機械工程專業(yè)英語-第一章-正交投影與基本視圖

1、Unit1 Orthographic Projection and Basic Views,The Definition of ProjectionTwo Kinds of Projection MethodsBasic views,I. The Definition of Projection,When an object is in the sun or under the light, there will be shad

2、ow on the floor or on the wall. This phenomenon is called projection. Through scientific studies and summarization, the relationship between shadows and objects is discovered, thus creating the projection methods.,,As sh

3、own in figure 1-1, suppose point S is the centre of projection. A projective line going through point S and a spatial point A intersects the projection plane P at point a. Point a is called the projection of the spatial

4、point A on projection plane P. Similarly, point b and c are respectively the projections of point B and C. Therefore, the projection of a point is still a point.,Figure 1-1 Definition of Projection,,When points a, b ,c a

5、re connected by lines, then triangle △abc becomes the projection of the spatial triangle △ABC on the projection plane P (see figure 1-2).,Figure 1-2 Perspective Projection,II. Two Kinds of Projection Methods,Perspective

6、projectionIf projective lines are all from the same point—the center of projection, the projection drawing of the object on the projection plane is then called perspective projection (See figure 1-2).,,As shown in figu

7、re 1-2, the size of the projection triangle △abc changes according to the distance between the projection center point S and the spatial triangle △ABC or the distance between the spatial triangle △ABC and the projection

8、plane P. Therefore, perspective projection does not show the original size of an object.,2. Parallel projectionParallel projection can be regarded as a special case of perspective projection. Suppose the projection cen

9、ter S is at infinity, the projection lines then can be viewed as parallel. Therefore, by drawing the parallel lines on the projection plane, the object can be drawn out. This method is called parallel projection. (The me

10、thod of drawing the parallel lines on the projection plane in order to draw out the object is called parallel projection.) (See figure 1-3),,In engineering drawings, the principles of perspective projection are often use

11、d for scenograph. Creating vivid and concrete visual images, this method is often used for drawing buildings. However, due to the complexity of drawing and poor measurement, it is seldom adopted in mechanical drawings.,B

12、ecause projection lines are paralleled, even if the distance between the object and the projection plane is changed, the shape and size of the projection remain unchanged. Oblique projection—the projective lines interse

13、ct the projection plane at an oblique angle to produce the projected image, shown in Figure 1-3 (a)Orthographic projection—the projective lines intersect the projection plane at a perpendicular angle to produce the proj

14、ected image, shown in Figure 1-3(b),As shown in Figure 1-4, when several projection planes intersect one another and form right angles, the resulted projection drawings are called multi-planed orthographic projection. Th

15、ey are also called orthographic drawing or orthographic projection in brief.,Orthographic projection has quite a few advantages. It can fully and truly reflect the shape and size of an object with good measurement; and t

16、he drawing is simple. Therefore, it is the most widely used drawing method in mechanical engineering, which is also the focus of this course. The drawings of machine parts by projection are called views. Views just show

17、 the visible parts of the work; the invisible part is drawn out only when it is necessary.,III. Basic views,According to the regulations of the national standard Mechanical Drawing, the six views resulted from the projec

18、tions of the machine parts on six basic planes are called basic views. As shown in Figure 1-5(a), these six basic planes are the surfaces of a regular hexahedron, the method of drawing used here is orthographic projectio

19、n and the first-angle projection (the machine parts are between the observer and the corresponding projection plane). As shown in Figure 1-5(b), the front plane remains unchanged after projection and the other projection

20、 planes are unfolded onto the same surface as the front plane. The arrangement of the basic views is shown in Figure 1-5(c).,Basic views and the directions of projection are specified as follows:The main view----project

21、ed from the front to the back.The left-side view---- projected from the left to the right; placed on the right to the main view. The right-side view---- projected from the right to the left; placed on the left to the m

22、ain view.The top view---- projected from the top to the bottom; placed above the main view.The bottom view---- projected from the bottom to the top; placed below the main view.The rear view---- projected from the back

23、 to the front; placed on the right to the left-side view.,That national standard stipulates six basic views does not mean that all machine parts should be shown by six basic views. On the contrary, the number of views sh

24、ould be as small as possible on the condition that all parts of the work can be fully and clearly shown.In addition to six basic views, national standard also stipulates other ways of drawings such as oblique view, part