。
3.相交：三对同面投影都分别相交，且投影的交点符合一点的三面投影特性。
4.交叉：既不符合平行两直线的投影特性，又不符合相交两直线的投影特性。
III. The relative positions of two lines
1. There are three types of relative positions of two lines: parallel, intersecting, and cross (neither parallel nor intersecting, i.e. not in the same plane, the two lines are said not to be coplanar, also known as heterogeneous)
2. parallel: each of the three pairs of congruent projections is parallel to the other.
3. intersect: all three pairs of congruent projections intersect each other, and the intersection of the projections conforms to the three-sided projection property of a point.
4. Intersecting: the projection properties of neither parallel nor intersecting pairs of lines.
参考资料：文字：百度；图片：微博；翻译：百度翻译
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